Anyone know about peltier devices? Can you drive them at half voltage?
I wish to make a container that is limited to ideally between 18 and 20
centigrade and apparently they do not like to be cycled much for temp
control,I would rather have it on constantly but at less than full
capabilities.
On further musing I would probably still have problems as the system is
dependent on ambient temp so I would probably have to have a temp sw to
switch on only when ambient is higher.
Anyone have ideas on how to keep between those temps using peltier?
It is for storing chocolate so it could probably be alright at lower
temps but not higher.

On 17/12/2015 01:08, F Murtz wrote:
Anyone know about peltier devices? Can you drive them at half voltage?
I wish to make a container that is limited to ideally between 18 and 20
centigrade and apparently they do not like to be cycled much for temp
control,I would rather have it on constantly but at less than full
capabilities.
On further musing I would probably still have problems as the system is
dependent on ambient temp so I would probably have to have a temp sw to
switch on only when ambient is higher.
Anyone have ideas on how to keep between those temps using peltier?
It is for storing chocolate so it could probably be alright at lower
temps but not higher.

Without looking anything up, from memory the cooling effect is dependent
on the current. There is a potential barrier to overcome sort of akin to
a diode. However they are rather lossy, with a high series resistance
where I2R losses can be significant.

Most decent peltier devices come with a series of graphs to give you
cooling capacity for a specific temperature difference across the device
for a specific I/V drive.

Is there one you have in mind?

Fredxxx wrote:
On 17/12/2015 01:08, F Murtz wrote:
Anyone know about peltier devices? Can you drive them at half voltage?
I wish to make a container that is limited to ideally between 18 and 20
centigrade and apparently they do not like to be cycled much for temp
control,I would rather have it on constantly but at less than full
capabilities.
On further musing I would probably still have problems as the system is
dependent on ambient temp so I would probably have to have a temp sw to
switch on only when ambient is higher.
Anyone have ideas on how to keep between those temps using peltier?
It is for storing chocolate so it could probably be alright at lower
temps but not higher.

Without looking anything up, from memory the cooling effect is dependent
on the current. There is a potential barrier to overcome sort of akin to
a diode. However they are rather lossy, with a high series resistance
where I2R losses can be significant.

Most decent peltier devices come with a series of graphs to give you
cooling capacity for a specific temperature difference across the device
for a specific I/V drive.

Is there one you have in mind?

Thinking of this, only a few $AU on ebay

12V 60W TEC1-12706
12v 60w

"F Murtz" wrote in message
b.com...
Anyone know about peltier devices? Can you drive them at half voltage?
I wish to make a container that is limited to ideally between 18 and 20
centigrade and apparently they do not like to be cycled much for temp
control,I would rather have it on constantly but at less than full
capabilities.
On further musing I would probably still have problems as the system is
dependent on ambient temp so I would probably have to have a temp sw to
switch on only when ambient is higher.
Anyone have ideas on how to keep between those temps using peltier?
It is for storing chocolate so it could probably be alright at lower temps
but not higher.

Donkeys years back I did something similar using a homebrew proportional
controller. As I remember, very simply -
Feed a diff amp with (1) actual (sensor) temp and (2) a required temp with
perturbation e.g, a 1Hz triangular waveform, value set by the hysteresis you
want. If the actual temp is high you hit the peltier with full whack, if the
temp is low you hit the peltier full whack in reverse (+ & - supply). Temp
settles somewhere up/down the triangle.

On Thursday, 17 December 2015 02:59:08 UTC, F Murtz wrote:
Fredxxx wrote:
On 17/12/2015 01:08, F Murtz wrote:
Anyone know about peltier devices? Can you drive them at half voltage?
I wish to make a container that is limited to ideally between 18 and 20
centigrade and apparently they do not like to be cycled much for temp
control,I would rather have it on constantly but at less than full
capabilities.
On further musing I would probably still have problems as the system is
dependent on ambient temp so I would probably have to have a temp sw to
switch on only when ambient is higher.
Anyone have ideas on how to keep between those temps using peltier?
It is for storing chocolate so it could probably be alright at lower
temps but not higher.

Without looking anything up, from memory the cooling effect is dependent
on the current. There is a potential barrier to overcome sort of akin to
a diode. However they are rather lossy, with a high series resistance
where I2R losses can be significant.

Most decent peltier devices come with a series of graphs to give you
cooling capacity for a specific temperature difference across the device
for a specific I/V drive.

Is there one you have in mind?

Thinking of this, only a few $AU on ebay

12V 60W TEC1-12706
12v 60w

Yes I've messed with one of those, I used an arduino PWM output which runs at about 500HZ I changed the pulse width to change the amount of cooling or heating. Using a TP31 transitor.
I used a CPU cooler on the other side as I found that I couldn;t get much cooling or heating for any lengh of time without finding a way to 'disipated' the heat or cold from the other side of the device.

On 17/12/2015 12:58, whisky-dave wrote:
On Thursday, 17 December 2015 02:59:08 UTC, F Murtz wrote:
Fredxxx wrote:
On 17/12/2015 01:08, F Murtz wrote:
Anyone know about peltier devices? Can you drive them at half voltage?
I wish to make a container that is limited to ideally between 18 and 20
centigrade and apparently they do not like to be cycled much for temp
control,I would rather have it on constantly but at less than full
capabilities.
On further musing I would probably still have problems as the system is
dependent on ambient temp so I would probably have to have a temp sw to
switch on only when ambient is higher.
Anyone have ideas on how to keep between those temps using peltier?
It is for storing chocolate so it could probably be alright at lower
temps but not higher.

Without looking anything up, from memory the cooling effect is dependent
on the current. There is a potential barrier to overcome sort of akin to
a diode. However they are rather lossy, with a high series resistance
where I2R losses can be significant.

Most decent peltier devices come with a series of graphs to give you
cooling capacity for a specific temperature difference across the device
for a specific I/V drive.

Is there one you have in mind?

Thinking of this, only a few $AU on ebay

12V 60W TEC1-12706
12v 60w

Yes I've messed with one of those, I used an arduino PWM output which runs at about 500HZ I changed the pulse width to change the amount of cooling or heating. Using a TP31 transitor.
I used a CPU cooler on the other side as I found that I couldn;t get much cooling or heating for any lengh of time without finding a way to 'disipated' the heat or cold from the other side of the device.

