Saturday, April 16, 2011

The V terminals are reading +5 volts. What are the S terminals for?

Q Clayton, KE4RTM, asks, “I have a simple question regarding a 5-V Lambda power supply. On the output side there are –S and –V, ground, and +S and +V terminals. The V terminals are reading +5 volts. What are the S terminals for?”

A On a power supply with +/–V and +/–S terminals, the S terminals are very likely “sense” inputs. In circumstances where the current draw might cause a significant voltage drop in the cable you are using to connect the power supply to the load, you would connect the sense terminals to the load via separate wires. The sense terminals would read the voltage at the load and relay that information back to the regulator circuit. If the voltage at the load drops, the sense circuit detects this and adjusts the output of the supply to a higher voltage so that the voltage on the load comes back up to the proper supply voltage.

Because there is very little current draw in the sense circuit itself, the wires connecting the S terminals to the load can be small gauge, even if the wires that supply power from the V terminals are quite large.

For many applications, the voltage drop is not so critical, so the manufacturer often includes shorting bars that connect the V and S terminals together at the supply.

From QST March 2001