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Subject: Re: DC/DC converter for LCD Panel Meter; another question
References: <3D76CBF2.E4B6E9E5@bellatlantic.net> <3D7B7C6D.2AFA9279@bellatlantic.net>
Date: Tue, 10 Sep 2002 02:09:31 GMT
NNTP-Posting-Date: Mon, 09 Sep 2002 22:09:31 EDT
Mark Holden wrote:
> I forgot to mention each has a Vreg downstream of the converter; 9V DC/DC
> converters would have made life easier [one regulator for all the inputs],
> but I didn't find them.
> > 2) A 12 volt supply at + - 20% is really poor. Use a zener to
> > get much better regulation. The DPM's I've used require
> > very little current so a zener in shunt is fine.
> Will do as you suggest here; it's a lead/acid battery system, so the voltage
> can be anywhere from 10 to 14.4 volts.
I was suggesting a zener regulator for the supply to the
DPM, but you already have that according to this note.
A zener regulator for the DC-DC converter may not be needed,
and I don't know what the converter draws, so I don't know
if using a zener in shunt to regulate the input to the
converter is a good idea.
> > 5) Expecting a DPM to survive a 100 amp fault somewhere in
> > the system that also blows the supply for the DPM may be too
> > ambitious.
> But surely, I should be able to protect the supply and the DPM? I really
> don't know what the transients might have been. I also have a couple of 100
> amp loads that switch on and off with solenoids.
Originally, you did not specify that a transient did the
damage. Yes - you can provide transient protection for the
supply (DC-DC converter). Get some 1.5KE15A transient diodes
from Digi-Key. Put one across the input to each DC-DC converter
"backwards" so that current does not flow through the diode.
Put a fuse between the battery and the DC-DC converter, before
the diode. The diode will absorb brief transients (spikes above
15 volts) and will short out on a longer duration overvoltage,
blowing the fuse.
> but a 100
> > amp fault somewhere in the system could put too much voltage
> > on whatever the DPM is measuring for it to survive. This of
> > course would depend on what the DPM was measuring and where
> > the fault occurred.
> They measure battery voltage [via a dedicated wire]
Use the technique mentioned above with the transient diodes
> as well as various
> ampages via shunts [battery bank and three separate alternators].
Fuse the wires at a current lower than the shunt rating, and
set the DPM to equal or above the shunt rating. Get transient
diodes of the proper voltage rating, and install them across
the sources whose current you are monitoring.
Finally, consider the economics. How much do you want
to spend to protect a cheap DPM? Fusing the lines I mentioned
should be done in any event, if it hasn't been done already.
The transient diodes are cheap - less than a dollar a piece
for the 15 volt units from Digi Key. From what you said
earlier, they are probably more expensive than that for you,
but still a lot less than the cost of the DPM. But my guess
is that it would be cost prohibitive to make them totally
> Thanks a lot for your help, Mark Holden
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