Why are the Triac Snubber Resistors Failing in this circuit.

I am not an expert in this but here is my speculation:

Typical 1/8th watt metal film resistors may be rated for 200-250 V and your inductive spike might be higher.

Typically these resistors are internally "spiral cut" (to automate trimming to specific values) and a high voltage spike can cause flash-over between the cuts.

This could evaporate the material locally causing an open circuit without burning the whole resistor.

Hi Doug,

That makes good sense. Here is a picture of some of the little failed beasties. The next time I have a test jig set up I will try to see what the spike looks like with my scope.

John

I don't work with mains voltage and learned something :-)  A quick look shows that it is possible to get a 500 V, 2 Watt metal film resistor for less than 4 cents.  How would one calculate or estimate the required voltage rating?

I suppose one could hook up their oscilloscope as John suggests and see :-)

The calculations show that under the line voltage (120) with no spikes the resistor itself has only 0.6 volts rms across it. This however hides the reality of the large swings in voltage as the capacitor charges and discharges. Doug's speculation seems to be a good one to me. I am going to set up an experiment and look at the situation with the scope tonight.

John

Yes, I agree.  I was wondering if there was a rule of thumb or way to guesstimate the voltage rating needed to handle the spikes.  I did a quick search last night and did not see anything.

If you have a lot of familiarity with snubber circuits and the load, then you may be able to make an educated guess. There are calculations available to work out what the peak back emf will be, but they require know about the inductance and physical characteristics of the motor and its winding. How fast the semiconductor is switched off also affects the voltage seen.

Remember that the calculations made are based upon rms, which will not be the peak voltage of an AC supply.

If you are really up for the theory and the maths, then there is this paper from ABB on snubber design, more into thyristors but the principles will be the same. Then again, as admitted within the paper, the easiest and usually most accurate is to measure it.

ABB Snubber Circuit Design

Kind regards

Thanks Donald,

this pretty much confirms why I should not be designing high voltage stuff

Me neither!! I get sweaty palms above 12V!!

Neil

Tonight I set up a circuit using the motor and snubber circuit from the machine I am working on. Instead of a Triac I am using a mechanical snap action switch to turn the motor on and off. Here is a picture of the setup.

The motor has two windings with a 6 uF capacitor between them. 120 volts is hooked up across one of the windings and the second winding gets the out of phase voltage through the capacitor. This causes the motor to rotate in one direction. If the other winding of the motor is hooked up across the 120 volts the motor will rotate in the opposite direction. For my first test I did not hook up the motor capacitor. I put the voltage across one winding and looked at the spike that was generated. This should give the worst case spike, much the same as if the motor was under a heavy load. Here is what I found:

As you can see there is a 337 volt PP spike with a max voltage of 179 volts across the 150 Ohm resistor.

I next hooked up the capacitor so that the motor would rotate. Here are the most severe spikes from a series of tests starts.

We are still seeing a max voltage spike of between 150 and 160 volts into the 150 ohm resistor.

I can see now that the voltages could have very easily exceeded specifications on the little resistors that were used on these boards. This would further support Doug's hypothesis.

John