Gas Valve Solenoid

Discussion in 'HVAC Heating & Cooling' started by Thatguy, Dec 21, 2008.

  1. Thatguy

    Thatguy Homeowner

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    Anyone know the resistance, or current draw @ operating voltage, for a 28mV gas valve solenoid?
    How 'bout the short circuit current for a 28 mV thermocouple?
  2. jimbo

    jimbo Plumber

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    Honeywell lists the specs for the Q340 as: open circuit voltage ~30mv; resistance .02 ohms. That would make the theoretical short circuit current 1.5 amps. I suspect the normal operating current is not in that range, as of course the load will not be zero ohms. If the load is in the range of even just a few ohms, that would give operating current in the range of 1 ma.
  3. Thatguy

    Thatguy Homeowner

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    So far, so good

    I have this test from a Johnson Controls Tech. Btn

    Open circuit mV for TC 20-28, 25 to 35
    min load voltage 8 mV
    open circuit min 4 mV due to reduced flame
    main burner on in 4 seconds at 4 mV.

    Now all I need is solenoid resistance to figure operating current, and then pass/fail specs for a solenoid tested independently of a TC.

    Seems that some TCs which meet the 28mV open circuit spec still cannot open a good valve.
    Last edited: Dec 21, 2008
  4. Bill Arden

    Bill Arden Computer Programmer

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    if it's a 30mv open and could be 28mv under load then the current would be in the range of (2mv / .02 ohms) = 100mA

    Since the standard seems to be based on a constant voltage source, the test should also use a constant voltage source.

    Also keep in mind that it's better to measure values than have a pass/fail result since it takes a lot less samples to create standard deviation statistics.

    If you are going to test a lot of valves... I would suggest making a low voltage power supply that you can vary during the test in order to measure the cut-in and the cut-out voltages. and also keep an eye on currents.
  5. Thatguy

    Thatguy Homeowner

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    Thanks, folks. I'm looking into making a test circuit for gas valves using a small battery and probably a resistor or IC voltage regulator, so that the voltage/current is predictable and repeatable.
    Seems some people like to test these valves with an AA battery.

    I don't mess with gas; this is to help an HVAC guy with the electrical part of the test.
  6. Redwood

    Redwood Master Plumber

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  7. Thatguy

    Thatguy Homeowner

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  8. hj

    hj Moderator & Master Plumber Staff Member

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    t'couple

    What is happening with the valve. I have never had a "good" valve that a "good" themocouple would not keep open. I have a Baso tester that I can vary the mv output to see when the solenoid drops out, and also the t'couple to see what its output is and how long it takes to fall to the dropout range when the pilot is extinguished.
  9. Redwood

    Redwood Master Plumber

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    Uh yea my bad!
  10. Thatguy

    Thatguy Homeowner

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    You have a link to your tester?
    Do you know how much current a 28mV valve takes, or its DC resistance? The guy I'm helping wants to know if he can use an AA battery to test it, and if I can find these values I'll be in a good position to say.
    Is there any way a bad or shorted valve solenoid can damage a good thermocouple?
  11. gdog

    gdog write diagnostic firmware for embedded industrial

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    What is the specific part number for the valve and TC you want to test? Is the data sheet online? If so, please point us to it.

    Why are you testing these components? To determine if it's bad onsite, or is this for some kind warranty validation check, or something else...?
  12. jimbo

    jimbo Plumber

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    Not sure about that one, but I would suspect that a battery could damage a good valve!
  13. jimbo

    jimbo Plumber

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    I found a Robertshaw reference on thermopile operated valves (750mv) and it indicates the resistance of the valve coil at about 11 ohms. A 30mv valve would likely be different.
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