Why are the Triac Snubber Resistors Failing in this circuit.

Question

I have recently been repairing power supply / control circuit boards for a small machine that cuts tape to a specified length. Here is a picture of the circuit board in question along with my notes so far:

Here is the schematic that I have reverse engineered from the circuit board:

I have been working my way through a box of circuit boards, that were replaced in the past as black boxes, and repairing them. I have noticed that on almost every board the 150 Ohm 1/8 Watt snubber resistors are open or showing meg ohms of resistance. I have done the math for the circuit and with our local 120 volt 60 Hz line voltage the .1 uF cap in series with the 150 Ohm resistor should have 4.5 mA of current and a corresponding 3 mW of power delivered to the resistor. Since the resistor appears to be an eighth Watt there should be plenty of room for it to handle this voltage, current and wattage.

Unfortunately I am finding almost 100% of these 150 Ohm snubber resistors failed. There are more of the same 150 Ohm 1/8 Watt resistors used on this board in other parts of the circuit but the only ones that are failing are in the snubber circuit.

I am hoping that someone with more experience than me can tell me what is going on in this circuit that is taking out the snubber resistors. The Triacs drive a Variable direction motor and a large solenoid so the loads are definitely inductive. The little resistors do not look heat damaged. They also do not look to be of high quality if that may be a contributing factor.

Any insights you may have will be appreciated.

John

dougw · Accepted Answer

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.

jw0752 · Answer

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

jw0752 · Answer

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

three-phase · Answer

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

three-phase · Answer

Nice set of tests John.

As you have used a mechanical switch and not the triac for the switching, this will absorb some of the energy available and reduce the spike. This is just the nature f the mechanical switch, it cannot switch as fast and as cleanly as a triac can.

If you can set up the test, then try and capture the pulse as the motor is switched off, it sounds like these were readings as the motor was switched on.

I think you have shown, that Doug probably has it right.

Kind regards.

fmilburn · Answer

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?

jw0752 · Answer

Hi Donald,

I ran the experiment with the Triac instead of the switch and I got just the opposite situation than what you predicted. The spikes were much less frequent and when I did get one it was never as bad as when I was using the mechanical switch. Here is a picture of the scope with the worst spike that I saw using the Triac.

I believe that this is caused by the Triac turning on more cleanly and seldom when the line voltage is at its peak. The Triac will ultimately turn off, after the trigger has been removed, as the voltage passes through zero volts which has minimal effect. I also added a MOC 3041 Optoisolator to the circuit and ran the test again. Now my max spikes were in the single volts PP range. Of course this optoisolator has a zero crossing circuit so that the Triac is intentionally turned on when the power is at its lowest point. The Optoisolators on the circuit board that was causing the failed resistors do not include a zero crossing circuit.

John

Why are the Triac Snubber Resistors Failing in this circuit.

Top Replies