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Very often I see high wire gauges (i.e., very thin cables) used for the wires that connect a test circuit's voltage input terminals to the sense screws of a power supply. I would have thought that you would want to use very thick low resistence cables to get an accurate read at the sense screws of the actual voltage that the circuit is getting.

It seems to me that by using thin wires of any length you are introducing parasitic resistence into the equation and skewing the voltage that the sense screws are receiving. Yet this seems to be common practice, thick cables for the power cables coming out of the supply and much thinner cables for the two sense connections.

What is the reasoning behind this?

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  • \$\begingroup\$ The reason is that the voltage sense function requires very little current. If the draw is only 1ma, even a wire with a 1 ohm resistance will only drop 1/1000 of a volt. Meanwhile, main power wires carrying 10 amps will see a 10 volt drop on 1 ohm wires so larger wires are required. \$\endgroup\$ – DoxyLover Jul 13 '15 at 22:02
  • \$\begingroup\$ The voltage sense circuit probably draws very little current, so voltage drop would be negligible. \$\endgroup\$ – user3169 Jul 13 '15 at 22:03
  • \$\begingroup\$ This is odd, because in testing with an HP 6653A power supply, I see a voltage drop of something like 20-30 mV (at 5V) when using 18 gauge cables instead of 12 gauge cables and a run of about 5 feet (for each of the two sense cables). \$\endgroup\$ – Michael Goldshteyn Jul 14 '15 at 3:24
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As mentioned very little current needs to flow in the sense wires so thin wire is adequate. It is used to save the cost of thicker wires as they would work just as well though using the excess copper from a thick sense wire in the power wires would be more productive in terms of power transmission. Same gauge is sometimes used for inventory reasons on the wire or terminals.

Some clever supply designs use just one sense terminal and expect both power wires to be of similar resistance and double the voltage drop measured in the wire that is being monitored to estimate the load voltage. The three wire RTD sensor arrangement in particular does this to save on the 4th conductor and the fact that all three wires are thin already and of the same length.

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