Tag Archive for dummy load

Panel Pooper

Institutionalized Dumpster Dive

Over the weekend, I had occasion (in other words I made an occasion) to visit A1 parts and surplus in Etobicoke. I have mentioned it before as a candyland for the junk enthusiast and this is essentially correct. Despite its remote location, it is well worth the visit, just be prepared to dig and expect no help from the rather cantankerous guy at the counter. Total comic book guy for the electronics store which I find endearing rather than offensive. Anyway, if you ever end up there, be prepared to wade through tons of unsorted junk, find a few gems, and score a few surplus deals.

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Lipstick on a Dummy

Or how I just love to overcomplicate things

Lights, buttons knobs and dials have always fascinated me. In fact, it says so in my baby book pretty much exactly. I think that was genesis of my love for technology, electronics, sci-fi and general science and nerdom. After all, what could be cooker then “techy shit” as gmunk puts it?

With every project I do, I tend to dream up ways of making the interface and functionality as flexible (read: complicated) as possible. I want to feel like I’m flying the frickin’ Enterprise when I do my thing.

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Dummy load acquires stability

Just a quick test and a quick solution

Had a few minutes to just check up on why the load would change when the voltage does. A simple test of trying a higher voltage power supply seemed to do the trick. Stayed within 10mA of where it should, and probably did better than that, but my multimeter can’t measure that. The new power supply is a 200mA 12V wall wort which is very unregulated. I measure 16V off the bugger with no load. Makes me wonder if what is printed on the box is merely a suggestion. Still, nice to have the headroom. Now the opamps have enough swing to really kick that MOSFET into regulating the current flow reliably.

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Dummy load get’s a tune up and another test

Improvements and fine tuning

As mentioned in my last post on the dummy load, though I am very pleased with the results, there is always room for some fine tuning and improvement. Also, some parts needed to be bought. First off, here’s a revised version of the schematic:

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Encouraging results

Some quick tests work out quite well

Had a tiny pocket of time with which to test a few things with my aforementioned dummy load I just build. In looking through my collection of junked wall wort power supplies I found a low power one, a 9V 210mA. I wanted to check and see if the voltage headroom for the opamp would be worse. As it turns out, the stated 9V is actually 11.3ishV but whatever. I knew the supply would only need to deliver a few milliamps so I ventured to see how much exactly.

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Constructive Feedback

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-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)


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.


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.


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.


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.

Caveman Dummy Load

Okay, so I thought I’d start with the fun post :) . For the truly nerdly, electronics is always entertaining, even if you can’t directly see what’s going on. For everyone, however, the fun bit comes in with a bit of mad science. We want fireworks, smoke, sparks, and flames. So long as we don’t get hurt our burn our house down, we can enjoy a bit of drama and cackle evilly as we dump 3A through something and feel the power coursing through it.

I mentioned in the last post about the need for an immediate, low-tech dummy load that doesn’t involve me sourcing parts, puzzling over schematics, waiting for online deliveries, or rushing to the shops to grab that one part I’m missing (I’m always missing one!). With that in mind I had a problem with my power supply project. I’m busy designing away the various modules, simulating what would happen and punching the math to make sure it doesn’t blow up on me. I need to test the damn thing as I build and although I can simply check the output voltage of each stage I prototype up, it doesn’t tell me anything of what I will do when i put a load on it. Will it be stable? Will it melt into a pool of toxic goo? The only way to know for sure is to find out. I take a calculated risk as everyone does quite literally by simulating my circuits but it doesn’t take into account the real world of component tolerance and the million weird and wonderful little variables that are assumed not to exist in mathematical simulations. Besides, it’s more fun to build stuff up and see it in action :) .

My inquisitive searches popped up a number of interesting ideas that don’t involve me making yet another complex project like the electronic dummy load I want to build. I needed something simpler and nothing can be simpler in electronics than a resistor. I mentioned in my last status post that the logical dummy load is a resistor, but of course finding one accurate enough that can dissipate the required power without melting has been challenging. Most power resistors have really wide tolerances, are bloody expensive, and not so easy to find. Keeping a stock of all the required values would then be a challenge even if I could find the right ones. So, like any enterprising maker, I squared my shoulders and proclaimed “I shall make my own!”.

What is a resistor anyway

First, I needed to examine what a resistor is before I could actually build one. Put simply, it is nothing more than a length of wire trimmed to a known resistance. All wires have resistance although it’s usually assumed to be negligible and with good reason – it usually is. This is because the highly conductive copper we usually use for wires are designed to have very low resistance so we get our signals through and don’t waste so much power in our projects. A wire’s resistance is a function of it’s material’s resistivity (a constant that is different for every conductive metal, copper is 1.7 x 10^-8 Ω m by the way) multiplied by it’s length, and all divided by the cross-sectional area of the conductor in question. So it’s easy to see from that statement that the length of the conductor increases the resistance, and the cross-sectional area decreases it. So the thicker the conductor, the lower the resistance, the longer the the conductor the higher.

It seems simple now, doesn’t it? just make it thin and cut it to the desired resistance, done. One problem – the current it’s resisting has to go somewhere, the laws of physics prevent energy from just ceasing to exist. As expected, it’s dissipated as heat. With enough current flowing through it that could be disastrous. Not only would it heat anything touching it, but could melt itself making is a very dangerous thing from both an electrical and “burn down your house” standpoint.

