Short/Overcurrent circuit protection - Page 1
Forum section: electronics / projects

author: sasa
author_rank: Regular Contributor
timestamp: September 05, 2017, 06:02:38 pm
content: I would need a short/overcurrent circuit protection for low voltage (3.3V and 5V circuits).This lad make quite interesting circuits, among it is this one I need:However, it use relays which is not easy and cheap to obtain. Is there any similar circuit available without using relays?It is enough just to shut down power, no need for reset button. I will not going into circuit protection details as I just need to have basic protection during breadboard testings, avoiding USB computer port (which have short protection) and using solar panel or batteries as main power supply during breadboard experimentation outside.

author: Zero999
author_rank: Super Contributor
timestamp: September 05, 2017, 06:14:52 pm
content: That relay seems fairly generic to me.If you filled in the location field in your profile, I might be able suggest where you can get one.

author: sasa
author_rank: Regular Contributor
timestamp: September 05, 2017, 06:41:51 pm
content: No need for that, thank you, I'm aware of all electronic shops in the country...I simply refuse to pay ripoff of 5 Euro for something worth less than 1 Euro...If there is no way to avoid relays, I will live without.Thank you for your time.

author: Zero999
author_rank: Super Contributor
timestamp: September 05, 2017, 07:34:28 pm
content: Of course you have the right for that information to remain private, as long as you're aware that it may sometimes reduce the helpfulness of some of the replies.To me the obvious solution would be to power the relay coil and associated transistor, from the same contacts which switch the load, via the normally closed switch. In other words, connect VCC on R1 to pin 4 on K2, via U3. Q2 could also be eliminated, if pins 5 & 6 on U2 were reversed. I'll draw a schematic if you're struggling to figure it out.Like most circuits on instructables, it contains errors. The LM358's common mode range doesn't include the positive supply, which will make the current sensing amplifier unreliable and U3 is drawn as a normally open switch, when the description says normally closed.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 05, 2017, 09:47:21 pm
content: This might be of interest, then?http://www.seventransistorlabs.com/Fuse/Note: it's so fast that you often cannot charge electrolytic capacitors. A roughly current limited supply (say with a series resistor) can come in handy. Or use two in parallel, one with a resistor to precharge, the other straight through. (I should implement that as a built-in option some day.)Tim

author: sasa
author_rank: Regular Contributor
timestamp: September 06, 2017, 11:07:51 am
content: Thank you, but no. No need for any more sophisticated device, than cheap DIY circuit.If there is no other way to avoid relay, I'm completely capable to make MCU based solution, which is probably all together cheaper than only relay itself: any small and extremely cheap MCU, small MOSFET (target circuits use max. up to 350mA), current sensing circuit and reset button.It is overwhelmed solution, but easy and cheap, at least for me.Thank you for your time.

author: Blinkenlights
author_rank: Contributor
timestamp: September 06, 2017, 12:38:20 pm
content: I actually prefer current limit to "circuit breaker" style limiting.However, if you like this circuit, why not fix it up?Replace the Relay with a heavy high-current PFET - but change the 1N4007 to a 1k resistor to ensure the gate goes fully off.To fix the LM358 Common mode problem change R3 and R5 to 3k3 1%, and while you are at it make R2 and R4 20k 1% also.The Tactile switch, normally closed type, is not easy to get. Better to use a microswitch and use the NC contact.As the relay is now missing, the other end of the tactile/microswitch should go to Q1 collector.R7 and R8 need to change to 47k to avoid turning the PFET on. That should do it - I think it will work with no relay.

author: Zero999
author_rank: Super Contributor
timestamp: September 06, 2017, 05:39:17 pm
content: Quote from: sasa on September 06, 2017, 11:07:51 amIs is the specific relay, used in the video that you want to avoid or is it relays altogether?As I said before, it can be done using a SPDT relay.What voltage and current range do you require?

author: sasa
author_rank: Regular Contributor
timestamp: September 06, 2017, 06:51:55 pm
content: Quote from: Blinkenlights on September 06, 2017, 12:38:20 pmThank you. I will try to redraw schematic accordingly and see if it will work.Quote from: Hero999 on September 06, 2017, 05:39:17 pmAs mentioned, targeted circuits on breadboard works under small voltage and current (3.3V - 5V and up to 350mA, i.e. less than USB current supply which is usually used), In that case using relay is not necessary at all.

author: Zero999
author_rank: Super Contributor
timestamp: September 06, 2017, 10:07:19 pm
content: Will the whole thing ever be powered from 3.3V or is there always 5V available, even if the voltage being switched is only 5V?It can be done without a relay. A transistor could be used to switch the current. It's just that a relay typically has a lower voltage drop and lower leakage current, than any transistor.The LM358 could be replaced with a 5V rail to rail op-amp, such as the MCP602 and an RS flop-flop (74HC74 or make one with logic gates) to switch a P-MOSFET, driving the load.

