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