Peltier control?

[droptail]

I am going to start a project using Peltier modules [Thermoelectric cooling (TEC)], and was researching control options.
A search here didn't turn up anything. Anyone done a similar project willing to share their approach?

Thanks

[Charles Linquis]

I have controlled them with filtered PWM output. What voltages/currents are you dealing with?

[droptail]

I have controlled them with filtered PWM output. What voltages/currents are you dealing with?

I'm looking at a pair in parallel of 40mm, 15Vmax, 9Amp variety.
I understand operating at 50% capacity improves efficiency.

[Charles Linquis]

Sure, they aren't very efficient in the first place and very few devices of any kind are most efficient at 100% capacity. And if you want long life and you are cooling something with them, make certain that you do a good job of heat removal from the 'hot' side.

A couple of other questons:

I assume you need proportional control (Y/N) ?
Does the Peltier GND terminal need to be isolated from the PIC (and rest of the circuit) GND
Do you really have 15V available, or is it 12?
Military, commercial or home use?

[ivanrosales]

James, IMHO your best bet is using PID, "Master" Henrik Olsson have a neat routine in this post: http://www.picbasic.co.uk/forum/showthread.php?t=5874&highlight=incpid
You should give it a try.

Regards.
Ivan.

[droptail]

Charles,
Do you mean variable control? That would be ideal, but I may only need to voltage divide from 13V to 7.5V for the 50% efficiency.
I'm not aware of any needed isolation requirements.
The modules are rated at 15Vmax, I am working with a 13V power supply.
Personal use.

Are we talking MOSFETs using PWM control?

Ivan,
I have downloaded the file and am looking at the link.

Thanks

[Charles Linquis]

Yes, I'm talking about PWM control using FETs. Many devices (such as brushless fans), AND Peltiers don't like to see a "chopped" waveform, so you should filter the PWM signal to pure DC. Also the PWM frequency should be above 20KHz, so that it doesn't generate any noise in the audible range. Basically, the simple circuit where you connect a FET Drain to the load's (-) terminal, the FET Source to GND, the FET gate to a PIC pin and the load's (+) terminal to 12V just won't do.

At current levels like this, you will need a FET driver, a big FET, an inductor, a Schottky diode and some capacitors that can handle high ripple currents.

I asked about the use because I just got an RFQ from a military customer that is using Peltiers. I didn't want to compete against myself.

[ivanrosales]

James, we are talking about two different issues, control and drive... as per control (this was your original question) I still recommend you implementing some sort of PID. For drive, you have to deal with two 9 Amp loads which is a lot so Mr. Charles approach is the right way to go. Some time ago I repaired a small peltier commercial fridge (used for an energy drink promotion) and it had a good EMI filter, a small control board and two solid state relays, very simple indeed...

[ivanrosales]

One more thing, needless to say that the aforementioned fridge had a huge aluminium dissipation at the back (shared for the relays and the peltier plate)...

[droptail]

My thought was to operate them at a max of 50%, about 7V/4.5A each, (9A total).
I prefer to connect them together in parallel, and drive or control as one device.

I was also considering a simple voltage divider to go from the 14V to the 50% state I prev mentioned to keep it real simple, but don't think the power resisters wouway to go would be practical/doable.

I won't know if this is an option until I receive my test modules soon and play with them. I would then have some "control" of them them based on on/off.

Thanks for the replys

[Charles Linquis]

Your choice. I can give you a "plug and play" circuit that will give you total proportional control, if you choose to go that route.

[BrianT]

I use an H bridge with 2 Pchannel and 2 Nchannel FETs on a 13.5 volt rail. The TEC pulls about 8 amps when flat out. The power supply can deliver 15 amps and the FETs can switch 75 amps. The H bridge is controlled via a dedicated 16F88 which reads a Microchip MCP9800 to give me 1/16 degree resolution and drives the H bridge. Because the FETs have just simple resistive gate drive via 2N7002 as level shifters there MUST be delays inserted between turning the FETs On or Off otherwise there is a short but deadly overlap where both FETs on the same side of the H bridge are conducting simultaneously. This leads to the FETs getting very hot or worse, it burns the traces off the PCB. My application is fully proportional and must both heat and cool. The drink cooler examples I have seen are simple cool-only with a thermostat to drop the drive once below the setpoint. Drive is flat out or off.

You MUST ensure the entire surfaces of each side of the TEC are thermally bonded to the cool chamber or the heatsink otherwise any small unbonded area will get very hot and burnout the TEC in no time.
TECs are poor coolers but good heaters. My device works between 1 Celsius and 45 Celsius -roughly Ambient +/- 25 Celsius. In cooling mode I must blow lots and lots of air over the heatsink to get down to the lower temperatures and supply high currents. When heating the chamber the average drive currents are about 1/4 of the cooling currents.
TEC life is severely degraded by thermal stress. If your controller drives full heat then full cool so that the average gives you the setpoint you are after the life will be short. The thermal stresses will break the bonds between the magic metal elements. I run a fixed frequency 50 Hz software PWM and vary the duty cycle from 0 up to about 19500 microseconds. I am only running a Proportional controller, not a full PID. Seems to regulate to about 0.1 Celsius but is slow getting to the setpoint which does not matter in my application which is left on for days on end.

HTH
BrianT

[Salman1]

hi..

i have already made hardware and there is no H-bridge used. the peltier module is connected with two PWMs i have to use 2 stage PID which has 2 outputs. the question is how it would like i have tried to make condition that if the current temperature is more or less than setpoint then switch PWM but it doesnt look like stable it switch and some how ocsillate. i have read about dead zone can any body give me more ideas how i can make it better or use and other condition?