# Noise Triggering SCRs

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#### Steve Tomporowski

It's a simple DC motor control circuit using two SCRs to supply full wave rectified voltage to a DC motor. A uP controls the phasing for speed control through an opto to the driver circuit which is referenced entirely to the motor (no ground or earth). The problem is the noise test which injects 150 nsec pulses of 500, 1000, 2000 and 4000 volts into the AC input. At 2k and 4k, the motor is being disturbed, so I think the pulses are triggering the SCRs. What is the normal noise suppression that is done on these devices? An links on this?

Thanks...Steve

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#### Roger Irwin

Start by using an ordinary Tranzorb/VDR combination on the input to clamp overvoltages.

Then add Snubbers. A resistor in series with a capacitor in paralell with the SCR. The SCR's will have a max value for dv/dt. Set up an oscilloscope to measure this (not an easy measurement, seek help if nexperienced!). One
channel should be watching the tension across the whole network, the second channel just looks at the capacitor voltage. Proceed as follows:-

If the capacitor is charging quickly relative to the impulse, then you need to increase the value.

If the capaitor is not charging but the impulse is still too quick, decrease the resistor value to let more current into the cap.

High voltage polycarbonate capacitors are ideal, use a VDC rating more than twice the RMS voltage and in any case inferior than the tranzorb rating.

Keep an eye on the resistor power rating, bearing in mind that it will always be disipating when the circuit is under tension.

BTW, it may be the micro that is being disturbed, techniques for identifying and correcting such problems are more involved.

It's a simple DC motor control circuit using two SCRs to supply full wave rectified voltage to a DC motor. A uP controls the phasing for speed
control through an opto to the driver circuit which is referenced entirely to the motor (no ground or earth). The problem is the noise test which injects 150 nsec pulses of 500, 1000, 2000 and 4000 volts into the AC input. At 2k and 4k, the motor is being disturbed, so I think the pulses are triggering the SCRs. What is the normal noise suppression that is done on these devices? An links on this?

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#### Steve Tomporowski

Thanks for the guidelines. Actually your last suggestion was close to correct. The micro was isolated from the SCRs and their triggering by an opto. It was the two transistor triggering circuit that was firing on noise. Just a couple of small caps at the base of each transistor gobbled up the noise without slowing down the real trigger pulses. I used .001 and .01 (depending on the pullup resistors!). Passes tests fine now and still works!