Wiring an ammeter

pheran

pheran

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Apologies if this is a very basic question but I want to install a panel ammeter to enable me to monitor power consumption from my 24v domestic battery bank (2x12v batteries in series). To avoid a constant drain and because I only need to take the occasional 'snapshot' the circuit will need to be switchable. Can anyone suggest a suitable circuit please. I get confused over all this talk of shunts etc. Many thanks.
 
At it's simplest the ammeter is an instrument through which all the current must flow.

However inside that instrument there is probably a sensitive meter connected in parallel with a "shunt". The shunt is a very low value resistance through which most of the current flows . The remaining small proportion of the current flows through the sensitive meter.

Once you grasp this concept you can see that the shunt can be placed in the main wiring where you want the current measured but the sensitive meter can be placed in an instrument panel where you can see it. Because only a small current passes though it it can be connected to the shunt with fairly light wires.

Look a t the section about "Warning lights and instruments" at http://www.tb-training.co.uk/cover.html

To satisfy the pedants out there I have to add that not all ammeters are as I have described above but I suggest you will want one with a remote shunt as described.

You'll find a lot on the above site informative, especially the basics if you are uncertain about electrics.


You may also be interested in The 12volt Doctors Practical Handbook which you can down load from HERE
 
[ QUOTE ]
because I only need to take the occasional 'snapshot' the circuit will need to be switchable.

[/ QUOTE ] Missed that. An ammeter does not consume any current itself (unless you are looking at digital meters) so switching it does not come into the equation.

A voltmeter consumes a small current so sometimes these are switched, but in most cases the consumption is so small it can be permanently connected after the isolation switch. When the isolator is on the voltmeter reads, when it is off it doesn't.
 
That is well explained Vic. The shunt does send only a small portion of the current to the remote meter. Or you could also say correctly that the shunt drops a voltage which is measured by the remote meter.
Now if you are using a moving coil analogue meter that voltage can be quite small and the current a percentage of the measured current.
Hoever if you are using a digital readout these measure voltage and all the ones I have seen require 200mv (.2volt) to reach the max readout number.(1999) This unfortunately requires a significant resistance in the wiring. So typically he will want the amp meter to read to a max 20 amps (19.99 amps) The shunt will then drop (waste) .2 volt at 20 amps or .1volt at 10 amps. That voltage drop may or may not be of concern for the electric devices running on the system.

The way around this is to find (buy) (or build) a meter with an amplifier which will provide the 200 millivolts from a much smaller volt drop in the shunt (I have never seen one on sale) or by fitting a switch in the system which will bypass the shunt so minimising the volt drop. You open the switch to measure the current.
And of course you must provide power to the digital readout. Usually by a push button switch.

Alternatively buy a moving coil analogue meter. Which takes no power to operate and a much smaller shunt volt drop. But does not give useful readings at small currents.

I have made shunts out of a piece of stainless steel sheet 20g typically 2 inches 50mm between bolt holes and and 1/2 inch 10mm wide.Connect a digital meter or 50 microamp moving coil meter to the shunt but using separate holes near to the holes for the main current.
You need to have an amp meter to put in series as a temporary measure to calibrate the shunt. For measuring measuring more current (getting a greater reading) you make the shunt longer or narrower. Wider shorter for less reading.

If you buy a moving coil meter already set up for amp meter ie tyically 5 amp max then you either run the main power wires to and from the meter or you may be able to open up the meter to find the shunt. A short piece of heavy wire connected across the terminals inside. This can be removed and mounted near the battery or power wires. However if you do this put fuses in both thin wires that go to the meter. Fuses should always be used with a remote shunt. good luck olewill
 
[ QUOTE ]
Hoever if you are using a digital readout these measure voltage and all the ones I have seen require 200mv (.2volt) to reach the max readout number.(1999) This unfortunately requires a significant resistance in the wiring. So typically he will want the amp meter to read to a max 20 amps (19.99 amps) The shunt will then drop (waste) .2 volt at 20 amps or .1volt at 10 amps.

[/ QUOTE ]


Or use a Hall effect shunt that measures the magnetic field around the cable. They have no connection with the cable, carry no current, have no volt drop and cannot be overloaded. They can also drive analogue meters.

Normally digital meters are 200 m volt, or 2 volt scale. Use the 2 volt scale, shunt output is 100 amps per volt, meters then reads upto 200 amp.

Brian
 
There are some detail here .

They are stable, we have been making them for nearly 20 years, mainly for production boats. As they cannot be overloaded it allows them to monitor net amps, whilst using the battery for engine starting.

Brian
 
Thanks Brian.

My main shunts are 500 A 50mV with an instrument amp giving an amplification of 10 to drive my standard DVM's

My main battery connections are with 40 mm x 20 mm buss bars to reduce the volt drop when my bow thruster is operated.

As you can see your units will not fit my main power feed connectors so I may have to make another plan.
 
Anything is possible, there are units waiting to go into production, time /forums/images/graemlins/frown.gif .

Drop us a photo af your installation, and I will comment.

Brian
 
I have used one of these AMP25 sensors. It's nominally +-25A but can read up to 60A. The big limitation is sensitivity to low current - it can't reliably measure less than about 0.1A.
There is also this range from Allegro but I haven't used them.
Derek
 
I just use the existing neg common battery lead as a shunt, and measure the millivolts across it.
(Having shifted the system feed wires from the battery post to the engine connection)
 
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