Why use 4 terminal current sense resistor?

[treez]

I have a 1mR resistor through which up to 10A flows.

I wish to amplifiy the voltage across this resistor with a x100 differential amplifier.

Do i really have to use a 4 terminalo resistor, or can i just use a two terminal one and connect it diligently ? (ie with careful PCB layout)

 

[Tahmid]

Do i really have to use a 4 terminalo resistor, or can i just use a two terminal one and connect it diligently ? (ie with careful PCB layout)

As long as the resistor can handle the power rating, a 2-terminal one should be fine. Be careful with layout.

 

[betwixt]

Tahmid is not wrong but there is a big advantage to using a 4 terminal resistor.

Consider that the resistor is very low value, only 1/1000 Ohm. Any wiring to the resistor that carries the 10A will also have a small resistance, while very tiny it could still be many times higher than 1m Ohm. If you are absolutely certain that your differential amplifier is connected only across the resistor and no other wiring is in series with it, you are OK, otherwise the voltage dropped will be higher than you expect and your amplifier will give an erroneous result. The 4 terminals are there so you ensure the measurement is only across the resistor element itself.

Brian.

 

[bigdogguru]

In support of betwixt's explanation, the use of a four terminal resistor for current sensing is often referred to as Kelvin Sensing/Configuration:



**broken link removed**

Four-terminal sensing

**broken link removed**

BigDog

 

[treez]

Thanks, the problem with using 4 terminal sense resistors of 1mR, is that they are never better than 5% tolerance.......thats too im-precise.

 

[bigdogguru]

Ohmite offers the 610 series four terminal devices with 5mΩ resistance at 1% tolerance rated for up to 32A.

There are other similar four terminal devices available in the 1% tolerance range.


BigDog

 

[FvM]

A rough hand calculation clarifies why 4-terminal connection of a 1 mOhm resistor is required to achieve a somehow acceptable accuracy.

4-terminal connection doesn't necessarily mean, that the resistor itself must have four terminals. That's a matter of resistor design. But you'll mostly want to use a PCB footprint with dedicated sense terminals and also a 4-terminal resistor symbol to clarify the intended circuit conection and allow for separate logical nets for source and sense.

 

[Tahmid]

Thanks, the problem with using 4 terminal sense resistors of 1mR, is that they are never better than 5% tolerance.......thats too im-precise.

Take a look at some of the resistors offered by RHOPOINT: **broken link removed**

Here's one: **broken link removed**
Datasheet: **broken link removed**

Hope this helps.
Tahmid.