Current limiting circuit  - Page 1
Forum section: electronics / projects

author: joniengr081
author_rank: Regular Contributor
timestamp: September 16, 2025, 09:30:55 am
content: I am struggling to find a current limiting circuit that can limit the current to maximum 5 mA. The voltage level is 5 V. Kindly share the circuit and if you have a possibility then please explain a bit how the circuit works. Thanks.

author: inse
author_rank: Super Contributor
timestamp: September 16, 2025, 09:44:35 am
content: Please specify allowable voltage drop and current limit precision to enable proposals.The range might be between primitive four-parts circuit and complex ones including reference and OPAMP

author: Ian.M
author_rank: Super Contributor
timestamp: September 16, 2025, 09:46:30 am
content: Most primitive would be a 1K resistor!

author: inse
author_rank: Super Contributor
timestamp: September 16, 2025, 10:00:06 am
content: Correct 😆

author: joniengr081
author_rank: Regular Contributor
timestamp: September 16, 2025, 10:14:20 am
content: I would assume 5 % voltage drop is acceptable. The current precision could be +/- 1 mA is possible.

author: temperance
author_rank: Super Contributor
timestamp: September 16, 2025, 10:17:59 am
content: Does the current limit reside in the supply line or the return line?

author: joniengr081
author_rank: Regular Contributor
timestamp: September 16, 2025, 10:23:02 am
content: In the supply line to the load.

author: Ian.M
author_rank: Super Contributor
timestamp: September 16, 2025, 10:31:51 am
content: Lets confirm: You want a high side 5 mA +/- 1 mA current limit for 5 V DC, supply positive, dropping no more than 0.25 V (5%) before the limit?Is the 5 V to the load switched or continuous?To ease implementation, are there any other supply rails more positive than the 5 V rail, and if so, can the 5 V rail sink current?

author: joniengr081
author_rank: Regular Contributor
timestamp: September 16, 2025, 10:39:00 am
content: I would say that +5 V to the load is continuous. There is only +5 V and Gnd available on the board.

author: Ian.M
author_rank: Super Contributor
timestamp: September 16, 2025, 10:57:28 am
content: Quote from: joniengr081 on September 16, 2025, 10:39:00 amHmmm. That + the max. 5% drop requirement pretty much pushes you to theclassic OPAMP + MOSFET constant current sink, 'flipped' to use a P-MOSFET and be a current source. You need to keep the sense voltage a little off the rail so for 5 mA, a 22 ohm sense resistor would be appropriate, giving 0.11 V drop across the sense resistor. Here are some suitable MOSFETs.https://mou.sr/3JXSxRVYou'll also need a single supply RRIO OPAMP, and a potential divider to bias its +in 0.11 V below the 5 V rail.This type of OPAMP + MOSFET current sink/source is very accurate, but the down-side is its relatively slow acting - if the load is removed, the OPAMP rails trying to drive the MOSFET hard on to get enough current through the sense resistor, so if the load is suddenly reconnected it lets through a surge which can be many times the set current, till the OPAMP can get off the rail and slew its output far enough to charge the MOSFET gate capacitance and get it back onto its Miller plateau linear region. In extreme cases, if a low impedance load is connected with the circuit already powered, the surge may blow the MOSFET.

author: inse
author_rank: Super Contributor
timestamp: September 16, 2025, 02:44:27 pm
content: I was thinking if it was possible to bypass the external MOSFET and use a RRIO OPAMPs output directly.The current is only 5mA, so there is possibly a type that can drive this..

author: Ian.M
author_rank: Super Contributor
timestamp: September 16, 2025, 03:12:07 pm
content: Quote from: inse on September 16, 2025, 02:44:27 pmThe problem with that is how do you sense the load current to close the feedback loop without an extra OPAMP as a differential amplifier? I suppose you could use a low-side shunt, or with some loss of accuracy and an OPAMP with a common mode input range exceeding its positive supply, you could put the shunt in the OPAMP's positive supply.*IF* there was a higher voltage rail, a Howland current pump could work . . .

