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.
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.