current limiting is not happening in LM723???

[message4guru]

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 in transistor? Because of 5V across it?

Totally confused . Help me in this regard...

 

[message4guru]

Dear sir

I will Check that sir.. But if it is went to breakdown after testing. What should be done to avoid Breakdown??

 

[FvM]

5V across pins 2/3 means the current limiting transistor is broken. Or the measurement incorrect. You should be able to detect a diode junction between pins 2 and 3 at the unconnected IC.

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One problem of the circuit is that it misses any base-emitter pull-down resistor. Shut down in current limit is very slow.

The circuit is apparently incomplete, no reference path, no frequency compensation. If you manage to get the circuit oscillating, it may quickly blow the current limit transistor. You may want to place a series current limiting resistor for pin 2, e.g. 100 ohm.

 

[message4guru]

Dear sir,

You are correct , my transistor went wrong... Sir u could give a complete circuit. so that i will study a circuit and understand things..

 

[message4guru]

Dear sir,,

You may Give some Circuit Collection of LM723. thank you sir..

 

[E-design]

You should really study the data sheet to understand the operation.

Below s a typical circuit with NPN pass transistors and current limit set to 600 mA with R3.
The voltage is variable between about 3 - 30 V with R6. Resistor R4 is not part of the circuit. It is just a test-load in the simulation.

 

[schmitt trigger]

Google LM723 power supply. There are literally hundreds of circuits available on the web.


But I agree, you could have damaged the base emmiter junction of the internal current sense transistor.

 

[E-design]

The b-e connection has an internal limiting resistor in series with the base of 400 ohm to be able to withstand some abuse. So it must have been quite an overload if this junction got blown.

You may still be able to use the part by connecting an external small NPN with collector tied to the comp pin. It is worth a try if you don't have a replacement on hand.

 

[message4guru]

Thank you sir...

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Many Circuit are incomplete or It does not have Reason For using Particular Value for the circuit... So All Google contents are not reliable.....

When it Comes to high current power supply, You can give some suggestions to built the circuit...

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Yes sir, other part of LM723 is working, Thank you sir for reusing the IC.