# PID temperature controller for TEC/Peltier

#### drfreaky

Joined Jul 5, 2009
17
Hi,

I am new to this forum, I need suggestions and ideas to built a power conditioning circuit for a peltier element.

My circuit is an analogue PID and the set point is set manually while the feedback is using a measured thermocouple as the feedback to the setpoint

Power = 130W and above
Voltage = 2 -15V
Current = 2 - 10A

Any help appreciated

Last edited:

#### drfreaky

Joined Jul 5, 2009
17
Anyone? Help

#### THE_RB

Joined Feb 11, 2008
5,430
Post your circuit? And what help do you need?

#### drfreaky

Joined Jul 5, 2009
17
I would like to have a constant current source but the voltage can be adjusted by the input voltage for example i would like to drive the peltier element at 9A thus the voltage can be varied from 8V to 10V to get a max. temperature difference

At the moment, i am using a bipolar transistor circuit but its only having a current regulation therefore if the Vinput changes the current changes and hence also the voltage

and here it the attachment and hope you can help

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#### beenthere

Joined Apr 20, 2004
15,808
Component values? Transistor types? Operating voltage? Temperature to maintain?

#### drfreaky

Joined Jul 5, 2009
17
The peltier element should have a resistance of +/-1.25 ohms and i am using 4 of them so i guess it should be around +5 ohms and connected in series (to get the same current). Or worse case scenario i need to build for individual 4 circuits with the same feedback

I havent look at the transistors yet, but i am thinking to use a transistor that can with stand up to 10-15A max value

for the resistor to the amplifier i am thinking to use a 1 ohm resistor since it works like a shunt an the current is set by the resistor but at this moment varying the input voltage would vary the voltage supply to the peltier element

the max value that the peltier element can go is up to 15V and i am using a 30V supply to supply current and voltage to the transistor. the problem is that i need to operate the peltier element at 9A and if i can vary the voltage from maybe 9V to 11V with constant current source.

Here i have attached the datasheet for an example i dont know if i am intrepeting it right but i am hoping its right, from the datasheet page 2 at 9.2A a larger temperature difference can be achieved by varying the voltage with in +/-1V when it is at 9V.

I hope you guys can help cause i am still having problems designing this power circuit

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#### Frakk

Joined Jul 6, 2009
40
I would use a pwm modulated FET to drive current to the peltier. Much more efficient than a transistor regulated current source and high current FETs are fairly cheap too. The thermal sensor would adjust the pwm duty cycle therefore regulate the cooling/heating effect.

If the temperature doesn't have to be very stable you could use a simple comparator with hysterisis to turn the FET on if the temp is above or below the set reference temp.

#### drfreaky

Joined Jul 5, 2009
17
I would use a pwm modulated FET to drive current to the peltier. Much more efficient than a transistor regulated current source and high current FETs are fairly cheap too. The thermal sensor would adjust the pwm duty cycle therefore regulate the cooling/heating effect.

If the temperature doesn't have to be very stable you could use a simple comparator with hysterisis to turn the FET on if the temp is above or below the set reference temp.
Any Schematics or designs that i could see to help me understand? I am not really familiar with PWM and i always thought that PWM are for digital control whereas my circuit is analogue.

#### Frakk

Joined Jul 6, 2009
40
First I would get familiar with the concepts before any schematics. Search for PWM, there is a very simple and effective circuit using an inverter with schmidt trigger, however you will need a voltage controlled method.

Is temperature stability a big issue with your application? If it is not, I would use a comparator that turns the peltier on and off when the temperature falls out of your range. They are very simple to use and you will find a lot of info about how to connect them properly.

Driving the mosfet and setting the temp with a PTC or NTC resistor is also straight forward and they are explained on a lot of websites.

#### THE_RB

Joined Feb 11, 2008
5,430
It's bad to turn peltiers on/off the thermal cycling destroys them. If I remember the freq right you are supposed to PWM at higher than 4kHz(?) to avoid the thermal shock.

But then it probably doesn't need a full PID, which are mainly needed when there are large sudden changes in load and the device needs to respond quickly. The slow response and fairly fixed load of a thermal system can be controlled easily enough just with PD or even just P.

