Peltier device... regulating temperature

[rowan194]

I'm considering adding temperature regulation to a 12V portable fridge.
The circuitry doesn't appear to be much more than a hot/cold switch
which switches polarity to a peltier device. Will switching the device
on and off with a certain "minimum" period (to avoid rapid cycling) be
sufficient or should I use PWM?

Also, the peltier device inside the fridge has 4 wires - red&black,
plus two yellow. Any ideas what the latter two may be used for, perhaps
an internal thermostat?

Thanks in advance...

 

[rowan194]

Also, the peltier device inside the fridge has 4 wires - red&black,
plus two yellow. Any ideas what the latter two may be used for, perhaps
an internal thermostat?

Further note: the yellow wires are marked "KA" and "KB" on the PCB.

I've drawn out the circuit, KA-KB appears to be the path between -ve
and the element when the switch is in the "hot" position, so I guess
it's a cutout switch to prevent it getting too warm. The only other
components are a couple of LEDs with limiting resistors, and a bridge
rectifier to ensure the polarity of the fan is always correct.

http://satin.sensation.net.au/rowan/fridgecircuit.gif

Since I'm only interested in cooling I could probably get rid of the
PCB and hardwire the fan, peltier device and thermostat... or break the
circuit on the cold side of the switch and place the thermostat inline.

That's assuming it's OK to use a thermostat....

 

[rowan194]

The fan in this thing is so noisy that I'm not sure a thermostat is
going to be of much use. At 22C ambient the interior of the fridge is
about 6 - 7C, which means that even on a reasonably mild day it would
be running continously.

The fridge is under my desk, after a couple of hours of sitting at the
computer my ears are actually hurting because of the noise......

I presume the fan itself is a vital part of cooling, to keep the
temperature on the warm side as low as possible.

 

[Mauried]

um I did a search with google on peltier controller and found
this http://www.oatleyelectronics.com/pdf/k140a.pdf
have a nice day

I'm considering adding temperature regulation to a 12V portable fridge.
The circuitry doesn't appear to be much more than a hot/cold switch
which switches polarity to a peltier device. Will switching the device
on and off with a certain "minimum" period (to avoid rapid cycling) be
sufficient or should I use PWM?

Also, the peltier device inside the fridge has 4 wires - red&black,
plus two yellow. Any ideas what the latter two may be used for, perhaps
an internal thermostat?

Thanks in advance...

Peltier fridges are sadly very inefficient and cant really cool all
that well.
The peltier elements that are usually used are at the best only around
50% efficient and have a maximum temperature differance between the
hot and cold side of the element of about 40D max.
This means that you have to keep the hot side as close to ambient as
you possibly can,and the small fans that usually come with the fridges
are too small to do this properly.
Its unlikley that if you fit a thermostat, it will ever switch off as
the peltier element simply wont be able to pull enuf heat out of the
fridge even on a mild day.

The only way Ive found to get any performance out of these fridges is
to add more peltoer elements , usually 4 are needed and water cool
them using water cooling kits made for cooling CPUs in PCs.
Does work, but is expensive, you are much better getting a real
portable fridge with a compressor.

 

[Friday]

Peltier fridges are sadly very inefficient and cant really cool all
that well.
The peltier elements that are usually used are at the best only around
50% efficient and have a maximum temperature differance between the
hot and cold side of the element of about 40D max.
This means that you have to keep the hot side as close to ambient as
you possibly can,and the small fans that usually come with the fridges
are too small to do this properly.
Its unlikley that if you fit a thermostat, it will ever switch off as
the peltier element simply wont be able to pull enuf heat out of the
fridge even on a mild day.

The only way Ive found to get any performance out of these fridges is
to add more peltoer elements , usually 4 are needed and water cool
them using water cooling kits made for cooling CPUs in PCs.
Does work, but is expensive, you are much better getting a real
portable fridge with a compressor.

Some friends experimented with one and found it would not cool a can of
beer to a drinkable temperature.

Friday

 

[rowan194]

Peltier fridges are sadly very inefficient and cant really cool all
that well.

[...]
Some friends experimented with one and found it would not cool a can of
beer to a drinkable temperature.

The fridge I have will chill a drink quite nicely (ice also forms near
the peltier device), but the fan is so noisy that it hurts my ears. I
tried replacing the fan on the back of the case with a smaller a low
noise PC case fan bolted "CPU style" to the heatsink and surprisingly
it still managed to cool to about 9C with the ambient in the low 20s.
The only problem is that there's a fair bit of radiant heat coming from
the back which I am not entirely comfortable with.

So it seems that even without moving a shitload of air through the case
it can still cool. Some ideas I've come up with...

1) Fixed speed control of the original fan, so that it's still moving
enough air to cool the heatsink sufficiently, but not enough to make
uncomfortable noise. (Even something basic like feeding it 5V instead
of 12V)

2) Temperature based speed control of the original fan... the hotter
the heatsink, the faster it turns.

3) Multiple smaller but quieter fans bolted to the heatsink, with one
running constantly and the second only switched in when the heatsink
gets too warm. Perhaps PWM on the additional fan to reduce the
possibility of a rapid temperature change.

4) Digital temperature sensors on the heatsink and inside the fridge
with an AVR controlling the speed of the fan(s)

I know that ultimately the fridge itself is quite inefficient, but it
was a present and I want to make use of it.

 

[Two Bob]

Peltier fridges are sadly very inefficient and cant really cool all
that well.
The peltier elements that are usually used are at the best only around
50% efficient and have a maximum temperature differance between the
hot and cold side of the element of about 40D max.
This means that you have to keep the hot side as close to ambient as
you possibly can,and the small fans that usually come with the fridges
are too small to do this properly.
Its unlikley that if you fit a thermostat, it will ever switch off as
the peltier element simply wont be able to pull enuf heat out of the
fridge even on a mild day.

The only way Ive found to get any performance out of these fridges is
to add more peltoer elements , usually 4 are needed and water cool
them using water cooling kits made for cooling CPUs in PCs.
Does work, but is expensive, you are much better getting a real
portable fridge with a compressor.

Bolt a second to the back of the first. The second will make the first more
efficient.