Path: utzoo!attcan!uunet!mcvax!ukc!strath-cs!al
From: al@cs.strath.ac.uk (Alan Lorimer)
Newsgroups: sci.electronics
Subject: Re: 7805 regulator
Summary: I'm not sure I understand the circuit.
Message-ID: <1138@stracs.cs.strath.ac.uk>
Date: 16 Jun 88 08:41:26 GMT
References: <592@picuxa.UUCP> <5770@bloom-beacon.MIT.EDU>
Organization: Comp. Sci. Dept., Strathclyde Univ., Scotland.
Lines: 46

In article <5770@bloom-beacon.MIT.EDU>, wolfgang@mgm.mit.edu (Wolfgang Rupprecht) writes:
> > I have a question about the MC7805 5V 1A regulator. Can you parallel
> > two of these devices together to derive a higher current?
> 
> You can always try to use a simple PNP buffer as such:
> (Any high-current silicon PNP will do.)
> 
> ~+8v in				    +5 out
>   o---+----\	    /-------------------+----o
>       |     \	   /			|
>       |      v  /			|
>       |    ---------	  +5v reg.      |
>       |        |	 +--------+    	|
>       | 2.7 ohm|	 |        |     |
>       +--\/\/\-+-------+ in  out+-----+
>       |        	 |  gnd   |	|
>       |         	 +---+----+	|
>      --- 0.1 uf //         |         --- 0.1 uf // 10uf tantalum
>      --- 10 uf tant        V         ---
>       | 		    	       	|
>       | 				|
>       v 				v

Seems a bit involved - I had to think for a while to figure out how this
was meant to work, I'm still not sure that is correctly solves the load
sharing problem though, is the load taken by the PNP transistor related
to the HFE of that transistor, and the 2.7 ohm resistor? Perhaps you
could explain the reasoning?

Since you want a higher current, you're probably going to need to use a
TO3 output device anyway - I know these are a bit more difficult to
mount mechanically, but why not just use an LM317K (3A) or 78H05 (5A?).
Alternatively, if you wan't really high output currents use an LM723
voltage regulator IC, with as many output transistors as you need.

If you are mounting the regulators on the same board as the components
you want to drive, then there is no problem, use the 7805s, and power
some of the board from one, and the rest of the board from the other,
but don't link the power supply lines - you then have full control over
the load taken by each of them.

Alan.
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