[SOLVED] LM723 based PSU, 6Amp load, 100mV drop per 1Amp load - Page 1
Forum section: electronics / projects

author: fivefish
author_rank: Frequent Contributor
timestamp: August 01, 2015, 06:26:13 am
content: Hi all, I needed a PSU capable of more than 1.5A the LM3xx regulators can give.So I tried an LM723 chip + an NPN pass transistor power supply circuit (based on datasheet). Was able to adjust it for 18V output and load it down to draw 6Amp. (Possibly more, but my bench PSU tops out at 6A)My "problem" is the load regulation. Output volts drop down by 100mV per 1A of current draw at the load, starting at 18Volts output at no load.So by the time I'm drawing 6Amps, my load voltage is down to approx. 17.4 volts.This is my 1st time using an LM723. Is there a way to improve this performance? How can I reduce that 100mv drop per Amp of draw?Or do you have other suggestions (other chips, regulator, circuit) that have better load regulation at a 6Amp draw? Thanks.

author: Joule Thief
author_rank: Regular Contributor
timestamp: August 01, 2015, 06:52:59 am
content: Are you measuring the 17.4v at the power supply output posts or directly at the load?

author: Kalvin
author_rank: Super Contributor
timestamp: August 01, 2015, 07:03:25 am
content: ^^ So, if you measure at the load, the wires may cause easily the voltage drop, as only 0.1 ohms for wiring causes 100mV@1A.

author: fivefish
author_rank: Frequent Contributor
timestamp: August 01, 2015, 07:15:54 am
content: QuoteHmmmm... yes, doesn't take much resistance to introduce a voltage drop at those high currents.But I'm measuring from output of PSU (breadboard).The DMM leads (from breadboard to DMM) is about 3 feet. These are the shortest DMM leads I have.Then from PSU (breadboard) to electronic load, another 2.75 feet of wire.And I measured the DMM leads I'm using and it's 0.04ohms round trip (black and red wires)

author: rs20
author_rank: Super Contributor
timestamp: August 01, 2015, 07:36:17 am
content: The leads to the DMM aren't carrying an amp, so they won't have a significant voltage drop. The length of the DMM leads is not a concern for that reason.If you had long leads to your loads, and you measured at the load, thenthatwould be the problem. But you're measuring it at the PSU breadboard, which is correct. Unless your breadboard has long traces that have a significant resistance themselves.

author: Joule Thief
author_rank: Regular Contributor
timestamp: August 01, 2015, 07:37:22 am
content: Try using your voltmeter to measure the voltage dropped across the 2.75 feet of wire to the load.Are you allowing 6 amps of current to flow thru breadboard contacts? This practice would suggest you are using relatively small wire to carry the 6 amp load. Both these issues can contribute to your voltage loss.

author: fivefish
author_rank: Frequent Contributor
timestamp: August 01, 2015, 07:42:48 am
content: I'm using a 2SD1047 for my NPN pass transistor (that's what I currently have in my spare parts bin). The Collector-Emitter Saturation voltage of this is 2.5V (Ic=5A, Ib=0.5A)Would I be better off using say a TIP3055, whose CE Saturation voltage is a low 1V (@ Ic=4A, Ib=0.4A), and 3V ( @ Ic=10A, Ib=3.3A)?I think I have a 2N3055 somewhere here, I'll try that.

author: Kalvin
author_rank: Super Contributor
timestamp: August 01, 2015, 07:43:36 am
content: What kind of breadboard you are using? Can you post a photo?The cheap breadboards are not typically suitable for high current circuits. If you do some measurements, you might find the missing 100mVs somewhere on the breadboard.

author: rs20
author_rank: Super Contributor
timestamp: August 01, 2015, 07:48:10 am
content: What's your input voltage? Is your input voltage sagging under the load? Just check that the output voltage isn't actually tied to the input voltage. You may also consider trying a darlington pair. The load regulation specs of the LM723 are only given at 50 milliamps (where the load regulation is 0.15% of vout for the record), and so to meet that spec, you need a transistor with a current gain of over 100 to get that particular spec at 6A -- and power transistors do not give you a current gain of 100.Also, the LM723 supports remote voltage sensing, i.e. you can connect R1 right close to the point where the voltage leaves the breadboard (and where you're plugging in the DMM) so that even if there is significant drop across the breadbroad traces, the LM723 will compensate.Quote from: Joule Thief on August 01, 2015, 07:37:22 amWhy? fivefish is already measuring at the breadboard, why do we care how much drop is happening past that point?

