Short Circuit protection with auto reset

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Mithun_K_Das

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I was working with some power unit. Usually I use micro-controller in this case. But I don't want to use MCU for this small circuit. So I designed one and its working well.



But the problem is if it once shorted, loads off. and never turns on the load until load terminal is opened. That means, once I short the terminals while the lamp was on, the circuit turns the light off. and can't turn it on until the load(lamp) is removed/open from the terminal.

What I was expecting, the circuit will auto reset the protection. But there should not be any complexity.

I want to fit the whole protecting circuit inside a 10mm dia tube. Is there any modification you can provide?
 

That circuit has a very high current limit, since the current is limited only by the ON resistance of the MOSFET and the battery impedance. Is that acceptable?

To automatically reset the circuit you can add an oscillator, such as a 555 astable, to drive a transistor connected between U2's base and ground. The period of the oscillator determines how often the reset pulse is applied. To minimize the pulse (reset) time (and short circuit current time if the output is still shorted), you can connect the astable output to the added transistor base through a series capacitor with resistor to ground. Alternately you could use another 555 connected as a one-shot, triggered by the astable. You can also use a 556 which is two 555's in one package.
 

Like crutschow suggests, using (2) 555 timers to create a one shot can give you a fixed reset time and interval for the protection to be applied. I Recently had to implement something similar on a high power amplifier to disable the inputs while the output current limit protection was engaged and for a certain time thereafter and used the same approach with 556 parts. IF you are using SMT parts it can be done in the space you require, as long as you don't design your RC timing circuits with more than about 10uF-22uF capacitance as it is harder to find SMT1206 or similar sized caps of higher value.
 

Why don't you just use a polyfuse with the correct current rating for the lamp? When the short is removed it will reset by itself.
 
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Using poly switch is the best solution. But its really hard to find in local market.

And if I've to use 555, then I could have use micro-controllers as well. so that is not a good choice.

I found one famous design, they are using one transistor(may be) and few diodes and resistors. But the transistor like device have no name on it. I don't know what it this device really. That is why I was asking for anything you know.

Thanks
 

Polyswitch/fuse is OK as long as you don't have to have a very exact/consistent tripping point. Ambient temperature will affect the trip point, as will normal variances in the switches'structure, and how many times it is tripped/reset. You'll have to look at the trip point carefully, and set it well below the max rating of the other components, because it could be sitting in a non-tripped state for seconds, or tens of seconds, near the trip point before it goes.

But, if a simple, doesn't have to be exact solution will work for this application then by all means do it as it will save you cost and some PCB space.
 

Can you give more details about the whole setup? What lamp, wattage, application, where mounted, etc.?
 

Actually this system will be used in a DC supply bus. The line have 12V dc and there is a number of different loads in different points. Each point should have a short circuit protection individually. This is why, I was planning to design something very small and effective. But as poly switch is not available in local market, so I have to use a transistor circuit with MOSFET.

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avg. load per terminal is 10A.
 

"I found one famous design, they are using one transistor(may be) and few diodes and resistors. But the transistor like device have no name on it. I don't know what it this device really."

Can you post the circuit? Perhaps we can reverse-engineer it here.
It may be using a PUT (programmable unijunction transistor).
 

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