author: joniengr081
author_rank: Regular Contributor
timestamp: September 16, 2025, 03:25:59 pm
content: Thanks for your comments. I am sorry I am not getting it completely.I am familiar with constant current source circuits, there are several integrated chips available for constant current source but my this application is maximum current limiter where the current can vary from 0 to 5 mA but not more than that given that the voltage is fixed.Kindly describe bit more or share the schematics for my understanding. Thanks in advance.

author: inse
author_rank: Super Contributor
timestamp: September 16, 2025, 04:34:00 pm
content: @ IanM: Dual OPAMPs are quite common, so probably that wouldn’t be an issue..@ Joniengr: there is no difference between a current limiter and a constant current circuit in your application.Both are limited by the signal source and the load.If you set your lab power supply to 5V and the current limit to 5mA and place a potmeter on the output, it will act like a current limiter.If you now set the output voltage to max, say 50V, it will behave more like a current source.But in principle they are the same.

author: Ian.M
author_rank: Super Contributor
timestamp: September 16, 2025, 04:47:02 pm
content: Quote from: inse on September 16, 2025, 04:34:00 pmUntil it is - usually due to excessive phase shift round the feedback loop causing instability. Also a differential amplifier will need an additional two pairs of well matched resistors, which many well be comparable in cost to a jellybean small signal P-MOSFET.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: September 16, 2025, 05:35:41 pm
content: If you could afford a 10% (0.5V) voltage drop before current limiting, than a selected JFET + a resistor is all what is needed. For a smaller drop a PNP pair of transistors (i.e.BC856BDW1T1G) might be sufficient (see the attachment) with ~0.2V voltage drop at 5mA.Note - you would need to experiment with the resistor values to get it right in real life and the limiting current value will vary with temperature and will change roughly proportionally to the supply voltage.CheersAlexP.S. - the temperature variation sim is added, doesn't look too bad. From 10C to 50C the current limit changes by about 0.6mA .

author: mikerj
author_rank: Super Contributor
timestamp: September 16, 2025, 08:28:22 pm
content: Quote from: Alex Nikitin on September 16, 2025, 05:35:41 pmA nice part for this is the BCM62B which is intended for current mirrors with suitably close matching. Only a few pennies more than the general purpose BC856BDW1T1G.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 17, 2025, 12:50:23 am
content: Quote from: Alex Nikitin on September 16, 2025, 05:35:41 pmTurns out you can improve the tempco, by adjust of the Ic current ratios.Testing with Vo = 500mV and ~ 5mA target limiting gave a null with an IC ratio of ~2.604That 'ideal' parabola null is only possible at one Vo, but it is still quite good over the limiting range. At 4V output, the current change is ~ 7.9uA or 0.15% for 0-50'CThere are quite a fewmatched pair PNP parts, some as good as 2mV VBE and 2% HFE

author: rfclown
author_rank: Frequent Contributor
timestamp: September 17, 2025, 03:55:37 am
content: Quote from: Alex Nikitin on September 16, 2025, 05:35:41 pmNice answer AlexFor the OP it is a good circuit. For me, I learned a few things about LTspice that I didn't know.First, I didn't know about the View Step Legend. Second, when I first tried your circuit the current limiting was fine... but the output voltage didn't hit bottom at 0.5 volts like shown in your display. It keep going down! I understand why a dc sim with a current source might do this, but yours shows more expected real life behavior. Only then did I poke around enough to see the "This is an active load" option on the current source. Third, I had never Googled to see how to change the plot background color. I've been using LTspice for many years and didn't know these things.

author: rfclown
author_rank: Frequent Contributor
timestamp: September 17, 2025, 04:09:09 am
content: Quote from: PCB.Wiz on September 17, 2025, 12:50:23 amAnother great reply.Looks terrific in simulation.