You shouldn't PWM a Peltier device directly, at least not if you're
using it for cooling. Cooling is proportional to current, but the
unwanted ohmic heating is in proportion to current squared (I*I*R).
Think about it - a 50% PWM at 1A peak in a 2ohm device will produce 1W
ohmic heating (I*I*R for half the time) whereas a continuous (100%) 0.5A
would produce 0.5W ohmic heating.

You can of course use PWM indirectly if you filter it to give a fairly
steady current.

Cheers
--
Syd

On Thursday, 17 December 2015 14:22:54 UTC, Syd Rumpo wrote:
On 17/12/2015 12:58, whisky-dave wrote:
On Thursday, 17 December 2015 02:59:08 UTC, F Murtz wrote:
Fredxxx wrote:
On 17/12/2015 01:08, F Murtz wrote:
Anyone know about peltier devices? Can you drive them at half voltage?
I wish to make a container that is limited to ideally between 18 and 20
centigrade and apparently they do not like to be cycled much for temp
control,I would rather have it on constantly but at less than full
capabilities.
On further musing I would probably still have problems as the system is
dependent on ambient temp so I would probably have to have a temp sw to
switch on only when ambient is higher.
Anyone have ideas on how to keep between those temps using peltier?
It is for storing chocolate so it could probably be alright at lower
temps but not higher.

Without looking anything up, from memory the cooling effect is dependent
on the current. There is a potential barrier to overcome sort of akin to
a diode. However they are rather lossy, with a high series resistance
where I2R losses can be significant.

Most decent peltier devices come with a series of graphs to give you
cooling capacity for a specific temperature difference across the device
for a specific I/V drive.

Is there one you have in mind?

Thinking of this, only a few $AU on ebay

12V 60W TEC1-12706
12v 60w

Yes I've messed with one of those, I used an arduino PWM output which runs at about 500HZ I changed the pulse width to change the amount of cooling or heating. Using a TP31 transitor.
I used a CPU cooler on the other side as I found that I couldn;t get much cooling or heating for any lengh of time without finding a way to 'disipated' the heat or cold from the other side of the device.

You shouldn't PWM a Peltier device directly, at least not if you're
using it for cooling. Cooling is proportional to current, but the
unwanted ohmic heating is in proportion to current squared (I*I*R).
Think about it - a 50% PWM at 1A peak in a 2ohm device will produce 1W
ohmic heating (I*I*R for half the time) whereas a continuous (100%) 0.5A
would produce 0.5W ohmic heating.

You can of course use PWM indirectly if you filter it to give a fairly
steady current.

Well I haven't seen any info that says I can't run it from PWM.
There's sources that say don;t run it in the 10s of Hz .

On 17/12/2015 15:38, whisky-dave wrote:
On Thursday, 17 December 2015 14:22:54 UTC, Syd Rumpo wrote:
On 17/12/2015 12:58, whisky-dave wrote:
On Thursday, 17 December 2015 02:59:08 UTC, F Murtz wrote:
Fredxxx wrote:
On 17/12/2015 01:08, F Murtz wrote:
Anyone know about peltier devices? Can you drive them at half voltage?
I wish to make a container that is limited to ideally between 18 and 20
centigrade and apparently they do not like to be cycled much for temp
control,I would rather have it on constantly but at less than full
capabilities.
On further musing I would probably still have problems as the system is
dependent on ambient temp so I would probably have to have a temp sw to
switch on only when ambient is higher.
Anyone have ideas on how to keep between those temps using peltier?
It is for storing chocolate so it could probably be alright at lower
temps but not higher.

Without looking anything up, from memory the cooling effect is dependent
on the current. There is a potential barrier to overcome sort of akin to
a diode. However they are rather lossy, with a high series resistance
where I2R losses can be significant.

Most decent peltier devices come with a series of graphs to give you
cooling capacity for a specific temperature difference across the device
for a specific I/V drive.

Is there one you have in mind?

Thinking of this, only a few $AU on ebay

12V 60W TEC1-12706
12v 60w

Yes I've messed with one of those, I used an arduino PWM output which runs at about 500HZ I changed the pulse width to change the amount of cooling or heating. Using a TP31 transitor.
I used a CPU cooler on the other side as I found that I couldn;t get much cooling or heating for any lengh of time without finding a way to 'disipated' the heat or cold from the other side of the device.

You shouldn't PWM a Peltier device directly, at least not if you're
using it for cooling. Cooling is proportional to current, but the
unwanted ohmic heating is in proportion to current squared (I*I*R).
Think about it - a 50% PWM at 1A peak in a 2ohm device will produce 1W
ohmic heating (I*I*R for half the time) whereas a continuous (100%) 0.5A
would produce 0.5W ohmic heating.

You can of course use PWM indirectly if you filter it to give a fairly
steady current.

Well I haven't seen any info that says I can't run it from PWM.

You just did, see above. Apart from that, you'd need to look. If only
there were some way of searching the Web for key phrases such "PWM Peltier".

There's sources that say don;t run it in the 10s of Hz .

Cheers
--
Syd

On Thursday, 17 December 2015 16:00:16 UTC, Syd Rumpo wrote:
On 17/12/2015 15:38, whisky-dave wrote:
On Thursday, 17 December 2015 14:22:54 UTC, Syd Rumpo wrote:
On 17/12/2015 12:58, whisky-dave wrote:
On Thursday, 17 December 2015 02:59:08 UTC, F Murtz wrote:
Fredxxx wrote:
On 17/12/2015 01:08, F Murtz wrote:
Anyone know about peltier devices? Can you drive them at half voltage?
I wish to make a container that is limited to ideally between 18 and 20
centigrade and apparently they do not like to be cycled much for temp
control,I would rather have it on constantly but at less than full
capabilities.
On further musing I would probably still have problems as the system is
dependent on ambient temp so I would probably have to have a temp sw to
switch on only when ambient is higher.
Anyone have ideas on how to keep between those temps using peltier?
It is for storing chocolate so it could probably be alright at lower
temps but not higher.

Without looking anything up, from memory the cooling effect is dependent
on the current. There is a potential barrier to overcome sort of akin to
a diode. However they are rather lossy, with a high series resistance
where I2R losses can be significant.

Most decent peltier devices come with a series of graphs to give you
cooling capacity for a specific temperature difference across the device
for a specific I/V drive.

Is there one you have in mind?

