Rebuilding The Elektor 400W Power Supply: Introduction

elektor-title-pictureEver since Elektor Electronics published the 400-Watt Laboratory Power Supply in October/November 1990, I have always had a fascination with the circuit and a desire to build it.  These magazines were one of the first Elektor issues I received as a kid and this design became one of the first complete circuits that I analyzed and (mostly) understood on my own.

The design is composed of two 0-40V, 0-5A, 200W isolated power sections that can be used independently, in series (0-80V) or in parallel (0-10A).  It always seemed like a nice, versatile power supply and the do-it-yourself nature was always alluring.  Unfortunately, I never bought the PCBs when they were available, not to mention that many of the semiconductors are no longer available.


In college, I started to breadboard the transformer pre-regulator.  However, this is the one part I never really fully understood and did not get to the point of applying power to it.  Recently, my interest was rekindled in this project so I pulled out the old pre-regulator board and started playing with it.  Nathan and I powered it up one day, and to our shock and amazement, it worked!  After a little more poking around, I think I can finally say that I understand its operation (more on that later).

The Design

The circuit is quite simple and elegant.  At its core, the power supply is a linear regulator with NPN output followers.  However, to minimize power dissipation in the NPN pass elements, the input voltage is regulated to be about 10V higher than the output at all times.  This means the transistors will dissipate at most 50W, regardless of output voltage.  This is accomplished using a pre-regulator on the line (high-voltage) side of the main transformer.  Until recently, I thought this design was proprietary and unique to this Elektor project, but I recently found an application note from SGS-Thomson that shows an almost identical circuit.  In a simple sense, the circuit is like a light dimmer, but for inductive type loads.  Feedback through an optoisolator completes the loop that keeps the voltage across the NPN pass devices around 10V.

The regulator itself consists of 2 feedback loops, one controlling the voltage and the other limiting the current.  The most restrictive of the two control loops dominates and takes over control.  Headers are also included to connect the two power supplies and allow them to track each other.  It’s been years since I untangled that portion of the circuit, so I won’t go into detail here.  The user interface is quite basic, consisting of analog potentiometers to set the voltage and current and panel meters to read the voltage and current.

Although the design seems pretty solid, there are a few places where some modern electronics could improve things.  Over the next few months (or as time permits) I will be prototyping and redesigning parts of the circuit.  My goal is to keep the same basic topology but integrate a more modern user interface, including digital set-points, an integrated ADC subsystem, and possibly networking via USB, Ethernet, and/or MRBus.  A new, power factor corrected front-end might also be investigated to replace the dimmer pre-regulator, but that will come later.

Next: Pre-Regulator


  1. I have actually started that project when I read it in Elektor in 1990. by buying components. I had a long pause and then, few years ago I made PCB’s (had to draw them, Elektor didn’t have in archives any more!!!). Latter I bought case and last year transformers. Now it’s time to finish it! I draw front and back panel so I can get it laser cut and now I am trying to adopt Sure displays to it.
    And all that in original schematics and components!

  2. Hi. I am building this source of power. But the output voltage does not exceed 30v. Can someone tell me how many volts is the primary transformer ?. Is it 22 or 44 ?. Thank you so much.

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