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Subject: Re: DC/DC converter for LCD Panel Meter; another question
Date: Sun, 08 Sep 2002 16:36:22 GMT
NNTP-Posting-Date: Sun, 08 Sep 2002 12:36:22 EDT
Mark Holden wrote:
> I have a similar problem with these meters; I'm running 5 of them [for
> various functions] in my "deep desert" motor home, and want to install
> another 3 on my houseship. I've blown several panel meters learning some
> lessons, and thought I had it right, giving each it's own dedicated DC/DC
> converter; but when I had a rather large [100ampish] fault elsewhere in the
> system, I lost yet another meter, even though it was isolated on it's own
> supply. actualy, the converter was also blown.
> without instruments, I limped home...
> what should I put in the circuit to protect everything? I blew a couple of
> those DC/DC converters. too, and they aren't very cheap, about $10.00 each
> around here [allegedly western Europe].
> the converters claimed to have thermal protection, etc., and be oh so
> robust; maybe I'm missing something.
> do I have to add a capacitor[s] or something on the input or output? do I
> need a V.reg on the input? the documentation that came with them just showed
> 12VDC in and 12VDC out, + - 20%.
> thanks, Mark Holden
Some points come to mind:
1) The DPM's I am familiar with require 9v, not 12. It is possible
you are feeding yours too much voltage.
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.
3) Make *SURE* your dc-dc converters isolate
4) You can home brew a dc-dc converter which will provide 2
isolated DC outputs for these DPM's for under $10 in parts
in the US - dunno what the parts would be in europe.
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
6) You might be better off using rechargeable batteries
to supply the DPMs. At about 1ma draw, the batteries will last
a long time on a charge. A battery supply will isolate
the supply side of the DPM from system faults - 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.
7) Some form of transient protection on the input and the supply
to the DPM may be needed.