author: sasa
author_rank: Regular Contributor
timestamp: September 07, 2017, 05:01:47 am
content: Quote from: Hero999 on September 06, 2017, 10:07:19 pmProtection circuit should always work on fixed 5V. It is only suitable replacement of USB power supply:1. Battery (9V) or solar panel (24V)2. If solar panel is used, a variable DC-DC converter is used to lower voltage not below 7.5V. It is MC34063 based and able to provide up to 1A, calculated for voltage ripple 10mV, 100KHz, anyway it is just a bridge here.3. Classical LM7805 linear regulator board in order to provide clean 5V.4. Protection circuit5. If testing device is powered with 3.3V, additional linear regulator board based on LF33CV is used.6. Testing device.

author: Zero999
author_rank: Super Contributor
timestamp: September 07, 2017, 07:48:56 am
content: Quote from: sasa on September 07, 2017, 05:01:47 amMore questions, I'm afraid.Will the over current protection circuit need to monitor the current from and switch a power source, higher than the 5V powering it? If so, then a relay of some form is unavoidable. The current sense will also have to be a hall effect sensor or one of those expensive proprietary current sensing amplifiers, which can work way outside its power supply range. Another option, is to simply use ORing diodes to power the current limiter off the highest voltage rail available.If the current limiter just works off 5V and circuit which is being protected, is run off 5V or 3V, then it's much easier.

author: sasa
author_rank: Regular Contributor
timestamp: September 07, 2017, 09:09:50 am
content: The 5V from LM7805 if only input voltage to protection circuit as well as output voltage and it is assumed constant.As well, nothing from testing device will flow forward, for instance from unprotected voltage doubler circuit or similar.Nothing fancy, just to cut off 5V supply from target circuit if short/overcurrent in testing circuit is detected. No need for reset switch or anything similar. Turn main mechanical power switch on LM7805 board OFF and then ON, power on modified target circuit should be returned.I could make easily MCU based SCP, OCP, OVP and OPP protection with no question ask. However if possible simple analog circuit as described without relay - better. My knowledge, as evident, in analog circuits is quite limited.

author: Zero999
author_rank: Super Contributor
timestamp: September 08, 2017, 11:06:48 pm
content: Here's how I'd do it. The voltage reference U2, could be the TL431 and U1 can be any decent rail-to-rail op-amp, which can work off 5V. The MCP602 will do.The reference is powered from the switched side, so the circuit will start of in the off state. When S1 is pushed, U2 will come on, U1b will go low and M1 will turn on. The current limit is set by R11.One of the advantages of using this circuit, as opposed to the one with the relay, is it will not sit there oscillating, even if the switch is pushed, whilst there's an overload.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 09, 2017, 07:30:20 am
content: Heh. It's a SPICE oscillator.Maybe not a IRL oscillator...What I mean by that is, if the op-amp SPICE models allow their outputs to fall unrealistically close to zero, it will oscillate. U1b will see ~zero and ~zero and chatter. IRL output saturation voltage may be enough to keep it sane. Then again, IRL mismatch in R1-R4 may randomly make the problem worse or better.Setting R1-R4 so there is a nonzero offset voltage, at zero load (when off), will solve that, well enough. Only a few mV is needed, but a few 10s or 100s of mV is no problem to add, and R11 can be offset by the same margin to remove the corresponding dead zone from its range.U1b should have a little bit of hysteresis (add a resistor in series with U1a output, and use a ~100x larger resistor from U1b output to +in), to ensure sharper switching and prevent chatter during commutation.C1 seems awfully small to start things up (does U2 start up fast enough?), but the whole circuit basically being a flip-flop, it probably doesn't matter.It may be a problem that there's no default load on it -- it could be triggered by static, or really, any kind of initial voltage on the load (say you plug in a still-charged electrolytic?).Quote from: sasa on September 06, 2017, 11:07:51 amIt does still seem to violate the OP's concern. My circuit isn't much more (it's the same basic circuit, with more refinements and optimizations), but I guess it's too fancy.Perhaps OP would be happiest with a fat power transistor and constant current source instead?Tim

author: sasa
author_rank: Regular Contributor
timestamp: September 09, 2017, 04:38:25 pm
content: Quote from: Hero999 on September 08, 2017, 11:06:48 pmThank you very much for the schematic and your time. However, that is not at all what I expected - only pointer on some similar circuit already available avoiding relay....What would happens on target circuit if power rail get shorted? If protection circuit is set to switch-off on 350mA, that would still be 1.75W. Perhaps some MCUs can handle short on power rails, however certainly not over maximum ratings (temp. and total power dissipation which is under 1W). I probably missing something obvious...Thank you very much for your effort and time.

