Troubles with current limiting - Page 1
Forum section: electronics / beginners

author: tiltit
author_rank: Regular Contributor
timestamp: November 25, 2016, 04:02:02 pm
content: Hi all, I've been building a dual tracking power supply so that I can experiment more easily with analog circuits.The power supply will be constituted of two separate boards, one for voltage regulation an one for current limiting. That way I can replace/improve one of them easily.I found schematic online that deals with the voltage regulation and it works really well.Now I am experimenting with current limiting circuits and I have come up with something that will source current irrespective of the input voltage(within limits).My problem is that if the load draws less than set current (for instance by adding another resistor in series with D1) the op-amp will still make sure there is 2V across R7 and that limits even more the current to the load.So my question is. How can I mitigate that?

author: Zero999
author_rank: Super Contributor
timestamp: November 26, 2016, 12:09:26 am
content: I'm not sure if I understand the question. You have a constant current source there, which should work quite well.

author: tiltit
author_rank: Regular Contributor
timestamp: November 26, 2016, 02:02:33 am
content: Yeah sorry, I know I have trouble explaining things.The current limiting works, what doesn't is that I would like it only kick in when the load wants to draw more than than the set current. Otherwise Q1 should just remain open and try not to affect the load.

author: Audioguru
author_rank: Super Contributor
timestamp: November 26, 2016, 02:55:06 am
content: The minimum supply for a TL072 is 7V because its inputs do not work correctly if they are within 3V from its negative supply (ground in your circuit and its output cannot swing closer than 3V from the positive or negative supply. If an input becomes within 3V from ground in your circuit then the output will suddenly go as high as it can.Use another opamp that does not have these problems.

author: tiltit
author_rank: Regular Contributor
timestamp: November 26, 2016, 03:07:19 am
content: The negative supply in my circuit is set to -5V I believe. I'll try -7V or something like that. The thing is I am just testing this circuit on a breadboard. And when it will work properly I will scale it up for use in my power supply. This circuit is meant to limit the positive voltage, I will have to then do another with a npn transistor for negative supply.I've often read online that the TL072 is old and antiquated, so if anyone knows of a good, easy to use and cheap op-amp please let me know.

author: not1xor1
author_rank: Frequent Contributor
timestamp: November 26, 2016, 06:10:04 am
content: The circuit works like a unity gain differential amplifier referenced to ground.The output of U1B, through Q1 emitter, has a value of Vdd - Vref (2V in your circuit) so across R7 you always get 20mA, regardless of the load and of the value of Vdd (until the load is lower than (Vdd - Vref - Vcesat)/R7 ).So the circuit works as it should, but to make it work properly in any condition (within the SOA of the components) you should ensure that R3,R6,R5,R4 have the same value (i.e. selected 1% resistors or .1% ones, assuming that Vin has a negligible output impedance) and you should not connect the positive supply of the opamp to the input voltage (Vdd in the schematic), but make it fixed.Anyway, the main problem with this circuit is that it is just a constant current source... you could use a lower value for R7 and a lower value for the reference to the input of U1B... and a mosfet as the pass element, but your output voltage will always be affected.You could put the current limit circuit before the voltage regulator, provided that the voltage regulator do not sink much current and doesn't behave oddly when the input voltage is less than set output voltage (+ the dropout of the pass element)

author: tiltit
author_rank: Regular Contributor
timestamp: November 26, 2016, 03:47:32 pm
content: Thanks for the input, I will try scale the circuit up within its working context and will post it latter on. But I will use BJT's because up until now, I’ve only had bad experiences with mosfets and op-amps oscillating.I am planing to use a BD 203 and a BD 20 for the current limiting up to about 1.2A.I'd like to ask why should the current limiting circuit be before to voltage regulation one? I was planing to have it after.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 26, 2016, 04:46:52 pm
content: Usually the separation is looking at an output stage (e.g. transistor based) and two control parts (usually OP based). Often the output of the control parts is combined with something like diode to give the minimum of both control values. The choice of the output stage often determines where to put the current measuring shunt resistors.For the output stage there are two main options:1) A stage that primarily controls the output current and thus has a high output impedance. This make current control easier and usually faster, but voltage control a little harder.2) A stage with low output impedance, like an emitter follower. This can make voltage control a little easier, but tends to make current control slow.Both ways are possible - the low impedance stage tends to be a little easier at low voltages (e.g. up to about 20-25 V) but more difficult for higher voltages, where the voltage swing of common OPs is not sufficient any more.Planing a dual supply, it could also be important if one can start with two completely isolated ones, or if one wants to use a common transformer winding / rectifier.

