Remote Sense RS Wires For Power Supplies


#1

RS or Remote Sense wires on select power supplies are a feature used to correct for the small voltage loss across the load wires or any other connections from the power supply output terminals to the load terminals. This feature is typically used in low-voltage high-current systems, circuits requiring precise voltage, or when long stretches of output wire are present. As load currents fluctuate the voltage drop across the load wires and connections will also change. By connecting the RS wires properly, the power supply will adjust the output voltage accordingly to help keep the voltage steady at the load. Every supply will vary, but it is common to see a supply to compensate the voltage output by about 0.5v max give or take such like that on the HRPG-300-12 model from Mean Well. The voltage change on well designed supplies is usually less than the voltage change to the output when adjusting the Vout potentiometer.
Wiring
Depending on your design, smaller wire gauge such as 22AWG is typically alright to use for the remote sense wires as these lead to a high impedance circuit so it will not draw a lot of current. The RS wires are simply run to the actual load terminals and bypass the actual load wires. Polarity must be uniform, meaning positive RS wire to positive load terminal, and negative RS wire to negative load terminal, or damage to supply can result. Sometimes a shielded cable is desirable especially in noisy environments or long RS cable runs.

A given supply will either require that the sense terminals be connected (either locally or remotely) for proper function, or not. If “not” a user-supplied local bypass typically won’t make much difference. This will all depend on internal RS circuit configuration of the power supply.

RS Bypass
A quick way to resolve remote sense floating issues and bypass this feature is to jumper the positive remote sense wire to the positive supply output and jumper the negative remote sense wire to the negative supply output otherwise known as local sensing or bypass. Do not mix polarities as this may damage the power supply.

Example Datasheet
https://www.meanwell.com/Upload/PDF/HRPG-300/HRPG-300-SPEC.PDF


#2

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There is often a 100 ohm resistor between the +sense and +ve and between the -sense and -ve outputs of bench power supplies. This protects the output from high voltage if the sense wires are accidentally disconnected. So why are these resistors low value like 100 ohms? What if it’s 1K?

I’m making a 6V power supply and can’t seem to see much difference in the output whether it is 100 ohms or 1k. The output voltage always falls if the sense wire gets disconnected, whether the resistor is 100 ohms or 1K.

Also, it seems 1K is better, if the actual power line gets disconnected. The 1k will survive, while the 100 ohms will burn up, possibly taking the sense wires with it.

So, what am I missing?

edit:

I think the question was not clear. There are no feedback resistors involved here. These are the resistors I am talking about: Rs and Rs shown below. They will burn up if they are low value and the output cable is disconnected (they are usually 100 ohm 0.5 watt; sometimes even 5 ohm or 10 ohm). But they will not burn up if they are 1K 0.25W. Image from here and here.


#3

Hi Catherine,
Thank you for your reply. I think your comment came from https://electronics.stackexchange.com/questions/373223/power-supply-remote-sense-protection-resistor-value

100 ohm or 1K ohm in series to a high impedance input will not affect too much. This is constant voltage, not constant current so resistors should not burn up unless polarities crossed.