ammeter shunt placement

Thread Starter

bwanna

Joined Nov 19, 2012
15
Hi,

This is a theory question regarding a shunt based ammeter.

Specifically, does it make a difference on whether the shunt is placed on the Pos vs Neg side of the circuit?

I've read that there is a difference in what is measured but am unable to find any details.

Thanks,
 

studiot

Joined Nov 9, 2007
4,998
Specifically, does it make a difference on whether the shunt is placed on the Pos vs Neg side of the circuit?
How are you connecting your ammeter and why do you think the shunt system makes a difference?
 

MaxHeadRoom

Joined Jul 18, 2013
19,712
Same current flows in each!
If the shunt is built in to the ammeter then it is only direction that you need to worry about, unless is a zero centre meter?
Max.
 

BillB3857

Joined Feb 28, 2009
2,516
Normally, the shunt will be placed in the position that will keep it, and its output voltage, as close to ground reference as possible. Consider a 500V DC source feeding a motor. Would you want the shunt in series with the 500V lead or in series with the ground lead.
 

studiot

Joined Nov 9, 2007
4,998
In my specific application it would be easier to put the Shunt across V+ right at the source
You keep referring to putting the shunt across a single terminal.

This does not make sense.

The installation diagram correctly shows the shunt in series with one of the supply leads. It does not matter which one.

In a vehicle the voltmeter section is often placed across the earthing strap to the battery, however there are arguments for placing ithe shunt in either the + or the - supply line.

Which one are you using as chassis or earth?

If you put in that one one you will always read the total current.

But say you have negtative earth, many arrangements use multiple connections to the positive terminal. If you put the shunt in the positive supply you could not then monitor total current.

So again I ask, what are your complete connections?
 

Thread Starter

bwanna

Joined Nov 19, 2012
15
Thank you all for your feedback. MCGYVR's link provided the explanation that I needed.

In short (no pun intended), the placement of the shunt on the Pos vs. Neg lines really doesn't seem to matter in the overall approach, especially if using a mechanical meter. However, the use of an electronic meter may dictate the placement of the shunt, depending on the design of the electronics in the meter. Shared supply or grounding could affect the meter's accuracy or even prevent it from working.

One good point was raised. For systems that have multiple connection points (for either the Pos or Neg sides) need additional consideration to determine EXACTLY what is being detected through the shunt. The ammeter (connected to the shunt) will not be measuring those parts of the system that bypass the shunt and connect directly to the Pos or Neg (depending on the setup).

Again, thanks for the feedback.

Cheers.
 

BillB3857

Joined Feb 28, 2009
2,516
The diagram shown in post #6 shows that there may be a strong possibility that the meter supply may not be able to share a connection with the shunt. Many of these meters require that the measured and source voltages cannot share a connection. I built a test fixture and needed to come up with a way to power the meters from an isolated source.
 

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Thread Starter

bwanna

Joined Nov 19, 2012
15
The diagram shown in post #6 shows that there may be a strong possibility that the meter supply may not be able to share a connection with the shunt. Many of these meters require that the measured and source voltages cannot share a connection. I built a test fixture and needed to come up with a way to power the meters from an isolated source.


That's what I think. I'm wondering if simply wiring in a 7808 regulator from the source to the meter would suffice to insulate it from measuring the amps from the source to the rest of the system?
 

t_n_k

Joined Mar 6, 2009
5,447
If there is an issue with power / common grounding: You could possibly use an op-amp configured as a differential amplifier to interface the shunt voltage to the (voltage) indicating module. For convenience the op-amp would be a uni-polar supply type with a common mode range including ground.
The op-amp can then be powered from the same source as the indicator module. A good op-amp with a low offset would be useful if you want better reading indication precision.
 

Thread Starter

bwanna

Joined Nov 19, 2012
15

studiot

Joined Nov 9, 2007
4,998
bwaana, there are a lot of posters working to try to help you here.

Each time someone offers something you tell us a little bit more about your setup and move the goalposts.

This is counterproductive communication and has lead to no solution after 17 posts and wasted others' time.

I have asked twice now for a proper description of what you are trying to do.
 

inwo

Joined Nov 7, 2013
2,419
This may be the one OP has.
http://www.ebay.com/itm/DC-50A-Red-LED-Digital-AMP-Ammeter-Panel-Meter-With-Current-Shunt-/110956430319?pt=LH_DefaultDomain_0&hash=item19d584c3ef

Has to be wired in low side. IMO

Now this one, I don't know?
http://www.ebay.com/itm/Mini-Red-LED-DC-Amp-Current-Meter-Ammeter-0-50A-No-need-Isolated-Power-C27-/180907264399?pt=LH_DefaultDomain_0&hash=item2a1ee9c98f

Specifies,"0-50A-No-need-Isolated-Power-This meter can be powered by the same source being measured(DC4.5-28V)., "
Good candidate to use with the same shunt.

Yet diagram shows connection in the low side?

Is it or is it not isolated?
These sellers generally don't know squat about what they sell.:(
 

Thread Starter

bwanna

Joined Nov 19, 2012
15
bwaana, there are a lot of posters working to try to help you here.

Each time someone offers something you tell us a little bit more about your setup and move the goalposts.

This is counterproductive communication and has lead to no solution after 17 posts and wasted others' time.

I have asked twice now for a proper description of what you are trying to do.

In my first post you'll notice I stated this was a " theory" question. As all good discussion goes, as questions were raised I responded. If you feel your time is being wasted, please feel free to not engage. Personally, I've been enjoying the dialogue and appreciating the good thoughts and suggestions.

Cheers
 
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