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Home built bench power supply

M31b-front.jpg
This adjustable power supply is a basic μA723 application. I built it in 1982 and refurbished and improved it in 2004.
This home-built bench power supply can be adjusted from 4-25 V and it can deliver 5 A. It has two large meters for output voltage and current. The + and - output receptacles are floating with respect to the case. A third receptacle is connected to the mains ground and metal case. Normally, it is connected to the “-”, but it can also be connected to the plus or left floating, although the voltage between any output and ground should not exceed 100 V.

    A look from the back, showing the bulky double heat sink.

A look from the back, showing the bulky double heat sink.

The circuit is based upon the μA723, a basic power regulator IC from the early 1970-ies. I boosted it with 4 2N3055 power transistors, mounted on 2 heat sinks piggybacked at the back to get a robust and reliable power supply that can deliver a lot of current without running too hot. The transformer is a 24V, 150 VA halogen transformer.

I built this device in 1982. At the time, I had access to a small metal workshop and I could made the chassis and case from sheet aluminium. The 1982 version had a single meter that could be switched between voltage and current. Originally, I built the control circuitry on a small piece of perfboard, the emitter resistors and driving transistor on a tagstrip, electrolytics fixed to the bottom.


    A look inside.

A look inside.

When I rebuilt the power supply in 2004, it hadn't been used for a while. The control pot was worn or dirty so the output voltage was jittery. I wanted to make some improvements.

I decided to give it two separate meters instead of a single one. I customised the dials by using letter transfers. It required some redesign of the front to accomodate two meters and three receptacles, but everything found a logical place. No place was left for a max-current control, but I didn't need that for this power supply. I made a new aluminium front plate to mask the large hole in the old front. Rubbed some letter transfers on it and covered it with a transparent coat. Instead of perfboard and mounting strips, I used a circuit board from an old 723 based computer power supply that could also hold the electrolytics. That looked a lot neater, though the circuitry was practically unchanged. Instead of passing the mains cord through a hole in the chassis, I used an IEC mains receptacle.


    Standing on the workbench while the meters are callibrated.

Standing on the workbench while the meters are callibrated.

This resulted in a robust and very usable power supply, with a single disadvantage, that it cannot be set to 0 V. But I have other power supplies that can.

Copyright © 2008 by Onno's E-page         published 2008-09-05, last updated 2015-03-08