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Thursday, 16 November 2017

Another PSU!


I've been fiddling today and have made myself another bench PSU.

I was looking for a 28V supply the other day, so I've knocked up something to give me a 28V, 12V and variable output.

The 28V line is a simple LM317 - there are plenty of calculators for that voltage regulator on line.

The 12V line is simply a 7812 regulator, I think I need to add a big heatsync through because the power dissipation at 500 mA will be 22 - 12 = 10V * 0.5 = 5W so its going to run quite hot.

The variable supply gives an adjustable output between about 2.5V and 15V; it's based on my favorite the old lm723:

The first pass transistor is a BD139 and then the main one is a TIP3055.

The current limiting is dictated by the voltage between pins 2 and 3 so we have a 0.1R resistor there on the main high current path, the voltage difference needs to be 0.6V to initiate the current limit so using ohms law we can easily calculate that the current limiting will happen at 0.6V/0.1R = 6A.

There's a really nice toroidal transformer which will deliver plenty of current.

It's all a bit of a bodge:

The case looks OK:

It's not bad for a few hours work. The bench looks like a bomb has hit it:

good, egh?


  1. LM723, I have been seeing it since my introduction to electronics in 1968 or so. of late i try to use LT1083, a Low drop device that could offer 7amps with minimumm in/ out differential.

    In your 723 design for 12V derivation, With extra filtering, perhaps 12V could be managed fby a smps buck pcb. They are cheap enough. I suppose it helps to overcome large heatsinks.
    We can have extra filter to remove the rippleas pwerhaps you may need it for RF ring or some such. If it is FM even that could be avoided.

    the best
    Advance greetings for the oncoming Christmas season.


    1. Thanks Sarma, always good to hear from you. Mark