Go Down

Topic: Peltier Reliability issues (Read 12192 times) previous topic - next topic

mdbonneaux


H-bridge product page

Rough sketch of it.  I'm using 2 pins in my setup- one to send the signal for the current to go one way (hot) and the other to go the other way (cold).  When one signal is on, the other is off.  I have the "to peltier" boxes colored as the wires as inserted.  There are 4 total inputs- the left 2 (white boxes) have just one control wire that turns the fan on the peltier's heatsink on or off (it's not since I'm not trying to make it cold).  I also have a power input missing; it's 5V in only and unused.

So as far as the amperage of the peltier goes, regardless of if that's the problem or not, by it trying to draw more power than the PSU and the bridge are able to supply, it's wearing them out significantly?

MarkT

Are you measuring the temperature of the heatsink?
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

mdbonneaux

Are you measuring the temperature of the heatsink?
... This I do not remember.  I remember that I have had a temp zone box on it with the camera but I do not remember what the result was.  I'll try to test this tomorrow.  Any explanation on how it could be causing my issue though?

jremington

#18
Nov 04, 2014, 12:24 am Last Edit: Nov 04, 2014, 06:19 am by jremington
Ah, yes, the venerable L298 motor driver.

The L298 driver chip can supply about 1 ampere per channel on average before it begins to overheat and shut down, despite the optimistic claims of the data sheet. As even the data sheet makes clear, the absolute upper limit for current is 2.5 amps/channel, and for only very short times, repetitively. See this test: http://www.ruggedcircuits.com/the-motor-driver-myth/ 

However, you can parallel the bridge outputs to approximately double that.

If you want a motor driver than can handle the current your Peltier requires, this is one suggestion (I use these and am happy with their performance) http://www.pololu.com/product/1451

Of course, you will need a beefier power supply too. Then, you can expect fully predictable performance from your system.

DVDdoug

Quote
1) feedback is being accomplished with a infrared camera and calibrated blackbody that's accurate to .001*C.  I set the PWM of the peltier, wait for it to level, and use the camera to match the blackbody to the peltier.
Apparently, you are not using this feedback to control the PWM.    Apparently, you are sending PWM values of "500" or "1000" instead of varying the PWM value depending on the feedback.

BTW - What do these values of 500 & 1000 mean?   The Arduino normally uses PWM values of 0-256.    If you are trying to send values above 256 the last 8 bits might be getting used.   In that case, the 8 least significant bits converted from a decimal number would look rather random.


Quote
2) @DVDdoug- could you maybe explain peltiers a bit more to me?  How would this having a 6A rating be causing this effect?
You are trying to "pull" 6 Amps from a 3 Amp power supply!   ...What happens if you plug too many toasters & hair dryers into the wall and pull too much current?     Your power supply might not have a circuit breaker like your home, but the voltage might drop, or the power supply might overheat, etc., or it might "safely" go into current limiting and cut-back the voltage.

From Ohm's Law, current is Voltage/Resistance.    The amperage depends on the voltage applied and the resistance of the Peltier.   If the power supply cannot supply the full 6A at 12V, the voltage must drop, since Ohm's Law is a law of nature and it's always true.



LaserLance

#20
Nov 04, 2014, 03:57 am Last Edit: Nov 04, 2014, 04:23 am by LaserLance
There's a lot of great advice here.

I work with high-powered lasers, some of which are air cooled. In the air-cooled lasers, we use Peltier devices and fans to manage the temperature of the laser diodes since the wavelength of light that is output by the laser diode is proportional to its temperature. This is important because the absorption wavelength of the gain medium is very narrow, so temperature needs to be regulated to within about 0.01°C in order for the laser to be efficient. With this in mind, I have a lot of experience with this type of system and I think that you are on the verge of solving the problem.

We use a PID controlled, PWM output, temperature controller that can source up to 25-Amps of current at 24VDC. The Peltier devices will respond faster if you can give them more current, assuming that the fan you have is sufficient in removing heat from the hot side of the device. You must have some sort of feedback, like a thermistor connected to a ADC pin of the Arduino. Of course you also need to have some sort of calibration table for the thermistor that you use. From there, a PID controller can be employed that will allow you to fine tune the output of the controller based on the input from the thermistor. One of your biggest problems is that you have neither a power supply nor a controller that can provide the sufficient current to the Peltier device. While your particular setup may not require 25-Amps, I think that 1-Amp of output is a bit low for a stable Peltier device.

Also consider that the Peltier device is a differential temperature device in that the environment temperature in which the device is installed will have some effect on the device's ability to heat, or cool. By using a stronger fan blowing across the heatsink, that has a constant RPM, you can make the system more stable this way and help it to heat, or cool, better.

This is one of the controllers that we use. You may be able to look at the datasheet to understand how a precision Peltier temperature controller works. http://www.ovenind.com/bv/Products/5R7-001-RoHS-Temperature-Controller-with-RS-232__5R7-001.aspx

Note: The MCU they use in this device is a ATMega2560. ;)

Laser-Lance.