author: fivefish
author_rank: Frequent Contributor
timestamp: August 01, 2015, 07:48:39 am
content: Quote from: Joule Thief on August 01, 2015, 07:37:22 amYes, possibly.... the hookup wires I'm using to connect from breadboard to pass NPN power transistor I measure to be 0.2 ohms a piece. (one wire going to Collector, and another wire going to Emitter)... and this CE junction will be passing 6Amps (which means, also going through those 0.2R breadboard wires).

author: fivefish
author_rank: Frequent Contributor
timestamp: August 01, 2015, 07:52:43 am
content: rs, yes I tried also a darlington configuration.... it was same result 100mV drop/Amp vs. the single pass transistor.Things I haven't tried yet....- using a different pass transistor- using multiple pass transistorshfe I'm using is 60-200 (2SD1047). I don't have problems reaching 6Amp, it's just that there's the 100mv drop per Amp.

author: rs20
author_rank: Super Contributor
timestamp: August 01, 2015, 08:00:25 am
content: Quote from: fivefish on August 01, 2015, 07:48:39 amBut while it's not ideal that you have these losses in your wires,all these losses can be detected and compensated for by connecting your R1 close to the output terminals-- provided you have sufficient input voltage that you don't experience drop-out.Quote from: fivefish on August 01, 2015, 07:52:43 amIf you've tried something as radically different as a darlington configuration and ended up with the same results, then clearly your transistor and the LM723 are doing just fine. The symptoms you are describe sound exactly like you have 0.1 ohms of resistance between where R1 connects, and where you're measuring with your DMM.Don't forget that voltage drop can happen on your ground return wires too!I suggest you connect some fly leads directly to the R1 lead connected to the output, and the R2 lead connected to ground (whichshouldbe extremely closely connected to the V- of the LM723). If you connect your DMM to those fly leads instead of the breadboard outputs, I think you'll see that 100mV drop/amp reduce greatly.

author: fivefish
author_rank: Frequent Contributor
timestamp: August 01, 2015, 03:06:07 pm
content: Guys thanks for the clues.... found where the voltage drops are happening.It's from the (2) breadboard wires going to the Collector, and to Emitter of the power transistor.Did some slight changes to test theory, and managed to lower drop from 600mV to 470mV @ 6Amps, or approx 77-79mv per Amp vs 100mV/Amp before.I can improve this further on this prototype but now I know on the final PCB design,these traces and wires going to the Collector and Emitter of power transistor should be short and thick to reduce these voltage drops.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 01, 2015, 07:03:05 pm
content: With correct wiring the voltage drop should be very much lower (e.g. less than 1 mV/A). It's not so much about using thicker and shorter wires, but about having the right connections. Typicaly there should be at least to wires going to the output post: one carring the load current and one to sense the voltage, with very little and mainly constant current.Short wires to the power transistors help to keep the circuit stable - that is not oscillating.

author: fivefish
author_rank: Frequent Contributor
timestamp: August 01, 2015, 10:31:48 pm
content: Thanks kleinstein... manage to get load regulation down to 323mV drop for a 6Amp load (less than 54mV drop per Amp) at 18V output,Just by moving a few wires on the breadboard. On a proper PCB, these figures should go down further I would think.

author: Andy Watson
author_rank: Super Contributor
timestamp: August 01, 2015, 10:41:44 pm
content: Quote from: fivefish on August 01, 2015, 10:31:48 pmNot necessarily. As Klienstein pointed out, it is not so much the resistance of the wiring or the PCB traces that will significantly improve the regulation, but the point at which yousensethe output voltage, i.e. you should have sense wires as close as possible to the point of delivery to the load.