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Need help with resistor watts

  1. Jun 4, 2008 #1
    I have an oven and the heater element is ran by a watlow controller. It is going out evey two or three years. The watlow controller runs a relay to turn element on and off. The relay on the watlow needs a load of 100ma min. to keep contacts clean. I have the resistance of the coil for the on off relay at 1218 with 120vac going into it for 98ma. Since it is a relay running a relay I have snubber or QUENCHARC in parallel on the on off relay. The snubber is a .1uf cap in series with 100ohm 1/2 watt resistor. I figured I would need a 1k resistor in parallel with the on off relay. The 1k should give me 120 ma but my power dissipation would be 14.4 watts. that would give me a total of 218ma excluding the snubber. What resistor wattage should I use or did I get the whole thing wrong. My background is mechanical but I took EE a long time ago
    Last edited: Jun 4, 2008
  2. jcsd
  3. Jun 4, 2008 #2
    I think you made a mistake that you add the snubber circuit at the input side of the relay (the coil side) where the peak current is low because of the 1218ohm coil reistance. So, your calculation for the snubber circuit might be wrong. The purpose of the snubber circuit is to protect the motor (output side) where the peak current is high, or the snubber resistor value will be low. Honestly, I never design any snubber circuit. What I remember is just in theory.
  4. Jun 4, 2008 #3
    The Quencharc or snubber is used to protect the relay in the watlow controller. I have a watlow output relay going to the coil on a mercury displacment switch. I have to per manufacture put the snubber on the coil on the merc switch. so that would put the snubber in parrarell with the 1k resistor and merc switch coil. One thing I could not understand is what watt resistor I would need. I figured up 549 ohms for the resistor and coil witch would put me at 218ma. I do not know how to figure the snubber into this to make sure the resistor and current values are correct. $1200.00 for the controller got to get it correct first time.
  5. Jun 4, 2008 #4


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    Welcome to PF.

    Looks like you have the E=IR part ok.
    Not so sure about the rest of it.

    For example: What is the coil current for the 120VAC relay?
    You should be able to find this on the relay spec or measure it.
    The DC resistance of the coil is fairly useless by itself.

    The snubber will also pass current.
    Look up computing capacitor impedance.
    One issue here is that the relay and snubber will interact so you need to know more about the relay coil and include it in the calculation.

    If your watlow 100ma min is specified in DC you might want to see if there is a corresponding AC spec or ask them.

    Since the watlow people seem to have told you about the snubber, then this is likely sufficient for your needs.
    If your concern is maximizing contact life then it would be best to double check it with them.
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