author: sasa
author_rank: Regular Contributor
timestamp: September 09, 2017, 05:05:54 pm
content: Quote from: T3sl4co1l on September 09, 2017, 07:30:20 amProbably not. If power rail is shorted, that may still be fatal for ICs on target circuit.Thank you very much for your comment, as well.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 09, 2017, 06:57:24 pm
content: So basically, you don't know what you want (you can't set a limit of 350mA and be within absolute ratings for a 100mA device; but in practice, it will be fine for tens or hundreds of us); what you think you want, is impossible (example: if an MCU is rated at 100mA, why would you supply it with 350mA capacity?); and you're not going to pay for a solution that could possibly solve your problem.So why even ask?Tim

author: Zero999
author_rank: Super Contributor
timestamp: September 09, 2017, 07:16:15 pm
content: Quote from: T3sl4co1l on September 09, 2017, 07:30:20 amI didn't bother simulating it, because I don't trust the SPICE models for this kind of thing.QuoteI agree, it could do with some tweaking. I would recommend both linear current limiting (200% of the set level) and a cut-off but that's probably too complex, for the original poster's needs.Quote from: sasa on September 09, 2017, 04:38:25 pmIt is similar to the original schematic but the relay is replaced with a MOSFET. A voltage reference has also been added, so the current limit doesn't change, with variations in the power supply.QuoteIt will pass more than 350mA for a tiny fraction of a second, before cutting the power. It may be too fast to enable the capacitors to charge. A low pass filter may be required, to avoid nuisance tripping.QuoteThen set the current limit to a lower level, if the power dissipation is too high. If the circuit needs the current, then one current limiter could be used to protect the high power part, with another, set to al lower trip point, for the sensitive circuitry.

author: sasa
author_rank: Regular Contributor
timestamp: September 09, 2017, 07:25:23 pm
content: Quote from: T3sl4co1l on September 09, 2017, 06:57:24 pmI was absolutely clear - target circuit draw maximum 350mA.As well I had clearly wrote subject - short and overcurrent.Above all, I have asked pointer on eventual existed solution if know, not to waste anyone time.Internet is full of rubbish and this is reputable place for such asking... or not?What I want is absolutely possible, with MCU and I will performed that.You may consider this thread closed.Thank you.

author: sasa
author_rank: Regular Contributor
timestamp: September 09, 2017, 07:31:46 pm
content: Quote from: Hero999 on September 09, 2017, 07:16:15 pmThank you very much again.

author: Zero999
author_rank: Super Contributor
timestamp: September 09, 2017, 09:48:36 pm
content: Quote from: sasa on September 09, 2017, 07:25:23 pmIt is possible with an MCU but is certainly no simpler. Like it or not, this is an analogue design and will need analogue components. You still need a current sense amplifier, U2.1 in the original circuit or U1a in my schematic, to measure the current. The comparator and voltage reference or potential divider could be replaced with the MCU's ADC or internal comparator. Then you need a switch, whether it be a relay, or a transistor, to interrupt the current.One problem with the MCU idea is, there's a risk the code could fail, leaving the switch on, although it's not a safety critical fuse and relays and transistors can also fail short circuit, so it could be sait it's an acceptable risk. You could use the watchdog timer to reset the MCU and start with the output off, until the button is pressed, to give some fault tolerance.QuoteSo you appear to have given up. I'm disappointed.

author: sasa
author_rank: Regular Contributor
timestamp: September 09, 2017, 10:40:40 pm
content: Quote from: Hero999 on September 09, 2017, 09:48:36 pmCertainly not. I consider that all info you have gave me is enough to make my own designs, when find more free time.Actually, I have few ZXCT1081 and one MCP601. Thus also may take a time to get another one or even MCP602.The most of time being hobbyist in electronic I have made simple MCU based projects, including firmware. That is not an issue.Thanks again.

author: Zero999
author_rank: Super Contributor
timestamp: September 10, 2017, 08:49:42 pm
content: I've just realised, the previous circuit I posted, used the wrong values for the current determining resistors. The gain of 10 and a 200mV reference, derived from U2 would have given I = 0.2(10×0.5R) = 40mA. That should be a lesson to me, not to design circuits at 12am!Here's the corrected schematic, with the ZXCT1081 used for the current sensing, rather than the op-amp.

author: sasa
author_rank: Regular Contributor
timestamp: September 10, 2017, 09:35:33 pm
content: I would also consider using NTC thermistor as an inrush current limiter and PTC as a resettable fuse. I believe NTC would be helpful in short circuit condition.

author: Zero999
author_rank: Super Contributor
timestamp: September 10, 2017, 10:59:57 pm
content: Quote from: sasa on September 10, 2017, 09:35:33 pmA PTC fuse is a good idea, because it will act as a fail safe in case the current limiter fails to disconnect the load.An NTC resistor is useless for short circuit protection, because its resistance gets lower, as it heats up, so it's useless for short circuit protection. NTC resistors are used to limit the power on surge current, because they have a high resistance when cold and decrease their resistance, as they heat up.