# Simple voltage regualtor

Discussion in 'Electronic Basics' started by Captain Dondo, Oct 24, 2003.

1. ### Captain DondoGuest

I need to power some meters in my car. They require 5 VDC at < 50ma -
some as low as 5 ma. I need less than 200 ma total.

I've looked at the various 5vdc regulators, but all seem to say then need
something like 7 VDC to produce 5vdc. Is it safe to run them with 13.8
(or even higher) voltages like you might see in a car? Charging systems
can produce - what - 14.5 VDC or something like that.

Any basic circuits or guidance would be welcome.

TIA,

-Dondo

2. ### John PopelishGuest

Almost all 5 volt regulators can handle 15 volts in, as long as you
heat sink them well enough to carry away the waste heat. a 10 volt
drop and a .2 amp current through them will produce 10*.2=2 watts of
heat.

Since the battery voltage can have big, nasty spikes, occasionally, it
is also a good idea to put a spike clamp (18 volt, 1 watt zener diode)
across the input (after the fuse) and an inductor (a dozen turns
through one of those big ferrite beads they put on monitor cables
works well) or low value resistor (10 ohms 1 watt, say) in series with
the positive supply line, followed by a big capacitor (220 uf, say) to
ground. Add a .1 uf mylar cap across the 5 volt output right at the
regulator, also.

3. ### Peter BennettGuest

The common three terminal regulators like the 7805 need _at least_ 7
volts to operate correctly, and will survive (and work correctly) with
input voltages up to 35 volts or so.

4. ### BaphometGuest

Captain

An LM309 can handle up to +35 volts input at 1000 ma. output
http://www.national.com/pf/LM/LM309.html

5. ### Robert MonsenGuest

If you want to avoid some of the heat dissipated by a linear regulator, use
a switching regulator, like the ones at www.maxim-ic.com

They have ones that will take you from 12V to 5V, which would probably be
what you want.

http://www.maxim-ic.com/PowerSupplies.cfm

There are lots of other manufacturers as well. You may be able to get these
parts mail-order at www.DigiKey.com or www.mouser.com or www.arrow.com

You will probably need a few external components as well (unlike a 7805...)

Regards
Bob Monsen

6. ### Watson A.Name - Watt SunGuest

They work fine up to their maximum, which is greater than the car
voltages. www.epanorama.net or www.fairchildsemi.com and search for
the data sheet for LM7805 etc.
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7. ### Captain DondoGuest

Everything else I understand, but.... Connect the zener from hot to
ground? Is that what you're saying? And if so, which end goes where?
Shouldn't there be a resistor in line with the zener to limit current? I
must not understand how zeners work (no surprise there, it's been 20+
years since my high school electronics....)
-Dondo

8. ### John PopelishGuest

The striped end (cathode) goes to positive. Those transients can
produce high voltages, but the total energy in them is not very
large. The zener can survive them with only the resistance of a fuse
in series. The zener will shunt all currents trying to raise the
voltage above +18 or below -0.7