Sense Resistors - What's the Difference?

number1cruncher

number1cruncher

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So I'm building a 116 based bldc controller. It calls for a 4-5mOhm current sense resistor. I'm familiar with the typical "shunt" that looks like a thick metal jumper. However, when ordering at Mouser.com, I found this and ordered it(in addition to the typical shunt: http://www.mouser.com/Search/ProductDetail.aspx?R=14AFR004Evirtualkey58810000virtualkey588-14AFR004E . It looks like an oversized resistor, but was labeled with fragile stickers all over its packaging. Does it work the same way as a normal shunt?

Anyway, I chose the typical shunt, since this other one was labeled as fragile and my rides can be quite bumpy. If anyone has any idea what the difference is and if it would(would not) have worked, it might be a good teachable moment some, including me.

Thanks! :mrgreen:
 
I dunno about the fragility; based on the spec sheet linked on that page, it is a ceramic-potted type, and those are typically pretty tough, even under vibration. You might want to ask Mouser why they labelled it that way?

However, anything that has leads entering a body that isn't fixed to the same surface as the leads are soldered to is eventually going to break the leads off (at the body entrance usually) from vibration/impacts like potholes and whatnot. That's one reason the simple wire type shunts may work better.

Another reason the wire may be better is under high current applications, it can directly dissipate heat from it's surfaces, whereas the ceramic-potted ones must first dissipate from the shunt into the ceramic, then from there to the air around them, and so it is faster for it to dissipate heat directly thru the leads into the PCB (which the wire-type also does). I'm not sure which one would work better at heat dissipation, but I'd bet on the wire-type.
 
amberwolf said:
Another reason the wire may be better is under high current applications, it can directly dissipate heat from it's surfaces, whereas the ceramic-potted ones must first dissipate from the shunt into the ceramic, then from there to the air around them, and so it is faster for it to dissipate heat directly thru the leads into the PCB (which the wire-type also does). I'm not sure which one would work better at heat dissipation, but I'd bet on the wire-type.
Click to expand...
Actually, the ceramic resistor will dissipate its heat better (assuming same amount of heat). The ceramic has a much lower thermal resistance than air so it acts as a heat spreader, increasing the surface area of the resistor. This lowers the thermal resistance of the resistor to the air so it's more effective at getting rid of its heat.

Imagine holding your finger 1/8" away from the wire resistor. It might be bit warm there, but certainly not hot.
Now imagine holding your finger against the ceramic resistor, 1/8" away from its wire core. It's going to be hot enough there to burn your finger. This is the heat being effectively moved to the larger ceramic portion of the resistor. From there, it's easier to radiate into the air.
 
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