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Supposing I want to measure 1-5A over a range of 1-40V on an ammeter, should I change the shunt resistor every time the voltage changes so that the voltage drop across the shunt resistor stays the same?

If yes, how do ammeters on lab bench power supplies work over a wide range of voltages?

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  • \$\begingroup\$ I thought I said it was frowned upon. Edit your original question. \$\endgroup\$
    – D.A.S.
    Commented Apr 8, 2018 at 5:27
  • \$\begingroup\$ electronics.stackexchange.com/questions/367147/… \$\endgroup\$
    – D.A.S.
    Commented Apr 8, 2018 at 15:54
  • \$\begingroup\$ Hey, sorry Tony. I didn't notice that! I wanted to ask this as a general question while the other one was specific to the project I am doing. \$\endgroup\$ Commented Apr 9, 2018 at 7:13
  • \$\begingroup\$ I answered both the specific and general case in my previous answer. Pd Rating is always for max current. \$\endgroup\$
    – D.A.S.
    Commented Apr 9, 2018 at 13:42

3 Answers 3

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Supposing I want to measure 1-5A over a range of 1-40V on an ammeter, should I change the shunt resistor every time the voltage changes so that the voltage drop across the shunt resistor stays the same?

No. The meter connected to the shunt is measuring the voltage across the shunt and not the voltage from the PSU. The voltage across the shunt will be proportional to the current through the shunt. If this stays within the shunt specification then all is well.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. PSU with shunt and ammeter.

If yes, how do ammeters on lab bench power supplies work over a wide range of voltages?

Again, the meter is reading the voltage drop across the shunt and not across the load.


From the comments:

If the voltage changes, wouldn't the voltage drop across the shunt also change?

Yes, of course, as defined by Ohm's law. So you or the power-supply designer should choose a shunt that is capable of measuring the range of currents of interest. For example, a 5 A bench power supply will give out 5 A max so a 5 A shunt is all that is required.

If the voltage drop across it changes, how would it measure current correctly?

Because if the current through the shunt drops the voltage across the shunt will drop in proportion. V=IR means that for a fixed resistance (the shunt in this case) voltage across the shunt is proportional to current through the shunt.

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  • \$\begingroup\$ If the voltage changes, wouldn't the voltage drop across the shunt also change? \$\endgroup\$ Commented Apr 9, 2018 at 7:09
  • \$\begingroup\$ Yes, of course, as defined by Ohm's law. So you or the power-supply designer should choose a shunt that is capable of measuring the range of currents of interest. For example, a 5 A bench power supply will give out 5 A max so a 5 A shunt is all that is required. \$\endgroup\$
    – Transistor
    Commented Apr 9, 2018 at 7:55
  • \$\begingroup\$ If the voltage drop across it changes, how would it measure current correctly? \$\endgroup\$ Commented Apr 9, 2018 at 9:57
  • \$\begingroup\$ Because if the current through the shunt drops the voltage across the shunt will drop in proportion. \$ V = IR \$ means that for a fixed resistance (the shunt in this case) voltage across the shunt is proportional to current through the shunt. \$\endgroup\$
    – Transistor
    Commented Apr 9, 2018 at 10:03
  • \$\begingroup\$ Oh, so the voltage can change! I didn't know that all along!! Thanks for shedding light on that. So, if I am using an ammeter designed for 75mV drop, I just have to select a shunt that drops 75mV at the maximum current I desire to use, right? Then, it will drop lower voltages at lower currents thereby showing the right amperes? \$\endgroup\$ Commented Apr 10, 2018 at 11:11
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They work over a wide voltage range because the current shunt is not voltage sensitive. The circuits reading the current often do so through precision resistors that divide the voltage down to safe levels.

This way the common mode voltage seen by the circuits is within a safe range, but precision instrumentation amps see the Vdrop across the shunt, and scale the gain and the display to make sense.

Most desktop equipment has a auto-scaling display and together with automatic gain adjust they can cover many decades of values with just one precision shunt. In this type of equipment resistors and shunts are typically in the 0.025% to 0.1% tolerance range.

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You must always choose shunt based on voltage and power dissipation with heat rise due to thermal resistance in open air so <=100mV . If enclosed then even less.

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