Thinking of this, only a few $AU on ebay

12V 60W TEC1-12706
12v 60w

Yes I've messed with one of those, I used an arduino PWM output which runs at about 500HZ I changed the pulse width to change the amount of cooling or heating. Using a TP31 transitor.
I used a CPU cooler on the other side as I found that I couldn;t get much cooling or heating for any lengh of time without finding a way to 'disipated' the heat or cold from the other side of the device.

You shouldn't PWM a Peltier device directly, at least not if you're
using it for cooling. Cooling is proportional to current, but the
unwanted ohmic heating is in proportion to current squared (I*I*R).
Think about it - a 50% PWM at 1A peak in a 2ohm device will produce 1W
ohmic heating (I*I*R for half the time) whereas a continuous (100%) 0.5A
would produce 0.5W ohmic heating.

You can of course use PWM indirectly if you filter it to give a fairly
steady current.

Well I haven't seen any info that says I can't run it from PWM.

You just did, see above. Apart from that, you'd need to look. If only
there were some way of searching the Web for key phrases such "PWM Peltier".

I did I have done.

http://electronics.stackexchange.com...eltier-element


Most of the microcontrollers I am familiar with do PWM in the hundreds of cycles per second range. No thermoelectric module is going to be able to distinguish that from a steady voltage.

Also, (google it) there is a paper out there where they tested PWM cycling with rates of 1/10s all the way up to 1000/1s rates and the peltiers did not exhibit any decline in performance over thousands of hours. The one that cycled every 10 seconds did exhibit temperature fluctuation due to the slow response time.

In any case, PWM is utterly safe for controlling a peltier.




There's sources that say don;t run it in the 10s of Hz .

Cheers
--
Syd

On 17/12/2015 16:55, whisky-dave wrote:



On Thursday, 17 December 2015 16:00:16 UTC, Syd Rumpo wrote:
On 17/12/2015 15:38, whisky-dave wrote:
On Thursday, 17 December 2015 14:22:54 UTC, Syd Rumpo wrote:
On 17/12/2015 12:58, whisky-dave wrote:

snipped

Yes I've messed with one of those, I used an arduino PWM output which runs at about 500HZ I changed the pulse width to change the amount of cooling or heating. Using a TP31 transitor.
I used a CPU cooler on the other side as I found that I couldn;t get much cooling or heating for any lengh of time without finding a way to 'disipated' the heat or cold from the other side of the device.

You shouldn't PWM a Peltier device directly, at least not if you're
using it for cooling. Cooling is proportional to current, but the
unwanted ohmic heating is in proportion to current squared (I*I*R).
Think about it - a 50% PWM at 1A peak in a 2ohm device will produce 1W
ohmic heating (I*I*R for half the time) whereas a continuous (100%) 0.5A
would produce 0.5W ohmic heating.

You can of course use PWM indirectly if you filter it to give a fairly
steady current.

Well I haven't seen any info that says I can't run it from PWM.

You just did, see above. Apart from that, you'd need to look. If only
there were some way of searching the Web for key phrases such "PWM Peltier".

I did I have done.

http://electronics.stackexchange.com...eltier-element

You do know that you're meant to read this stuff? Your own link says,
"Peltier devices are one of the few things you do not want to run with
pulses, particularly in cooling applications...", and then goes on to
explain why.

Most of the microcontrollers I am familiar with do PWM in the hundreds of cycles per second range. No thermoelectric module is going to be able to distinguish that from a steady voltage.

Rubbish. What magical property does the Peltier module have which
allows it to integrate PWM? Peltier modules have no reactance other
than strays.

Also, (google it) there is a paper out there where they tested PWM cycling with rates of 1/10s all the way up to 1000/1s rates and the peltiers did not exhibit any decline in performance over thousands of hours. The one that cycled every 10 seconds did exhibit temperature fluctuation due to the slow response time.

Link?

In any case, PWM is utterly safe for controlling a peltier.

It's safe, of course, but quite inefficient when cooling, for reasons
explained ad nauseum. Best stop digging.

Cheers
--
Syd

On 17/12/2015 16:55, whisky-dave wrote:
On Thursday, 17 December 2015 16:00:16 UTC, Syd Rumpo wrote:
On 17/12/2015 15:38, whisky-dave wrote:
On Thursday, 17 December 2015 14:22:54 UTC, Syd Rumpo wrote:
On 17/12/2015 12:58, whisky-dave wrote:
On Thursday, 17 December 2015 02:59:08 UTC, F Murtz wrote:
Fredxxx wrote:
On 17/12/2015 01:08, F Murtz wrote:
Anyone know about peltier devices? Can you drive them
at half voltage? I wish to make a container that is
limited to ideally between 18 and 20 centigrade and
apparently they do not like to be cycled much for temp
control,I would rather have it on constantly but at
less than full capabilities. On further musing I would
probably still have problems as the system is dependent
on ambient temp so I would probably have to have a temp
sw to switch on only when ambient is higher. Anyone
have ideas on how to keep between those temps using
peltier? It is for storing chocolate so it could
probably be alright at lower temps but not higher.

Without looking anything up, from memory the cooling
effect is dependent on the current. There is a potential
barrier to overcome sort of akin to a diode. However they
are rather lossy, with a high series resistance where I2R
losses can be significant.

Most decent peltier devices come with a series of graphs
to give you cooling capacity for a specific temperature
difference across the device for a specific I/V drive.

Is there one you have in mind?

Thinking of this, only a few $AU on ebay

12V 60W TEC1-12706 12v 60w

Yes I've messed with one of those, I used an arduino PWM
output which runs at about 500HZ I changed the pulse width to
change the amount of cooling or heating. Using a TP31
transitor. I used a CPU cooler on the other side as I found
that I couldn;t get much cooling or heating for any lengh of
time without finding a way to 'disipated' the heat or cold
from the other side of the device.

You shouldn't PWM a Peltier device directly, at least not if
you're using it for cooling. Cooling is proportional to
current, but the unwanted ohmic heating is in proportion to
current squared (I*I*R). Think about it - a 50% PWM at 1A peak
in a 2ohm device will produce 1W ohmic heating (I*I*R for half
the time) whereas a continuous (100%) 0.5A would produce 0.5W
ohmic heating.

You can of course use PWM indirectly if you filter it to give a
fairly steady current.

Well I haven't seen any info that says I can't run it from PWM.

You just did, see above. Apart from that, you'd need to look. If
only there were some way of searching the Web for key phrases such
"PWM Peltier".

