Current limiter - Page 1
Forum section: electronics / projects

author: electronics man
author_rank: Frequent Contributor
timestamp: February 19, 2014, 09:06:11 pm
content: I have a current limiter circuit which consists of a diferential opamp across a current shunt resistor, the output of that opamp goes to a comparator whith a 10 turn pot on the other input, so when the comparator senses that both inputs are more than or equal to one another is turns on and switches on a MOSFET to pull the adjust of an lm317 low to tun the output off and stopp the current, the problem is it oscillates as I expected. This is because as the output is turned off the current drops so the MOSFET switches off, this then means the current increase so the circuit triggers again, my question is how can I make the circuit stay low until the over current stops.Daniel

author: tszaboo
author_rank: Super Contributor
timestamp: February 19, 2014, 10:06:55 pm
content: step 1: post schematic.

author: electronics man
author_rank: Frequent Contributor
timestamp: February 19, 2014, 10:29:00 pm
content: sorry its not that great it is just a section from a larger schematic. the output of that opamp just connects to the mosfet

author: dannyf
author_rank: Super Contributor
timestamp: February 19, 2014, 10:43:56 pm
content: You don't want it to be a limiter; you want it to be a latching switch.

author: electronics man
author_rank: Frequent Contributor
timestamp: February 19, 2014, 10:45:55 pm
content: yeah i suppose but not one that will stay on for too long so not a relay

author: miceuz
author_rank: Frequent Contributor
timestamp: February 19, 2014, 11:49:08 pm
content: I've seen an excellent post on the topic somewhere in electronic.stackexchange.com. Comparator is your problem. I'd suggest putting another opamp instead of comparator and setting a finite gain for that opamp.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 20, 2014, 12:03:14 am
content: Sounds like a bad recipe for a good oscillator. (What happens when it exceeds current? Turns off. So current is zero? Let's turn it on again! Repeat at probably 100kHz or so, or until circuits fry...)Tim

author: electronics man
author_rank: Frequent Contributor
timestamp: February 20, 2014, 12:21:35 pm
content: When dave made this circuit for his usupply he put a unity gain opamp between the differential amp and the high side of the current shunt.

author: codeboy2k
author_rank: Super Contributor
timestamp: February 20, 2014, 01:30:09 pm
content: Some DC-DC controllers go into a hiccup mode during over current shutdown to avoid this very oscillation problem; granted its still an oscillation, but its very slowBasically, the output switch controller is gated via the hiccup controller. When the hiccup controller turns on the gate and allows the output switch to turn on, and another over-current event is detected , it resets the hiccup timer and the gate turns off. More sophisticated controllers even increase the timeout each time to some max, and reset the timeout to zero after its working again (the cause of the overload is removed).

author: electronics man
author_rank: Frequent Contributor
timestamp: February 21, 2014, 05:52:56 pm
content: could someone sugest a diferent soluution mabe a single chip would be nice but it must be variable whith an external pot.

author: dannyf
author_rank: Super Contributor
timestamp: February 21, 2014, 06:19:57 pm
content: A potential solution was suggested to you earlier: use a latching switch.A schmitt trigger that can never be reset for example.

author: ConKbot
author_rank: Super Contributor
timestamp: February 21, 2014, 06:26:07 pm
content: Quote from: T3sl4co1l on February 20, 2014, 12:03:14 amOr put an inductor in series and make a buck converter that only turns on above the current limit, and make the oscillation useful

author: electronics man
author_rank: Frequent Contributor
timestamp: February 21, 2014, 06:28:46 pm
content: Quote from: ConKbot on February 21, 2014, 06:26:07 pmwhat value would i need.

author: matkar
author_rank: Regular Contributor
timestamp: February 21, 2014, 07:09:43 pm
content: If you're searching for an integrated solution you could find something useful browsing web pages from different chip producers.Depending on your current / voltage needs you might find something on TI site:http://www.ti.com/lsds/ti/power-management/power-protection-distribution-monitoring-products.pageThese two might do the trick:TPS2421-1TPS2421-2Look at Maxim site for suitable solutions as well:http://www.maximintegrated.com/products/power/power-switching/Try other producers too.

