EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Gurumurthy on June 13, 2017, 06:08:46 am
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I designed a Lab power supply with LM723(for 0-30V @ 5Ampere),Initially I limited the current at .65 ampere(I limit=o.65V/1 ohm). But the problem is ,the IC is not performing the current limiting operation, eventhough it had potential difference is 5V across its base to emitter transistor which does this current limiting function.
What could be the problem?
1)is with current sense resistor precision?
2) is it with Base emitter junction breakdown with transistor? Because of 5V across it?
Totally confused . Help me in this regard...
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The current shunt precision should not be a problem. The current limit with the LM723 is not really accurate, as the reference voltage used is just the VBE of the internal transistors. Anyway the current limit should engage, even if maybe a few percent off.
Once you get 5 V across the shunt, chances are very high that the internal transistor for the current limit is broken due way to much base current. However normally the circuit should prevent this. So maybe one of the power transistors of the regulator as a whole is broken too.
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Without base-emitter shunt resistors, the output transistors may have responded too slowly to prevent damaged to the low impedance base-emitter junction of the current sense transistor. Even with the shunt resistors, a high current design might cause damage anyway. I would include a resistor in series with the base of the current limiting transistor which is pin 2 on the DIP package.
Commercial high current power supplies based on the 723 do this in one form or another; either they add the series resistor or they implement foldback current limiting which includes it anyway. Foldback current limiting only requires adding two resistors and is well worth it in a high voltage or high current design.