# choosing shunt resistor.

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#### electricity86

##### New Member
I have a shunt resistor - 5mOhm, 3W rating, 1% tolerance- and I pass through it up to 17Arms current, meaning it consumes ~1.5W.

It gets very hot when 17Arms flows in it.

If i chose a same shunt but with 5W rating, will it help conducting the heat?

#### ericgibbs

##### Well-Known Member
I have a shunt resistor - 5mOhm, 3W rating, 1% tolerance- and I pass through it up to 17Arms current, meaning it consumes ~1.5W.

It gets very hot when 17Arms flows in it.

If i chose a same shunt but with 5W rating, will it help conducting the heat?

Its Watts = V * I, I^2 * R, = 17^2 * 0.005 = 1.445W

Are you sure its a 5milliOhm resistor.???

##### Banned
Use a smaller value shunt resistor 1m maybe, that will reduce your power consumption dramatically but require more gain on your opamp, you don't want shunt resistors warm if you can help it. But if you're stuck with that particular shunt value the 3W you're using is fine, just make sure it has some space on the PCB and if possible attach a heatsink to it.

Very hot is relative as well. If you can still touch it it's not 'very' hot.

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#### electricity86

##### New Member
Thank you for you comments.
I must stick with the 5mOhm value.
what do you mean by attaching a heatsink to that?
What is a heatsink?

It gets so hot that you cant touch it for more than a split of a second.

#### ericgibbs

##### Well-Known Member
Thank you for you comments.
I must stick with the 5mOhm value.
what do you mean by attaching a heatsink to that?
What is a heatsink?

It gets so hot that you cant touch it for more than a split of a second.

Heatsink can be an aluminium plate, with 5mR bolted firmly to it, to carry away the heat.

#### electricity86

##### New Member
Heatsink can be an aluminium plate, with 5mR bolted firmly to it, to carry away the heat.

Thank you.
My shunt is on air, meaning only its 2 pins touches the PCB, and its body is on air.
In that case, will a heatsink help?

You can think of it like that:
____ח____
where the: ח, is the shunt and the ____ is the PCB.

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#### ericgibbs

##### Well-Known Member
Thank you.
My shunt is on air, meaning only its 2 pins touches the PCB, and its body is on air.
In that case, will a heatsink help?

You can think of it like that:
____ח____
where the: ח, is the shunt and the ____ is the PCB.

Yes, thats one way to do it.

#### Boncuk

##### New Member
If you can find an outdated transistor heatsink used for metal can packed transistors like BC160 you could use it on the resistor.

It looks like attached.

Boncuk

#### Attachments

• Heatsink.jpg
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#### electricity86

##### New Member
I'm not sure that i understood your idea.

How a transistor can be used for heatsinking?

Moreover, its an air-shunt, so how can you sink the heat that its body absorbs, if its body is high above?

##### Banned
Wrap something around it..... A heat sink is an aluminum sleave that spreads heat out over it's surface allowing it to dissipate heat. They weren't saying use a transistor to dissipate heat just that some older style transistors use heat sinks that will work on a shunt resistor.

#### Willbe

##### New Member
It gets so hot that you cant touch it for more than a split of a second.
Then it's over 71C; if it boils water it's over 100C.
1.5w for >51C rise above ambient is a thermal resistance of 51/1.5 = >34 degrees C per watt.
Some of these resistors can stand way more than the 175C for semicond. junctions so it may be OK for this resistor to get this hot.

Thermal resistance in electronics - Wikipedia, the free encyclopedia

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##### Banned
Make sure you have decent air flow around the resistor though. You did already say it was away from the board which should be fine, I see power resistors mounted on stand about 2cm's from the board on many charge control circuits. I don't know how much that temperature increase effects the linearity of your current reading though.

#### ericgibbs

##### Well-Known Member
I'm not sure that i understood your idea.

How a transistor can be used for heatsinking?

Moreover, its an air-shunt, so how can you sink the heat that its body absorbs, if its body is high above?

When you fit a power resistor to a pcb, its a good idea to mount the resistor above the pcb surface, leaving a gap for cooling air.

If possible, when the leads of the resistor are long, use a small screwdriver as a mandril and wrap the end wires of the resistor around the screwdriver blade to form a coil.
That is, dont crop the resistor wires short.

This small coil will help to cool the resistor and what is important is to keep the heat from the pcb track.
Many power resistor connections fail at the pcb track/solder joint.

OK.

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