The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
Subject: Help with 78 series regulator
Date: Tue, 21 Jan 2003 12:38:43 -0800
Organization: MindSpring Enterprises
X-Server-Date: 21 Jan 2003 20:54:53 GMT
X-Newsreader: Forte Free Agent 1.92/32.572
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. 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??
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!
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup