Bench PSU idea for maintaining correct voltage levels at a distance - Page 1
Forum section: electronics / testgear

author: akis
author_rank: Frequent Contributor
timestamp: June 03, 2014, 10:30:13 pm
content: I have almost completed building a new bench PSU making use of this idea. The leads are usually 1m-1.5m long, I have half a dozen pairs, and 2-3 bench PSUs. all used interchangeably. It appears that at higher currents there is significant voltage drop on the leads - and what the PSU meter reads is not the real voltage 1.5 meters away.This is not usually a big problem but recently I was working on projects utilising Li-Ion batteries and there even 100mV matters.My idea, which I have almost implemented, is to include a third lead, a thin lead, to parallel the output lead and sense the voltage right at the crocodile clip, even 2 meters away. That will then be the feedback loop on which the bench PSU will regulate on.I have not tried it yet with long leads, only with about 15cm long, and my only fear is will I get oscillations or anything at 2 meters?Has anyone else tried this idea?

author: Dave
author_rank: Super Contributor
timestamp: June 03, 2014, 10:39:55 pm
content: This is commonly known as remote sense. Many higher end power supplies offer this.You need two additional wires, not just one.Here's a nice article about remote sense.

author: alexwhittemore
author_rank: Frequent Contributor
timestamp: June 03, 2014, 10:46:47 pm
content: Yeah, there are a bunch of name brand units with sense inputs. For example, theAgilent E3632A. If you look closely at the picture there, you'll see + and - terminals, but also +S and -S, which are feedback (sense) terminals to do a kelvin measurement like you describe. The thing is, this design (really, capability) is pretty rare in that the situation you describe is pretty rare. It's only if you have really long runs or really thin wires carrying relatively high currents that it'd even become an issue. For example, the E3632A presumably has the sense terminals because it's got mA/mV setting resolution and can push 7 or 4 amps, which is to say it's conceivable that a kelvin application might make sense. On the other hand, my Agilent U8001A doesn't have sense leads because it's got 10mV/mA setting resolution and can only push 3A. Some quick math, my 1m banana wire here is 25mOhm or so, so to get 10mV drop on that would require a half amp. In typical use case of this supply (and most), it's rare that it'd matter.Here's an interesting tidbit though: while it typically won't matter, the supply DOES have an output spec to meet - it needs to be spot-on at the terminals. With my supply set to 10V, it's only 1-2mV off. In the case of the U8001A, the supply actually uses kelvin sensing internally. There are two heavy 1 foot or so leads to the output jack board, and two smaller sense wires feeding back to the regulation circuitry at the back of the unit (where the heat sink lives). In theory, you could break those out to panel jacks and mod kelvin sensing into this supply. Of course, I bet the supply I linked above is designed for worst case where a dumb user has disconnected feedback, whereas the U8001 may have undefined behavior where, say, it rails all the way high even when 1V is set on the output, lighting your DUT on fire because you didn't think to connect sense. Suffice it to say, I won't be doing that mod

author: akis
author_rank: Frequent Contributor
timestamp: June 03, 2014, 10:59:06 pm
content: OK so it seems I completely ignored/neglected the minus lead and only accounted for the voltage drop on the plus lead....Now with one sense lead I do not need low value sense resistors actually in my design they are 10k so that even if you do not connect the plus lead there will be very little current flow.However the accuracy will only go half way in fixing the problem due to the missing sense minus lead. Doh....

author: retrolefty
author_rank: Super Contributor
timestamp: June 03, 2014, 11:05:20 pm
content: That is also a pretty common feature on industrial fixed DC power supplies like Lambda and the such. Shown below one either shorts the +S to +V and -S to -V locally at the terminal strip or connect them remotely at the load to compensate for the voltage drop of the wires supplying the load.http://www.gibuys.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/d/s/dsc05781_1.jpg

author: georges80
author_rank: Frequent Contributor
timestamp: June 03, 2014, 11:59:28 pm
content: Remote sense with good resolution (and accuracy) is a great way to simplify efficiency calculations.I have a Rigol DP1116A which can optionally run with remote sense and I made a pair of kelvin clip leads. Great resolution and accuracy.I also have a Maynuo 300W DC load and can optionally run remote sense on it as well. Also has great resolution and accuracy.Since both can calculate and display watts dissipation, it makes life very simple to sweep a range of input voltages and output loads and log input and output power for my LED drivers (some can operate at 100W+). Enter the data into excel to graph efficiency curves for various operating points. Beats having to wire up 4 accurate meters and log 4 parameters for each point.cheers,george.

author: gxti
author_rank: Frequent Contributor
timestamp: June 04, 2014, 12:43:14 am
content: 3 wires would work pretty well in theory as long as you can assume the lead losses are approximately the same. The tricky part is subtracting the measured loss out a second time. But if you can run 3 wires you can almost certainly fit a fourth so you might as well go all the way.

author: Psi
author_rank: Super Contributor
timestamp: June 04, 2014, 12:54:55 am
content: ah, but now you need sense wires for your sense wires. That few nA of current through the sense wires is still causing a vdrop error for the remote sense system

author: jamesglanville
author_rank: Contributor
timestamp: June 04, 2014, 01:21:32 am
content: Have you considered significantly increasing the size of the cables? It's not perhaps as elegant as kelvin sensing, but it is relatively cheap, and a hell of a lot simpler.

author: PA4TIM
author_rank: Super Contributor
timestamp: June 04, 2014, 06:34:11 am
content: Sense is not complicated. See the read out of the psu as a multimeter. Instead of measuring straight at the output terminals of the psu, you measure at the end of the leads that feed the thing that needs power.

author: macboy
author_rank: Super Contributor
timestamp: June 04, 2014, 01:01:17 pm
content: Quote from: PA4TIM on June 04, 2014, 06:34:11 amIt's not just the display that needs to use remote sense, it is the voltage regulator itself. In order words, you want the regulator to control the voltage directly at the load, rather than at the power supply terminals. Remote sense gives you an accurate voltage set point, not simply an accurate reading on the display (but it gives that too).

author: babysitter
author_rank: Frequent Contributor
timestamp: June 04, 2014, 01:28:33 pm
content: The manuals of about every remote-sensing PSU contains warning that the inductance of the force+sense wiring can turn your regulator setup into a power oscillator - check yours!

author: macboy
author_rank: Super Contributor
timestamp: June 04, 2014, 01:50:54 pm
content: Quote from: babysitter on June 04, 2014, 01:28:33 pmYou should twist the supply wires together, and twist the sense wires together (better yet, use a real twisted pair, like a pair borrowed from a CAT-5e network cable). This nearly perfectly cancels the inductance of the wires to help ensure stability. A tradeoff is increased capacitance between the sense wires, and this could (I think) result in slightly slower response to load transients.

author: robrenz
author_rank: Super Contributor
timestamp: June 04, 2014, 02:11:59 pm
content: Valuable insights here from the HP6114A/HP615A manual

author: PA4TIM
author_rank: Super Contributor
timestamp: June 04, 2014, 02:42:44 pm
content: Quote from: macboy on June 04, 2014, 01:01:17 pmYes, I know. Build and repaired enough PSU's and Vrefs with sense. I do not find the principle complex. In a good psu and every SMPS you have feedback that measures the voltage. Instead of doing that at the output of the source you now do that at the input of the load.Building it can give more troubles because the longer wires add inductance. You do not want the PSU to run out of Phase margin and starts to oscilate.

author: akis
author_rank: Frequent Contributor
timestamp: June 05, 2014, 08:39:20 am
content: The initial version which I have already built works with 1 sense wire only and does half the job, but at least it is better than nothing, especially at 5A. The reason I am keeping it is I spent time making the PCB, drilling, and so on.It works by simply sensing the voltage on the load by means of an extra wire rather than sensing the voltage somewhere inside the PSU case. The input resistance is into the MOhms range so there is no possibility of burning resistors as someone mentioned above. Which I still cannot understand why you'd need to short the load with low value "sense" resistors in the fist place.For sensing + and -, all I need is a differential amplifier and reference to the ground (the ground inside the PSU case) so that the voltage sense feeds off of that value. That is one extra op-amp and 4 resistors, all 100K, I still cannot see why and how I would need low value resistors that could somehow be shorted and burned.

author: macboy
author_rank: Super Contributor
timestamp: June 05, 2014, 12:17:57 pm
content: Have you considered what happens when (not if, but when) the sense wire becomes disconnected from the load accidentally?For this reason, you should have a resistor inside the power supply, between the output terminal and the sense line. If the sense line goes open, then the regulator will sense the output voltage through that resistor. If the sense line is connected, then the much lower resistance of the sense wire will effectively "override" the resistor. So what value resistor? It depends on the input impedance of the sense circuit, I have seen 100 Ohm in a commercial product.

