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The Zener pre-regulator returns, and is improved

So nice to have finally set up my workbench again and I’m a flurry of pliers and screwdrivers. Following up on a semi-meh-kinda success (but sort-of fail) is a resounding success! Just what the engineer ordered.

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Zener Pre-regulator further testing

Finally I was able to do some electronics shopping and got the few (trivial) parts that I needed. They weren’t even fancy, just some jumbo crocodile clips and a few resistors and miscellaneous parts. Took me only a month!

Anyway, as I returned to my bench, I noted my pre-regulator already set up and remembered I needed to run some tests. Last time I had worried I had used a PNP instead of NPN darlington transistor by mistake and found that I was indeed using NPN and it was working just fine. I also noted some disturbing voltage readings when heavily loading it.

In the last post, the pre-regulator worked just fine with a bit of voltage variation over the range of loads I could be using. If using a high resistive load, the voltage could climb too high above 30V, dangerously increasing the voltage differential the poor LM317 has to deal with (which could cause an overload even with a pass transistor taking most of it), or too low when using a low resistance load (higher current) dipping below my absolute minimum required of 26V.

The maximum user selectable voltage output of my power supply will be 24V (actually 48V as it is ±24V and there is a negative mirror of the positive side, we’re just dealing with one side for now) so the LM317 needs at least 2V of dropout (or headroom) to output a good clean 24V which is how I arrive at 26V. Of course, electronics being electronics, I know better than to call it a day once I get 26.0V, a bit of padding needs adding.

I had previously measured 25.something volts when using a ~3A load (my home made resistor) which is below the minimum I need. Most frustrating. I postulated a bit of capacitance could beef it up so I gave it a go and tried a bunch of caps before settling on 100µF which gave me a nice 27.4V which is just plain ideal! Problem solved there. For now. I checked with higher resistance loads (up to 10kΩ) and found it gave me an output of 30.4V which is still acceptable to me.

Of course, this is only one piece of the puzzle. I’m going to have to check again to see how well it fares with the current limiter, then voltage regulator, then load after that. It is more than likely some tweaking will be necessary to keep it all nice and stable and happy.

Other measurements

Of course, just getting the right voltage and current out isn’t the whole story. My DMM updates kind of slowly, and at the 200V range I have to use, it gives me only one decimal point to work with below 1V so it’s hard to see quick or small variations in voltage over time. Naturally, I fired up my ancient oscilloscope to take a peek at what it’s actually outputting. To my understanding, no power supply is perfect, and some ripple is always present. What is acceptable ripple I have no idea but I always assume the less the better. Considering a 30V output, the unfiltered ripple would be 30V which is insane. Thanks to the huge filter caps I have on the thing I can reduce that a fair deal. This is the first time I tried to measure the ripply on my power supply.

powerripple

The result seems to be approximately 6mV in a curve that screams to me capacitor which is not surprising. A quick google search turns up a ripple value for your average computer PSU of 120mV on the 12V line which is twenty times what I measured from my home made power supply! Having measured my own hacked ATX PSU it was also noisy as hell, which I hear is endemic to switch-mode supplies. Sure I had my 6mV of ripple, but at least it was clean ripple! A nice clean waveform. It seems I’m on the right path. More measurements to come later as I add other modules. The later stages and in particular the voltage regulator *should* show even less ripple. I remember a section on the LM317 on ripple rejection…

I will also probably double check my results. I do remember I saw a Fundamentals Friday episode of Dave Jone’s excellent EEV blog on the very subject of measuring ripple and noise of a power supply, found here.

Another consideration was, with how hard I’m driving my transistors by pulling 3A through them, was heat. I have zero idea on how to calculate how big a heat sink I need for a particular application and naturally assume the bigger the better. I’m sure Dave Jones has something on that also come to think of it. Time to get back to class. In the meantime, because I hate leaving the bench when I can melt things straight away, I used my indefatigable logic to conclude that I will drive it as hard as I plan to run it and take a bunch of measurements. Then I will plan for at least 20% margin above that.

I used the thermocouple that came with my multimeter to measure the case temperature of the TIP142 transistor and ran it through a 3A load. The temperature climbed steadily after a few minutes to about 120°C which is bloody hot! I’m uncertain whether or not it would have climbed higher than that given time, but it was climbing very very slowly at that point. 120°C is exactly 80% of the TIP142′s max junction temperature of 150°C which sounds just fine but I’m not really satisfied. If I left the thing running for three hours would it approach or exceed 150°C? I have no idea. Better over-safe than melted and sorry.

I will definitely be using a larger heat sink for all six pass transistors used in my design, and of course there will also be forced-air cooling as well. Pretty much all semiconductors tend to start acting badly as they approach their maximum rated temperatures so it’s a good idea to keep them as cool as possible. Easily done.

If you are a budding hobbyist reading this I recommend building your own power supply, you’ll learn a hell of a lot in the process.

Update

Have a good look at the pic of my oscilloscope reading above. Note that it is set to 2ms/division. So each of those squares on the x-axis is 2ms. Note that there are four (close enough) divisions for each wave. 4 divisions at 2ms/div is 8ms. Now think of this. The AC cycle in our houses, which I have converted to DC in my power supply, is 60Hz or oscillating 60 times per second. Because I use a full-wave bridge rectifier, that frequency doubles because it inverts the negative voltage to positive voltage so it oscillates at 120Hz. So what is the wavelength? 1 second / 120 times a second is 0.0083333ms. So it will take 8 and 1/3 ms for each wave which is exactly what you see on my ‘scope :) .

