Hi Guys,

I hope that I am posting this in the right place. I posted in the arduino forum already but it was not answered and swept away by the vast amount of post per day there. So I thought this forum might be the better place for this question anyways... however I hope I am not breaking any rules by reposting the same problem here.
I want to use this Peltier
http://uk.rs-online.com/web/p/peltier-modules/4901395/
with this power supply
http://uk.rs-online.com/web/p/embedded-switch-mode-power-supplies-smps/7002381/
and control it with an arduino.
so my research of the depth of various forums so far points me to High power

H-bridge motor drivers. (or just the chip they employ?? as at least on RS they only sell those and not the drivers i find on ebay..)
however I am not good enough with electronics to assess what exactly I need. I have a vague idea that the bridge should be chosen quit strictly to my appliance to not loose a lot of power or fry my circuit and as Peltiers are quit different things than dc motors I am not sure if I can just take the motor drivers one to one (or if I for example should use a capacitor in paralell to my peltier, but what effects has that on the h bridge? and so on)...

what the peltier will need to do is to ramp a temperature in a very precise way, I don't really know yet how often that means switching between heating and cooling or freewheeling but i think the more flexible on that end the better for temperature control.

So I think I have posted all the relevant info now, there is some more project description in the arduino forum post but I don't think it absolutely necessary stuff. here the link for hte interested though: (http://forum.arduino.cc/index.php?PHPSESSID=o2rtjjkjbr5efjlfukju3vhlu2&topic=189913.0)

I would be very thankful for any help.
Already a "Buy this thing for these reasons" with a link would be enough
(although i might come back with a sketch of how i plan to connect it up before actually doing things )
All the best
sven

 

You could use an old Atx psu for the suppy, and use a pwm circuit from the arduino, or use a555 timer chip as a pwm source with an Irf640 mosfet. For temp sensing i would use an op amp comparator with thermistor, and feed the signal to the pwm circuit. If you can set up the Arduino as a pwm source and temp sensor in one, then do that, or use the method i suggested.

 

Hi Dave,

thanks for the help. Doe thermistors have the range I am interested in? I will cool the device down to 77 K and they should not break by doing so. also I will have to be able to measure temperature from around 180 K to 350 K.
I know RTD would do the trick but those seem to be a bit on the expensive side of things.
Apart from that, any ideas on how to actually drive the peltier?
All the best,
Sven

 

Ok then at those temperatures -70C to +80C ,i would use TMP 36 sensor or Vishay do Thermistors with an op amp comparator replace the ntc sensor R8,with the TMP36, omit Ub, Uc, Ud, and connect the output of Ua to pin 4 of the 555 timer.

Note as the temperature rises the TMP36 output will increase, so the opamp output will go to zero, as it cools, it will go high, if you need the opposite use an NPN transistor to invert the signal, alter the temperature switch level by changing the values of R4/R5/R7 . The Thermistor will do the opposite its output will increase with a decrease in temperature.

As for driving it use a 555 timer pwm , you can switch the pwm on/off by applying 12volts to pin 4= on/ ov = off.
You set the pwm output by altering the potentiometer P1, which varies the heating control of the Peltier so this could be connected to the output of the temp sensor opamp on pin 1,. if 555 timer pin 4 is at 12volts heater is on, if pin 4 is at zero volts heater is off.

Also you can use an lm324 pwm

 

Hi Dave,

thanks for the help. Doe thermistors have the range I am interested in? I will cool the device down to 77 K and they should not break by doing so. also I will have to be able to measure temperature from around 180 K to 350 K.
I know RTD would do the trick but those seem to be a bit on the expensive side of things.
Apart from that, any ideas on how to actually drive the peltier?
All the best,
Sven

I can't see a peltier creating such a wide range of temperatures myself and I certainly can't see it managing -266degC unless that's not what you meant when you wrote 77K. I would think 350K (or was it +77degC) would be possible.

 

Hey I actually meant 77 Kelvin. However the peltier will not cool down to 77 K. Liquid Nitrogen will, after that the sample gets Illuminatet by a xenon flash (or maybe highpower LEDs if they are strong enough) and then either the peltier or the room will heat the whole thing up until 200K where the experiment starts and I want to use the peltier to have a very controlled heating gradient (something lie .5 or 1 degree per second) up to about 350 K. Then I will start from the beginning. As long as the peltier can stand the 77 k its all easy peasy (sort of) In some faq for peltiers I read that they should be ok as long as you heat them not to quickly so there is not to much stress through different expansion of the different materials....

Do I need to no anything special about using a ATX psu for testing purposes in the beginning? I found some youtube links on that...

cya sven

 

I think the peltier will be OK as long as you are careful about gradients and heating (or cooling) rates. I suppose you've confirmed they actually work at 77°K?

A challenge will be to establish a protocol for heating at the rate you want. It may be simple to describe but implementing it will be tricky. You may need to learn about PID control concepts and then tune the code for your apparatus. PWM duty cycle is your gas pedal.

 

A peltier creates a temperature difference and needs to have a heatsink on the reference side (ie the side you dont really care about opposite the side that you are trying to set the temperature on). How are you going to dump the reference side heat when the controlled side is well below room temperature? As you suggested a bridge control it sounds like you want to flip the peltier from heat to cool how will the reference side work?

 

I don't think you will get a TEC to go down to 77°K.
Maybe a stacked TEC will. You will also need to water cool the outside TEC.

 

Once again, I wont use the peltier to go down to 77 K. I use liquid nitrogen (boiling point 77 K) to cool the device. the side of the peltier that I don't care about will be a large enough copper block (I think of something like 5 times the heat capacity of the sample plate) As the peltier will be pumping heat out of this block during the experiment and it has a larger heat capacity it should always stay below the temperature of the sample plate. so cooling this will not be a problem as long as it is just for correcting the heating gradient and not cooling down by massive amounts.
However wayneh is probably right that the control programming might not be easy (i guess this depends a littlebit on the heat capacity of the system in relation to the peltier and I have no clue exactly how to do it) I have heard about PID but never done it. I think there is a arduino library for PID though.
however as a first try I would just try something simple like substracting the real temperatur from the should be temperature and use this error with an arbitrary multiplicator (defined through testing) as a value for the amount of heating or cooling I need...
but if anybody has experience in control of gradients I am more than happy for help. but i fear most people only want to cool or heat at maximum rate until a given value...