PWM Question

Discussion in 'General Electronics Chat' started by DougB, Dec 22, 2010.

  1. DougB

    Thread Starter New Member

    Sep 11, 2010
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    I am using a PWM to power/regulate a Thermal Electric Cooler.With respect to the PWM only my normal supply voltage to the PWM is ~20VDC. (2.5 to 30 is an acceptable range). The PWM is calibrated to limit the maximum output to 5.75 VDC. Max allowable current to the TEC is 2.8A.

    I need at times to use a battery to power the PWM. Lets assume the need for cooling is the same with respect to the TEC regardless of the source voltage to the PWM. Assuming a battery output of 12 VDC, what effect would this have on the calibration of the PWM. Would it still limit output to 5.75 or would it likely be some other value. Would it affect the current output as well?

    Hope this doesn't seem like a stupid question. All I have is some old schematics for this power supply designed by a fellow 10 years ago. He is not available to answer any questions. I fix aircraft for a living. Reading schematics I can handle. Understanding what goes on in the black box is way above me.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Welcome to AAC!

    A schematic is worth a thousand words, can you show yours?
     
  3. DougB

    Thread Starter New Member

    Sep 11, 2010
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    The PWM is a kit produced by a company called Velleman. The kit # is K8004. I dont know if that will help much. I could produce a schematic of the control cct that provides the control voltage into the PWM for system regulation but I am not sure that this would help a lot either.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Uhh, schematics are pretty much required for anyone to offer meaningful help. Otherwise we are just offering guesses. Schematics are the language of electronics. You will notice no one else has jumped in yet, we are waiting for you to provide basic information.

    TECs are pretty much current devices. Increase the current, the effect increases, until you cross the threshold that causes damage. Other folks on this site claim PWM is bad for TECs. I'm not so sure myself, but I would admit that while I have played with them I am no expert.
     
  5. nerdegutta

    Moderator

    Dec 15, 2009
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  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Add one more battery for 24 volt nominal.

    Add a voltage regulator (set for 20V) and use that when the circuit is run off of battery. Your timing and output voltage levels should remain very close to where they were before.
     
  7. DougB

    Thread Starter New Member

    Sep 11, 2010
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    Thanks Nerdegutta, I see the schematic for the PWM is found on the illustrated assembly document you published the link for. The only external input to the system is a reference voltage supplied by an LM35 temp sensor and its associated circuitry. If it would help to post that schematic I can do that. Kermit2, the PWM is only one power consideration for my project. Using a 24 volt alternate supply would cause me more grief than I already have. I am almost in over my head already.
     
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