Zener Pre-Regulator Build-test

The magic smoke appears but fails to ruin the day!

So after a couple of minutes getting all the bits on the breadboard and blowing the breaker once through my own idiocy, I managed to get the thing together. The results are, well, exactly as expected. In fact, I got a steady 30.0V with no load. I was expecting around 29.3 due to drop-out from the transistor but it makes zero difference.

28 and something volts while pulling 1.3A, not bad. The value kept climbing also.

28 and something volts while pulling 1.3A, not bad. The value kept climbing also.

I left it on, and let it run and no problems whatsoever.

I decided to try a dummy load to see if it catches on fire or melts or something. The only low-value high power resistor I had that wasn’t 1Ω or less was a 22Ω 10W one so I figure I’d give it a try. It ran and did it’s thing for a bit until I saw smoke escape and quickly shut it down. A few more (very careful) power up tests revealed it was the resistor that was smoking! A quick calculation revealed that it was dissipating on the order of 40.9W! Yeeouch that’s hot! I recorded a temperature of 150°C on the thing.

Close up of my pre-regulator

Close up of my pre-regulator

The other parts faired well, the zener and biasing resistor held up fine, though the darlington transistor heated up like crazy. Still not in danger of melting it. It’s designed to run up to 150°C and needless to say, I will have a giant heat sink on it in addition to forced-air cooling.

I count this a success.

Room for improvement

Though it was easy to see that it “worked”, what’s more difficult to tell was how it works over time without that 22Ω resistor becoming lava on my breadboard. Because of this, I was only able to keep it powered up for less than a minute at a time while I feverishly took measurements. I would have ideally liked to be able to safely set the load and forget about it melting on me while I conduct voltage, current, and temperature measurements over time.

It is apparent, I neat an adjustable dummy load. It is the only way I can accurately test and calibrate my power supply without melting anything. Fortunately, Dave Jones did a video on it and I hope to build one soon myself. Valuable piece of kit that. Dave, as usual, is a life-saver.

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  1. […] my testing of the pre-regulator, it quickly became obvious that stocking a whole bunch of high power resistors just isn’t […]