True or False ?

Thanks John.I had a question on your capacitor rated voltage,why does the dc voltage need to be 4 time higher then the ac rated vale and what is the pupose on the 47 ohm resistor ? Regards.

A capacitive arc snubber connected across a contact can reduce the flash each time the contact opens. By diverting the load current to the capacitor for a brief moment, it allows the contact to get enough air between the points that and arc is not drawn between them. However, that same capacitor can dump a big current into those points when the contacts close, causing them to pit and possibly weld together. For this reason, such snubbers also include a resistor in series with the capacitor.

This sort of thing works best with inductive loads, like motors, where the inductance causes the current to decrease more slowly than the voltage does. Resistive loads, like heaters do not produce the extra voltage at the moment contacts in series with them open, so a snubber may not help much. You may just need a bigger contactor.

But if you want to try it you will need a 1 microfarad capacitor rated for use with 240 volts AC or more (or 1000 volts DC) and a 47 ohm, 2 watt resistor. Connect them in series and across the contact points.

Here is a data sheet for an example of a capacitor type rated for across the line, AC operation (class X), available from Digikey:

Thanks John and Pooh for the reply and explaination in this issuse.

Line voltage can have some nasty spikes riding on it, occasionally. Class X, line rated capacitors are made with generous over voltage ratings and safe failure modes (the metalizations tends to evaporate around a hole burned through the film, rather than short and explode).

You can get some idea of the real voltage capability of a Class X1 capacitor by looking at some of the details on the data sheet I referenced.

DC rated capacitors are not generally made with such over voltage in mind, so you have to get the safety factor by using a much higher voltage rating than the normal peak line voltage, and you are still not necessarily getting the graceful failure modes.

The resistor is a current limit mechanism for when the contacts close near peak line voltage (1.414 times 240 VAC RMS = 339 volts). There will be a very large current pulse as the capacitor dumps all that voltage while the contacts are bouncing together, and that inrush can damage the contacts and/or the capacitor. The 47 ohm resistor will limit that peak current to 339/47=7.2 amps (in addition to the current of the heater).

The 4x is misleading. The cap is subjected to pulse operation which makes rather strenuous demands of it. Voltage 'derating' is helpful for pulse operation.

To limit the current. Re-read what John said.

Using a cap without that R can result in some 'interesting' results.

Graham

The capacitor or capacitor / resistor will have no effect on a resistive (heater) load

-- Dan Hollands