# What is the use of SENSE + and - in a PSU?

I was in search of a good power supply for my radio. Apart from the batteries I have tried all the AC wall adapter or the SMPS based supplies seem to introduce noise in the radio. The SMPS being the worst.

I have an old PSU as in below pic unused in my collection. As per the data sheet its a high quality voltage source which made me think of using it for this purpose.

It has pins A,1,2,3,4,5,6,7 which corresponds to Adjust, - Sense, B-, +Sense, B+, Not Used, AC, AC. AC is rated for 115V. Its max current is 150mA, which is well within my current draw.

What is the use of the _ and + SENSE pins? I've seen this on an old PSU unit I had with me back from the 1960s. Will it improve my output in any manner?

Datasheet here.

Update: So, I unpacked the PSU today and did some adjustment and checking now.I connected the (-SENSE & -B) and (+SENSE & +B) together and adjusted the voltage to 8.8V. Now, I tried removing the SENSE lines from both +B and -B terminals and measured the voltages across the each terminals: -SENSE & -B: 13.6V, +SENSE & +B: 8.1V, +B & -B: 22.4V, -SENSE & +SENSE: 0V . So, as our members have explained the sense lines seem to be very important in this case.

• I suspect that A is not a pin but is instead an adjustment screw. – Andrew Morton Nov 9 '17 at 20:55
• @AndrewMorton Yes the A is actually a screw to adjust the voltage. – The_Vintage_Collector Nov 9 '17 at 20:57
• Usage of the connections is well described in the answers, just wanted to add that this is known as a "Kelvin connection." – B Pete Nov 9 '17 at 21:14
• @BPete, interestingly, this product must be for some very fine engineering if they offer the 4-wire hookup for a 150 mA supply. – Ale..chenski Nov 9 '17 at 21:30

Cables have some resistance, and when you pass current through them, they have a voltage drop. This causes the load to "Receive" less voltage than what is actually set on the power supply.

Sense wires measure the voltage at the load (sense wires pass very little current so they don't suffer that much voltage drop), and the PSU corrects the voltage based on the feedback they get.

Hopefully, this schematic will explain it better:

simulate this circuit – Schematic created using CircuitLab

Sense pins are for remote voltage feedback. Without connecting Sense wires to corresponding B wires the PSU won't work properly, if at all.

If you are using a very long cable between the PSU and your load, the main power wires (B+ and B- in this case) can suffer a substantial voltage drop. When you connect the Sense- to B- and Sense+ to B+ at the POINT OF LOAD, it closes the feedback loop, and difference amplifier inside the regulated PSU will compensate the drop of voltage along main feed wires. As result, the PSU will maintain its rated voltage level right at the destination point.

NOTE: the sense wires don't carry much current, and can be made of a fairy fairly thin wires.

NOTE2: If the connection to load is not that long and/or voltage drop of little concern, the Sense-/+ can be connected to B-/+ right at the output strip connector of the PSU. Many PSU came with this kind of jumpers out of the box. And the local jumpers can be removed if necessary to make the 4-wire connection to load. Alternatively, to be safe out of the box, the Sense pins might have a weak internal connections to corresponding B pins. In case of using the external 4-wire hook-up, the feedback wires simply override the internal connections, and the point-of-load regulation will take place.

• So making a parallel connection between +SENSE & B+ and -SENSE & B- should improve the regulation. Right? – The_Vintage_Collector Nov 9 '17 at 20:56
• @RahulSalin Yes, Sense wires are connected in parallel to B wires. Proper polarity is extremely important. – Ale..chenski Nov 9 '17 at 20:57
• fairy thin wires - ah, a connection with magic smoke. – Andrew Morton Nov 9 '17 at 21:00
• @RahulSalin: Making a parallel connection near the load will improve regulation. The reason some supplies bring out sense wires rather than simply including an internal parallel connection is that a connection within the supply would be separated from the load by a length of cable. – supercat Nov 9 '17 at 21:01

The sense lines tell the power supply what the voltage is at the point of load, so if you have a 20ft cable that connects the power supply to the load for some reason, you can run separate sense lines and hook them up right at the load. This will remove the voltage drop caused by the wiring resistance, and the supply will margin up the voltage at the output until the voltage at the load is at the commanded point. The key here is to run separate lines, as no current will flow through the sense lines. If you're not going to this trouble, then you can just hook the sense lines up directly to the output of the supply.

Another use is if you want to put a multimeter in series to measure the load current - most have fairly high resistance on the current range and the voltage drop can stop the device operating. This is particularly common on devices that operate on a low voltage.

The danger is that if the sense wires become disconnected the PSU will go to it's maximum voltage probably destroying the load. Some PSU with sensing I have used in the past have an internal diode between the sense terminal and the output terminal. That way the voltage only increases by 0.6V if the sense wire becomes disconnected. Of course this also limits the voltage drop on each output lead to 0.6V.

• About how many ohms do you think a multimeter's shunt will be? – Bort May 31 '18 at 13:47
• @JohnM a conventional meter is usually less than 1ohm, more on the scales of 0.1Ohms, is that something that you'd want to put in series with a power supply? No, you'll burn out the meter or short the supply (whichever comes first). You need to put a resistor in series with the supply and meter to measure max currents. – laptop2d May 31 '18 at 15:42