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  1. #1
    Join Date
    Mar 2001
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    137

    Hmm

    Hi All:

    I just changed a time delay on a Lincoln pizza oven. The time delay keeps the blower on for 20 minutes after the switch to the oven is turned off so the oven can cool down.

    There was a resistor across the terminals of the time delay. Does anyone know the function of this resistor?

    Also, on Coleman mobile home furnaces they used to put a resistor across the terminals of the blower relay.Why?

    I have always wondered about this and finally after 38 years decided to ask.

    Thanks;

    Terry

  2. #2
    Join Date
    Dec 2003
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    the resistor is the control for how long the delay is- the 20 minutes corresponds to the particular resistance value of that resistor- if you used a different size resistor, you would get a different delay time. Don't know about the coleman blower relay, tho'.

  3. #3
    Join Date
    Mar 2001
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    137
    Rimk:

    Can you be specific about the resistor? I mean can you draw me a time delay circuit using the resistor?

    I would think if you wanted a 20 minute time delay you would use a "timing processor" and have it programmed for 20 minutes. (microchip)

    They use a "bleed resistor" across the terminals of a start capacitor to keep the relay contacts from arcing when the relay contacts start to close.

    Anyway, not meaning to argue, just curious as to how the resistor is "actually" used and the purpose for it.

    The old time delay had no such resistor and has worked well for several years. However, when the time delay failed, the time delay would then "jump in and out" and failed to time the 20 minutes after the main switch was turned off.

    Thanks;

    Terry

  4. #4
    Join Date
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    The resistor on the timer is in series with a capacitor and determines how long it takes the capacitor to charge up.

    When the capacitor is fully charged it fires the gate on an SCR (Silicon Controlled Rectifier) which completes the circuit and turns on the device it is controlling.



    The resistor across the relay protects the circuit board the relay is wired to from back emf by bleeding off the voltage when the relay shuts down so the voltage does not reach the circuit board.

    Norm

  5. #5
    Join Date
    Mar 2001
    Posts
    137

    Talking

    NormChris:

    Thanks;

    I got it now.

    Terry


  6. #6
    Join Date
    Sep 2002
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    Originally posted by Terry
    NormChris:

    Thanks;

    I got it now.

    Terry

    This is the sort of stuff that appears on RSES CM and CMS exams. It is also useful to know.

    Norm

  7. #7
    Join Date
    Aug 2005
    Posts
    833
    Originally posted by NormChris



    The resistor on the timer is in series with a capacitor and determines how long it takes the capacitor to charge up.

    When the capacitor is fully charged it fires the gate on an SCR (Silicon Controlled Rectifier) which completes the circuit and turns on the device it is controlling.



    Norm
    So is the time delay set by how long it takes to charge up the capacitor? He said it was a time delay on turning OFF the blower.
    eventu rerum stolidi didicere magistro

  8. #8
    Join Date
    Sep 2002
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    South Dakota
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    Originally posted by cg2
    Originally posted by NormChris



    The resistor on the timer is in series with a capacitor and determines how long it takes the capacitor to charge up.

    When the capacitor is fully charged it fires the gate on an SCR (Silicon Controlled Rectifier) which completes the circuit and turns on the device it is controlling.



    Norm
    So is the time delay set by how long it takes to charge up the capacitor? He said it was a time delay on turning OFF the blower.

    The internal electronics of the circuit can be wired to work either way. Direct or reverse acting. I provided the simpler explanation of direct acting.


  9. #9
    Join Date
    Aug 2005
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    833
    So is this the way the Mars TDRs work? When you set the dip switches or turn the dial you are changing resistance in series with a capacitor?
    eventu rerum stolidi didicere magistro

  10. #10
    Join Date
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    Originally posted by cg2
    So is this the way the Mars TDRs work? When you set the dip switches or turn the dial you are changing resistance in series with a capacitor?

    Yup! If it is an off cycle delay an integrated circuit reverses the operation to cut off the circuit after the delay.

  11. #11
    Join Date
    Aug 2005
    Posts
    833
    WOW! now I know how the magic black boxes work! Trying to get to take the CM test this year, maybe it will come in handy. Still, voltage-in, no voltage-out, you replace it, right?
    eventu rerum stolidi didicere magistro

  12. #12
    Join Date
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    Originally posted by cg2
    WOW! now I know how the magic black boxes work! Trying to get to take the CM test this year, maybe it will come in handy. Still, voltage-in, no voltage-out, you replace it, right?
    Voltage in and no voltage out unless there is a solid state timer in the middle!


  13. #13
    Join Date
    Aug 2005
    Posts
    833
    You mean in the middle of the black box?
    eventu rerum stolidi didicere magistro

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