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.
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.
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.