#### drfreaky

Joined Jul 5, 2009
17
It's bad to turn peltiers on/off the thermal cycling destroys them. If I remember the freq right you are supposed to PWM at higher than 4kHz(?) to avoid the thermal shock.

But then it probably doesn't need a full PID, which are mainly needed when there are large sudden changes in load and the device needs to respond quickly. The slow response and fairly fixed load of a thermal system can be controlled easily enough just with PD or even just P.
Any schematics or circuit design that i can look at to help me understand?

#### drfreaky

Joined Jul 5, 2009
17
Any suggestions that i can look at?

#### drfreaky

Joined Jul 5, 2009
17
http://en.wikipedia.org/wiki/Comparator
http://en.wikipedia.org/wiki/Voltage-controlled_oscillator
http://en.wikipedia.org/wiki/Pulse-width_modulation
http://homepages.which.net/~paul.hills/SpeedControl/Mosfets.html

Without more information about your project and what it needs to do specifically it is hard to show you an example. I asked if temperature stability a big issue?
I dont think temperature stability is an issue i guess i just need to cool and heat up the peltier to a certain temperature and i am planning to do it with a PID. Say if i use and H- Bridge to cool and heat up the peltier unit can i still use a PId controller?

#### Frakk

Joined Jul 6, 2009
40
Yes you can use an h-bridge to do that. You will need 2 comparators and use them as window detectors. Your "window" will be the set temperature, if the voltage falls below, the h-bridge will heat the peltier, if it goes above, it will cool it down. As RB posted it might not be a good idea to do this with a peltier, I would definetly check the datasheet or other sources.

#### THE_RB

Joined Feb 11, 2008
5,430
You can do it with one comparator. Connect one input of the comparator to a voltage divider using a potentiometer, so you can set the temperature. Connect the other input of the comparator to a voltage divider which is a resistor and a thermistor (not thermocouple). This is a basic on/off temperature controller using a comparator, there are tons of circuits on the net you can look up.

Now to make it oscillate you add negative feedback with a resistor from the comparator output back to the - input of the comparator, and put a small cap from that input to ground to set the oscillation frequency by the cap value so it oscillates at about 5kHz, and aim for oscillation amplitude (on that pin) of about 1/2 of the supply voltage. You can set that by the value of the feedback resistor.

Now it oscillates as a 5kHz PWM temperature controller with the duty cycle controled by the difference between the control voltage you set on the potentiometer and the (average) voltage created by the thermistor on the other input. It acts like a crude P of the PID, the D is taken care of by the thermal lag of the whole device and generally I is not needed in temperature controllers.

#### drfreaky

Joined Jul 5, 2009
17
You can do it with one comparator. Connect one input of the comparator to a voltage divider using a potentiometer, so you can set the temperature. Connect the other input of the comparator to a voltage divider which is a resistor and a thermistor (not thermocouple). This is a basic on/off temperature controller using a comparator, there are tons of circuits on the net you can look up.

Now to make it oscillate you add negative feedback with a resistor from the comparator output back to the - input of the comparator, and put a small cap from that input to ground to set the oscillation frequency by the cap value so it oscillates at about 5kHz, and aim for oscillation amplitude (on that pin) of about 1/2 of the supply voltage. You can set that by the value of the feedback resistor.

Now it oscillates as a 5kHz PWM temperature controller with the duty cycle controled by the difference between the control voltage you set on the potentiometer and the (average) voltage created by the thermistor on the other input. It acts like a crude P of the PID, the D is taken care of by the thermal lag of the whole device and generally I is not needed in temperature controllers.
Thanks but now i am thinking to use a chip to power drive my peltier elements i think i need around 30-50V and current of up to 10A as i am thinking to connect them all in series. Each of the element is around 100 - 130W

My circuit would response to value changes as i am thinking to set my temperature at constant 25C therefore if there is any changes to the surface area (not the peltier element) that i am cooling down, my controller would automatically read this error and automatically heat or cool down the peltier element to adjust to the set point temperature.

Temperature changes are due to intensity of light or room temperature

So any suggestions?

#### Frakk

Joined Jul 6, 2009
40
We already suggested how it can be done. What kind of chip are you thinking about to drive the peltiers? Can you tell us what you are going to use this for? Keeping the temperature of what?