Electronics > Beginners

PS limiting current for no reason?

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AngraMelo:
Ok, so I have been in this project for a while. You guys already helped me with the protection circuit (the crowbar is not there anymore, Im using now an opamp that controls a transistor that controls a beefy relay. When the regulated voltage that comes from the output of the PS goes into the non inverting input of the op amp is within the parameters 13-14V, it turns on the transistor which turns on the relay and the binding posts of the PS are "on" if something shorts out and the output voltage goes higher than 14V the opamp output goes to ground which wont turn on the transistor which wont turn on the relay so the binding post are "off") It is working so thank you all for helping me!

That is not the issue anymore.

The issue is, it is limiting the current to around 7.5A. The voltage on pin 10 of the 723 is slowly dropping (around 30mV per second) so what happens is: if I connect a load that requires 10A, it provides only 9A and slowly drifts down to 7.5. What I think it is happening is that the 723 cannot drive Q1 which is an 3055, so when pin 10 drops the voltage given that it cant provide enough base current to Q1, that drops the voltage on the base of the power transistors limiting the current.
Am I right? What do you guys think?

UPDATED THE SCHEMATICS

AngraMelo:
So I measured the current on the emitter of the pass transistor Q1 and it goes as high as 100mA and starts to go down. So something is not right. the 723 should be able to drive that Q1.
The 723 can output 150mA so if we consider a low gain of 20 for the 3055, if the 723 outputs 100mA Q1 would be able to provide 2A, which is way more than the 4 transistors will need on their bases to let a 10A current to pass.
Am I crazy here?

rstofer:
I know the first line in the datasheet says 150 mA output current but I'll be darned if there is any other number anything like that anywhere in the datasheet (other than the marketing blurb in the upper right corner).  There are power dissipation curves and how much the regulator dissipates depends entirely on the operating voltages and the output current.

You didn't provide the PS output voltage (or I missed it) and, near as I can tell, you're going to have to go back and calculate the power dissipation.  This will be critical when outputting low voltage because there is more drop across the regulator.  You would still need to output high current into Q1 if you are driving high current from the PS and you would have a maximum voltage drop across the regulator.

I would measure the base voltage droop on Q1 and maybe put a finger on the regulator.  If the regulator is protecting itself, the output voltage will decrease.  There may also be a temperature rise.

ArthurDent:
Just for the heck of it put another 0.1 ohm resistor in parallel with R7-R10 and let us know what the output current is.

Also replace Q1 with a 2N6576 or similar.

AngraMelo:
I tried putting another 0r1ohm resistor and no cigar.
Im putting 28V and the output is supposed to be 13.8V.
When I connect any significant load, like for example 2A, the output drops about 0.5V and the current never exceeds 9amps.
I tried putting a 2222 in series with a bd139 and then to the 3055 (Q1)to minimize the current at the output of the 723 and the result was the same.
Im completely lost

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