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question about shunt resistor

jon5500

Oct 31, 2020
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In the process of finishing a 0-12 VDC @ 0-10 A switching power supply. Fairly new at this and wanted to ask about the shunt resistor. It will be in the ground line. Plans mentioned two options: either a custom shunt made from constantan wire or similar, or simply two 5W 0.1 ohm power resistors in parallel. Since I wanted to keep costs down, I am going with the power resistors, but I have a concern: I will be running 12 gauge wire for the output lines, but when the ground wire reaches the shunt, even the two resistors in parallel combined are far less than 12 gauge. Should I be concerned, or is the tiny resistor/ 12 gauge wire distance (basically just for the 12 ga wire to resistor wire connection) irrelevant to heating and losses?

Not sure I was totally clear on what I'm asking, so here is a diagram I made which explains it better:

shunt drawing.jpg

https://i.imgur.com/pYSRxiX.jpg

Thank you in advance!
 
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davenn

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Sep 5, 2009
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Fairly new at this and wanted to ask about the shunt resistor. It will be in the ground line.

Why would you want to put it to ground ??

A shunt resistor goes across the terminals of the ammeter IF the ammeter is not able to measure expected currents from the PSU to whatever load

So say your ammeter was a 100mA FSD (full scale deflection) meter then you would need a shunt across it so that it could read say 1A or 10A

Ammeter Shunt –Construction and Calculation | Electrical Concepts (electricalbaba.com)

I have to assume that is what you are referring to as it's the common usage of the word shunt resistor in electronics

If you are referring to something completely different, then you need to clarify what you are talking about
 

jon5500

Oct 31, 2020
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This is the circuit and R13 is the shunt. Current is monitored by the TL494.

Screenshot from 2020-12-02 15-33-34.png
 

Bluejets

Oct 5, 2014
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even the two resistors in parallel combined are far less than 12 gauge

The route length and the resistance creates such a low voltage drop at that current and hence a low power disipation.
Well taken care of with the two parallel 5w resistors.
 

jon5500

Oct 31, 2020
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The route length and the resistance creates such a low voltage drop at that current and hence a low power disipation.
Well taken care of with the two parallel 5w resistors.

Ok, thanks.
 

Frankchie

Nov 14, 2017
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jon5500,
10 amps through two parallel .1 ohm resistors = .05 ohms * 10^2 = 5 watts. The two 5w resistors in parallel can handle 10 watts so as already mentioned you should be okay.
Be advised 5 watts generates a lot of heat, even 1 or 2 watts gets very hot. If touched it can burn you. It's probably best to mount the resistors in an uncongested area, maybe even elevated off any PC board to maximize air flow and minimize heating other components.
 

Bluejets

Oct 5, 2014
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10 amps through two parallel .1 ohm resistors = .05 ohms * 10^2 = 5 watts.

Diagram resistance is 0.025R
To obtain this result then two 0.05R resistors are required.
Think Op has made a boo boo somewhere.
 
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Frankchie

Nov 14, 2017
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Diagram resistance is 0.025R
To obtain this result then two 0.05R resistors are required.
Think Op has made a boo boo somewhere.
Good point.. The schematic also says it can supply up to 20 amps. That calculates to 10 watts dissipated so some bigger wattage resistors are required.
 

jon5500

Oct 31, 2020
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Sorry, not quite the schematic but close. Didn't realize I posted the wrong one until pointed out here. What I'm finishing will supply up to 10 amps and use the paralleled 0.1 ohm resistors.
 
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