Mad Teddy's incredible old power supply

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My incredible old power supply

My Dad was a civil engineer, but he was also a qualified electrician, having trained while in the British Royal Air Force during WW2. He had a hobby interest in things electrical - which is presumably where I get my own predisposition to such things.

I started to become interested late in primary school. I'd see my Dad tinkering with some intriguing-looking thing or other, and want to know how it worked. So we developed a shared interest.

Later, in high-school, when I found some interesting project books in the school library, he helped me to build some of them. Of course, batteries don't last long; hence the need for a power supply. So he built one!

Here it is. (You can click on it to see a bigger version.)

It is shown set to its maximum voltage (approximately 20V AC), switched on with no load.

It plugs into the 240V mains. There is a small torch globe mounted behind a green transparent cover to indicate power-on. Above the voltmeter are two terminals, between which is connected a piece of 5-amp fuse wire. The lower meter is a 5-amp FSD (full-scale deflection) ammeter.

You may have noticed a strange little symbol on each of the meter faces. It's a stylized "JLW", my Dad's initials, which he liked to put on things he'd modified in some way, as a little personal touch.

The control on the left is a five-position rotary switch. The settings are 1V, 7V, 14V, 17V, and 20V (approximately). The control on the right is a rheostat - a wire-wound forerunner to the modern potentiometer.

When I was young, I didn't understand the intricacies of how the controls worked. I would use the rheostat to achieve a fine-control on the output voltage, as set by the rotary switch. One day, when my Dad caught me with the rheostat turned right down (anticlockwise), he nearly freaked out. As a result of this, I came to vaguely understand that the rheostat could get HOT, and possibly burn out, at high-current levels. Oh dear! Luckily, no harm done. My electrical knowledge is at a much higher level now, of course, and I treat the old thing with a great deal more respect! (In fact, I hardly ever touch the rheostat at all - I normally just leave it turned fully clockwise.)

To be honest, I don't know the exact details of the circuit. A while ago, I decided it was high time I did; so I carefully unscrewed the front panel and tried to get a look inside. However, I reluctantly gave up on the idea; I was afraid that disturbing the forty-odd-year-old wiring might cause serious damage. Without learning much at all, I replaced the cover and decided to leave well enough alone. If it ever stops working, I'll bite the bullet then.

The power-supply provides AC only - it does not have a built-in rectification system. A few years ago, when I wanted a DC supply, I built a stand-alone full-wave bridge rectifier from parts of an old car alternator that somebody had junked:

AC from the power supply is input at the left (yellow terminals); DC is output at the right (red = positive; black = negative). Smoothing is achieved very simply by a 2200 microfarad, 50V capactior across the output terminals.

Here is a close-up of the business part of the device (love the blobby soldering!).

Alternator armatures have three windings, to produce three-phase AC output. To rectify this to DC, which the car needs, there are six strongly-built diodes included. The benefit of three-phase rectified output is that it is quite smooth. For more information on this and related matters, see this link:


I'm not an expert on alternators. I do know that, in addition to the six "on-board" diodes, at least some alternators also include three "bead"-type diodes.

My home-made bridge rectifier consists of three "on-board" diodes, one of which (on the extreme right) is no good (which is presumably why the alternator was junked in the first place), and two "bead"-type diodes, which you can see in the lower-central part of the picture. The "on board" diodes are concealed beneath the dark brown (Bakelite or similar) insulating plate; you can only see the top connections to them. The four good diodes are soldered together in the standard bridge configuration.

Theoretically, a smoothed full-wave rectifier like this provides a DC voltage which is nominally equal to the input AC voltage times the square root of 2 (~ 1.414). So I should be able to get about 28V DC out of this. In fact, it provides about 27V DC, which is not too bad at all.

So: that's the power supply that you may see here and there in photographs of various projects in this site.

I'm well aware of how lucky I am to own such a thing. Modern power supplies cost quite a lot; and from what I've seen of them in catalogues, many of them don't appear to be as robust as my old clunker. Few, if any, claim to be able to provide 27V DC at 5 amps without having a seizure!

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UPDATE, Sunday, 1st March 2009

I have a sad story to tell.

A while back, I accidentally shorted the output leads from my old bridge rectifier. I saw some smoke emanating from its right-hand bead diode...


... so I bought a commercial bridge rectifier and made up the unit you see here.

It works just as well as the old one. Some day, I'll attempt a repair job on the old one (I have a couple of spare bead diodes from another old alternator ). In the meantime, the new one fulfils the function admirably.


My Dad died in 1981, at the age of 57, from cancer. I was 28 years old at the time.

We didn't always get on too well. There was a classic "generation gap"; he had grown up during the depression years, and I grew up in the 1960's. It was a recipe for anguish, all over the western world.

It's possible, if one is not careful, to dwell on the negatives too much. At this time in my life, I think I've dealt reasonably well with the main issues, and am now able to see the whole era in its historical perspective. Sometimes, of course, I still feel flashes of resentment.

But I've come to realize that it wasn't all bad. As I've moved into my middle years, and to some extent returned to my roots, I can see that there were some good aspects to our relationship.

He taught me to play chess. He helped foster my interest in science and technology - even though, I'm sure, he was sometimes exasperated with my inability to grasp things which, to him, had become so familiar as to be obvious. On a few occasions, as I moved into young adulthood, he helped me to deal with stupid bureaucrats. I think we had - sort of - made a kind of peace before his death.

So, as I prepare this website, it's only right that I acknowledge a debt. If it hadn't been for my Dad's good influence, the website probably would never have existed.

Saturday, 11th October 2013

Did you click on that "roots" link, above? If you didn't, please do so now. For as long as it lasts, it's a YouTube video of Dusty Springfield's lovely version of the Carole King and Gerry Goffin song "Goin' Back".

Just a few of the lyrics:

...But thinking young and growing older is no sin,
And I can play the game of life to win...

... And every day can be my magic carpet ride,
And I can play hide and seek with my fears -
And live my days, instead of counting my years.

Let everyone debate the true reality;
I'd rather see the world the way it used to be.
A little bit of freedom's all we lack,
So catch me if you can - I'm goin' back...

While you're about it - do make a point of clicking on the previous link ("anguish") which will take you to another memorable song on YouTube, also highly relevant in the context.

Okay, you can put your hanky away now! Back to business!

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