author: babysitter
author_rank: Frequent Contributor
timestamp: June 05, 2014, 12:24:29 pm
content: Akis:Your 3-wire sense scheme is okay, I just wanted to point you at the inductance-sponsored loss of phase margin. Short distances might be okay, bigger ones - check for oscillation. Maybe you add a switch on your PSU and some capacitors to be able to select a output capacitor that prevents oscillation while trading off regulator speed.Also, put a Resistor between your Sense plus and Force plus ; this will prevent that your sense when not connected sees 0V and force goes way up to RAIL trying to get Sense to a reasonable voltage. (This, however, needs to be small enough to let the voltage rise not too high, but also high enough to not burn when you only connect sense+ but not force plus. The only condition I can see something burning besides overvoltage.)You could have a look if you can put the panel meters input to sense, then a sense minus might make a bit moresensein that you get the right number shown even when the regulator doesnt regulate to your remote sense minus.

author: akis
author_rank: Frequent Contributor
timestamp: June 05, 2014, 12:38:27 pm
content: It is already very similar to how you describe it.There is a relay which disconnects the output from the load and there are two momentary switches on the front of the PSU for "channel on" and "channel off". When "off" the voltage output is sampled from inside the PSU so it can be set to an appropriate level and read at the PSU voltmeter. When "channel on" the voltage is *additionally* sensed at the output load. Because it is "additionally" it does not matter (much) if it is disconnected. So there are three cases: sense wire disconnected, sense wire connected to PSU output terminal (like a local loopback) and sense wire connected on the load, far from the PSU. None of these cases is catastrophic.The op-amp which performs the voltage regulating senses the voltage inside of the PSU through a 100K resistor, and voltage on the sense wire through a 10K resistor. The 10K resistor overrides the 100K resistor, but only if the sense wire is connected, otherwise the 100K internal resistor takes over. We do not want the sense resistor to be too low, because we do not want a low resistance path through the sense wire.So, again, I completely fail to understand the purpose of anything so low as 100R - unless it is to eliminate noise picked up at the leads. I think a 2K-10K resistor is a good compromise between noise and protection against currents flowing through the wrong wires and in potentially the wrong direction (imagine if your load happens to be a battery with a higher voltage than the output of the PSU). You cannot allow any path for current to flow and destroy internal components.My PSU's output has a polarity protection diode but the sense wire circuit is only protected by means of the 10K resistor.However I JUST thought about a bad condition: sense wire connected to earth! In that case the output of the PSU will go full voltage trying to raise the output which *appears* always to be low. That is catastrophic.So there should be *another* mechanism to not allow a voltage drop on the wires more than say 1V - so that even if the sense wire senses more than 1V, we limit to 1V.Hmm, I had not thought about that.

author: madshaman
author_rank: Frequent Contributor
timestamp: June 05, 2014, 12:46:44 pm
content: I don't know if anyone's mentioned, but you can always use your meter for sensing.Yeah, I know, it won't be in the feedback loop, but if your device has power requirements such that this setup wouldn't work (e.g. it draws current erratically, which causes the power leads to heat and cool), then it really needs its own onboard supply.Also, as mentioned, many bench supplies can be easily modded to bring out sense leads.

author: akis
author_rank: Frequent Contributor
timestamp: June 05, 2014, 12:53:23 pm
content: Quote from: babysitter on June 05, 2014, 12:24:29 pmSeems our posts crossed. I realised the condition you describe as I was typing my previous message. The way it is arranged if the sense wire connects to 0, then there will be an effective potentiometer sensing 1/11 of the output voltage: 100K to the internal output and 10K to the load. So suppose we ask for 30V, it will sense 2.9V and will raise the output to the max. The only solution that comes to mind is to apply a limit for the voltage drop on the sense wire.However an easier solution, and more practical too, is to connect the sense wire and the output wire together at the crocodile clip, so it is impossible to play silly buggers with the sense wire. There is no reason having a sense wire loose on the desktop detached from the main output wire.

author: robrenz
author_rank: Super Contributor
timestamp: June 05, 2014, 02:16:12 pm
content: From theHP power supply handbook