Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!uwm.edu!rpi!wrf
From: wrf@mab.ecse.rpi.edu (Wm Randolph Franklin)
Newsgroups: sci.electronics
Subject: Re: How to save your car's electrical stuff?
Message-ID: <2574133F.4AAC@rpi.edu>
Date: 29 Nov 89 17:34:53 GMT
References: <6709@merlin.usc.edu> <2571CD5D.238F@rpi.edu>
Distribution: usa
Organization: Rensselaer Polytechnic Institute, Troy NY
Lines: 51

In <2571CD5D.238F@rpi.edu> wrf@mab.ecse.rpi.edu (Wm Randolph Franklin) writes:
>In <6709@merlin.usc.edu> cyamamot@girtab.usc.edu (Cliff Yamamoto) writes:
>>This got me thinking about the audio/ham-radio gear I have installed in my
>>car.  Can anyone confirm if the following would work:
>>
>>  (-) 12 Vdc (+)  30 Amp
>> +--- battery ---o-fuse-o--+----> to your favorite & expensive equipment
>> |                         |
>> |                         -
>> |                         ^ 30 Amp +15 Vdc Zener diode  ( just match the
>> |                         |                               diode rating with
>> +-------------------------+----> car chassis              the fuse rating )
>>
>
>That looks reasonable with  a few  caveats.  The fuse  should be smaller
>than  the Zener, and  be  quick blow, since you don't  want the Zener to
>blow first.  If you  want to be fancy, run  the  car's ignition off this
>circuit also, or have some other indicator, so you  know when there's an
>overvoltage.

I just  realized a serious omission  in my answer  - I assumed something
that might not be obvious.  What your proposed circuit does is this:

Your favorite equipment draws  an insignificant current  compared to the
battery's capability, so  before the fuse blows you  essentially have  a
Zener in series with the battery.  Now  if the voltage rises,  that poor
little Zener  is trying all  on  its own  to drag the  battery's voltage
down.  Unless it's a 200A 2000W Zener (do they exist?) it'll blow pretty
fast.  Your circuit works, if it does, because the fuse will blow first.
If the Zener blows first, you have no protection, and no indication that
the protection failed.

That's why  I said to use a  small,  quick blow  fuse.  Standard voltage
regulation circuits use a  resistor in series with  the Zener, and maybe
use the output to control a power transistor instead of feeding straight
to  the  load, if the load  is significant.   The  problem  with using a
resistor is that all the load current goes through it  so your equipment
doesn't get the full  voltage.  It's probably  better to use a regulator
chip, like the LM385.

Here's a crazy idea that should work, but which I haven't seen anywhere.
If the voltage is high for awhile  you might want to  know about it even
before   the fuse would  blow  normally.  If  you  use a   big  zener or
regulator, and wrap them together, as the voltage  rises,  the regulator
will get hotter and the fuse will tend to blow earlier.

-- 
						   Wm. Randolph Franklin
Internet: wrf@ecse.rpi.edu (or @cs.rpi.edu)    Bitnet: Wrfrankl@Rpitsmts
Telephone: (518) 276-6077;  Telex: 6716050 RPI TROU; Fax: (518) 276-6261
Paper: ECSE Dept., 6026 JEC, Rensselaer Polytechnic Inst, Troy NY, 12180


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