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Li'l PowerHouse Switchmode Power Supply

This highly efficient design can deliver from 1.23V to 40V at currents up to 1.2A.

By Peter Smith and Leo Simpson

Click for larger image

This New Power Supply has a big power output for its size. It can give DC voltages up to 40V and the output current can be as much as 1.2A, depending on the voltage setting. And it can be varied right down to less than 1.5V while still giving out 1.2A. This is great for testing battery circuits that operate at 1.5V, 3V or whatever.

In times past when we have looked at doing a compact power supply, the natural approach would be to produce an analog design with an analog meter on the front panel. An example of this was our dual tracking 18.5V supply published in the January 1988 issue. The analog approach has the virtue of simplicity and it gives good results.

Main Features

  • Output voltage continuously variable from 1.23V to 40V
  • Output current of 1.2A from 1.23V to 30V
  • LCD panel meter for voltage & current
  • 10-turn pot for precise voltage adjustment (optional)
  • Adjustable current limit
  • LED current overload indication
  • Output fully floating with respect to mains earth
  • Load switch
  • Low output ripple
  • Short circuit and thermal overload protection
  • Minimal heatsinking

But that’s in the past. And it is boring. Nowadays we can do a lot better with a switchmode design. It is more efficient so big heatsinks are unnecessary and you can get a higher maximum DC voltage output for a given secondary voltage from the power transformer. And you can get a lower minimum DC voltage at full current with having problems with the high power dissipation of a conventional series regulator.

In fact, this circuit will have losses of less than 10W (including transformer losses) under worst case conditions, meaning that it does not need any heatsink apart from that provided by the back panel of the case. By contrast, if we had gone to a conventional regulator using the same power transformer, it would have losses (ie, heat) of around 30W when delivering 1.5V at 1.2A and it would need a fairly substantial finned heatsink on the rear of the case.

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