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From: firstname.lastname@example.org (Byron A Jeff)
Subject: Re: Help with 78 series regulator
Date: 21 Jan 2003 17:06:01 -0500
Organization: College of Computing, Georgia Tech
In article , bc wrote:
Hi. I E-mail a copy of this posting as you requested.
-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.
Just one resistor? This is your mistake.
Note that the 7805's basic job is to keep the voltage between the output and
common at 5V. The single dropping resistor will cause the common pin to rise
therefore raising the output voltage.
But with a single resistor, that voltage is dependant on the current flowing
through the resistor which is based on the current flowing through the circuit.
-The circuit seems OK on no load ( well, a 500R resistor actually )
Because it's a constant current.
-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.
Again it's a constant current.
-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.
But it isn't a constant current. So the voltage is all over the place...
-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.
Because it isn't constant current. Nor should it be.
-Why does the regulator completely lose the plot when it's connected to the
-flash circuit if it's perfectly happy with a resistor??
Because it isn't constant current, therefore it cannot regulate.
-Can anyone help?
Sure. It's pretty simple. Change the common/adjust pin so that it see 1V when
the output is 6V. The regulator will pull the output to exactly 6V because
remember that its job is to maintain a 5V differential between the common and
the output pin.
It's as simple as adding another resistor to the common pin circuit, creating
a voltage divider:
Simply make R1 5 times the value of R2 and the common pin and the Vout will
maintain a rock steady 6V.
BTW this is exactly the same process as the LM338 except the difference in
voltage between the adjust/common and Vout is 1.2V.
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