Update part two

To give you a comparison, in the video linked above about power supply ripple and noise, Dave Jones compares the data sheets for two power supplies, one a high current switchmode, the other the Rigol DP832 commercial bench supply. The former had a 10mV rms ripple rating, and the latter had a 2mV peak-peak rating. If i am reading my oscilloscope correctly, I am showing 6mV peak to peak to I am actually very happy with that result. When I have more of it built, I will have to of course test it under a wide variety of loads and get a more accurate picture of the ripple coming out of it but it is encouraging.

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.

IMG_0558
IMG_0559
IMG_0563

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.

IMG_0565

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.

Zener pre-regulator

The need to drop some volts

I mentioned in a couple of previous posts that my giant toroid transformer that I want to use for my power supply was a little too beefy for my purposes. I mentioned in the most recent blog post that transformers are often rated at just below normal mains voltage to provide a “guaranteed minimum” and that once you combine this fact with your rectifier and filter, you actually end up with a higher voltage than you originally intended. Most of the time, this is great for ensuring you can overcome that pesky dropout voltage (around about 2.5V for most regulators to be on the safe side) but in some cases, it can get dangerously close to frying said regulators by exceeding the max input voltage (maximum voltage differential).

In my case, the chunky toroid, after rectification and filtering with no load gives me a rather beefy 42.6V when on paper it should have been 30V. Given that most common regulators have a maximum input voltage of 35-38V, it becomes obvious that I had to step it down a tad. There were a few options to do this:

  • Unwind the secondary of the transformer a few winds
  • Use series diodes to drop the voltage by 0.7V each
  • Construct a pre-regulator to drop the voltage to safe levels

I have examined each of these in turn and came to to the following conclusions:

Thing of beauty, don't want to wreck it

Thing of beauty, don’t want to wreck it

The toroid is a lovingly packaged beastie, professionally wound in crazy spirals, wrapped in plastic and nicely presented with leads. Since I barely know what I’m doing, it would be unwise in the extreme to mess with it. Why break something to make it work when other solutions can work just as well?

Isn't that gross looking? Embarrassed to say I made that...

Isn’t that gross looking? Embarrassed to say I made that…

I did try the series diodes, and was able to get a voltage drop of 4.2V by bodge soldering 6 of them in series. As you can see from the photo, there are quite ugly and apart from appearances do not reassure me that they will hold together well and 4.2V is not nearly enough of a drop to be useful to me. Essentially, I made a mess of 6 diodes to only get down to 38.4V, barely outside the red danger zone. Also, the legs on the high-power diodes are quite chunky and would be a severe pain in the ass to get into a pre-drilled circuit board and I will not have them floating in the air inside the case in case heat/cracked joint/whatever causes them to fall apart starting a fire. Also, when I’m long dead and someone opens the case to see what I’ve built, do I really want them to see that bodgy mess? No way man.

The third option is one I considered before and after thrashing about with the other ideas, it has been revealed to be the best one. Difficulty is, with voltage regulators having the limitation mentioned above I couldn’t use them. As mentioned in a previous post, I am going to go with a simple, yet high powered idea of using a darlington transistor biased with a zener diode. I was able to acquire some 30V zeners (part: 1N4751A) so these two, plus a couple of biasing resistors is all I need.

Here, I ran up a simulation to illustrate:

Pre-regulator test circuit

Please ignore the 2N3904/3906 transistors, they are mere placeholders for the TIP142/147 darlingtons I intend to use since CircuitLab didn’t have either in their box of parts. Only me building it up would determine whether or not I have to adjust any of these values/parts or not. I like living dangerously.

I chose a 10Ω load resistor to simulate a current output of 3A at around 30V. The actual output voltage doesn’t need to be precise as it will be further regulated later, so long as it’s above 26.5V or so it will be fine.

The tricky bit is biasing the thing. I first assumed that I could use a 10kΩ resistor to power the zener and bias the transistor. I quickly figured out that this isn’t enough, especially when under load. The zener needs a minimum of 5mA to get that nice voltage drop going on and the transistor needs a proper current to regulate the voltage.

Through fiddling about, I settled on what I think is a good compromise. My zeners are 1W so there’s one limitation I have to consider. Though I am well within the max collector/base voltage limit of 100V, the max base current is 500mA, which will be hard to come close to, but worth noting.

Through messing about with the load, I noticed that it had a hard time regulating itself at higher currents and I kept having to drop the biasing resistance to keep it working. This presented a problem of higher currents both for the base and the zener. Though the base can handle it and then some, the zeners were fast approaching their power limit.

The compromise is the 470Ω biasing resistor. It seems, at that level, to keep it’s regulation up to 3A more or less which is what I chose as my maximum current output anyway. Any higher than that my current limiter will drop the voltage to keep it from melting itself.

This also keeps the zener/base current steady at 26.84mA. Though this seems like peanuts, given the voltage drop across the zener the power dissipation would be about 800mW or 80% of it’s maximum.

Though I realize some of that current is going into the base of the darlington transistor, I really don’t want to try lowering the resistance any further. Always better to have some headroom in case Murphy and his law show up. Ohm, and his law, also indicate that the resistor will dissipate a third of a Watt (0.338 to be exact) so 1W 470Ω resistors would be nice to have. As it turns out, I have one.

If I need to get higher power parts, this is also possible. A 30V Zener diode 5W (part: 1N5363B) exists and is cheap, and i’d nead a helluva resistor to go with it, probably a 10W 100Ω. I think I can avoid doing that though.

In a few minutes, I’ll head down to the lab and build it up.