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Old 16th February 2006, 12:08 PM   #1
AndrewT is offline AndrewT  Scotland
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Question Diac & Triac relay operation?

Hi all,

I have been trying to understand how a Diac triggered Triac operates to switch on a relay.

I attach my interpretation of the operating voltages that I have fathomed out so far. (zipped from MSword).

My biggest problem is I cannot measure the Diac voltage.
The DMM samples various voltages all over the waveform.
The scope cannot display at the very low repetition frequency of about 3 seconds. I can just make out a sawtooth waveform but that's mostly guesswork.

Please advise if any of my thoughts are right or correct me if wrong.

I believe the Triac only cuts off if it sees a reduction to almost zero current.
Can I presume that pulling the gate back to ground will NOT switch off the Triac?
Attached Files
File Type: zip diactriacschmvolt.zip (18.4 KB, 24 views)
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Old 16th February 2006, 12:22 PM   #2
Eva is offline Eva  Spain
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To test the diac I would use something made with potentiometers, so that you can generate the ramp manually. You can attach a emitter-follower to the potentiometer with a collector resistor to limit the current.

Once a triac has ben triggered and latched, you can't do anything through the gate to turn it off. However, if a gate current has been flowing just before the turn-off event and the zero-current period is short, the triac may retrigger itself. Triacs are slow devices.

If you don't have experience with triacs, you should read about their four working regions and the inconveniences of using positive gate trigger pulses with negative MT currents. If unipolar triggering has to be employed, negative pulses are preferred.
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Old 16th February 2006, 12:35 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi Eva,
thanks for your reply.

I don't want to test it. I'm trying to understand what I have inside my Sugdens.

I would like to add some speaker protection but before considering modifying this part I need to know more.

These amps are destined to drive my sub-bass units (4 off @10ohm) and will be located up in the loft and rarely inspected. I do not want to lose 4 expensive 15inch Tannoys.

I downloaded a series of tutorials and now need to go and look for -ve triggering advantages.
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