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Reply-To: "Mark Holden"
From: "Mark Holden"
References: <3D76CBF2.E4B6E9E5@bellatlantic.net> <3D7B7C6D.2AFA9279@bellatlantic.net>
Subject: Re: DC/DC converter for LCD Panel Meter; another question
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Mon, 09 Sep 2002 21:41:08 GMT
NNTP-Posting-Date: Mon, 09 Sep 2002 23:41:08 MET DST
Organization: chello broadband
wrote in message
> 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.
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.
> 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.
Generally speaking, double what they are in the US. And also, I already got
the replacements [didn't yet install them].
> 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
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.
> 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 -
I was running them on alkalines for a while, but they are expensive there
[India], and I get counterfeits [fake Duracells] sometimes, which don't last
long, so I decided to "do it right".
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] as well as various
ampages via shunts [battery bank and three separate alternators].
> 7) Some form of transient protection on the input and the supply
> to the DPM may be needed.
Would zeners be suitable for this?
The DPMs are 200mV; is there something that can protect from transients that
low? I suppose I could put protection before the dividing resistors for the
voltmeter, but it was the ammeters that gave the most grief. They are on 1
A/1Mv shunts, so should be good to 200A.
Thanks a lot for your help, Mark Holden
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