Need help with analog power supply problem

Thread Starter

Jack_K

Joined May 13, 2009
140
I have a home made PS that has a problem. It is a variable PS that I have been using for several years. It suddenly developed a problem I can't figure out. The schematic is attached, but doesn't show the transformer, rectifiers, and capacitors, since that portion works fine.

I have verified all resistors are ok and all transistors and the diode check ok with an analog ohmmeter. The LM317 is in the TO-220 package and has a heat sink on it.

The PS powers on and the voltage is adjustable. It will power a 200 ohm load with no problem, but, as the resistance is decreased, the voltage drops until it finally reaches zero. If I remove the two 2N3055 transistors, it does the same thing. I suppose that means the LM317 is bad. I don't currently have another to try replacing it.

Suggestions?

Correction to schematic. It's not a 2N2905, it's an MJE104. Shouldn't matter.
 

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Last edited:

ebp

Joined Feb 8, 2018
2,332
[EDIT] lost my first sentence somehow
Since you can get a small amount of current over (I presume) the full normal output range, it is unlikely the LM317 is bad.

Try removing the 47k resistor and shorting the 120 ohm resistor at the LM317 input. That will allow you to test the 317, which should be able to deliver up to at least one ampere. Be careful it doesn't overheat, though it is thermally protected and should survive even if it does get really hot.

If the 317 is OK, I would suspect the 2N2905. If its current gain has dropped it would not supply sufficient base drive for the 3055s. However, drop in gain isn't often something that happens unless the base-emitter junction is frequently subjected to reverse breakdown, and I see no way that should be happening in your circuit.

[EDIT 2]: Sorry, I missed your note on the MJE104, but as you say, it shouldn't matter in general terms.
 
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Thread Starter

Jack_K

Joined May 13, 2009
140
Did you measure the resistor?
Yes I did. I replaced the MJE104 with a 2N2905. Same results.
I'm going to replace the 120 ohm with a 27 ohm 2 watt, not that it will fix the problem. Just think it's better.

I guess it's time to replace the 2N3055s.
 
Last edited:

ericgibbs

Joined Jan 29, 2010
9,563
hi J,
You say the rectifiers and smoothing are OK.?
What happens to the DC input voltage to the regulator circuit, as you increase the load current.?
E
 

ArakelTheDragon

Joined Nov 18, 2016
1,327
Is it not normal for a current source, if you decrease the resistance, that to decrease the voltage? Can you check if the current stays the same if you reduce the resistance?
 

ebp

Joined Feb 8, 2018
2,332
The lower the value of the resistor at the position of the 120 ohm resistor, the higher the output current at which the 2N3055s begin to conduct. With 120 ohms, the threshold is going to be something around 5-10 mA. At 5 mA, the voltage across 120 ohms would be 0.6 V, which is likely enough for at least a few microamps of driver (2N2905) collector current.

I would suggest setting the output voltage and load to some combination that yields proper regulation at the output, then checking the voltage across each of the resistors. It will be very low across the 0.1 ohm resistors, but the others should all have some reasonable fraction of a volt. The 470 should have around 0.6 to 0.7 V across it. If it is much more than that there is a problem with the 2N3055s. If it is much less, there is a problem with the 2905.
 

Thread Starter

Jack_K

Joined May 13, 2009
140
I would suggest setting the output voltage and load to some combination that yields proper regulation at the output, then checking the voltage across each of the resistors. It will be very low across the 0.1 ohm resistors, but the others should all have some reasonable fraction of a volt. The 470 should have around 0.6 to 0.7 V across it. If it is much more than that there is a problem with the 2N3055s. If it is much less, there is a problem with the 2905.

The power supply worked for over 10 years with the values I posted. It suddenly started doing this. The 470 has 0.6 v across it. I'll replave the 2N3055s when I get some.
 

Thread Starter

Jack_K

Joined May 13, 2009
140
I would suggest setting the output voltage and load to some combination that yields proper regulation at the output, then checking the voltage across each of the resistors. It will be very low across the 0.1 ohm resistors, but the others should all have some reasonable fraction of a volt. The 470 should have around 0.6 to 0.7 V across it. If it is much more than that there is a problem with the 2N3055s. If it is much less, there is a problem with the 2905.

The power supply worked for over 10 years with the values I posted. It suddenly started doing this. The 470 has 0.6 v across it. I'll replave the 2N3055s when I get some.
I retraced the layout since I don't have the original schematic. I also replaced the 120 ohm with a 27 ohm 2 watt. It still does the same thing.

Here's the corrected schematic.
 

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ericgibbs

Joined Jan 29, 2010
9,563
hi Jack,
Ref my post #9, have you confirmed that the Vinput into regulator section is 'OK' at the higher output current setting.?
E
 
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