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Subject: Re: Help with 78 series regulator
Organization: Nobody nowhere
X-Newsreader: Microsoft Outlook Express 6.00.2720.3000
Date: Tue, 21 Jan 2003 22:42:04 -0000
NNTP-Posting-Date: Tue, 21 Jan 2003 22:42:08 GMT
"ånønÿmøu§" wrote in message
> On Tue, 21 Jan 2003 18:39:37 -0800, "bc" wrote:
> >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.
> >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
> >disposable cameras except that this one is from a more expensive camera
> >operates from 6V. The reason I'm using a 5A regulator is that these
> >pull about 3 amps on power-up.
> >When I connect my flash circuit to the 78H05 the output simply goes
> >It rises quickly to 10V ( the capacitor charges VERY quickly) and the
> >transistor gets VERY hot. I darent leave it connected too long or
> >will burn out, I fear.
> >Why does the regulator completely lose the plot when it's connected to
> >flash circuit if it's perfectly happy with a resistor??
> A regulator of this type can sink little to no current at its output.
> So, if it encounters a higher voltage at the input, it can't regulate
> it to what ever voltage its rated at. You maybe forced to try putting
> a diode in the regulator's output and or add a drain or bleeder
> resistance to the regulator's output.
> If your pulling 3 Amps through anything... its going to get hot!
Use a bigger wattage transistor and a heatsink
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