Current limit not working? - Page 1
Forum section: electronics / beginners

author: OutbackBob
author_rank: Contributor
timestamp: March 08, 2015, 02:10:28 am
content: Hi All,I've been a long time viewer, but finally decided to actually get my hands dirty and build something, which as per Dave's advice, I thought best to start with a bench PSU.I just built this up:http://electronics-diy.com/30v-10a-variable-bench-power-supply.phpMy voltage control is working perfectly, however the current control I'm not so sure about.I have Dave's words ringing in my head "... I hope you build it and it doesn't bloody work!"So I powered it up with voltage and current limits set to minimum, and voltage adjustment works fine, first time, which I was pretty happy about. It goes down to about 2.7V and up to about 14V which sounds about right as I have a 16V supply.Next, I made sure my current limit was at minimum and then connected a 1ohm 5W resistor on the output as my load. I was measuring the voltage, expecting to see it drop as current limit kicked in, but nothing happened except for the resistor getting HOT!I switched out to measure the current going through the resistor and sure enough it was about 2.7A. I checked and double-checked that everything was plugged in right and it looks good, so went to bed in disgust.Woke up this morning with a thought that due to my minimum voltage being 2.7V, the current limit is not going to be able to lower it any more than that, so it probably was "working", just not how I assumed it would. With this in mind, I fired it up and bumped the voltage up to 12V and applied the same 1ohm 5W resistor. This time I saw the voltage drop down to 2.7V, confirming that the circuit is working, you beauty!My questions are:- how do I know what the range of my current limit will be based on the values in the circuit? Is it just the 500R pot that determines this or does R7 and R8 come in to play as well?- I get the feeling that this is going to be really coarse current limiting in the range of 2.7-10amps, which makes it useless for a lab power supply does it not? 2.7A running through a microcontroller project is still going to kill it I would assume?- Do I need a powersupply that will regulate down to 0V for a current limit that will actually protect my circuitry in the event of shorts?Anyone have a good precision powersupply project they can recommend? I liked the idea of a 10A one as I am also in to RC Helis, so need a fairly beefy supply for charging batteries, however maybe I can't expect to get the best of both worlds, ie. high current but with precise control?Cheers,Bob.

author: TerminalJack505
author_rank: Super Contributor
timestamp: March 08, 2015, 04:39:15 am
content: This threadhas some discussion about the exact schematic that you have posted.The LM723, it seems, is a poor choice for a lab supply.The thread has a link to another project that looks much better.

author: mij59
author_rank: Frequent Contributor
timestamp: March 08, 2015, 05:22:49 am
content: Schematic is just wrong, try something like thishttp://chemelec.com/Projects/LM723-PS/LM723-0.png

author: OutbackBob
author_rank: Contributor
timestamp: March 08, 2015, 06:19:35 am
content: Hmmm, looks like its back to the drawing board to find a more modern design.I liked the look of Dave's uSupply, but got scared off after all the talk of chips blowing.I do like the idea of including a micro for control for the extra learning exercise.Open to suggestions if anyone has good schematics.Cheers.

author: minsik
author_rank: Contributor
timestamp: March 08, 2015, 06:52:40 am
content: Hi. Admirable attempt. However from my experience to build an adjustable, current limited and protected supply true 0-30V 20-25A and yet used for many small projects I came up with the default LM317 datasheet cct with some upsizing mods, with current limit cct as the best and simplest to action. In 1983-4 I spent 6 months in the shed building and testing this beast. As it lasted over 30 years it was well worth it and cost a fraction of the available lab supplies at the time.LM317 with output direct to the output terminals, (closest) using 4* parallel mj4502 pnp (easier to saturate series pass transistors. Matching the current share resistors .1ohm 10W, make your own with nichrome wire and pass 1-2A through and measure the voltage drop to match them) then the lm301 or TL071 with a negative supply to make sure the output is fully zero when current limited and adjustable from zero volts not 1.2.I used it in tv and two way service (powering hf and mobile two way usinf all the grunt, and general breadboard ccts requiring 10-100ma protection) for nearly 20 years and several times a day with out too any problems. Had an old bw tv transformer rewound secondary and another smaller transformer to provide the higher supply to the lm301 and -V rails.Used a push button soft start primary with a series degaus PTC thermistor pulling in the primary feed relay to limit mains current in case of major s/c anywhere and it was the output overvoltage protection too. Quality wire wound pots and large voltage and current meters on the front panel (dual scales with range switching).Still have it the shed somewhere and its still working.Did have a rebuilt to replace the mj4502's and lm301 at one abusive smoke out, the rest was on some vero pcb. The rectifier diodes were press stud car alternator diodes mounted on double sided pcb material as the heatsink.Cct supplied is as close as I could currently find.Happy building.Minsik

author: TerminalJack505
author_rank: Super Contributor
timestamp: March 08, 2015, 07:57:27 pm
content: Since you already have most of the parts, what you might do is ditch the LM723 and provide the same functionality that it does with discrete parts. If you look at the "equivalent circuit" diagram in the datasheet then you'll realize that all the part does is package-up a voltage reference, error amplifier, series-pass transistor, and current limit capability (via an NPN transistor.)By using discrete parts you can get down to (or near) 0V on the output and you can get much better current limiting.The reason the LM723 can't get down below ~2V is due to the fact that the error amplifier's output can't swing down to ground. A modern rail-to-rail op amp can help in this regard, although you still might want to have a -1V to -2V rail to truly get to 0V on the output.The way that your posted circuit implements current limiting is not nearly good enough for a lab power supply. All that it is doing is sensing the voltage across the parallel resistors R5 and R6 via the 500R pot (R11.) When the sensed voltage is large enough (~0V65) to turn on the internal NPN transistor then it will pull down on the series-pass transistor's base, decreasing the output voltage.If you do the math (I = 0V65 / 0R235) then you'll notice that the minimum current that you can "program" is ~2.75A. It is actually just a coincidence that the minimum settable current has the same value as the minimum settable voltage.So far as implementing better current limit functionality, you'll probably want to use a high-side current sense IC (something like anINA169INA193) and feed its output into the non-inverting input of an op amp that has the inverting input "program" the maximum current. The output of the op amp would then be fed to an NPN transistor whose collector is connected to the series-pass transistor's base.