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over current protection needed

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Advanced Member level 2
Oct 7, 2001
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over current protection, tutorial, transistor

I need a simple and reliable over current protect circuit. Circuit must cut the line current when it exceeds 2A @28V.


Can you do any changes to the soiurce of the 28 V? Many power supply ICs have over current built in. Look at the data sheet for the IC in your supply and it will tell you what resistors to put on which pins.

Here is a link to a current limiting circuit **broken link removed**

Why not use a PolySwitch device? They are used in almost every PC nowdays to protect the ps from shorts on the keyboard/mouse connector, scsi, usb and so on. The main advantage over a fuse is that they reset by themself after they have tripped (when voltage is removed for a few seconds).

See for examples.


You have similar device from Bourns, which they call Multifuse.

Some ting’s to remember for Polyswitch/Multifuse
You have a small hold current that’s flows trough the device.
There is a maximum interrupt current/voltage.
It’s has a temperature dependent trip current



What sort of PSU do you use ??
If you want to build it from scratch, I would use the linear regulator
type 723. This chip is very reliable and very cheap !!



As you can see there are a variety of methods. Each has cost and performance combination different from any other.

The selection depends upon how fast must the load be disconnected and how critical is the current threshold.

Re: summary

Actually i won't cut off the main current. I only control one of the terminals. At first i thoguht a transistor based solution. Then i decided to use opamp. But i couldn't be sure and asked you. This polyswitch or similars looks like to be suitable if they are fast and cheap enough.

Re: summary

I think polyswitches or similars aren't fast enough. They need seconds to cut off the current. (am i wrong?) Are there any other suggestions or circuits?


You are right. Things that require heat take time to work. You might want to try an ordinary fuse. They can blow in milliseconds if they are fast acting. Watch out for the load initial current if you have to charge capacitors in it. In that case a slow blow fuse will work.

Tell us more about your load and source. Will the load only draw large currents when some bad thing happens? Can your source put out more current for a short time?

Re: right

Source can supply more than 4A steadily and put up with larger currents for a short time. And it's protected by two different types of fuses which activate at 4A . Load draws small currents in normal operation. If something goes wrong it can draw 2,3... times more than normal current or there can be short circuit. What i want to do is, allowing faults to a certain limit. When limit is reached, circuit must cut off that channel's current without affecting other channels. I want this protecting circuit to be resettable so i can't use a fuse.

stuck with electronic

In this case you are stuck with the electronic method. Can the reset be a bag of fuses near by? You might look into circuit breakers. I have only seen these on the power mains and do not know if they are available for lower voltages and currents.

You can look at Hot Swap IC from manufactures like Maxim, Linear, and TI.

There is a simple current limiter with two transistors.
The first transistor you connect the base and emitter? over a current shunt resistor.
This transistor shunts the second transistor which is your switch.

Or a simple crowbar with a tyristor and a polyswitch.

And when You put a small thyristor between pass transistor base and GND, triggered by sense transistor collector, this circuit remain off state after overcurrent.
Possible reset push button parallel the thyristor anode-cathode.

Overcurrent protection

Hi Seyyah, can you please post some schematic in PDF or GIF or
BMP format and I will help you.


Hi flatulent :D

Ok I upload again!!

I work in the tutorial in how to calibrate it, and some modification for more current !!!! 5 to 6 amp

in this new schematic i make a correction, in the power transformer.

this power supply is for shortcircuit overcurrent protection ideal for breadboard use, I use it and work fine.

Hi all :D

This is the instructions and MOD for the Power Supply


Re: Overcurrent protection

This is the general schematic for Tornado

Re: Overcurrent protection

how is this? I think it can work and it is simple. What do you think? Does this have any weakness according to you or does it need a revision?

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