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From: John Popelish
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X-Mailer: Mozilla 4.7 [en] (Win98; U)
Subject: Re: Help with 78 series regulator
Date: Tue, 21 Jan 2003 23:17:42 GMT
NNTP-Posting-Date: Tue, 21 Jan 2003 18:17:42 EST
> 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
I suspect the flash gun draws current as fast spikes, instead of as a
steady current. This drives regulators nuts. Try adding a capacitor
in parallel with the load. A microfarad of mylar or a couple hundred
microfarads of electrolytic should help. You should also have both a
.1 microfarad ceramic or mylar right between across the power input ot
the regulator, and another big electrolytic ( thousand microfarads, at
least) in parallel with that, unless you have a big battery feeding
the regulator. These caps absorb the high frequency energy from the
load, and the regulator only has to deal with the more slowly changing
And, if you want ot increase the output voltage you should use two
resistors in a voltage divider configuration. One connects from
output to reference pin, and the other between reference pin and the
negative rail. This acts as a minimum load on the output, and keeps
the output voltage more constant as load changes. A 500R and 100R
respectively should work better than just a single resistor you are
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