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#218605 - 07/08/17 01:48 PM Re: Linear Power Supply Quits Under Load [Re: Potseal]  
JoeTestingEngr  Offline
Joined: Nov 2005
Posts: 790
Chicago, Il.
That should be a good linear supply to learn on. I should have had the datasheet in front of me when I responded. The current is sensed between pins Current Limit (2) and Current Sense (3). Check that you have <<0.6V between pins 2 and 3 to verify that you aren't in current limit. Then, use your DMM on DC, then AC Volts, with black on Pin 7 (V-). Pins 11 and 12 have to be stable and several volts above your final output voltage. If that's OK, the (typical) Reference Voltage at Pin 6, should be 7.15VDC. As with any Op Amp, the 723's main purpose in life, is to keep its inputs, pins 4 and 5, equal. VRef, or another stable reference voltage, is fed into one input, while a divided down sample of the output, is fed to the other. On supplies <7V, you would see the reference voltage divided down. It's all of the inputs and outputs available on the 723, that make it useful for both positive and negative supplies. Output current is limited only by your filtered source, and room for driver and output transistors, and emitter balancing resistors.
Of course, you don't want the 723 to work too hard if it's driving other transistors. So you would keep the PDIP well under its 150mA and 660mW maximum ratings. That's where the Alphas, or emitter current gains, of the driver and output transistors come in. A 5 amp supply with its 723 sourcing <= 50mA, would have to use transistors with a combined gain of at least 100. That's why you will see a driver transistor driving the bases of multiple, emitter balanced, output transistors.
Now, looking at the datasheet for the On Semi 2N3055, that I just downloaded from Digi-Key, the features say it all. The DC current gain ranges from 20 to 70 at 4 amps of collector current. You can see that a transistor with a gain (Beta) of 20, (Alpha) of 21, would need a base current of almost 230mA, to get an emitter current of your supply rating of 4.8A. That's more than the 723 has to offer, and why your supply will be using an external driver transistor. The 2N3055 isn't a common driver for such a small supply so I wonder if that third one might be part of the current limit or a crowbar circuit.
Good luck,

Last edited by JoeTestingEngr; 07/08/17 02:03 PM. Reason: typo

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#218749 - 09/15/17 11:37 PM Re: Linear Power Supply Quits Under Load [Re: Potseal]  
JoeTestingEngr  Offline
Joined: Nov 2005
Posts: 790
Chicago, Il.
Did you ever get back to troubleshooting the linear supply?

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