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