[Moved] LM317 High Current Power Supply

[Suhas Anand]

Load current flows through R3 and RV1 right? That is how we desing the value of R3.

 

[FvM]

No, only R1.

 

[D.A.(Tony)Stewart]

I see a problem with all the R ratios affected by Q1 current gain and Vbe for varying voltages at high current.

Correction
If R1=1k , after 1.3mA quiescent current of U1 is drawn, the bypass base drive begins to pullup and over-rides the regulator.

R1 forces pullup when current of U1= Vbe/R1
10k is far too big and should be <<10 ohms and P= 1.4^2/R1

Design U1 power dissipation is never exceeded for selected thermal resistance Rja. and resulting temp. rise.

- the intent of the bypass Q1 is to boost/divert power dissipation BUT keep the output voltage regulated by U1

 

[Suhas Anand]

Ok. I will try doing that. If it doesnt work out i guess i ll have to go with the circuit with 2n3055 transistors.

 

[D.A.(Tony)Stewart]

I corrected my previous advice.

 

[Suhas Anand]

How 1.3mA?

 

[Audioguru]

How 1.3mA?

The current in the LM317 is about 10mA with no load. With no load the 10mA flows through R1 which causes the darlington to turn on (because 10mA x 1k= much more than its Vbe of 1.3V) when it MUST NOT turn on.

Therefore the value of R1 at 1000 ohms is much too high, it should be 10 ohms. If R1 is 10 ohms then it keeps the darlington turned off when the load current is zero or low and when the load current exceeds about 1.3V/10 ohms= 130mA. The LM317 passes up to 130mA.
Then the darlington begins to turn on and pass any load current that exceeds 130mA.

 

[ALERTLINKS]

A relevant thread,
https://www.edaboard.com/threads/269989/

 

[Suhas Anand]

Thanks all. I will try replacing r1 and get back!

 

[Suhas Anand]

Hello,
I tried replacing R1 by 10ohms. Same problem

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Can some one tell me how to calculate values of R1 and R2 ?

 

[D.A.(Tony)Stewart]

Hello,
I tried replacing R1 by 10ohms. Same problem

- - - Updated - - -

Can some one tell me how to calculate values of R1 and R2 ?

Test transistor to ensure it is not defective and shorted across Vce. Obviously if R1=0 Ohms The transistor cannot conduct and Ice is open circuit.

Validate your measurements.

 

[FvM]

I tried replacing R1 by 10ohms. Same problem.

I'm not sure if we are talking about the same circuit.

 

[Suhas Anand]

I'm not sure if we are talking about the same circuit.

I am talking with respect to the circuit which I posted. Post no 1.

 

[Audioguru]

Q1 should not turn on if the load current is 120mA or less.
Since it turns on when you turn down the output voltage then I betcha that you connected the collector and emitter pins of the darlington transistor BACKWARDS, and the darlington transistor is a cheap fake one that is missing the diode from collector to emitter, or maybe it is not a darlington.

Oh, is D1 a zener diode instead of a normal diode?

 

[FvM]

I am talking with respect to the circuit which I posted. Post no 1.

Thanks for confirming. I agree with Audioguru, there's no obvious reason why this circuit shouldn't work.