19 Jan
2010
19 Jan
'10
3:37 p.m.
Date: Tue, 19 Jan 2010 16:39:49 -0500
From: ploopster at gmail.com
To:
Subject: Re: Altair 680 power supply...
Chuck Guzis wrote:
But I thought part of the problem was that the regulator with the
slightly higher output voltage would drive the voltage difference
against the output of the lower-voltage regulator? Wouldn't isolation
diodes prevent that from happening?
Peace... Sridhar
No
It would still be a supplied by the regulator with the largest voltage
out.
I did mention that these regulators are designed to run in the
current shut-down mode, indefinitely unless the power spec is
exceeded. That means they can be paralleled with care.
The regulator, with the slightly higher voltage, will current limit,
and then start loading up the next regulator.
It's not the best for the life of the regulator but still within the
design spec of the regulator.
Dwight
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Couldn't you get around at least part of the problem with isolation
diodes?
No sir. Consider that the output of one 7805 might be 5.05 volts and
of the other, 4.95 (manufacturing tolerances being what they are,
that's not unreasonable). The bulk of the current will be conducted
by the 7805 with the higher voltage output. One might put a resistor
in series with the output of each one to balance the load a bit, but
that would play hob with the voltage regulation. (On the other hand,
this technique is used when paralleling rectifiers to balance the
current.)