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Subject: Help with 78 series regulator
Date: Tue, 21 Jan 2003 18:39:37 -0800
Organization: University of Leicester, UK
NNTP-Posting-Date: 21 Jan 2003 18:33:42 GMT
X-Newsreader: Microsoft Outlook Express 5.50.4807.1700
I have a bizarre problem with a 78series voltage regulator.
The regulator is a 78H05 ; a 5V 5A TO3 can.
I have incorporated this IC in a standard circuit, except that I have
incorporated a small resistor in the COMMON connection to increase the
output from 5V to 6V.
The circuit seems OK on no load ( well, a 500R resistor actually )
If I add a load of 1R0 ( a 50W component!) I get a current of approx. 5Amps
and the o/p is 5.7V. Seems good to me.
Next I tried to power up my test circuit. This is a blocking oscillator.
It's actually the circuit for a photographic flash-gun and consists of a
tiny transformer operating @ high frequency which
charges a capacitor to 300V. Its just like those little flash circuits from
disposable cameras except that this one is from a more expensive camera and
operates from 6V. The reason I'm using a 5A regulator is that these things
pull about 3 amps on power-up.
When I connect my flash circuit to the 78H05 the output simply goes potty.
It rises quickly to 10V ( the capacitor charges VERY quickly) and the
transistor gets VERY hot. I darent leave it connected too long or everything
will burn out, I fear.
Why does the regulator completely lose the plot when it's connected to the
flash circuit if it's perfectly happy with a resistor??
Can anyone help?
Please email any responses as I dont get to come here very often
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