🚀 go-pugleaf

In article <1991Feb11.051608.2280@athena.cs.uga.edu> mcovingt@athena.cs.uga.edu (Michael A. Covington) writes:

>>We want to carve about 4.6 volts off that 13.6.  If we call it 4.5, and you
>>can come up with a 4.5 volt zener, then just put the zener in series with the
>>load:
>>
>>+ 12V o----------------|<-----------o + 9.1V
>>			4.5V
>>			zener
>>			diode
>>
>   NO! You'll fry a Discman.
>   The voltage in the car is not a constant 13.6V;
>   it varies from 11 to 17 volts!!!!!  (Measure it if you don't believe me.)
>
>   You don't want a constant voltage DROP, you want a constant
>   voltage OUTPUT.  A 7809 regulator chip will do the trick.

Well, on your car, perhaps.  ;^)

Seriously, Mike, I'm going by experience here...so your mileage may vary.

I've used the chain-of-seven diodes for quite a few years, and never any
damage to any electronics.  Some years back, I used a 7806 in TO-3, nicely
mounted on a heat sink, to power a little tape player.  It worked just fine --
for about seven months.  Then something went wrong with the '06.  Fortunately,
I was wearing the headset at the time.  The volume increased significantly,
and the motor speeded up.  I didn't know what the problem was, but I had the
instinct to yank the plug out of the cassette player.  It survived without
damage; I didn't.  I burned my hand when I thoughtlessly picked up the regu-
lator.

I've observed battery voltages below 12V on occasion -- when the motor wasn't
running, and when the battery was getting a bit low.  I've never -- NEVER  --
observed anything that would push a meter up to 17V on any vehicle I've worked
with.  Measured with a 'scope, things might be different, but it's likely the
power supply filter caps would swallow such transients without difficulty.

In any event, I've never had any equipment failures of this sort, nor of the
voltage reducer, either.

One could easily add a voltage clamp if this is worrisome, however.  Across
the output, put a string of diodes (or a zener) with a conduction voltage of
say, 10 to 10.5 volts (16 or 17 rectifiers, if you're doing it that way).

If the output voltage rises enough to turn that diode/diode-string on, it
will clamp the output voltage at that point, and will blow the fuse if the
voltage tries to rise much higher.

Messy?  Sure, but unless you have a pretty shaky voltage regulator, it should
give you sufficient reduction to operate your equipment, and sufficient saf-
ety with the shunt circuit to protect it.  Twenty-four rectifiers take up a
bit of room, but what's your equipment worth?

A safety shunt of this sort is also a good idea with IC regulators.  I've seen
those fail often enough (not just the tale told above) to make me pretty wary
of relying only on God and Mammon to see that they don't wind up with their
pass transistor shorted.  The same thing applies to discrete circuitry.

Note that I have not installed a shunt on the reducers I use:  risky, perhaps.
But, again, I've not had any problems with those circuits...ever.


						d

PS -- Now, watch me smoke a nine-volt appliance later this week....

						d

PPS -- Recently, someone's .sig said: "If there's one thing software types are
	sure of, it's that smoke is a hardware problem."

						d


--
    "Got to slap these Goddamn Third World nations around, Flynn," he said,
    "until they learn some manners."     -- Gregory MacDonald, from _Flynn_
  Duke McMullan n5gax nss13429r phon505-255-4642 ee5391aa@triton.cirt.unm.edu