I did I have done.

http://electronics.stackexchange.com...eltier-element



Most of the microcontrollers I am familiar with do PWM in the
hundreds of cycles per second range. No thermoelectric module is
going to be able to distinguish that from a steady voltage.

Wrong.

Also, (google it) there is a paper out there where they tested PWM
cycling with rates of 1/10s all the way up to 1000/1s rates and the
peltiers did not exhibit any decline in performance over thousands of
hours. The one that cycled every 10 seconds did exhibit temperature
fluctuation due to the slow response time.

In any case, PWM is utterly safe for controlling a peltier.

It is utterly safe. However it's not a smart way to run a peltier as
already explained to you.

First hit will assist you:
http://lmgtfy.com/?q=peltier+pwm+vs+constant+current

Constant current is best. It you really need to drive it from a PWM,
make the frequency as high as you can and stick a big inductor in series
to make the current as constant as possible.

If you don't understand the different between average current and rms
current in terms of resistive heating may I suggest you go away and find
out. There is a very big difference.


There's sources that say don;t run it in the 10s of Hz .

Cheers -- Syd

On Thursday, 17 December 2015 18:06:41 UTC, Syd Rumpo wrote:
On 17/12/2015 16:55, whisky-dave wrote:



On Thursday, 17 December 2015 16:00:16 UTC, Syd Rumpo wrote:
On 17/12/2015 15:38, whisky-dave wrote:
On Thursday, 17 December 2015 14:22:54 UTC, Syd Rumpo wrote:
On 17/12/2015 12:58, whisky-dave wrote:

snipped

Yes I've messed with one of those, I used an arduino PWM output which runs at about 500HZ I changed the pulse width to change the amount of cooling or heating. Using a TP31 transitor.
I used a CPU cooler on the other side as I found that I couldn;t get much cooling or heating for any lengh of time without finding a way to 'disipated' the heat or cold from the other side of the device.

You shouldn't PWM a Peltier device directly, at least not if you're
using it for cooling. Cooling is proportional to current, but the
unwanted ohmic heating is in proportion to current squared (I*I*R).
Think about it - a 50% PWM at 1A peak in a 2ohm device will produce 1W
ohmic heating (I*I*R for half the time) whereas a continuous (100%) 0.5A
would produce 0.5W ohmic heating.

You can of course use PWM indirectly if you filter it to give a fairly
steady current.

Well I haven't seen any info that says I can't run it from PWM.

You just did, see above. Apart from that, you'd need to look. If only
there were some way of searching the Web for key phrases such "PWM Peltier".

I did I have done.

http://electronics.stackexchange.com...eltier-element

You do know that you're meant to read this stuff?

Yes I did. It gives two sides to it, those that thibnk you can and those that can't see why you can't use PWM.



Your own link says,
"Peltier devices are one of the few things you do not want to run with
pulses, particularly in cooling applications...", and then goes on to
explain why.

But doesn;t explain it.


Most of the microcontrollers I am familiar with do PWM in the hundreds of cycles per second range. No thermoelectric module is going to be able to distinguish that from a steady voltage.

Rubbish. What magical property does the Peltier module have which
allows it to integrate PWM? Peltier modules have no reactance other
than strays.

it doesn't need a magical property any more than a light dimmer does.


Also, (google it) there is a paper out there where they tested PWM cycling with rates of 1/10s all the way up to 1000/1s rates and the peltiers did not exhibit any decline in performance over thousands of hours. The one that cycled every 10 seconds did exhibit temperature fluctuation due to the slow response time.

Link?

yes I want a limk that explain why PWM can't be used.


In any case, PWM is utterly safe for controlling a peltier.

It's safe, of course, but quite inefficient when cooling, for reasons
explained ad nauseum. Best stop digging.

there was NO explanation shown that made sense.

On 18/12/2015 12:45, whisky-dave wrote:
On Thursday, 17 December 2015 18:06:41 UTC, Syd Rumpo wrote:

Your own link says,
"Peltier devices are one of the few things you do not want to run with
pulses, particularly in cooling applications...", and then goes on to
explain why.

But doesn;t explain it.

There are two scenarios when driving the Peltier at say 1/4 max power.

The device has to pump both the heat from the cold side kI and any
resistive losses generated inside the device I^2R to the hot side.

So if you want to run it at I for 1/4 of the time and 0 for 3/4

Heat output = kI/4 + I^2R/4

But if you run it at I/4 continuously (ie filter the PWM drive to DC)

Heat output = kI/4 + (I/4)^2R = kI/4 + I^2R/16

In other words the pulse mode driven PEC is more inefficient turning a
greater proportion of the higher drive current into waste heat.

Driving it with DC you can either get the cold side colder for the same
input power or use less input power to obtain the same cooling.

--
Regards,
Martin Brown

On 18/12/2015 12:45, whisky-dave wrote:

snipped

Yes I did. It gives two sides to it, those that think you can and those that can't see why you can't use PWM.

Riiight.

Cheers
--
Syd

On 18/12/2015 12:45, whisky-dave wrote:
On Thursday, 17 December 2015 18:06:41 UTC, Syd Rumpo wrote:
On 17/12/2015 16:55, whisky-dave wrote:



On Thursday, 17 December 2015 16:00:16 UTC, Syd Rumpo wrote:
On 17/12/2015 15:38, whisky-dave wrote:
On Thursday, 17 December 2015 14:22:54 UTC, Syd Rumpo
wrote:
On 17/12/2015 12:58, whisky-dave wrote:

snipped

Yes I've messed with one of those, I used an arduino PWM
output which runs at about 500HZ I changed the pulse
width to change the amount of cooling or heating. Using a
TP31 transitor. I used a CPU cooler on the other side as
I found that I couldn;t get much cooling or heating for
any lengh of time without finding a way to 'disipated'
the heat or cold from the other side of the device.

You shouldn't PWM a Peltier device directly, at least not
if you're using it for cooling. Cooling is proportional to
current, but the unwanted ohmic heating is in proportion to
current squared (I*I*R). Think about it - a 50% PWM at 1A
peak in a 2ohm device will produce 1W ohmic heating (I*I*R
for half the time) whereas a continuous (100%) 0.5A would
produce 0.5W ohmic heating.

You can of course use PWM indirectly if you filter it to
give a fairly steady current.

Well I haven't seen any info that says I can't run it from
PWM.

You just did, see above. Apart from that, you'd need to look.
If only there were some way of searching the Web for key
phrases such "PWM Peltier".

I did I have done.

http://electronics.stackexchange.com...eltier-element



You do know that you're meant to read this stuff?

