triac

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ptonsparky

ptonsparky

Tom
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NE (9.06 miles @5.9 Degrees from Winged Horses)
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EC - retired
Managed to capture a snapshot of the voltage and current waveform on a portion of a control circuit that has been giving the owner fits for some time. A very noticable notch appears in the sine wave both upper and lower at about the same angle. The "control contact" for a Franklin Submonitor is a triac, so I learned late yesterday, and in this installation it is in the neutral of the control circuit. Is this how an overloaded triac would react?
 
ptonsparky said:
Is this how an overloaded triac would react?
Click to expand...
Not necessarily, depending on the length of the notch. A triac doesn't begin conducting until it is triggered, which must occur each half-cycle. You seem to be detecting the delay between the beginning of the half cycle and the triggering pulse.
 
Dipping back into the deep recesses of memory... where is the notch? Is it at the zero crossing or part way through each half cycle?

IIRC, a zero crossing notch is usually caused by a flaky trigger circuit (late triggereing), but also some older-design TRIACs would produce a zero-crossing notch no matter nicely you triggered them. This was a big problem with stage lighting dimmers in the '70s.

A notch or other blip part way through the half cycle wouldn't come from the TRIAC, but likely some reactive component elsewhre in the circuit.
 
I can't see exactly what it looks like, or where you are measuring it, but it sounds like it might be harmonic distortion - maybe from some power supplies from a computer or other electronics.

What causes this is that as the AC voltage crosses through zero, the load sucks down the voltage on the input filter of the power supply. Then when the AC voltage suddenly exceeds the input filter voltage, a large current spike flows in to recharge the filter.

I can't say if the distortion and spike might be giving your triac any problems or not.
 
I'm with Steve66 on this; notched voltage "sine" wave is now almost normal due to the ubiquity of stuff that has rectification in it, including computers, UPSs, high frequency fluorescent lights, VFDs, the list just keeps getting longer.

However, I fail to understand the problem; could you perhaps post a diagram...?
 
That looks suspiciously like the notch pattern of a GTO thyristor being fired for voltage control. A DC drive somewhere else perhaps? Maybe a really old AC drive using a Current Source Inverter? Maybe an old UPS or other battery charger?

It's on a control circuit for a Franklin with a SubTrol on it you said. It's not likely the SubTrol, they use an SMPS inside for control power. Is there a battery backed alarm in the control panel though?

I would start by disconnecting everything and seeing if it is on the incoming line. If not, start adding things back in until you see the problem again. At least you will know where to start looking.
 
New installation year ago. No drive. 480v submersible with PF correction capacitors and Franklin Submonitor. Radio control. Hi $ surge suppression on the 480. Small stepdown for lighting & control.

One half block from my shop. I don't think it has run a whole week since installed. Making more money doing follow up than if I had installed it.
 
Picture sure helps.
Note that the notch in the voltage waveform occurs when the current waveform goes through zero.

A Triac will turn off when the current goes through zero (not the voltage).
It must then be triggered on again.
 
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