Shunt Resistance

Hi I was trying to create a current measurement circuitry. For testing purposes I used a 1ohm resistor in series with a 1k resistor with a source voltage of 15V. Therefore the current flowing through the circuit should be approx 15.6mA and the voltage reading across the 1Ω resistor should be about 15.6mV.

But the Voltage reading was about 17mV which corresponds to 17mA. I am not sure if I am doing this right. Could somebody lead me in the right direction.

How close to 1.00Ω was your 1Ω resistor? How close to 1,000Ω was your 1KΩ resistor? Next, how close to 15.000 volts was your 15 volt source? Starting to see what can be happening here?

Things can get a little tricky. For example if I want a precision resistor as a current shunt I would be doing a 4 wire kelvin measurement on that resistor. Also, in real world shunt use the shunt needs to remain cool and not heat changing resistance. For your test to work things really need to be very tight tolerance including the meter you are using to measure the voltage drop across your shunt resistance.

Ron

Reloadron said:

I would be doing a 4 wire kelvin measurement on that resistor.

Click to expand...

This is an important point - with a current measurement on a low shunt resistance, you need to be account for the solder junction resistance, lead resistance, etc. Best way to handle it is with that "4 wire" measurement.

Hi,

Lets see, if the resistor was really 1.090 ohms instead of 1.000 ohms you'd measure 17mv instead of 15.6mv, so the resistor could be a little higher than normal. I'd check the tolerance of the resistor.

The other thing to think about is insertion loss. But that's probably not an issue here.

Jeswin doesn't know about tolerance:
1) The 15V could have 10% tolerance so it could be anywhere from 13.5V to 16.5V.
2) The resistors also have a tolerance and can be 20%, 10% or 5% from their assumed value.

I see that no one bothered to check the original math on this post. That was odd.

1 ohm in series with 1K is 1001 ohms. With 15v applied, that's a current of 14.985ma which would produce 14.985mv across the 1 ohm resistor.

1 ohm inserted into the single 1K ohm circuit results in only a 0.1% deviation in values.

All this 4-terminal stuff that's being discussed is referring to measuring that 1 ohm resistance accurately, Jeswin. All the discussion is confusing, making one think that there's a 4-terminal current measurement going on here. If that 1 ohm resistor is properly inserted into the circuit using good soldering techniques, I doubt that any of the solder resistance or lead resistance (insertion loss as Mr Al says) is of any consequence.

My observation would be that the current sense resistor is off-value. With only a little over 200µw being dissipated, heat is hardly a consideration for pushing the resistance value off. For accurate measurement with a low temperature coefficient (room temp is a bigger player on the value than is the current through the resistor for this particular circuit), I'd suggest using a 1 ohm, 1% metal film resistor for the current sense resistor. Make sure the power supply is right on 15.00v as Ron suggests. For purposes of experimentation, use a 1K 1% metal film resistor for the load as well to minimize error, since you are trying to check theory against reality. If then the voltage across the 1 ohm resistor is off from the theoretical 14.985mv by more than maybe 0.2mv, I'd start taking a close look at my DMM accuracy since you are changing ranges to measure between the supply voltage and that millivolt voltage drop. Measuring low voltages with a standard handheld DMM can be a bit tricky. You are looking at the lower end of the 200mv range on most DMMs and the lower you go in the measured voltage, the higher the inaccuracy will be. You can get into thermoelectric error simply from test probes creating a thermocouple effect as they contace the measurement points.

For an exhaustive and interesting treatise on low voltage and current measurements, look at Keithley's web site ( http://www.keithley.com ) for one of their books on such measurements. Go to the "Knowledge Center" button and there you will see the "Low Level Measurements Handbook", available to view online for free or as a free paper copy upon request. I advise going for the paper copy. It's a good book.