Techie Ammeter problem

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ean_p

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We've a 140 - 140 VDO ammeter with external shunt fitted in to the system. Now it all works fine and dandy except that small current draws are imperceptable on the scale as the smallest increament is 20. The overall sizing is good as it shows level of charging and discharging when using the inverter and shore supply charger etc. So is there a technique where by the 'value' of the scale can be decimated so as to represent one tenth of the shown value until say a level of 15amps is reached ie +60 on the meter is 6 amps and so on until at full scale deflection it reverts back to full face values. Don't really want to change to a digital unit as its all part of a matched instrument display. Any suggestions very welcome!

cheers

Ian P.
 
Aren't ammeters just millivolt meters measuring across a known ohm value shunt ? I would have thought getting another, different value, shunt and switching between the two should work. Just a guess though - I could well be talking b*ll*cks.
 
You are correct. The meters that incorporate a shunt are actually measuring millivolts across the shunt. You will need another shunt. Contact VDO (Birmingham) and see if they can provide your needs. Might be an idea to measure the volts drop across the shunt at a known current, measure the resistance of the shunt (which could be very difficult unless you have a calibrated and very accurate ohmmeter). Cheers
 
What you say will work, but the problem is that the 'low current' meter will be overloaded when large currents are passing. It might be possible to make an arrangement with diodes that avalanche into forward flow and protect the meter, but I think you might be better off saving for a digital autoranging meter that will change its range to suit the current flowing. Either that or a Sterling or similar digital battery monitor.
 
Two shunts would be needed the new one having say 10 times the resistance as the original to give a scale length of 14 amps and placed in series with it. It would have to be capble of withstanding the maximmum current that is expected though. The meter would then have to be swichted from being across one shunt to being across the other, observing the polarity, by a two pole 2way switch. The danger is of course of blowing the meter movement if the low range is selected when a high current passes. To some extent this can be avoided by using a switch biassed to the high range. Then the low range is only selected briefly in order to read it.

BUT I read the question as wanting a meter with a non-linear scale without any switching and the associated pitfalls being involved.

What is required is a meter with a logarithmic scale. They exist but I doubt if it is possible to modify the existing set up!
 
Would suggest leaving the setup as is and adding a digital meter for the low range say scaled for 20 amps. just put it across the existing shunt and use a divider or amp to get the correct scaling. When drawing higher currents the digtal meter will overload but it normally won't bea problem.
 
Many many thanks for some very good replies, and a whole boat load of 'food for thought'...

best regards

Ian P.
 
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