I have been working on an Arduino-based system to control the temperature of a laser diode thermoelectrically. I am using a laser mount from Thor Labs which has a Peltier module and a thermistor built into it. The max current and voltage rating for the Peltier module is 3A and 3.75 V respectively.
I have to implement a PI control loop within the Arduino to control the PWM signal to maintain a set temperature for the Peltier module. To drive the Peltier module, should I use a circuit with MOSFETs in H-bridge configuration or shall I use one of the motor drivers like L293N for the Arduino? After looking at other forums on similar topics, I found people suggesting both.
I will also have to convert the PWM signal to a current signal since Peltier modules are current-driven devices. Using a direct voltage to drive them might destroy them. I will also have to use an inductor or an RC low pass filter to smooth the signal before passing it to the Peltier for better operation.
The PI loop will read the temperature of the laser diode mount and the PWM would be adjusted according to that.
Any suggestions on how should go about making the driver circuit for the Peltier?
Normally, you don't use PWM for heating/cooling because nothing heats-up or cools down that fast (except maybe a LASER diode ).
You've probably noticed how the heater in your house works... It turns-on full-blast 'till it gets to the target temperature, then it shuts-off. Your refrigerator works the same way, cycling on & off.
There is usually some hysteresis. That is, a heater doesn't turn on until you are slightly below the target temperature, then it stays-on 'till you are slightly above the target. (That's so your heater doesn't cycle on & off several times per second.) You may not need to do that, but there's probably not any harm in slightly over-cooling your LASER.
I will also have to use an inductor or an RC low pass filter to smooth the signal before passing it to the Peltier for better operation.
You wouldn't want to do that either. The "R" would have to be small in value (for low voltage drop) but physically big to dissipate the heat, and that means your "C" has to be big, and the thermal-mass will automatically "smooth" the temperature change anyway.
That won't work. The ancient L293 is extremely inefficient and can't come close to handling 3 Amperes. Consider a modern motor driver, like this one, keeping in mind that most motor drivers require more than 5 to 6V for motor power.
Using a direct voltage to drive them might destroy them.
Not if you use a low voltage power supply (say, 3.5V). Since you are cooling only, you can simply use a logic level MOSFET as a switch.
Normally to use PWM to drive a peltier, you need an inductor in series with the peltier to smooth out the pwm to provide a constant current .
The inductor needs to have a saturation current higher than the maximum current the peltier will draw.
Peltier is solid state, you can feed it intermittent power same as a led.
Get a logic level (referring to the gate, otherwise 10V is needed) MOSFET that can carry > 3A. Ebay had IRLZ44's cheap this summer, check out the datasheet, you get a few for $1.. I bought almost $10 worth at 14.5 cents each.
Bigger concern for you may be your code. If you don't block, you could be taking samples per millisecond.
There seems to be some confusion as to whether PWM damages Peltier elements. If that is your only concern, go ahead and use PWM.
Here is a scientific approach to the question. The paper's conclusion is that the damage done over thousands of hours of operation is small and not significant.
However, this site points out that with regard to power input, PWM makes less efficient use of available power. Again, if power cost is not a concern, go ahead and use PWM.
I'm not sure whether the scientific tests cover the essential parameters. In fact we have small temperature jumps from the PWM frequency, and possibly much larger jumps from one stable temperature to another one. Since PWM frequency doesn't cost money with an Arduino, and the high frequencies allow for small flattening inductors, this problem is almost academic. More attention should be paid on the start of cooling, and on temperature changes causing mechanical strain. The Peltier should not be stressed by too high temperature jumps at either side, i.e. the temperatures should be given some time for settling on a new setpoint. The PWM duty cycle should not change abruptly, but should increase or decrease with an acceptable slope.
I'm not sure whether the scientific tests cover the essential parameters.
Then do the experiment and settle the issue. It would be very straightforward, albeit time consuming and the results would provide a useful contribution.
When there's parts and assembly needed, I don't think that's only academic.
An inductor and flyback diode would do? I'd have to cookbook... whut's a fly-back di-ode? Wiki-time!
Oh... right, so the inductor can cycle the current from its own field collapsing.
How does Peltier efficiency with PWM compare to efficiency of a linear PS?
Suppose you used much lower pulse frequencies, into seconds per cycle like microwave ovens do?
Hi,
An important parameter you must check about your Peltier device is the Max surface temperature - differential you are allowed.
Exceeding this will cause the element to fail.
So if you are cooling you need to also monitor the HOT side of the Peltier element and shut down if that figure is approached.
If you find it is reached in normal opertation, you will need a bigger heatsink, and/or ventilation fan.
Can you post a link to the data/specs of your Peltier Device please.
Have you worked out how much heat energy you will have to dissipate?
GoForSmoke:
Suppose you used much lower pulse frequencies, into seconds per cycle like microwave ovens do?
That's not a good idea, because every temperature cycle stresses the element, by different expansion of the stacked materials over temperature. Such stress will at least reduce the efficiency and lifetime of the element.
If the hot side is in contact with a thermal mass, even a heatsink, there won't be much thermal shock on the wafer.
Project is to control temperature of a heat-producing device, a solid heatsink makes sense.
Within a Peltier, the heat travels between the junctions as electron states in current not as molecular movement. The heat is taken up by electrons crossing from the more highly conductive material on one side of the junction to the less highly conductive material (heat raises electrons above the step) and then heat is released when those electrons cross back to the first material and lose the excess energy. Between junctions the energy transferred is not heating the material between, Seebeck knew this first!
The semiconductors don't get hot except through much slower conduction from the hot side. That is my concern about thermal stress, would cycling power cause an appreciable fast temperature drop in the wafer. Mostly ever not.
I agree that smoothing power to the wafer should be best but not about necessity. A low-bypass filter with components able to take the wattage, how much there?
Hi,
The PWM frequency is higher than the reaction of thermal mass of the device to cause thermal shock.
Car/Truck 12/24V fridges use PWM control on their Peltier elements along with a fan to cool the heatsink.
One of our calibration pots at work is a small volume cell to hold liquid with a Peltier element connected.
If I remember, the PWM frequency is 24kHz with an inductor in series with the Pelter element, it heats to 50DegC and goes down to -10DegC.
It uses a PID controller.
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