Installing a second shunt

[Slim]

I presently monitor my 12 volt electrics via a Mastervolt MICC and a Smartguage. My problem is that the MICC is showing it's age and sometimes it's readings are 'suspect' i.e it will show a discharge when it should be showing a charge or the amps shown + or- are obviously wrong. Rebooting the instrument temporarily returns matters to normal. I've checked all the wiring and am satisfied that it's not a poor connection.

My question is, can I fit a second shunt in series with the present one to support a new ammeter ( I don't want to disturb the existing one as it would be difficult to get to). logic tells me I can but !!!!!!!!!!!!!!!!!!!!

 

 

 

Frank

[Skye]

I presently monitor my 12 volt electrics via a Mastervolt MICC and a Smartguage. My problem is that the MICC is showing it's age and sometimes it's readings are 'suspect' i.e it will show a discharge when it should be showing a charge or the amps shown + or- are obviously wrong. Rebooting the instrument temporarily returns matters to normal. I've checked all the wiring and am satisfied that it's not a poor connection.

My question is, can I fit a second shunt in series with the present one to support a new ammeter ( I don't want to disturb the existing one as it would be difficult to get to). logic tells me I can but !!!!!!!!!!!!!!!!!!!!

 

 

 

Frank

 

I would have thought if you put an additional shunt resistor in series with the existing one, if the value of the resistance was the same then the volt drop across it would be half the original so your reading would be half what it is now. I believe you need to put it in parallel with the original.

[Graham Bowers]

I presently monitor my 12 volt electrics via a Mastervolt MICC and a Smartguage. My problem is that the MICC is showing it's age and sometimes it's readings are 'suspect' i.e it will show a discharge when it should be showing a charge or the amps shown + or- are obviously wrong. Rebooting the instrument temporarily returns matters to normal. I've checked all the wiring and am satisfied that it's not a poor connection.

My question is, can I fit a second shunt in series with the present one to support a new ammeter ( I don't want to disturb the existing one as it would be difficult to get to). logic tells me I can but !!!!!!!!!!!!!!!!!!!!

Frank

Yes, series.

Placing two identical shunts in parallel will result in the overall resistance of the pair being half the resistance of each. The ammeter would then indicate (in error) only half the true current was passing. Qualitatively it is easy to be seduced in to thinking half the voltage will be dropped across a pair in series. The trouble is that in that thinking, the resistance of the load has been forgotten - the reality is that you have three resistances in series, shunt 1, shunt 2, the load. The volt drop across a shunt is negligible in comparison with that across the load.

Can you not place your ammeter in parallel with the first shunt? OK, you'd need to know the resistance, so the ammeter (millivoltmeter with Amps written on it) was correctly calibrated. I know you said it was difficult to get to the first one, but I,m not sure if that,s to remove it, or just to connect to it. Oh, for that to work, I,m assuming the other instrumentation already connected to it is very high impedance, so can be ignored.

Hope this helps

Graham.

[RobinJ]

I presently monitor my 12 volt electrics via a Mastervolt MICC and a Smartguage. My problem is that the MICC is showing it's age and sometimes it's readings are 'suspect' i.e it will show a discharge when it should be showing a charge or the amps shown + or- are obviously wrong. Rebooting the instrument temporarily returns matters to normal. I've checked all the wiring and am satisfied that it's not a poor connection.

You say you have checked all the wiring?

Just make sure you know where the earth/ground/hull connection is! Mastervolt often fit an internal earth on the shunt, if you have a hull connection on the battery side, this could confuse it?

Shunts have such a small resistance they are unlikely to change much, however it is worth removing the shunt and checking the bolts, making sure they are as clean as possible (this would affect the reading of large loads rather than small ones?).

[by'eck]

I presently monitor my 12 volt electrics via a Mastervolt MICC and a Smartguage. My problem is that the MICC is showing it's age and sometimes it's readings are 'suspect' i.e it will show a discharge when it should be showing a charge or the amps shown + or- are obviously wrong. Rebooting the instrument temporarily returns matters to normal. I've checked all the wiring and am satisfied that it's not a poor connection.

My question is, can I fit a second shunt in series with the present one to support a new ammeter ( I don't want to disturb the existing one as it would be difficult to get to). logic tells me I can but !!!!!!!!!!!!!!!!!!!!

 

 

 

Frank

 

Since the resistance of a shunt is extremely small, and the resulting volt drop in the order of millivolts, a second in series will work with almost the same accuracy as a single. This since the current flow through each will be identical and only tempered by the volt drop of the redundant one reducing the flow minutely.

 

However its adding complexity, reducing reliability and inevitably adding some extra resistance into the circuit however small. It most definitely falls into the bad practice category.

[smileypete]

My question is, can I fit a second shunt in series with the present one to support a new ammeter ( I don't want to disturb the existing one as it would be difficult to get to). logic tells me I can but !!!!!!!!!!!!!!!!!!!!

Not ideal, but yes.

 

cheers, Pete.

~smpt~

[Skye]

Now I've had chance to think about it, to answer you question yes you can, but putting shunts in series is bad practice but they will work depending on the type of meter fitted. What you need to do is find out the route course of your problem and sort that, not find out great ways to monitor it

[Justme]

The existing set up is giving iffy readings, but is that due to the meter or the shunt?

 

The shunt could have overheated at some point & is now faulty.

 

I think I would want to remove it.