Optoisolated TRIAC Trigger

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eeboy 16 years ago

I am using a MOC3023 optoisolated triac trigger

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to control the HV pulse to a xenon flash tube directly. Essentially one end of the TRIAC ties to the trigger transformer and the other to ground. A .047uF cap feeds the primary of the trigger transformer with 350VDC. This cap is charged via a

1M resistor so, after the cap has discharged into the transformer, there is no low impedance path through the transformer. The application seems to work well in my limited testing.

The datasheet warns in several places that the device should not be used to drive loads directly but used only as a trigger device. I've also seen many other xenon trigger circuits which use a similar optoisolated TRIAC trigger device to drive an SCR (which is then connected to the trigger transformer). I presume this was done because the optoisolated TRIAC trigger wouldn't survive.

Any thoughts as to the longevity of using the triac trigger directly?

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Jan Panteltje 16 years ago

On a sunny day (Sat, 12 Dec 2009 07:48:13 -0600) it happened "eeboy" wrote in :

trigger

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to control the HV pulseto a

transformer and the other to ground. A .047uF cap

via a1M resistor so, after the cap has discharged

The application seems towork well in my limited

todrive loads directly but used only as a trigger

optoisolated TRIAC triggerdevice to drive an SCR (which

the optoisolated TRIACtrigger wouldn't survive.

In my datasheet it is listed for 1.2 A max surge for 10 mS, 2 A for 10 uS, If you stay below that it should be OK. Also stay below the other maximum specs. But I would expect a much higher peak current in the case of a discharge. Can you measure the peak current and how long it lasts?

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Phil Allison 16 years ago

"eeboy"

** It will work until it stops.

Try drawing a spark to common from the secondary of that trigger tranny.

Then you may see why adding an SCR is a good idea.

..... Phil

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eeboy 16 years ago

Assuming the dielectric of the trigger transformer doesn't breakdown, the parts have adequate physical separation and any residue (flux, etc) has been cleaned from the board... why would this happen? I am sure I am missing something simple here.

J

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eeboy 16 years ago

This is one of my problems. I don't have high voltage probes. Perhaps a high impedance resistor divider (>10M) would provide sufficient protection without loading the circuit?

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Phil Allison 16 years ago

"eeboy"

** Whaaaaaaaaaaaaaatttttt ????

You sure DO have some problems - pal.

.... Phil

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Phil Allison 16 years ago

"eeboy"

** If you think the trigger coil or strip on the flash tube can never spark or become grounded - fine.

But Murphy is laughing his head off.

.... Phil

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Robert Baer 16 years ago

These iso-optulated triac drigger devices have a rather limited current rating (see data sheet), and if driven so as to conduct more than rated current, they will not turn off. Have not tried over-voltage.

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Phil Allison 16 years ago

"Robert Baer"

** Not relevant to the case of discharging a 47nF to trigger a flash.

** Please do.

..... Phil

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Robert Baer 16 years ago

Using a high value resistor is OK for a DC divider, but it gives poor AC (read "pulse") response. So a small, adjustable HV capacitor in parallel is required: a gimmick will do in a pinch.

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J

Jan Panteltje 16 years ago

On a sunny day (Sat, 12 Dec 2009 20:40:09 -0600) it happened "eeboy" wrote in :

impedance resistor divider (>10M) would provide

on

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To measure *current* all you need is a scope and small value series resistor?

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Optoisolated TRIAC Trigger

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