Wiring ammeter shunt

PabloPicasso

PabloPicasso

Well-Known Member
Joined
12 Feb 2010
Messages
2,927
Visit site
If I get one of these. http://www.ebay.co.uk/itm/221575421830 How do I wire the ammeter's shunt. Is it in line between the domestic battery and the fuse. board?

How much current are the ammeter and shunt likely to draw?

If it's significant Could I have a 1 off 2 switch so I have the option of running the electrics whilst bypassing the shunt?
 
assuming you want to measure flow out of and into your domestic bank, it goes between the battery negative and the rest of the world. It needs to capture all currents.

It will draw insignificant current.

Tony.
 
PabloPicasso said:
If I get one of these. http://www.ebay.co.uk/itm/221575421830 How do I wire the ammeter's shunt. Is it in line between the domestic battery and the fuse. board?
Click to expand...

Depends what you want to measure. If you want to measure the current drawn by the fuse board, wire it in the negative wire to the fuse board, as shown in the diagram in the eBay ad.

PabloPicasso said:
How much current are the ammeter and shunt likely to draw?
Click to expand...

The eBay ad says less than 60mA.

PabloPicasso said:
If it's significant Could I have a 1 off 2 switch so I have the option of running the electrics whilst bypassing the shunt?
Click to expand...

Yes, you can bypass the shunt if you wish.
 
The schematic drawing in the listing shows where the shunt goes. The shunt draws no current and the meter a very small current, in the spec, 60mA, ie 60/1000ths of an Amp. I wouldn't bother bypassing it.
 
elton said:
If you can't answer those questions, why are you fitting a meter? You won't understand what it says.
Click to expand...

I thought the point of fora such as this was to help and encourage understanding.

Yes, they are basic questions but demonstrate a desire to move forward and that is to be admired.

Tony.
 
Jabs said:
I thought the point of fora such as this was to help and encourage understanding.

Yes, they are basic questions but demonstrate a desire to move forward and that is to be admired.

Tony.
Click to expand...
Electrical theory isn't something that can just be "picked up" off a forum. It isn't difficult to get a good grasp of basic DC electrical theory, but neither is it something that can be learnt in dribs and drabs.
 
elton said:
Electrical theory isn't something that can just be "picked up" off a forum. It isn't difficult to get a good grasp of basic DC electrical theory, but neither is it something that can be learnt in dribs and drabs.
Click to expand...

I have to say that I agree. I often cringe at the thought that some people asking "electrical" questions are then going on to do actual wiring. Even I as an experienced Broadcast TV Engineer had a lot to learn when I bought my boat, my book of choice being Calder. See link bottom left of my Bavaria Forum "Support Us" page.
 
JumbleDuck said:
The entire point of the shunt is to draw current! OK, I know what you mean, it doesn't cause a current to flow when nothing else is running.
Click to expand...

A shunts job is to produce volt difference in two wires, the current that goes in one end comes out the other.

Brian
 
For safety-wire an inline fuse (1A) in red the PWR + wire at the point that it connects to the + (pos) bat. cable junction.
I would install a switch in the red PWR + lead also.
Personal experience with "always on" devices such as this has stung me in the past if boat not used for a few days.(Low Bat).

Lets give him a break re-his electrical expertise.He had enough smarts to ask how to do it,before he got in a mess.
I cringe at times also when it is obvious the poster is after the fact &/or doesn't plan to follow good advice-or---is given poor advice.

I am more concerned about ones ability to do the actual physical wiring & connections in a safe manner.

It's his boat & his $$. Therefore,his choice.

I thought forums were to learn & help. Peace/ Len Certified Electronic Engineering Technologist.
 
