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