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From: "petrus bitbyter"
Subject: Re: Help with 78 series regulator
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Tue, 21 Jan 2003 20:02:32 GMT
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Organization: chello broadband
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"bc" schreef in bericht news:firstname.lastname@example.org...
> 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 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
> 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 thin it's not that bizarre. That common connection draws some current.
And, as you experienced, this current depends on the output current. So
instead of adding a constant voltage of about 1V to your output, you add a
voltage that varies with the load (and with the variation of the load.) To
overcome this problems I advise to replace the small resistor in the common
connection with a 1N400x diode. Furthermore you should decouple the output
carefully with both an elco and a ceramic capacitor. Otherwise you risk an
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