author: tiltit
author_rank: Regular Contributor
timestamp: November 26, 2016, 07:34:28 pm
content: Ok, so I now have a schematic for the current limiting. In the attachments I added the schematic of the voltage regulator board that was more or less copied from here (http://www.joebrown.org.uk/wp/?p=7294). It work fine and I’ve already put it together.The outputs of that board should then go into the current limiting board.Will it work or is it complete bullcrap.

author: Audioguru
author_rank: Super Contributor
timestamp: November 27, 2016, 02:54:30 am
content: An M317 and LM337 need to have a minimum load of about 10mA or the output voltage will rise as described in the datasheet. Your circuit has 1k as its load which is a current of only 1.25mA when the voltage is low and the load draws a very low current. Usually the resistor between the output and the ADJ pin is 240 ohms for an LM117 or is 120 ohms for an LM317 so that the current is always at least 1.25V/120 ohms= 10.4mA but yours is 900 ohms. With 120 ohms then the trimmer must also have its value reduced.

author: tiltit
author_rank: Regular Contributor
timestamp: November 27, 2016, 03:09:08 am
content: Thanks for that information, I had no idea that that resistor sets the output current.That means that I will need a 2.5k trimmer. Damn, I was hoping to use those that are in my junkbox.

author: not1xor1
author_rank: Frequent Contributor
timestamp: November 27, 2016, 05:21:19 am
content: Quote from: Audioguru on November 27, 2016, 02:54:30 amusually LM317/377 work fine with just 5mA of load, 10mA is the worst caseif tiltit has an oscilloscope he can easily check if his ICs work with a 220? resistance, as shown in the datasheet examples

author: not1xor1
author_rank: Frequent Contributor
timestamp: November 27, 2016, 05:42:52 am
content: Quote from: tiltit on November 26, 2016, 07:34:28 pmyou could implement current control by adding a few components before the LM317/337but that would cause a different kind of problem: when the positive output is affected by the current limit the negative output is affected as well, while the opposite will not be true as the LM317 doesn't track the LM337 outputBTW do you need a fixed or a variable current limit?BTW do you need variable or fixed current limit

author: Kleinstein
author_rank: Super Contributor
timestamp: November 27, 2016, 08:44:30 am
content: It is not a good idea to have separate power stages for voltage regulation and current limiting. Using a voltage regulator and separate current limiter behind it can not work well, as there will be some voltage drop in the current limiting circuit. The suggested current limiting circuit is not working - it is more like a try to get voltage an current regulation - but more like mixing bits an pieces that don't work together.For a start, just use one side - adding the second polarity is relatively easy (just swap NPN-PNP and similar). Many commercial supplies even have two identical, insulated parts, with a switch / relays to do the optional connections to get a +- supply.

author: tiltit
author_rank: Regular Contributor
timestamp: November 27, 2016, 05:19:56 pm
content: Thanks for all the feedback. So it ultimately is bullcrap.QuoteI wanted to have a variable current limit, but most importantly I do not want the lm317/337 to blow up when the outputs are shorted. (bad stuff happens).QuoteI think that is what I will try to do. I already have a Korad 3005 power supply and my plan was to build dual tracking one to play around with audio circuits and the like. That is why the tracking part is important to me.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 27, 2016, 05:58:48 pm
content: The LM317 and similar have an internal current limiting (at about 1.5 A, with fold-back at higher voltage drop) and thermal protection. So just from a short they will normally not blow up.If it just for a crude (not fast) current limiting one might get away having an additional current limiting before LM317/LM337.However a complete lab supply circuit (based on OPs and BJTs) is not that much more complicated.

author: tiltit
author_rank: Regular Contributor
timestamp: November 27, 2016, 07:52:56 pm
content: I've been looking into single supply op-amps for a single rail power supply that would go down to 0 Volts. I could then make two of them and somehow connect them together.Options I've found are the LT1078, AD820 and OP90. Would any of these be suitable for such a project?LT1078 would be best because it's the cheaper option.QuoteThat's good news for me, so I can use what I already have in the meantime. Thanks.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 29, 2016, 02:21:53 pm
content: Having OPs with single supply is one of the least problems: for the +-15 V supply there is a negative supply available. The LT1078 is a slow low power version - it might not work in all circuits. For a fist start one could use the LM358 - hard to find a cheaper single supply OP.