Shunt wanted for old EDMC Charge / monitor

[PembrokeshirePromise]

Hi,

Especially for Brian - who kindly suggested that if I posted some pictures he might be able to suggest some appropriate sized shunts. Basically I can find no info on the shunts I should use to use the current measurement facility of this box.
Original post here

Photos below showing various component numbers
(sorry for the size of the pictures - but they need to be quite large to see the component numbers)

Any ideas ?

Many Thanks
James

 

[William_H]

If they are using the LCD readout directly for current measurement then the LCD readout almost universally run to 200 millivolts(.2volt) for full indication. (199)
you have the choice of 2 amps max reading or 20 amps. My guess is you want 20 amps (19.99) so a simple calculation of R=E/I gives R = .2/20 which is .01 ohm. This is fairly low resistance but may be available from Maplin/RS etc. Power dissipation will be 4 watts at 20 amps so get a 5 watt resistor.
Now if you can't get a shunt you can make one from a piece of stainless steel plate perhaps 1cm wide 5cms long with a hole in each end to bolt the main wiring lugs and the sense wiring lugs to. This can be bolted on to a piece of fibreglass or plastic. You will need a multimeter or amp meter to check the calibration of the shunt. for more resistance and more reading on your LCD meter ( you can check it at a current as low as 2 amps for useable accuracy. You can make it longer or grind it narrower or even make cuts into the sides of the SS plate not adjacent to one another so that the path for current is longer. Use SS as it has a high resistance definitely don't use brass or copper or aluminiuum as it is too low in resistance
If you can check the calibration of the shunt a very elegant solution is to use the wiring already in situ as a shunt. So if you are measuring charge discharge current to a particular battery. Put one ampmeter sense wire on the battery terminal the other wire on the other end of the lead probably at the battery switch. The inherent resistance of the supply wire provides the resistance for the shunt. (it also means less total resistance and hence total voltage loss.) There may not however be enough resistance in the wiring. One solution is to make the wire longer or thinner. if you have enough reading and so want less resistance then you can shorten the wire or connect one of the sense leads at apoint along the wire. ie tapping at the centre will give half the reading on LCD. to be honest with 200 mv for the LCD you are not likely to get enough resistance to give a reading for 20 amps but would probably be good for 200 amps max reading. Which may be adequate. (make sure there are no switches in the wiring you are using as a shunt cos a switch off or poor contacts would really stuff it up.
It is possible that your charger use an amplifier of some sort for the meter in which case you will need less resistance which is good as it means less voltage drop. .2 volt is not really bad but it would be better not having that drop.
when you are checking the calibration you can use whatever current is flowing to calibrate as it is all a linear function so you can extrapolate.
PS these LCD readouts are available form maplins etc for about 20 squid.
Don't forget to put small fuses (in line) in both the sense wires. having got this far you can see that you can use a rotary switch 2 pole many position to allow you to monitor current in any dc circuit ie mast head light
good luck olewill

 

[PembrokeshirePromise]

olewill,

Many thanks for the excellent comprehensive reply - that certainly gives me some good places to start

Best wishes
James