LM723 30V-10A Power Supply Problem

[muneeb khan]

from http://www.thebackshed.com/windmill/articles/VariablePowerSupply.asp
I am making this power supply, the problem is that the voltage at output stays constant i-e 37V.(input voltage are 40V).
I have double check the connections but there is no mistake, i disconnected the 2N3055 part and made a simple circuit and it works fine, i also test the 2N3055 ther all are good. I am struck there, plz help me any suggestion will be appreciated.

[IanJ]

Hi,
Resize your image, it'll make it easier for folks to troubleshoot.
Ian.

[Strada916]

check the BD139 is ok. Sounds like the pass transistors are turned full on.

[muneeb khan]

i test BD139 with multimeter it show some value on all pins irrespective of the pins i suppose to be the base..
I think its not working properly i ll get a new one and share with you guys..

[Strada916]

remember BD139 is a Darlinton Transistor. But it should still behave like a transistor non the less.

[muneeb khan]

Hi thnks! the BD139 was faulty, i replaced it and its working fine but under load it again blown out
Please any help.

[Andy Watson]

Does it have to be this circuit? There must be 1000s of LM723 circuits out there - most of which appear to have been designed rather thrown together. I have very little confidence in this "design" because:
1. The main output transistors have a gain in the range 20 to 70. If you've been unlucky and obtained a batch at the lower end of the gain range you will be asking Q1 to deliver a drive current of 10A/20 = 500mA. Taking the worst case scenario of low regulated output voltage, i.e. most of the unregulated voltage will be across Q1, you will be asking Q1 to dissipate about 20W. I believe the BD139 is only specified for 12W (on a heatsink). It's going to fry!
2. I would specify 2N3055H, they are less susceptible to thermal run-away. And, belt-and-braces, add a resistor across base-emiiter (to deal with leakage from Q1).
3. The current limit, R11, appears to be adjustable from about 2.5A to infinity - a dangerous state. I would put some extra, fixed resistance on the upper end of R11.
4. If you want to see a regulated 30V output, R7 should be 3300 ohms.
5. C2 ? I can guess why it's been put there - but it doesn't inspire confidence!

[Dr. Frank]

Hi,

maybe it's not the nicest one, but the basic circuitry seems to be ok / standard.

But the dimensioning of the components is critical, i.e. that's not robust. The author of the circuit did not take care for that, obviously.

40V DC is the MAXIMUM voltage for the 723, and the heat dissipation for the BD139 is critical for high current, low output voltage.
Please give an indication, what is the idle state input voltage..
 
The input capacitor(s) is (are) too low, you'll need about 2200µF for each 1A output current.

I also would guess, that BD139 is destroyed by heat dissipation.

You do not give us information, under which condition the BD139 blows up, and if it is mounted on an appropriate heat sink.

Please check beta of the 3055s @ 2.5A each, and calculate worst case heat dissipation of BD139 from that.

Ordinary 3055 may have very low beta, the H types are selected for higher values. Should be at least 80 or more.

To improve current distribution between those four 3055s, you might add 4x 22 Ohm between BD139 emitter and each of the 3055s bases.
(beta goes down, if one of the 3055 conducts more of the current than the others.)
Frank

[qno]

You need a current limiting resistor in the base of the 139.
When output voltage is low and current is high a lot of heat is dissipated in the 139.
You can use darlington output transistors to limit the current needed do drive the output.
A collector resistor for the 139 can limit the power in the139.
This will affect max voltage under full load.

[muneeb khan]

Does it have to be this circuit? There must be 1000s of LM723 circuits out there - most of which appear to have been designed rather thrown together. I have very little confidence in this "design" because:

Thanks Andy! Please can you suggest me a circuit for 10A power supply that is better than this?

[muneeb khan]

I also would guess, that BD139 is destroyed by heat dissipation.

You do not give us information, under which condition the BD139 blows up, and if it is mounted on an appropriate heat sink.

thanks Dr.Frank The BD139 is mounted on the heat sink, as I connected a 9V dc motor, after a jerk voltage goes to constant 37V. And i checked the BD139 its was showing a short between Base and Emitter..

[electronics technician]

i'm new here and i'm going to try this project is there anything i should do to get it to last a long time and most importantly get it to work first time  thanks for the help oh and could i make this a dual supply +/- 

[electronics technician]

You need a current limiting resistor in the base of the 139.
When output voltage is low and current is high a lot of heat is dissipated in the 139.
You can use darlington output transistors to limit the current needed do drive the output.
A collector resistor for the 139 can limit the power in the139.
This will affect max voltage under full load.

what value resistor should you use? thanks

[redwire]

I note the pinout in the schematic is incorrect for the LM723, in the DIP or can package... http://www.ti.com/lit/ds/symlink/lm723.pdf
EDIT my bad, it's okay for the DIP.

[electronics technician]

can i add more (2N3055H) power transistors to get 20A if i put 2 or 3  bd139 transistor in parallel with the first one to get more than 10 amps now is this suggestion possible