ammeter shunt placement

Discussion in 'General Electronics Chat' started by bwanna, Jan 28, 2014.

  1. bwanna

    Thread Starter New Member

    Nov 19, 2012
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    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,
     
  2. studiot

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    Nov 9, 2007
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    How are you connecting your ammeter and why do you think the shunt system makes a difference?
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    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.
     
  4. mcgyvr

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  5. BillB3857

    Senior Member

    Feb 28, 2009
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    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.
     
  6. bwanna

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    Nov 19, 2012
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  7. studiot

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    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?
     
  8. bwanna

    Thread Starter New Member

    Nov 19, 2012
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    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.
     
  9. BillB3857

    Senior Member

    Feb 28, 2009
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    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.
     
  10. bwanna

    Thread Starter New Member

    Nov 19, 2012
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    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?
     
  11. BillB3857

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  12. t_n_k

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    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.
     
  13. ronv

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  14. bwanna

    Thread Starter New Member

    Nov 19, 2012
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    I like the DC/DC isolation approach. Was looking at this:

    http://www.digikey.com/product-deta...64467?itemSeq=143451733&uq=635265325653226813

    The challenge is that the system has a distributed grounding along multiple points. The power source is a single output. For this reason, I'm trying to find a way to have the shunt in series on the V+ output going to the many loads (having multiple points of contact eventually coming back to the V- (GND)
     
  15. BillB3857

    Senior Member

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    There is no place in the power distribution that ends up with a single wire going back to the source?????
     
  16. bwanna

    Thread Starter New Member

    Nov 19, 2012
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    You mean to GND. No. GNDing for this circuit is on a bus. Only the V+ has a single point.
     
  17. BillB3857

    Senior Member

    Feb 28, 2009
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    So how does the power source tie to the bus?
     
  18. studiot

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    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.
     
  19. inwo

    Well-Known Member

    Nov 7, 2013
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    This may be the one OP has.
    http://www.ebay.com/itm/DC-50A-Red-...319?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-LE...399?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.:(
     
  20. bwanna

    Thread Starter New Member

    Nov 19, 2012
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    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
     
    Last edited: Jan 29, 2014
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