Yes I did. It gives two sides to it, those that thibnk you can and
those that can't see why you can't use PWM.

That is so, so true.

Your own link says,
"Peltier devices are one of the few things you do not want to run
with pulses, particularly in cooling applications...", and then
goes on to explain why.

But doesn;t explain it.


Most of the microcontrollers I am familiar with do PWM in the
hundreds of cycles per second range. No thermoelectric module is
going to be able to distinguish that from a steady voltage.

Rubbish. What magical property does the Peltier module have which
allows it to integrate PWM? Peltier modules have no reactance
other than strays.

it doesn't need a magical property any more than a light dimmer
does.


Also, (google it) there is a paper out there where they tested
PWM cycling with rates of 1/10s all the way up to 1000/1s rates
and the peltiers did not exhibit any decline in performance over
thousands of hours. The one that cycled every 10 seconds did
exhibit temperature fluctuation due to the slow response time.

Link?

yes I want a limk that explain why PWM can't be used.


In any case, PWM is utterly safe for controlling a peltier.

It's safe, of course, but quite inefficient when cooling, for
reasons explained ad nauseum. Best stop digging.

there was NO explanation shown that made sense.

It would to someone versed in basic electronics.

On Thursday, 17 December 2015 19:36:16 UTC, Fredxxx wrote:
On 17/12/2015 16:55, whisky-dave wrote:

In any case, PWM is utterly safe for controlling a peltier.

It is utterly safe. However it's not a smart way to run a peltier as
already explained to you.

it was explained that it was not as efficinet where they advertised their controllers, yes, which are also way outside the student budget.



First hit will assist you:
http://lmgtfy.com/?q=peltier+pwm+vs+constant+current

Constant current is best. It you really need to drive it from a PWM,
make the frequency as high as you can and stick a big inductor in series
to make the current as constant as possible.

http://tetech.com/faqs/#19

The "ON" and "OFF" pulses occur so rapidly that the module does not have enough time to change temperature in response to each electrical pulse. Instead, the module assumes a temperature difference relative to Vaverage. When the controller is properly tuned thermal cycling is eliminated. Thus, these controllers do not degrade the reliability of a module from thermal cycling in the same way that a thermostatic or slow "ON-OFF" controller would.

as said the diferce is in teh speedd of switching.

One thing our students get confused by is that a square wave pulse that goes from 0V to a higher voltage is NOT AC. it's still DC as current only flows in one direction.


If you don't understand the different between average current and rms
current in terms of resistive heating may I suggest you go away and find
out. There is a very big difference.

0.707 compared to 0.636 isn't it for PWM square wave it would vary.

The RMS voltage is the voltage that would give the same heating effect as AC in a resistive load.
That's basiclly where RMS comes in as meaingful in power.





There's sources that say don;t run it in the 10s of Hz .

Cheers -- Syd

On Friday, 18 December 2015 14:28:01 UTC, Fredxxx wrote:
On 18/12/2015 12:45, whisky-dave wrote:

there was NO explanation shown that made sense.

It would to someone versed in basic electronics.

But you can't explain I see.

whisky-dave wrote
Fredxxx wrote
whisky-dave wrote

there was NO explanation shown that made sense.

It would to someone versed in basic electronics.

But you can't explain I see.

No need, Martin explained it very clearly.

On 18/12/2015 15:24, whisky-dave wrote:
On Friday, 18 December 2015 14:28:01 UTC, Fredxxx wrote:
On 18/12/2015 12:45, whisky-dave wrote:

there was NO explanation shown that made sense.

It would to someone versed in basic electronics.

But you can't explain I see.

I provided a link that did. I'm sure it gave a better explanation than I
could have done, or wish to have done.

Especially after you seemingly have difficult in basis comprehension
understanding Syd's reason why PWM is not an ideal way to drive a peltier.

--
A closed mouth gathers no foot.
A man with a closed mouth doesn't look stupid.

On 18/12/2015 15:23, whisky-dave wrote:
On Thursday, 17 December 2015 19:36:16 UTC, Fredxxx wrote:
On 17/12/2015 16:55, whisky-dave wrote:

In any case, PWM is utterly safe for controlling a peltier.

It is utterly safe. However it's not a smart way to run a peltier
as already explained to you.

it was explained that it was not as efficinet where they advertised
their controllers, yes, which are also way outside the student
budget.

So when you say "No thermoelectric module is going to be able to
distinguish that from a steady voltage." I'm pleased that you seem to
have learned something new.


First hit will assist you:
http://lmgtfy.com/?q=peltier+pwm+vs+constant+current

Constant current is best. It you really need to drive it from a
PWM, make the frequency as high as you can and stick a big inductor
in series to make the current as constant as possible.

http://tetech.com/faqs/#19

The "ON" and "OFF" pulses occur so rapidly that the module does not
have enough time to change temperature in response to each electrical
pulse. Instead, the module assumes a temperature difference relative
to Vaverage. When the controller is properly tuned thermal cycling is
eliminated. Thus, these controllers do not degrade the reliability of
a module from thermal cycling in the same way that a thermostatic or
slow "ON-OFF" controller would.

as said the diferce is in teh speedd of switching.

One thing our students get confused by is that a square wave pulse
that goes from 0V to a higher voltage is NOT AC. it's still DC as
current only flows in one direction.

Alternating does give the game away. However, there are many instances
where the voltage is not "alternating" in your eyes, where the current does.


If you don't understand the different between average current and
rms current in terms of resistive heating may I suggest you go away
and find out. There is a very big difference.

0.707 compared to 0.636 isn't it for PWM square wave it would vary.

The RMS voltage is the voltage that would give the same heating
effect as AC in a resistive load. That's basiclly where RMS comes in
as meaingful in power.

Your quoting numbers to 3 decimal places for an arbitrary PWM waveform
speaks volumes of your engineering skills.



There's sources that say don;t run it in the 10s of Hz .

Cheers -- Syd


I find it quite disturbing you might be teaching students anything like
this.

"Fredxxx" wrote in message
...
On 18/12/2015 15:23, whisky-dave wrote:
On Thursday, 17 December 2015 19:36:16 UTC, Fredxxx wrote:
On 17/12/2015 16:55, whisky-dave wrote:

In any case, PWM is utterly safe for controlling a peltier.

It is utterly safe. However it's not a smart way to run a peltier
as already explained to you.

it was explained that it was not as efficinet where they advertised
their controllers, yes, which are also way outside the student
budget.

