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Thread: How do I test Millivolt valve system - Pilot is on, but main burner not lighting

  1. #31
    Homeowner Thatguy's Avatar
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    Quote Originally Posted by mnalep View Post
    and after I put it back in the 2nd tome the reading was at the normal 750mV. Not sure why it was low the first time?
    I assume 750 mV is open circuit voltage. The voltage for a t'pile is related to flame temperature. There's probably a graph somewhere online for the relationship between temp. and voltage.
    A blue flame indicates more nearly complete combustion than a yellow flame.

    Two things define a voltage source: open circuit voltage and output voltage when current is being drawn.
    Voltages sources have a Thevenin equivalent circuit and it's pretty useful to figure out the two values in this equivalent circuit.
    Open circuit voltage is only half the story.

    You might want to buy a 3 ohm resistor and measure the t'couple voltage output when loaded down with this many ohms. Independent tests of each system component comes in handy.
    If the internal resistance of the t'pile is 3 ohms you will get 750/2 = 380 mV across the resistor.
    Last edited by Thatguy; 11-10-2010 at 10:55 AM.

  2. #32
    Plumber jimbo's Avatar
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    AH....my old buddy Thevinin, and his right hand man Kirchoff! I believe you are about right on the loaded source measuring about 380,

  3. #33
    Homeowner Thatguy's Avatar
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    Quote Originally Posted by jimbo View Post
    AH....my old buddy Thevinin, and his right hand man Kirchoff!
    I used to go drinking with these guys. . .

  4. #34
    DIY Senior Member mnalep's Avatar
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    thatguy,

    I'm not sure what the meaning of 'open circuit' is? I measured with the pilot flame on, and the leads unhooked from the gas valve. Is that what you meant?

    how would I physically hook up " a 3 ohm resistor and measure the t'couple voltage output when loaded down"? I've never heard of this, would you educate me please?

    where would i get a 3 ohm resistor?

    when you said "open circuit voltage and output voltage when current is being drawn. " do you mean with vs without the gas valve being hooked up to the leads of the pilot generator?

  5. #35
    Homeowner Thatguy's Avatar
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    Quote Originally Posted by mnalep View Post
    thatguy,

    I'm not sure what the meaning of 'open circuit' is? I measured with the pilot flame on, and the leads unhooked from the gas valve. Is that what you meant?
    Yes, it means no current is flowing out of the source. The current drawn by the voltmeter is in the microamp or nanoamp range and can be considered to be zero in this case.
    A wall outlet with nothing plugged in is open circuited.
    A short circuit means the maximum current available from a source is flowing.
    A closed circuit means some current is flowing.


    how would I physically hook up " a 3 ohm resistor and measure the t'couple voltage output when loaded down"? I've never heard of this, would you educate me please?
    Instead of hooking up the gas valve to the t'pile, hook each lead of the resistor to each lead of the t'pile. Then you'll have a simple series circuit with only two components, a source and a load.

    where would i get a 3 ohm resistor?
    I hate to send you to Radio Shack but there aren't many other places left where you can get components.
    http://www.google.com/imgres?imgurl=...1t:429,r:7,s:0

    when you said "open circuit voltage and output voltage when current is being drawn. " do you mean with vs without the gas valve being hooked up to the leads of the pilot generator?
    Yes, the t'pile can be loaded down with the valve or a resistor.

    This is assuming somewhere in this thread that the valve coil resistance of a working gas valve has been determined to be 3 ohms.
    Last edited by Thatguy; 11-11-2010 at 07:02 AM.

  6. #36
    DIY Senior Member mnalep's Avatar
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    thatguy, i've started reading that site, electronics-for-beginners.com, but McAfee Site Advisor (my antivirus) is saying there are potential security risks with that site. have you used it safely?

  7. #37
    DIY Senior Member mnalep's Avatar
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    ok, so an open circuit has no load device on it?

    I thought a short circuit was where a device, say, a toaster, was not getting any current? That sounds like the opposite of "maximum current available from a source is flowing" - wouldn't it?

    Isn't a short circuit when, say, a wire between the fuse panel and, for example, a toaster - has a bare spot in the rubber, and the copper wire is touching something metal - short circuiting the electricity away from the toaster?

    So a closed circuit has a load device? and 'some current' would still be enough to run the device properly?

    This is assuming somewhere in this thread that the valve coil resistance of a working gas valve has been determined to be 3 ohms.
    - I never tested the resistance(ohms) of the valve...it sounds like i'd need to?

    If the internal resistance of the t'pile is 3 ohms you will get 750/2 = 380 mV across the resistor.
    - so if I test for DC mV with a 3ohm resistor attached to the t'pile generator (and assuming the t'pile also had 3ohms resistance) - then I'd measure 380mV DC ?
    Last edited by mnalep; 11-13-2010 at 06:16 PM.