author: dzseki
author_rank: Frequent Contributor
timestamp: February 22, 2014, 02:52:16 pm
content: Once I inherited a DIY amplifier and there were some pcbs in series with every voltage lines, as it turned out they were electronic current limiters, I reverse engineered the schematics then, now I loked it up for you, I think it is a very clever circuit, it can be used in both polarity just keep in mind the current flow direction.

author: electronics man
author_rank: Frequent Contributor
timestamp: February 22, 2014, 06:45:17 pm
content: Quote from: dzseki on February 22, 2014, 02:52:16 pmthanks for that very helpfull.

author: dannyf
author_rank: Super Contributor
timestamp: February 22, 2014, 08:11:49 pm
content: QuoteBD243/BD740 form a PNP; Together with BD749, they form a SCR. BC237 is normally not conducting (that 250p capacitor is very important,). As such, the SCR is conducting hard.As the voltage drop over the scr goes up, the voltage across BC237's b-e junction goes up. Once BC237 starts to conduct, it diverts current away from the "SCR", thus turning it off. Once the scr is turned off, all voltage is drop'd on the whole device, further locking it up.aka, this is a latching switch.Because of that, it can be implemented a variety of ways, using SCR, mosfet, solid state relays, etc.

author: dzseki
author_rank: Frequent Contributor
timestamp: February 23, 2014, 07:29:25 am
content: Quote from: dannyf on February 22, 2014, 08:11:49 pmThanks for the clear explanation, I also figured out that the little capacitor has a great importance here, but never knew exactly why is this working at all -that's why I drawed the schematics then... And yes, it is latching. Once it goes out you have to remove the power completely to switch on again (I guess shoring in and out would do the trick as well, but this approach isn't very wise if you use it as a fuse)

author: ohmineer
author_rank: Contributor
timestamp: February 23, 2014, 08:32:52 pm
content: If there aren't special requirements, the simplest current limiter that I recall is based on 2 PNP transistors (or 1 PNP + 1 PMOS).PMOS (Or right PNP) is used as pass transistor.Current demand can increase until top left PNP BE junction is forward biased.Therefore, VBE voltage drop over R1 resistor (Sense resistor) set maximum allowed current.

author: SeanB
author_rank: Super Contributor
timestamp: February 23, 2014, 08:41:11 pm
content: True that limits current, but the pass element will get mighty hot when it is limiting and dropping most of the supply voltage. the earlier one at least latches into a low dissipation mode when it trips out until the supply is interrupted.

author: electronics man
author_rank: Frequent Contributor
timestamp: February 23, 2014, 08:51:23 pm
content: that is probably the best one i have seen so far, the pass element could be heatsinked, as they are on my BOM already and there will be a fan to keep everything nice and cool.

author: ohmineer
author_rank: Contributor
timestamp: February 23, 2014, 09:35:54 pm
content: Beware! I have just realised that there is a mistake in the screenshot I posted. Iout is not Vcc/R1. The correct expression would be Iout=Vbe/R1.The drawback of this approach is the change on Vbe with ambient temperature. In any case, I think that there are solutions to compensate for this, although sometimes it does not worth it.

author: electronic_eel
author_rank: Regular Contributor
timestamp: February 23, 2014, 11:20:26 pm
content: If you don't want oscillation, you wanta) latchinga1) forever until poweroffa2) automatic retry after a few milliseconds to secondsb) foldbackThe latching ones completely shut the output off when tripped. The foldback ones always let a little bit of current through. If the current drawn drops below a threshold, the limiter is reset and the full current is available again.Here is a discussion with some schematics:http://www.electronics-related.com/sci.electronics.design/thread/196166/electronic-fuse.php

author: dannyf
author_rank: Super Contributor
timestamp: February 24, 2014, 12:22:35 am
content: QuoteReplace Q1 with some leds and they act as over-current indicators.