So when you say "No thermoelectric module is going to be able to
distinguish that from a steady voltage." I'm pleased that you seem to have
learned something new.


First hit will assist you:
http://lmgtfy.com/?q=peltier+pwm+vs+constant+current

Constant current is best. It you really need to drive it from a
PWM, make the frequency as high as you can and stick a big inductor
in series to make the current as constant as possible.

http://tetech.com/faqs/#19

The "ON" and "OFF" pulses occur so rapidly that the module does not
have enough time to change temperature in response to each electrical
pulse. Instead, the module assumes a temperature difference relative
to Vaverage. When the controller is properly tuned thermal cycling is
eliminated. Thus, these controllers do not degrade the reliability of
a module from thermal cycling in the same way that a thermostatic or
slow "ON-OFF" controller would.

as said the diferce is in teh speedd of switching.

One thing our students get confused by is that a square wave pulse
that goes from 0V to a higher voltage is NOT AC. it's still DC as
current only flows in one direction.

Alternating does give the game away. However, there are many instances
where the voltage is not "alternating" in your eyes, where the current
does.


If you don't understand the different between average current and
rms current in terms of resistive heating may I suggest you go away
and find out. There is a very big difference.

0.707 compared to 0.636 isn't it for PWM square wave it would vary.

The RMS voltage is the voltage that would give the same heating
effect as AC in a resistive load. That's basiclly where RMS comes in
as meaingful in power.

Your quoting numbers to 3 decimal places for an arbitrary PWM waveform
speaks volumes of your engineering skills.



There's sources that say don;t run it in the 10s of Hz .

Cheers -- Syd


I find it quite disturbing you might be teaching students anything like
this.

I dunno what the prob is, my bipolar PWM drive (circa 1980) worked fine for
years and may still be working
I never researched losses, inefficiencies etc etc, someone wanted something
cooling fairly accurately, thats what they got

On 18/12/2015 23:03, brass monkey wrote:
"Fredxxx" wrote in message
...
On 18/12/2015 15:23, whisky-dave wrote:
On Thursday, 17 December 2015 19:36:16 UTC, Fredxxx wrote:
On 17/12/2015 16:55, whisky-dave wrote:

In any case, PWM is utterly safe for controlling a peltier.

It is utterly safe. However it's not a smart way to run a peltier
as already explained to you.

it was explained that it was not as efficinet where they advertised
their controllers, yes, which are also way outside the student
budget.

So when you say "No thermoelectric module is going to be able to
distinguish that from a steady voltage." I'm pleased that you seem to have
learned something new.


First hit will assist you:
http://lmgtfy.com/?q=peltier+pwm+vs+constant+current

Constant current is best. It you really need to drive it from a
PWM, make the frequency as high as you can and stick a big inductor
in series to make the current as constant as possible.

http://tetech.com/faqs/#19

The "ON" and "OFF" pulses occur so rapidly that the module does not
have enough time to change temperature in response to each electrical
pulse. Instead, the module assumes a temperature difference relative
to Vaverage. When the controller is properly tuned thermal cycling is
eliminated. Thus, these controllers do not degrade the reliability of
a module from thermal cycling in the same way that a thermostatic or
slow "ON-OFF" controller would.

as said the diferce is in teh speedd of switching.

One thing our students get confused by is that a square wave pulse
that goes from 0V to a higher voltage is NOT AC. it's still DC as
current only flows in one direction.

Alternating does give the game away. However, there are many instances
where the voltage is not "alternating" in your eyes, where the current
does.


If you don't understand the different between average current and
rms current in terms of resistive heating may I suggest you go away
and find out. There is a very big difference.

0.707 compared to 0.636 isn't it for PWM square wave it would vary.

The RMS voltage is the voltage that would give the same heating
effect as AC in a resistive load. That's basiclly where RMS comes in
as meaingful in power.

Your quoting numbers to 3 decimal places for an arbitrary PWM waveform
speaks volumes of your engineering skills.



There's sources that say don;t run it in the 10s of Hz .

Cheers -- Syd


I find it quite disturbing you might be teaching students anything like
this.

I dunno what the prob is, my bipolar PWM drive (circa 1980) worked fine for
years and may still be working
I never researched losses, inefficiencies etc etc, someone wanted something
cooling fairly accurately, thats what they got

No one here is doubting that a cheap and quick setup of PWM drive does
not work. It is a simple form of control ideal where efficiency is not
important.

What this thread has demonstrated is that someone who teaches students
doesn't seem to understand that PWM drive might be less efficient than
with a constant current.

"Fredxxx" wrote in message
...
On 18/12/2015 23:03, brass monkey wrote:
"Fredxxx" wrote in message
...
On 18/12/2015 15:23, whisky-dave wrote:
On Thursday, 17 December 2015 19:36:16 UTC, Fredxxx wrote:
On 17/12/2015 16:55, whisky-dave wrote:

In any case, PWM is utterly safe for controlling a peltier.

It is utterly safe. However it's not a smart way to run a peltier
as already explained to you.

it was explained that it was not as efficinet where they advertised
their controllers, yes, which are also way outside the student
budget.

So when you say "No thermoelectric module is going to be able to
distinguish that from a steady voltage." I'm pleased that you seem to
have
learned something new.


First hit will assist you:
http://lmgtfy.com/?q=peltier+pwm+vs+constant+current

Constant current is best. It you really need to drive it from a
PWM, make the frequency as high as you can and stick a big inductor
in series to make the current as constant as possible.

http://tetech.com/faqs/#19

The "ON" and "OFF" pulses occur so rapidly that the module does not
have enough time to change temperature in response to each electrical
pulse. Instead, the module assumes a temperature difference relative
to Vaverage. When the controller is properly tuned thermal cycling is
eliminated. Thus, these controllers do not degrade the reliability of
a module from thermal cycling in the same way that a thermostatic or
slow "ON-OFF" controller would.

as said the diferce is in teh speedd of switching.

One thing our students get confused by is that a square wave pulse
that goes from 0V to a higher voltage is NOT AC. it's still DC as
current only flows in one direction.

Alternating does give the game away. However, there are many instances
where the voltage is not "alternating" in your eyes, where the current
does.


If you don't understand the different between average current and
rms current in terms of resistive heating may I suggest you go away
and find out. There is a very big difference.

0.707 compared to 0.636 isn't it for PWM square wave it would vary.

The RMS voltage is the voltage that would give the same heating
effect as AC in a resistive load. That's basiclly where RMS comes in
as meaingful in power.

Your quoting numbers to 3 decimal places for an arbitrary PWM waveform
speaks volumes of your engineering skills.



There's sources that say don;t run it in the 10s of Hz .

Cheers -- Syd


I find it quite disturbing you might be teaching students anything like
this.

I dunno what the prob is, my bipolar PWM drive (circa 1980) worked fine
for
years and may still be working
I never researched losses, inefficiencies etc etc, someone wanted
something
cooling fairly accurately, thats what they got

No one here is doubting that a cheap and quick setup of PWM drive does not
work. It is a simple form of control ideal where efficiency is not
important.

What this thread has demonstrated is that someone who teaches students
doesn't seem to understand that PWM drive might be less efficient than
with a constant current.

In my application, I'd prolly still have needed bipolar. Otherwise, using a
variable constant current would necessitate waiting for the temp to
naturally change as opposed to forcing it. Hmmm LOL, the more I think of it
the less I know
Matches, bootlaces anyone?

On Thu, 17 Dec 2015 08:55:20 -0800, whisky-dave wrote:

On Thursday, 17 December 2015 16:00:16 UTC, Syd Rumpo wrote:
On 17/12/2015 15:38, whisky-dave wrote:
On Thursday, 17 December 2015 14:22:54 UTC, Syd Rumpo wrote:
On 17/12/2015 12:58, whisky-dave wrote:
On Thursday, 17 December 2015 02:59:08 UTC, F Murtz wrote:
Fredxxx wrote:
On 17/12/2015 01:08, F Murtz wrote:
Anyone know about peltier devices? Can you drive them at half
voltage? I wish to make a container that is limited to ideally
between 18 and 20 centigrade and apparently they do not like to
be cycled much for temp control,I would rather have it on
constantly but at less than full capabilities.
On further musing I would probably still have problems as the
system is dependent on ambient temp so I would probably have to
have a temp sw to switch on only when ambient is higher.
Anyone have ideas on how to keep between those temps using
peltier?
It is for storing chocolate so it could probably be alright at
lower temps but not higher.

Without looking anything up, from memory the cooling effect is
dependent on the current. There is a potential barrier to
overcome sort of akin to a diode. However they are rather lossy,
with a high series resistance where I2R losses can be
significant.

Most decent peltier devices come with a series of graphs to give
you cooling capacity for a specific temperature difference across
the device for a specific I/V drive.

Is there one you have in mind?

Thinking of this, only a few $AU on ebay

12V 60W TEC1-12706
12v 60w

Yes I've messed with one of those, I used an arduino PWM output
which runs at about 500HZ I changed the pulse width to change the
amount of cooling or heating. Using a TP31 transitor.
I used a CPU cooler on the other side as I found that I couldn;t
get much cooling or heating for any lengh of time without finding a
way to 'disipated' the heat or cold from the other side of the
device.

You shouldn't PWM a Peltier device directly, at least not if you're
using it for cooling. Cooling is proportional to current, but the
unwanted ohmic heating is in proportion to current squared (I*I*R).
Think about it - a 50% PWM at 1A peak in a 2ohm device will produce
1W ohmic heating (I*I*R for half the time) whereas a continuous
(100%) 0.5A would produce 0.5W ohmic heating.

You can of course use PWM indirectly if you filter it to give a
fairly steady current.

Well I haven't seen any info that says I can't run it from PWM.

You just did, see above. Apart from that, you'd need to look. If only
there were some way of searching the Web for key phrases such "PWM
Peltier".

I did I have done.

http://electronics.stackexchange.com...ow-to-drive-a-
peltier-element


Most of the microcontrollers I am familiar with do PWM in the hundreds
of cycles per second range. No thermoelectric module is going to be able
to distinguish that from a steady voltage.

Also, (google it) there is a paper out there where they tested PWM
cycling with rates of 1/10s all the way up to 1000/1s rates and the
peltiers did not exhibit any decline in performance over thousands of
hours. The one that cycled every 10 seconds did exhibit temperature
fluctuation due to the slow response time.

In any case, PWM is utterly safe for controlling a peltier.


Provided the on-current is within the maximum rating of the peltier
device, that's true but the PN junctions do exhibit the forward current
characteristic of a diode with a small ohmic series resistance. There's
usually a few hundred of these junctions in series (but in parallel for
the thermal path) so a 6A 12v device may have an effective total
resistance of 2 ohms (the forward volt drops at each PN junction are only
a few millivolts each).

As you increase the current, the I squared losses eventually start
adding more heat than the device can pump from the cold to the hot side.
If you set your on-current setting of your PWM controller to this
critical limiting value, no matter the duty cycle, each current pulse
will add as much heat as the junctions can transfer. This is an extreme
example of why you don't want to drive a peltier device directly from a
PWM current source.

There's sources that say don;t run it in the 10s of Hz .


And others that say 50Hz is fast enough (but only from the point of view
of thermal cycling stresses, ignoring the other implication of using a
high duty cycle PWM supply). Cycling the device 2 or 3 times a minute
accelerates thermally induced fatigue stress. The best way to control the
cooling is to regulate a totally smoothed DC current supply (for best
efficiency, by using a switching regulator).

--
Johnny B Good

On 19/12/2015 02:34, Johnny B Good wrote:
On Thu, 17 Dec 2015 08:55:20 -0800, whisky-dave wrote:

On Thursday, 17 December 2015 16:00:16 UTC, Syd Rumpo wrote:
On 17/12/2015 15:38, whisky-dave wrote:
On Thursday, 17 December 2015 14:22:54 UTC, Syd Rumpo wrote:
On 17/12/2015 12:58, whisky-dave wrote:
On Thursday, 17 December 2015 02:59:08 UTC, F Murtz wrote:
Fredxxx wrote:
On 17/12/2015 01:08, F Murtz wrote:
Anyone know about peltier devices? Can you drive them at half
voltage? I wish to make a container that is limited to ideally
between 18 and 20 centigrade and apparently they do not like to
be cycled much for temp control,I would rather have it on
constantly but at less than full capabilities.
On further musing I would probably still have problems as the
system is dependent on ambient temp so I would probably have to
have a temp sw to switch on only when ambient is higher.
Anyone have ideas on how to keep between those temps using
peltier?
It is for storing chocolate so it could probably be alright at
lower temps but not higher.

Without looking anything up, from memory the cooling effect is
dependent on the current. There is a potential barrier to
overcome sort of akin to a diode. However they are rather lossy,
with a high series resistance where I2R losses can be
significant.

Most decent peltier devices come with a series of graphs to give
you cooling capacity for a specific temperature difference across
the device for a specific I/V drive.

Is there one you have in mind?

Thinking of this, only a few $AU on ebay

12V 60W TEC1-12706
12v 60w

Yes I've messed with one of those, I used an arduino PWM output
which runs at about 500HZ I changed the pulse width to change the
amount of cooling or heating. Using a TP31 transitor.
I used a CPU cooler on the other side as I found that I couldn;t
get much cooling or heating for any lengh of time without finding a
way to 'disipated' the heat or cold from the other side of the
device.

You shouldn't PWM a Peltier device directly, at least not if you're
using it for cooling. Cooling is proportional to current, but the
unwanted ohmic heating is in proportion to current squared (I*I*R).
Think about it - a 50% PWM at 1A peak in a 2ohm device will produce
1W ohmic heating (I*I*R for half the time) whereas a continuous
(100%) 0.5A would produce 0.5W ohmic heating.

You can of course use PWM indirectly if you filter it to give a
fairly steady current.

Well I haven't seen any info that says I can't run it from PWM.

You just did, see above. Apart from that, you'd need to look. If only
there were some way of searching the Web for key phrases such "PWM
Peltier".

I did I have done.

http://electronics.stackexchange.com...ow-to-drive-a-
peltier-element


Most of the microcontrollers I am familiar with do PWM in the hundreds
of cycles per second range. No thermoelectric module is going to be able
to distinguish that from a steady voltage.

Also, (google it) there is a paper out there where they tested PWM
cycling with rates of 1/10s all the way up to 1000/1s rates and the
peltiers did not exhibit any decline in performance over thousands of
hours. The one that cycled every 10 seconds did exhibit temperature
fluctuation due to the slow response time.

In any case, PWM is utterly safe for controlling a peltier.


Provided the on-current is within the maximum rating of the peltier
device, that's true but the PN junctions do exhibit the forward current
characteristic of a diode with a small ohmic series resistance. There's
usually a few hundred of these junctions in series (but in parallel for
the thermal path) so a 6A 12v device may have an effective total
resistance of 2 ohms (the forward volt drops at each PN junction are only
a few millivolts each).

As you increase the current, the I squared losses eventually start
adding more heat than the device can pump from the cold to the hot side.
If you set your on-current setting of your PWM controller to this
critical limiting value, no matter the duty cycle, each current pulse
will add as much heat as the junctions can transfer. This is an extreme
example of why you don't want to drive a peltier device directly from a
PWM current source.

There's sources that say don;t run it in the 10s of Hz .


And others that say 50Hz is fast enough (but only from the point of view
of thermal cycling stresses, ignoring the other implication of using a
high duty cycle PWM supply). Cycling the device 2 or 3 times a minute
accelerates thermally induced fatigue stress. The best way to control the
cooling is to regulate a totally smoothed DC current supply (for best
efficiency, by using a switching regulator).

Agreed, though a switching regulator uses PWM drive to set average
voltage which is then filtered with and inductor / capacitor combination.

Hence my suggestion earlier of running the PWM at the highest frequency
possible and placing an inductor in series; to reduce minimise ripple
current.

On 18/12/2015 23:58, bm wrote:
"Fredxxx" wrote in message
...
On 18/12/2015 23:03, brass monkey wrote:
"Fredxxx" wrote in message
...
On 18/12/2015 15:23, whisky-dave wrote:
On Thursday, 17 December 2015 19:36:16 UTC, Fredxxx wrote:
On 17/12/2015 16:55, whisky-dave wrote:

In any case, PWM is utterly safe for controlling a peltier.

It is utterly safe. However it's not a smart way to run a peltier
as already explained to you.

it was explained that it was not as efficinet where they advertised
their controllers, yes, which are also way outside the student
budget.

So when you say "No thermoelectric module is going to be able to
distinguish that from a steady voltage." I'm pleased that you seem to
have
learned something new.


First hit will assist you:
http://lmgtfy.com/?q=peltier+pwm+vs+constant+current

Constant current is best. It you really need to drive it from a
PWM, make the frequency as high as you can and stick a big inductor
in series to make the current as constant as possible.

http://tetech.com/faqs/#19

The "ON" and "OFF" pulses occur so rapidly that the module does not
have enough time to change temperature in response to each electrical
pulse. Instead, the module assumes a temperature difference relative
to Vaverage. When the controller is properly tuned thermal cycling is
eliminated. Thus, these controllers do not degrade the reliability of
a module from thermal cycling in the same way that a thermostatic or
slow "ON-OFF" controller would.

as said the diferce is in teh speedd of switching.

One thing our students get confused by is that a square wave pulse
that goes from 0V to a higher voltage is NOT AC. it's still DC as
current only flows in one direction.

Alternating does give the game away. However, there are many instances
where the voltage is not "alternating" in your eyes, where the current
does.


If you don't understand the different between average current and
rms current in terms of resistive heating may I suggest you go away
and find out. There is a very big difference.

0.707 compared to 0.636 isn't it for PWM square wave it would vary.

The RMS voltage is the voltage that would give the same heating
effect as AC in a resistive load. That's basiclly where RMS comes in
as meaingful in power.

Your quoting numbers to 3 decimal places for an arbitrary PWM waveform
speaks volumes of your engineering skills.



There's sources that say don;t run it in the 10s of Hz .

Cheers -- Syd


I find it quite disturbing you might be teaching students anything like
this.

I dunno what the prob is, my bipolar PWM drive (circa 1980) worked fine
for
years and may still be working
I never researched losses, inefficiencies etc etc, someone wanted
something
cooling fairly accurately, thats what they got

No one here is doubting that a cheap and quick setup of PWM drive does not
work. It is a simple form of control ideal where efficiency is not
important.

What this thread has demonstrated is that someone who teaches students
doesn't seem to understand that PWM drive might be less efficient than
with a constant current.

In my application, I'd prolly still have needed bipolar. Otherwise, using a
variable constant current would necessitate waiting for the temp to
naturally change as opposed to forcing it. Hmmm LOL, the more I think of it
the less I know
Matches, bootlaces anyone?

As I have said a simple solution is often the best, even if not optimum.
It is also possible to have bipolar constant current source.