Last edited:
The question "How much power will the shunt use" is a bad question but what I think he means to ask is "How much voltage will the shunt drop" or waste. The answer is about 75 millivolts at full rated current (50 amps). So not much. So don't worry.
The meter itself will as said draw about 40 milliamps. so worth fitting a switch if it may be in a powered circuit for long periods. (days). ie about 1 amp hour per day.
Now I bought one of these. http://www.ebay.co.uk/itm/262455987311?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT.
Now the shunt must be in the negative circuit. It has 2 heavy wires for the measured current. 3 wires for the volt meter power supply (red) and power negative (black) So I connected the current wires in the negative circuit the pos supply to 12v + and pos volt meter also to the 12v+ve. I found that the amp meter measured some 130% too much.
I queried the seller who gave a wiring diagram showing a separate supply ie a small battery for the meter supply. A surprise as volt meter works fine measuring and supplied by 12v +ve.
I discovered however that all worked oK if I disconnected the small black wire (supply and measured volts negative)
The negative supply then comes via the amp meter wires in the negative line. So a bit ugly technically but OK in practice.
So yes wiring can be a bit tricky assuming your meter is similar in design to mine. (except yours has a remote shunt.)
Once wired try to check calibration of the map meter by running a load of known current draw. (Headlight bulb) just to check calibration. Accuracy is not normally so important for current. (10% would be fine) good luck olewill
 
Related question: (hope I don't get criticised for my ignorance)
Regarding cheap ammeters with internal shunt (and hence easy to install), that are only recommended for <10amps....if they are exposed to more than 10amps, say 30amps, will they be damaged, or simply be inaccurate?
 
Venus1 said:
Related question: (hope I don't get criticised for my ignorance)
Regarding cheap ammeters with internal shunt (and hence easy to install), that are only recommended for <10amps....if they are exposed to more than 10amps, say 30amps, will they be damaged, or simply be inaccurate?
Click to expand...

Probably not damaged by a moderate over load, just wont display more than the rated maximum, but there will be a limit.


A good place to learn the basics is Tony Brooks' Boat Electrical course notes at http://www.tb-training.co.uk/
 
Last edited:
William_H said:
The question "How much power will the shunt use" is a bad question but what I think he means to ask is "How much voltage will the shunt drop" or waste. The answer is about 75 millivolts at full rated current (50 amps). So not much. So don't worry......................
Click to expand...

In full pedant mode............... The question wasn't "How much power will the shunt use?". It was "How much current?"
 
halcyon said:
A shunts job is to produce volt difference in two wires, the current that goes in one end comes out the other.
Click to expand...

That's one way of looking at it. I prefer to think of it as something which diverts current away from somewhere else, but that's possibly because I still think of meters are electromechanical devices driven by current whereas most modern ones are driven by voltage.
 
Also forgot to mention that my amp meter and also I believe yours will not register amps in the other direction. So only suitable for either measuring amps into the battery from charging or amps out of the battery in discharge but not both. Would need 2 x amp meters to do both functions.
Sorry I did not notice the diagram for your meter so much of what I said earlier is irrelevant.
Yes agree with Vic the amount of overload the meter is capable of handling depends on the power dissipation capability of the shunt. I suspect that with a built in shunt the resistor would physically be as small as possible so much capability beyond the 10 amps of the meter. However with an external shunt usually bulky and so capable of dissipating a lot of heat so overload no real problem. olewill
 
Adding a shunt will reduce the current.

A shunt has resistance (from a circuit analysis point of view it's just a resistor).

(See JD in #17 for why it is called a shunt.)

To put some values on it;

Battery 12.0V
Load 1.2Ω

Current = 12.0V / 1.2Ω = 10.0A

Resistance for a typical 50A shunt is 0.001Ω

So if you add the shunt the load becomes 1.2 + 0.001 = 1.201Ω

Current with shunt = 12.0V / 1.201Ω = 9.99A

A small but real reduction in current.

Not an issue on a boat you may think and you would be right with regard to the OPs proposed shunt.

However, many of us use multi-meters on our boats, some have auto-ranging. Multi-meter's have different shunt resistances for different current ranges. For low current ranges shunt resistance can be large (EG 100Ω for a 2mA range). Electronic equipment may refuse to come out of standby, turn-on, change mode or just operate strangely because the multi-meter's shunt doesn't switch to a lower value quickly enough. Not an issue with a dumb load such as a bulb. The problem can be avoided by using the highest current range (10 or 20A which most commonly has a fixed shunt resistance and often a dedicated terminal.

For the OP; the current drawn by the electronic meter is the significant factor. 60mA is a lot if left permanently connected.
 
You must log in or register to reply here.

Share this page

Top