author: temperance
author_rank: Super Contributor
timestamp: September 17, 2025, 01:35:49 pm
content: Quote from: Ian.M on September 16, 2025, 03:12:07 pm

author: inse
author_rank: Super Contributor
timestamp: September 18, 2025, 07:28:01 pm
content: Just for curiosity I built PCB.Wiz‘s circuit but I used low VCEsat transistors PBSS5240 instead of BC858.And the circuit works perfectly!With a supply of 5,00V I get the following output voltages:0mA: 5,00V1mA: 4,99V2mA: 4,96V3mA: 4,94V4mA: 4,91V5mA: 3,80V5,6mA: 0V

author: Zero999
author_rank: Super Contributor
timestamp: September 18, 2025, 08:52:23 pm
content: Quote from: temperance on September 17, 2025, 01:35:49 pmCute, but it needs an op-amp which can handle input voltages up to the +V rail and draws negligible supply current, compared to the load current. I simulated it with an op-amp model which has a supply current of 1mA and it only delivered 4mA to the load.op-amp source.asc(1 kB - downloaded 5 times.)EDIT:Stick a MOSFET or BJT (if you're not bothered about accuracy so much) in the feedback loop of the op-amp.op-amp source MOSFET.asc(1.18 kB - downloaded 6 times.)

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 18, 2025, 09:59:15 pm
content: Quote from: Zero999 on September 18, 2025, 08:52:23 pmTrue, but there is a wide choice of Opamps today.A 3-PEAK LMV321B at appx 5c, has high drive and RRIO with 12.5 ohms drive, and it has 90uA of IQ, that is quite temperature stable.That gives smallest PCB solution.Of course, for higher currents or extreme precisions a PFET is a classic solution, and allows wider choice of cheapest RRIO opamp.

author: Benta
author_rank: Super Contributor
timestamp: September 18, 2025, 10:45:35 pm
content: Waaay too complicated.Vishay/Siliconix J511 will do the job, but it's obsolete. No problem, available from Linear Systems as J511 or SST511. Also from Interfet as SMPJ511.Works to below 1 V, then they become resistive. Brilliant little parts.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 18, 2025, 10:54:48 pm
content: Quote from: Benta on September 18, 2025, 10:45:35 pmHmm..$1.30 each, and very soft with 90% I at 3V (because it is a 50V part), and +/- 20% spread.Yes, it is simple but very costly and poor performance.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: September 18, 2025, 10:56:48 pm
content: Quote from: Benta on September 18, 2025, 10:45:35 pmWith the on resistance of 400 Ohm and voltage drop 1V at 2,5mA ?! The closest I could find (as I've mentioned earlier) to a reasonable solution with a single JFET would be the 2SK170B or a similar device + 22 Ohm resistor, it would work but with the voltage drop at 4mA about 0.4V.CheersAlex

author: Benta
author_rank: Super Contributor
timestamp: September 18, 2025, 11:32:55 pm
content: The OP asked for a current limiting circuit. No linearity or other stuff was required. Why all the over-engineering?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: September 18, 2025, 11:36:07 pm
content: Quote from: Benta on September 18, 2025, 11:32:55 pmOccasionally it is useful to read the whole thread before postingCheersAlex

author: Benta
author_rank: Super Contributor
timestamp: September 18, 2025, 11:56:46 pm
content: Quote from: Alex Nikitin on September 18, 2025, 11:36:07 pmQuite frankly, I mainly read the OP's responses, not your and others' suggestions.So my response/suggestion still stands.Youryou can put somewhere else.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: September 19, 2025, 12:02:47 am
content: Quote from: Benta on September 18, 2025, 11:56:46 pmQuite frankly,you didn't.CheersAlex

author: Benta
author_rank: Super Contributor
timestamp: September 19, 2025, 12:19:35 am
content: Try again. OP username is joniengr081.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: September 19, 2025, 12:21:35 am
content: Quote from: Benta on September 19, 2025, 12:19:35 amExactly.Quote from: joniengr081 on September 16, 2025, 10:14:20 amCheersAlex

author: temperance
author_rank: Super Contributor
timestamp: September 20, 2025, 02:37:55 pm
content: QuoteIt doesn't hurt to think of as many ways you can find to do the same thing. Some creativity can sometimes lead to interesting solutions. I find such things interesting. Maybe the OP will ignore all the noise or maybe the OP will get a better understanding about how to "think" about such things.

author: mikerj
author_rank: Super Contributor
timestamp: September 22, 2025, 03:41:22 pm
content: Quote from: Benta on September 18, 2025, 10:45:35 pmIt doesn't look useable with a 5v supply: