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Quick question - voltage regulators in parallel?

L

lain

Jan 1, 1970
0
What's the input voltage ?

Graham

Input: 12v
Output: 5v @ (ideally: 30amps, but...)

I doubt linear regulators will do that without active cooling, if
they'll do it AT ALL. I might end up going with switching after all...

- Eric A.
 
E

Eeyore

Jan 1, 1970
0
lain said:
Input: 12v
Output: 5v @ (ideally: 30amps, but...)

I doubt linear regulators will do that without active cooling, if
they'll do it AT ALL. I might end up going with switching after all...

2A load means 14W of dissipation which is a fair bit. You can 'get rid' of some of fairly inexpensively with a
power resistor in series with the feed. 2.2 ohms would drop 4.4V and offload 8.8W of that dissipation from the
regulator proper. Don't forget to provide a bypass cap at the regulator input terminal though ( use low-esr type
al-electro - say 10uF ).

Graham
 
E

ehsjr

Jan 1, 1970
0
lain said:
Unfortunately if this works I'll be making a lot of them, so the
one-off solution won't work.. however after digging around the
datasheet for this 7805 I see it says it's fine to go over 1A so long
as you provide adequate heatsinking... so I'll just slap a nice
heatsink on it & some thermal paste and hope for the best...

- Eric A.

Not gonna work for you. Your best approach is the
one redbelly gave in his response: the 7805 with
an MJ2955 PNP pass transistor - use a heat sink.

30 amps with a linear is possible with this method,
but it gets physically large - you'd need a BIG
heatsink and at least 4 pass transistors to be
safe.

You mentioned an automotive environment & big
caps. Forget massive capacitance. Someone mentioned
LC. I'll add this - use a TVS diode like a P6KE24
to ground with a voltage dropping resistor. The
general idea is like this:

+12 ---[L]---+----[1R]---+---[V/R]---[Load]---+
| | |
[C] [TVS] |
| | |
Gnd ---------+-----------+--------------------+

The LC acts to limit transients. The 1 ohm 25W power resistor
will drop 2 volts taking a little heat away from the regulator
under normal conditions. If a transient gets past the LC and
the TVS conducts, the power resistor will drop the voltage
and dissipate power so that the regulator & the TVS are
protected.

Ed
 
lain said:
Ohhh neat, never seen these before. But! - I'm on an extremely tight
budget and these linear regulators are only about $1 each, whereas the
PTH-series seems to run about $11 to $30 each (mouser.com being my
source), but I've got 'em bookmarked for future projects now, thanks!

-Eric A.

Eh, heatsinks aren't exactly cheap either. Unless you need the low
noise of a linear, I'd go with a buck converter.
 
Z

Zak

Jan 1, 1970
0
Why not look into the PTH series from TI? Switching regulator modules
in a variety of packages and input voltages. Plus they don't heat up
that much.

I remember building a switcher with a 7805, a transistor (PNP), coil and
diode. Worked reasonably well for what it was. I think it was the
'current boost' circuit with the coil between PNP collector and 7805
output, and the diode to prevent the collector going negative.


Thomas
 
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