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DC Power supply short circuit protection :/

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ibnul7

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20130830_100338.jpg20130830_100338.jpgHey guys, I have designed and built this constant current/constant voltage power supply. I know it's a bit unconventional, but I've used a darlington pair transistor with a zener diode to regulate the voltage. The LM317 regulates the output current. The circuit does work very well with all sorts of loads. I now want to add short circuit protection, so I was just wondering how I would go about doing it. So far I''ve tried placing a PTC thermistor in series with the 16v Vin, and it works okay, except that the power transistor gets extremely hot (so I'm guessing it didn't really shut off the circuit). Any advice will be much appreciated. Thanks :)
 

ibnul7

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so i simulated this circuit. the thing is that the two npn transistors will be using about 40W and 20W in the event of a short circuit. That's so high :/ any suggestions?
 

snishanth512

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Is there any use of PIN diode at Collector and Emitter of transistor (Q2)???
 

ibnul7

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tbh not really :/
 

E-design

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You show a DC input of 16V but also a 10V capacitor over the same point?

What are the voltage and current range you want this to work? The circuit will have very bad regulation as there is no proper feedback to compensate. At the moment, the output is only dependent upon the gain of the two transistors. That will change with temperature.

Which type of transistors are you using?
 

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ibnul7

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You show a DC input of 16V but also a 10V capacitor over the same point?

What are the voltage and current range you want this to work? The circuit will have very bad regulation as there is no proper feedback to compensate. At the moment, the output is only dependent upon the gain of the two transistors. That will change with temperature.

Which type of transistors are you using?

sorry, the input is actually 9v-13v now and that input cap is rated at 50v, just to be safe. The first signal transistor is a regular ss8050 https://www.fairchildsemi.com/ds/SS/SS8050.pdf
and the second one is a ksc5021f power transistor https://pdf1.alldatasheet.com/datasheet-pdf/view/53386/FAIRCHILD/KSC5021F.html

That simulation you posted is exactly what happens, although the voltage drop is always constant at 1.5ish volts. How do you suggest i add feedback?
 

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