  8. #38
    Homeowner Thatguy's Avatar
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    Quote Originally Posted by mnalep View Post
    ok, so an open circuit has no load device on it?
    Right; no current flows.

    I thought a short circuit was where a device, say, a toaster, was not getting any current? That sounds like the opposite of "maximum current available from a source is flowing" - wouldn't it?
    ?

    Isn't a short circuit when, say, a wire between the fuse panel and, for example, a toaster - has a bare spot in the rubber, and the copper wire is touching something metal - short circuiting the electricity away from the toaster?
    Yes, it's a path that causes current to flow in the wrong places and usually at values much higher than the design value.

    So a closed circuit has a load device? and 'some current' would still be enough to run the device properly?
    A circuit can be closed and still have abnormally high or abnormally low current.

    - I never tested the resistance(ohms) of the valve...it sounds like i'd need to?
    Yes.

    - so if I test for DC mV with a 3ohm resistor attached to the t'pile generator (and assuming the t'pile also had 3ohms resistance) - then I'd measure 380mV DC ?
    With one assumption, yes

    0.750 =open circuit t'pile voltage [Voc] in volts
    2.890 =presumed t'pile internal resistance [Rint] in ohms
    3.000 =load resistor [Rload] in ohms

    0.127 =current [I] = Voc/(Rint + Rload) in amps
    0.382 =voltage across load resistor [Vload] in volts = Rload x I

    Try electro tech forum. Those guys seem to know their stuff.

    My Apple doesn't much care about viruses. My other computer, which hasn't been online for more than a year, still warns me about potential viruses.
    Anti-virus s/w is a money machine. What's to keep them from generating new viruses so they can sell you more upgrades?
    Last edited by Thatguy; 11-14-2010 at 02:15 PM.

  9. #39
    DIY Senior Member mnalep's Avatar
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    thatguy, Thanks, I am starting to understand. So if I did all this and found .382v DC, would that mean my t'pile is good, or bad, or weak? Sounds like it might be weak - if it is supposed to generate 750mV closed circuit am I saying that correctly? Or is the 750mV stated for this part just the open circuit voltage rating with no load on it (gas valve, or, resistor)?

    By "electro tech forum" - do you mean "ELECTRICAL FORUM" on this website?

    I hear what you say about anti-virus s/w makers. Who knows what they are up to.

    I am surprised to hear its not an issue on Apple! I'd be creamed on PC without it. And I still have not found one I really like. McAfee is seeming effective, but updates daily and stops me from working for about 15 minutes whenever it updates. It's free with my ATT DSL internet though.

  10. #40
    DIY Senior Member mnalep's Avatar
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    btw, do you ever have a problem replying here - get a message like 'the token has expired'?

    This just happened to me, and even reloading the page did not help. I'vw begun copying my replies, before clicking on POST REPLY, in case this happens - to avoid losing a message i've taken time to type. But sometiem forget to copy firts and lose a message I've typed.

  11. #41
    Homeowner Thatguy's Avatar
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    Quote Originally Posted by mnalep View Post
    thatguy, Thanks, I am starting to understand. So if I did all this and found .382v DC, would that mean my t'pile is good, or bad, or weak?

    By "electro tech forum" - do you mean "ELECTRICAL FORUM" on this website?

    I hear what you say about anti-virus s/w makers. Who knows what they are up to.

    I am surprised to hear its not an issue on Apple! I'd be creamed on PC without it. And I still have not found one I really like. McAfee is seeming effective, but updates daily and stops me from working for about 15 minutes whenever it updates. It's free with my ATT DSL internet though.
    Pass/fail limits for these components seem pretty wide.

    If the load voltage is half that of the open circuit voltage then maximum power is transferred from source to load and this is desirable for a setup like this.

    Run some experiments. Your t'pile can be simulated with a D cell and two 6.2 ohm resistors and your valve with a 3 ohm resistor.
    Some t'couples only put out 28 mV but I can't find a spec on their internal resistance.

    http://www.electro-tech-online.com/
    Last edited by Thatguy; 11-14-2010 at 02:32 PM.

  12. #42
    Retired Defense Industry Engineer jadnashua's Avatar
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    For anything electrical to work, there must be a complete circuit from the source of power back to the power source. A thermocouple is a power source. When it is disconnected from it's load (the gas valve), and you measure it with a high impedance multimeter (this is almost like 'nothing', since it's internal resistance is generally quite high) you are measuring it as an open circuit. This is sort of like measuring the output of a D-cell when it isn't in a circuit. A closed circuit generally consists of a power source, and a load the power source is designed to run. A short circuit is where the power source is connected directly to the return on the power source, bypassing the load. A short-circuit is generally catastrophic - things overheat and can burn up.

    To test the thermocouple with a load, out of its normal circuit, you'd take the two leads and connect each to one end of the resister, then measure the voltage across the resister.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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