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Subject: Re: Photo Flash Trigger Circuit Help
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Wed, 13 Nov 2002 22:55:02 -0500
NNTP-Posting-Date: Wed, 13 Nov 2002 22:58:44 EST
Organization: Cogeco Cable
Hello John, thank you for your kind reply.
When you say a microfarad capacitor do you mean a 1 microfarad capacitor?
Also the SCR I am using is Radio Shack Part Number 276-1067 and the data on
the back says:
"DC gate trigger current: Typ. 0.6ma"
But I guess a little extra current flow never hurt.
The Zener is just one that I had in one of my parts boxes, I chose it for no
other particular reason but thanks for letting me know that I can safely use
a 1/4 watt as I do have a mind to make the circuit as small as possible.
I will try all the modifications you have suggested and let you know how it
Thank you again for your help I really appreciate it.
"John Popelish" wrote in message
> This circuit has two weaknesses that I can see. The 5.6 k resistor
> limits the gate current to about 17 ma peak and this may not be enough
> of a thump to cleanly trigger the SCR. If you parallel the zener with
> a microfarad in series with a 47 ohm resistor, the peak current will
> near 200 ma. This will also allow you to raise the 5.6 k to an even
> higher value to keep it from draining as much power from the 98 volt
> source. There is also no need for a 5 watt zener. A 1/4 watt would
> work as well, and give a better voltage regulation.
> The other problem is that the gate is very sensitive to leakage
> current and hum pickup without a bit of resistance from cathode ot
> gate to drain this away. With the above capacitor thumper, you could
> easily tolerate 100 ohms cathode to gate to make sure the SCR is
> completely off till the gate pulse arrives without loosing much of the
> gate drive pulse. This assumes that you are not using an SCR that
> already includes such a shunt resistor, internally.
> John Popelish
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