Tag Archive for darlington transistor

Zener Pre-Regular Revisit

In revisiting my power supply project I also revisited a number of unanswered questions. Chief among them is how on earth do I get rid of the excess voltage from the AC rectification? Previously, I had mentioned this was somewhat of a shock to me as a novice that a 30V AC tap from a transformer can gain 12.6V in the rectification to DC. I know part of that is the combining of the AC waveforms and the bumping up by the mammoth amount of capacitance I have on it to smooth it. I had originally tried some very dodgy and very ugly collection of series power diodes which plain just would not work. They were not only ugly, but ridiculously unsafe and would prevent the proper operation of the circuit at low currents and would be unreliable at high currents. Scrap that. I’ll consign that to the embarrassing fail bin.

The next idea was to pre-regulate the voltage down to a safe level for the downstream regulators and prevent unnecessary power dissipation. I chronicled before some shaky success but discarded that idea after it proved somewhat lacking and prone to smoke. I also thought that now three darlington transistors in the signal path was somehow wrong, there was something about it I didn’t like for some reason.

In poking about again with fresh eyes and a clear mind I decided once again to make a zener pre-regulator, having it control the base of a darlington transistor to set the output voltage to just shy of 30V. Most of the original concept stayed the same with a few little modifications to make it safer and include the proper ratings of components as well as ensuring that no datasheet “Absolute Maximum Ratings” were being flirted with. Schematic below. (please note the caveats at the bottom of this post)

pre-regulator-test-circuit

As I had been down this road before, I was tickled to discover that I already had everything I needed in my parts bins and with my 10Ω home made power resistor just completed I set to marrying it all together. Having only a few parts it was rather trivial to assemble it.

I test powered up the AC board as I had not touched it in a year and I got that delightful hum and that crazy 85.2V reading between the positive and negative rails. I kind of freaked at that moment, not only because 85.2 is a lot of volts but I realized I really need to be extra safe with this thing. Also I forgot that it was centre-tapped and i just measured the negative lead (-42.6V) unnecessarily. I cut the power to it and noticed my multimeter barely dipped. I realized that the 10 milliFarads of capacitance I had on the thing to smooth the power is not only extremely dangerous when charged, but would probably take a decade to discharge though the multimeter’s very high input impedance. Rather than touch the positive and negative wires together to discharge the caps instantly (which would have resulted in a very big and dangerous bang) I carefully placed them on a 30Ω power resistor I had to drain the caps quickly and gracefully.

This is why you are always told to never touch capacitors when opening up equipment as they could be charged still. They must be discharged. Smart is using a low value power resistor to “bleed” them dry of charge. Stupid is shorting the terminals with a screwdriver. For safety, I will include such a resistor – a “bleeder resistor” – to discharge the caps when it is switched off.

Anyway, with the AC board working great, it was time to hook up the latest candidate for a pre-regulator and try it with some loads.

The setup. From left to right: my 10Ω home made dummy load resistor, the zener pre-regulator, and the AC board

The setup. From left to right: my 10Ω home made dummy load resistor, the zener pre-regulator, and the AC board

It worked sort of fine though the numbers were of course somewhat off from my simulated circuit. For one thing the 5W 30V zener I was using led to a regulated voltage of 32ish volts which was higher than I wanted it to be. For my stuff to work well I needed it about 26-29V. I needed enough headroom for the eventual voltage regulator to make a nice steady 24V yet as low as possible to reduce the power it will dissipate due to the voltage differential. On a whim I whacked in the 1W 30V zener I had and behold – I got 28-29V. Perfect. Just what I wanted.

I tried a variety of loads including: a 1k resistor, 100Ω power resistor, 30Ω power resistor, and yes – my monster of a 10Ω resistor pictured here in glowing glory as it dissipates something like 90W of power.

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Overall I would call it a success with some caveats. I did notice a change in voltage depending on the load I was putting across it. This is not a huge deal as I do not need it to be an accurate voltage regulator, but i do need it to stay under 30V and above 26V, preferably with a bit of padding, no matter what load i draw from it. In the schematic above I added some capacitance to hopefully smooth it up a bit and keep it a bit more stable. I will test this tonight in the lab. I did get the disturbingly low reading of 25.6V (ignore my stupid multimeter it sometimes forgets decimal points) which will definitely need investigation as this is below my absolute minimum of 26V.

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Another problem, that I just noticed in fixing up the schematic to post on here, is I probably used the wrong transistor. On it, and from examples I had used to design it, I indicate an NPN darlington to be used and I had probably mistakenly used a PNP one. This worked just fine but I might investigate while i’m down there to see if indeed I did indeed use the TIP147 instead of the TIP142 and what, if any, effect swapping them would do.

Update

Well I just took a poke on the bench and I was indeed using the TIP142 NPN darlington like I was supposed to. I still need to investigate why the voltage dipped and if I can repeat that and take some careful measurements. I understand how to use the darlington as a current regulator, and the dip in voltage would suggest it’s limiting the current (which I do not want it to do at this stage). It makes a basic sort of sense by the 30V zener would net roughly 30V on the output (I guess) but I need to know the why and specifically the calculations involved. CircuitLab showed me that I would get around about 30V regardless of current draw, why this real-life dip I haven’t a clue – yet. I’ll try and repeat the experiment and isolate the conditions under which the voltage dips. I’ll try various other loads too to see if it goes outside the usable window. More to come.

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.