# question about sensing resistors

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#### Triode

##### Active Member
is a sensing resistor a special kind of resistor, or is it just a resistor that is being used to make current detection possible? The issue in particular is that I have this motor driver with a connection diagram calling for a 10 ohm current sensing resistor and I'm not sure if thats just requireing 10 ohms of resistence there or something specialized.

##### Banned
Nope, just 10 ohms of resistance, generally they're 'overbuilt' resistors, so they dissipate heat well, nothing really special about them though.

#### dknguyen

##### Well-Known Member
THey aren't anything special. They are usually just much lower resistance (milliohm range) than other resistors with much better heat dissipation/current capability. They might also be made to handle surges better and have lower inductance because they are often used in motor switching applications.

Nothing really says you can't use a regular low value resistor. It's pretty hard to find a 10ohm resistor labelled as current sense anyways since that's very high resistor for most current sensing.

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#### Russ Hensel

##### New Member
But keep in mind:
If resistor gets hot resistance may change, perhaps enough to matter, or not. Oversizing the resistor helps minimize this.

Wire wond resistors are fine for DC but may not work well as frequency goes up.

10 ohms seems high for current sense unless current is low. dknguyen is right on.

#### Triode

I wasn't looking at the diagram when i wrote that. It's actually 0.5 ohms, which sounds pretty normal. I guess I'm going to have to wait till i need to order a bunch of parts again to setup this motor controller, cause they don't sell these at any store I know of, and I'm not paying $7 shipping for a 5 cent part. Last edited: #### Russ Hensel ##### New Member I wasn't looking at the diagram when i wrote that. It's actually 0.5 ohms, which sounds pretty normal. I guess I'm going to have to wait till i need to order a bunch of parts again to setup this motor controller, cause they don't sell these at any store I know of, and I'm not paying$7 shipping for a 5 cent part.

You may be able to salvage some nicrome wire from an old toaster to get high current resistance wire for free. Hard to solder, may want to attach with nuts and bolts and solder to them.

#### Willbe

##### New Member
I wasn't looking at the diagram when i wrote that. It's actually 0.5 ohms, which sounds pretty normal. I guess I'm going to have to wait till i need to order a bunch of parts again to setup this motor controller, cause they don't sell these at any store I know of, and I'm not paying \$7 shipping for a 5 cent part.
Depending on the current you can use 5' of #30 awg wire wrap wire. It melts at 8A to 10A. Or 10' of #27. Or 20' of #24.

##### Banned
Sense resistor needs to be very accurate (low tolerance) and very small, so it wont affect the current that flew in the circuit before connecting the sense resistor.

Its purpose is to let you calculate the current through the consumber/appliance, accurately.

What did you mean dknguyen by saying its built well for heat dissipation?

#### Triode

##### Active Member
well a long coiled piece of wire, like some spare electromagnet wire i have, would dissipate heat well, and as long as i test the length I'm going to use with a multimeter first, I can make the tolerance very tight.

#### 3v0

##### Coop Build Coordinator
Forum Supporter
A DIY resistor is a bit much in my book.
Mouser has SMD sense resistors for about 50 cents. They are not tiny.

This is 2 Watt .001 Ohm
ULR2R001FLFTR

#### Triode

##### Active Member
Yeah, I normally order these kinds of things. I just wanted something to make the motor controller work for now so that I don't have to wait till next time I need enough parts to justify shipping on an order before I can learn how to use the controller.

#### Diver300

##### Well-Known Member
At 10 Ω it isn't very important, but it is quite common to have sensing resistors wired as 4 terminal resistors. That is where the wires that carry the current are separate from the wires that sense the voltage.

The reason is so that the resistance of any connections do not cause any measurement errors. Power Standards Lab - How 4-terminal resistors work explains why no.

#### 3v0

##### Coop Build Coordinator
Forum Supporter
I am trying to understand this.

So if one took a standard resistor and ran two wires from the sense circuit directly to the resistor you have in effect a 4 terminal resistor ?

At 10 Ω it isn't very important, but it is quite common to have sensing resistors wired as 4 terminal resistors. That is where the wires that carry the current are separate from the wires that sense the voltage.

The reason is so that the resistance of any connections do not cause any measurement errors. Power Standards Lab - How 4-terminal resistors work explains why no.

#### Triode

##### Active Member
I'm pretty sure that its just a two terminal resistor, since that's how it appears on all the circuit diagrams. That's interesting though, i never knew such a type of resistor existed.

#### dougy83

##### Well-Known Member
I am trying to understand this.

So if one took a standard resistor and ran two wires from the sense circuit directly to the resistor you have in effect a 4 terminal resistor ?
This is also important when routing PCB traces to the sense element - they should connect directly to the sense element pads, and not to some arbitrary point of some track that happens to connect to the sense element.

#### Hayato

##### Member
I am trying to understand this.

So if one took a standard resistor and ran two wires from the sense circuit directly to the resistor you have in effect a 4 terminal resistor ?
Well, the 4 terminal resistor could be 2 resistor packed in 1, so that they are connected in parallel, to improve the thermal dissipation by dividing the current.

#### 3v0

##### Coop Build Coordinator
Forum Supporter
According to this graphic from the wikipedia article there is only one resistor. As far as I understand the principal you need to hook the sense traces/wires directly to the sense resistor, not some common gnd etc.

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##### Banned
I'm not sure what Hayato is talking about, I'm sure it could be done but I've never seen two resistors packed in the same package like that before. If it's a sense resistor and has four terminals they're for kelvin sensing.