What resistor wattage should I use for a load of 100ma to keep contacts clean?

In summary, the conversation discusses the use of a snubber or Quencharc circuit to protect a mercury displacement switch connected to a watlow controller. The snubber is meant to be placed in parallel with the 1k resistor and merc switch coil, and its purpose is to protect the relay in the watlow controller. The proper wattage of the resistor needed for the snubber circuit is unknown and further research or clarification from the watlow company is recommended.
  • #1
I forgot ohms
7
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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
 
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  • #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.
 
  • #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 displacement 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.
 
  • #4
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.
 

1. What is a resistor wattage rating?

A resistor wattage rating is a measure of the power handling capability of a resistor. It indicates the maximum amount of power that the resistor can safely dissipate without overheating or failing.

2. How do I determine the wattage rating for a resistor?

The wattage rating for a resistor can typically be found on the resistor itself, either printed on the body or indicated by color-coded bands. If this information is not available, you can use the formula P=V^2/R, where P is power in watts, V is voltage, and R is resistance in ohms. This will give you the maximum wattage that the resistor can handle.

3. What happens if I exceed the wattage rating for a resistor?

If the wattage rating is exceeded, the resistor may overheat and potentially fail. This can lead to malfunctions or damage to other components in the circuit. It is important to always use resistors with wattage ratings equal to or higher than the calculated power in your circuit.

4. Can I use a resistor with a lower wattage rating?

Using a resistor with a lower wattage rating than required can lead to overheating, which can cause the resistor to fail. It is important to always use resistors with wattage ratings equal to or higher than the calculated power in your circuit.

5. Are there any safety precautions I should take when working with resistors and wattage?

Yes, when working with resistors, it is important to always follow safety precautions. This includes using resistors with the correct wattage rating, not exceeding the maximum wattage, and allowing for proper ventilation to prevent overheating. It is also important to handle resistors with care and avoid touching them while they are in use, as they can become hot. Always wear appropriate protective gear and follow proper handling procedures.

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