[SOLVED] Simple short circuit protection suggestions need - Page 1
Forum section: electronics / beginners

author: Chriss
author_rank: Frequent Contributor
timestamp: April 12, 2019, 11:15:54 am
content: Hi!I made a load switching driver with an ne555 and a mosfet.My question is:How can I make a simple short protection for the mosfet?If the load is in some way shorted out it should protect the mosfet.I could put a fuse in the circuit but I would use something more automated.The output voltage and current what the mosfet should handle is 12v/3A. Everything over 3A should trigger a circuit and brake the circuit tp the mosfet.Any ideas please.Than you.Sent from my GT-I8260 using Tapatalk

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2019, 11:31:57 am
content: A fuse would do little to protect the MOSFET, which would most likely blow first, protecting the fuse.What's the nominal load current?Please post a schematic.

author: MagicSmoker
author_rank: Super Contributor
timestamp: April 12, 2019, 11:37:23 am
content: Use a beefier MOSFET, insert a small resistance in series with it and the load to limit short circuit current to something the MOSFET can tolerate for a few dozen milliseconds, then use a fuse with a current rating just above the expected continuous load current rating such that it will blow when exposed to the resistor-limited short circuit current in less time than it takes the MOSFET to blow.Otherwise, the classic NPN transistor current limiter circuit works well. See, for example, partway down on this page:https://www.brighthubengineering.com/diy-electronics-devices/66646-basic-transistor-circuits-for-beginners-explored/

author: Chriss
author_rank: Frequent Contributor
timestamp: April 12, 2019, 11:37:48 am
content: That's for the quick reply.For now I'm unable to post a schematic of my original idea I build up because I'm out of reach of my pc.The nominal current of my load is around 1A.It is a solenoid of a petrol car injector.Sent from my GT-I8260 using Tapatalk

author: Psi
author_rank: Super Contributor
timestamp: April 12, 2019, 11:48:17 am
content: Depending on how simply of a solution you want, you could use a high/low side driver instead of a mosfet.If the driver has short circuit protection built in, as most do, then it's all sorted for you already.Some have an analog current sense output that might be useful, depending on what you're doing. You could even use it to disable the 555 output if Isense voltage indicates current is over a threshold. i.e. implement your own current limit which is lower than the chips own over-current protection.I'm not suggesting you use this one, it's overkill, but this is the kind of thing i mean.I used one of these in one of my products.https://www.digikey.co.nz/product-detail/en/stmicroelectronics/VN5E010AHTR-E/497-10505-1-ND/2260565(Note that over current protection in driver ICs is usually just implemented as inrush current protection in combination with over-temperature protection.)

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2019, 01:00:09 pm
content: Is it OK if the power needs to be reconnected to reset it?I would use a sensitive gate, low holding current SCR, as the trigger element.When the voltage across R1 reaches about 0.6V, the SCR triggers, cutting off the base drive to the emitter follower powering the 555 timer circuit. D1 protects Q1's base from reverse voltage, from C1 (the decoupling capacitor in the 555 circuit), when U1 triggers. It must be a sensitive gate SCR, with a holding current of under 10mA. A low powered SCR in a TO-92 package is ideal.

author: Chriss
author_rank: Frequent Contributor
timestamp: April 12, 2019, 03:09:07 pm
content: Thanks guys for the help.The last post from Zero999 with the schematic is the exact solution for my problem.Can you pls give some advice about a type of SCR I should look for?Could I use the 2N5061?Thank you.

author: Ian.M
author_rank: Super Contributor
timestamp: April 12, 2019, 04:15:55 pm
content: Zero999's circuit can be simplified slightly if one can use the 555 Reset pin rather than having to remove its power supply. Unfortunately a SCR has too high an on state voltage to directly drive a 555 Reset pin and pull it low enough to guarantee a reset. The solution to that is to do away with the SCR and replace it with a SCR-like PNP/NPN latching pair. That lets you use the collector of the NPN to pull Reset low which will get it within 0.3V of ground.LTspice SIM attached. D2 is a steering diode to let the 555 generate low duty cycle pulses.

author: Chriss
author_rank: Frequent Contributor
timestamp: April 12, 2019, 04:51:18 pm
content: How many wat's should be the Rsens resistor?I mean, When the Iload=2A a simple 1/4w or 1/2w resistor will burn I assume...

author: Ian.M
author_rank: Super Contributor
timestamp: April 12, 2019, 05:16:47 pm
content: That depends. For short enough pulses at a low enough duty cycle a 1/4W resistor would be fine, but if you want it to stand up to DC, it needs to be able to handle the max current continuously. Calculate its dissipation by V2/R, and take V as 0.7V which gives V2of 0.5. Therefore a 0.33R resistor (for approx 2A trip current) would need to be at least 1.5W, and it would be preferable to make it at least 2W.Consider paralleling higher value, lower wattage resistors to get the required wattage more cheaply. Caution: they will be a local hot-spot so you will need to derate them.

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2019, 08:02:26 pm
content: Ian,I wouldn't say your idea is simpler than mine, as it uses roughly the same number of components, just different. It's probably cheaper as another BJT will cost less than an SCR and is easier to get.Quote from: Chriss on April 12, 2019, 03:09:07 pmYes, that will do, but if you can switch the reset pin, just use Ian's circuit, with common transistors, such as the BC458, PN2222A, 2N3906 etc.

author: Ian.M
author_rank: Super Contributor
timestamp: April 12, 2019, 08:24:59 pm
content: I agree its not simpler - a SCR direct to the reset pin would be, but I explained why that's no good.One further note: Zero999's circuit can also be reset after a trip by a momentry switch across the SCR, and if you want a 'Tripped' indicator put a LED + series resistor in parallel with R3 (same in both).

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2019, 08:51:29 pm
content: Quote from: Ian.M on April 12, 2019, 08:24:59 pmOr a normally closed switch in series with R3, which will also work in your circuit too.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2019, 02:09:14 am
content: No need for a latch, you've got a 555 right there!If this is like a flyback driver, then something like this will do,https://www.seventransistorlabs.com/Images/555%20Boost.pdfTim