Paul__B

4) The peltier is about as flush with the heatsink as I can get it without using a press of some sort- I can fit about 3 or 4 sheets of notebook paper in between the two.
Eh?  If you can fit paper in between the two, then it is not making contact at all!

TomGeorge

Hi, have  you got a fan on the heatsink fins when you are cooling, the peltier cannot do it all on its own, it needs help with heat sinks and sources.
The fan also moves air around, so when heating you do not get a localized area of chilled air on the side you are pumping heat from.

Just a suggestion.
Tom...... :)
Everything runs on smoke, let the smoke out, it stops running....

mdbonneaux

Eh?  If you can fit paper in between the two, then it is not making contact at all!

@Paul__B, if you'll read some of my earlier posts, this gap is due to having a heat sink compound in between the peltier and the heatsink, not because "they're not touching."




@TomGeorge, the fan only runs when I'm trying to cool it and thus far all of my calibration testing has been for higher than ambient temps.  I've tested it for cooling functionality, I'm just not using that aspect of it right now.




Apparently, you are not using this feedback to control the PWM.    Apparently, you are sending PWM values of "500" or "1000" instead of varying the PWM value depending on the feedback.

BTW - What do these values of 500 & 1000 mean?   The Arduino normally uses PWM values of 0-256.    If you are trying to send values above 256 the last 8 bits might be getting used.   In that case, the 8 least significant bits converted from a decimal number would look rather random.

You are trying to "pull" 6 Amps from a 3 Amp power supply!   ...What happens if you plug too many toasters & hair dryers into the wall and pull too much current?     Your power supply might not have a circuit breaker like your home, but the voltage might drop, or the power supply might overheat, etc., or it might "safely" go into current limiting and cut-back the voltage.

From Ohm's Law, current is Voltage/Resistance.    The amperage depends on the voltage applied and the resistance of the Peltier.   If the power supply cannot supply the full 6A at 12V, the voltage must drop, since Ohm's Law is a law of nature and it's always true.



1) I'm using the Due set to 12-bit mode.  This allows a range of 0-4095 in each direction for me (hot and cold- 0-4095 on one pin to control it getting hot, and the same on another pin to make it cold).  Arduinos  based on the ATMega chips are limited to 8-bit resolution (0-255), but the Due is ARM based and allows this expanded range.

2) I'm well aware of Ohm's Law.  I frequently also use Wattage as a better estimate of how much juice is being used by a system than just V or A(I) when talking to my friends/family members who aren't as technical as I am  (I am by no means an expert, but I am more proficient than your typical consumer).  Going by your explanation, I'd be expecting the temp on the 0-250-500 cycle to be lower than or equal to what I get on the 0-500 cycle since the circuit would be fluctuating (hitting a peak and dropping until it can pick back up again over and over).  This isn't the case.  The exact opposite in fact.  The 0-250-500 cycle is getting HOTTER than the 0-500 cycle.

3) EXPERIMENTAL DATA TIME!  WOO!

I'm running it right now at full-tilt.  12V, PWM of 4095, and it's pulling... 1.65A.  I can take a pic and post it if you need me to.  It's not overloading.




@LaserLance- I'm going to be spending a bit looking over your post before I comment on it.  You seem to be doing something more fine-tuned than what I'm trying to do and have it working well (I only need accuracy to 0.1*C).

jremington

Good luck with your project!

mdbonneaux

BTW I do want to apologize to everyone if I seem angry.  I've been working on this off and on for months and I'm really tired of fighting with this, but dropping it entirely is not an option at this point.  I just want to figure this one issue so I can move on to calibrating the thermistors.

KeithRB

Then Paul_B is right, if you have a 3-4 sheet of paper gap, they are not touching at all. Heatsink compound is to fill the *gaps* between the device and the heatsink. The best thermal transfer is when the device has 100% metal-to-metal contact to the heatsink. In the real world, this can't happen, so we use heatsink compound to fill in the gaps, so you have something better than air transferring the heat.

The key is the spread the heatsink compound thinly - you should be able to see through it - and then torque it properly so that you get maximum metal-to-metal contact and the compound fills in the other gaps.

mdbonneaux

Then Paul_B is right, if you have a 3-4 sheet of paper gap, they are not touching at all. Heatsink compound is to fill the *gaps* between the device and the heatsink. The best thermal transfer is when the device has 100% metal-to-metal contact to the heatsink. In the real world, this can't happen, so we use heatsink compound to fill in the gaps, so you have something better than air transferring the heat.

The key is the spread the heatsink compound thinly - you should be able to see through it - and then torque it properly so that you get maximum metal-to-metal contact and the compound fills in the other gaps.
OK, is there a way this could be causing my problem?  I'll fix this later even if it isn't the cause though.

TomGeorge

Hi, heatsink compound is not as conductive as you think.
It should be used sparingly, I have a small straight edge and I use it to wipe the  compound over the surface.
It is surprising how little you need to use.

http://www.youtube.com/watch?v=zPJspil1HJo

Tom.......... :)
Everything runs on smoke, let the smoke out, it stops running....

Boardburner2

What pwm frequency are you using ?

For a peltier it should be below 3 kHz

Go Up