Copper, having such a low resistivity, is hardly ideal to use for a resistor. I would need many kilometres of the stuff to get what I want and/or have it so fine a gauge that it would melt from the current I’m intending to dump through it. As I contemplate this, I toot on my ecig and it hits me – the kanthal wire in the coil could work perfectly. I check a packet of the coil wire I have and sure enough it says clearly 18Ω/m. Perfect.

I laugh to think – all a resistor is a heater.

The build

Seems simple enough, I need a precise 10Ω resistor, I have a meter of kanthal which I know is 18Ω/m, and I know it will take a few amps dumped through it since I do this many times a day with my ecig. All I have to do is cut it and test, cut it and test until I hit 10Ω. Then I can mount it on… what to mount it on… oh jeez. Here I have a length of wire, that’s going to get really hot if I dump more than an amp through it (I intend to dump three) and I have to have some way of holding it down. A live wire with enough current to kill me and enough heat to burn me (and anything around it) very badly is never a great combination. I need something non-conductive yet heat resistant. I look about and grab a pencil, maybe this could work?


As you can see above it did work after a fashion. It measured 10Ω, I was able to put lower currents (<1A) through it without a problem. The above happened when I tried 1.33A from my little power supply and the pencil predictably started to burn. Yeah, that was a dumb idea. Also, my house now reeks of burning pencil.

I rummaged about for anything else I could find that might hold this hot potato and came up with nothing. I was hoping some sort of ceramic something might be floating about somewhere but no dice. In rummaging around I found a part that a friend gave me ages ago. It was a piece of a heater from an old car which is nothing more than a piece of plastic, some sort of heat resistant material and some brass strips. The nichrome heating coils were still on it so I cut them off and fitted on my coiled kanthal wire. Bingo, I had a solution.

In action

With my homemade resistor somewhat safely mounted in something that probably wouldn’t fly apart and melt, it was time for the real deal. Time to use it as my test load. I chose 10Ω as my intention was to test my newly prototyped pre-regulator with it and powers of ten make calculations really easy to do in my head. 30V over 10Ω is 3A or my target max current for my power supply. I will detail the results of the pre-regulator test in a following post about it specifically but the short of it is IT WORKS. Gwahahaha

Pretty isn’t it? that’s 3A of current running through it thereabouts and 30V across it. It was a lovely glow and did indeed give off a lot of heat. My chilly basement lab was quite cozy :) . The pre-regulator played nice too and gave me more or less expected results



It just goes to show that even complex problems can still have caveman DIY solutions in this modern age. I built me a dummy load with no controls, no readouts, no fuss, and no real expense. I will still build my electronic dummy load of course, since I do need a constant current sink that is finely adjustable for more accurate measurements and to calibrate my power supply.

Dummy loads

The need to build everything to have something to calibrate against

In my testing of the pre-regulator, it quickly became obvious that stocking a whole bunch of high power resistors just isn’t feasible. In order for that to work, I would have to have on hand a variety of high power resistors to suit various voltages and currents. In addition to that being a pain, it’s also very hard (and expensive) to find accurate resistors at high power and having to acquire a collection of them is just silly. Since my power supply must be capable of delivering up to 3A at 24Vdc, that means a potential power dissipation of 72 watts! Any resistor I find for that specification (which would be 8Ω) would essentially be a giant heating element and probably have a wide tolerance.

The best solution is to build yet-another piece of test gear: the dummy load. I mentioned before the ever-great Dave Jones of EEVBlog fame has a nice quick video on how to build one. Though this appears to work great, the specifications are a bit wimpy for what I need for this application, and limits possible future applications as well. Dave’s design seems to max out at about 1.335A and I would need more than double that to test my power supply. It also marks the re-appearance of another hassle which has plagued me in current measurements, and that’s finding an adequate shunt resistor(s). Dave’s design uses 10x 10Ω 1% resistors which certainly keeps the math easy, but I’m finding it really hard to find any of them at my local electronics shops! I’ve found 1Ω ones, 0.1Ω ones, but no 10Ω ones to parallel up to make 1Ω. Bummer. This is a job for DigiKey for sure.

Higher power design

Naturally, needing something with a bit beefier spec, I hunt through google looking for similar but higher power capability. I found a really nice one done by Paul Renato and he’s used a similar design to Dave’s but has improved on it quite a bit. For one, he’s upped the current sinking capacity to 7A (I had arbitrarily picked 5A for my needs so this is ample). He’s also made use of the two unused opamps in the LM324 to provide some overload and thermal protection. There are a couple of things I’m not clear on, namely how to select the proper MOV, thermistor, and schottky diode from his schematic. I may have to write him on it. Beauty of his design is he’s already gone to the trouble to whack in as much functionality as possible, which is usually something I do when presented with a project schematic. I can’t think of anything I would add to this one. Brilliant.

Here’s the full article and schematic

So yeah, more parts to acquire and more projects to build. This one definitely looks like a winner and not terribly complicated either. I need to go shopping for MOSFETs and some other choice bits. Ah damn those current shunts! I’ll get those too finally.


The diode close to the power supply appears to be a zener, not a schottky (got me symbols mixed), though why that’s there is a bit of a mystery to me. It could be regulating the 12V input since his photos show him using a wall wart to power the thing. It’s the only imaginable purpose I can come up with. The 10:1 voltage divider used to set the current seems to me to have a max of 1.2V in this case (setting 12A) though he indicates 0.7V (7A) should be max. I assume the overload circuitry is designed to catch the over-current, but I may just tweak the input to be a little friendlier as well as substituting the zener with a proper voltage regulator.