help on Protection circuit for power supply

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ronchiejames

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could somebody give me a circuit for power supply short circuit protection.
just made my power supply and accidentally got shorted and was roasted.
a working crowbar circuit or relay protection circuits would be appreciated.

i am using split LM317T and LM337T adjustable power supply working on a max of 3 amps.
 
Maybe my cct also give some ideas...
 

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Hey, Ron! nice modifications! I like it, just added a resistor.
 

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There are a few points that I don't get.

Reading from the datasheet of LM317 and LM337, both of them have built-in current limit and thermal protection. In the datasheet it has also mentioned cases about input and output shorts conditions which calls for protection diodes. So these regulators should survive an output short if the precautions of adding the protection diodes are followed.

Maybe ronchiejames would tell us the input voltages to them and whether those protection diodes are being fitted in this case.
 
I was wondering the same thing. LM317 and LM337 are, as you said, designed to be virtually bulletproof if you don't violate the datasheet limits.
 
Yes, agree Ron, but our circuit also protect the sensitive load: cannot remain only negative (or positive) output. (often destructive) And easy variable the current limit in few steps for experimenting. (by switchable CS resistors)
 
Sebi said:
Yes, agree Ron, but our circuit also protect the sensitive load: cannot remain only negative (or positive) output. (often destructive) And easy variable the current limit in few steps for experimenting. (by switchable CS resistors)
Click to expand...
Good point, Sebi.
 
thank you for your suggestions!

what is that schematic symbol that is ganged to the switch before the regulator?

if possible: can u pls put up some values of the resistors and transistors
still a newbie......... sorry if im not quite familiar with all the schematic symbols and only knows ohms law for calculations.
i can ionly mplement the circuit if it has part numbers and values......
 
The diode symbol with third leg (under relais) is a thyristor (SCR). All small types useable. The SCR remain conductive after positive impulse (the relay pull). The "C" capacitor value 100n to 10uF, depending what You need: quick or slow reaction for shutdown. Transistors: BC547 and BC557. (almost any type work) The current sensing (CS) resistor for 1A about 0.7ohm. You can modify with ohm's law. The relay voltage depend from transformer voltage, when the DC on buffer cap. too high, apply a serial resistor.
 
this was the circuit!

can u pls check my circuit if its ok and make some adjustments if necessary. POT stands for potentiometer.
 

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When either of those transistors conduct they will bypass their associated regulator and apply the unregulated voltage to the output.

You have C1 and C4 connected the wrong way round.
 
Just to be safe, I would use 120 ohms for R3 and R6 as recommended and shown in the datasheet. The more expensive and lower operating current LM117 and LM137 can use up to 240 ohms.
If you use 220 ohms, some LM317 and LM337 IC's will cause the output voltage to rise with a light load.
 
Hi David,
The transistors begin conducting when the current through the regulators causes enough voltage across the 1.1W? resistors to turn them on. The transistors are in the regulation loop so if they conduct too hard, the regulator reduces the output voltage and current. The booster transistors are shown in the datasheets.

C1 and C4 (and even C5) have the correct polarity. The LM337 is a negative regulator.
 
transistor conduction

The transistor(current boost) is only there to give the regulator the support if larger current is needed and not to let the regulator do a lot of work. So if the load needs current more than 700ma the transistor will turn on to provide more current, and it doesnt mean that regulator will no longer work cause it still needs to provide the 700ma to keep the transistor on.
The capacitors are connected correctly or else they are toasted. Actually this is a working circuit until it got shortcircuit and the LM317 part no longer works.

For the performance of the regulator is not really an issue cause it is not all the time that the load needs current greater than 700ma, what im most worried about is the protection from shortcircuits.
 
Re: transistor conduction

ronchiejames said:
what im most worried about is the protection from shortcircuits.
Click to expand...

When there is a short circuit, sufficient current passes through the 1.1W resistor into the LM317/LM337 to turn the booster transistor ON. Thermal protection of the regulator would not have cut in fast enough to save your two NPN and PNP transistor.
 
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