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| DC dummy load circuit calibration |
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| VEGETA:
I will need time to get heatsinks from my friend, so by the time PCBs arrive I will get them together. However, I could try one mosfet + opamp (lm358) with low currents (200mA or so) just to see how stuff works. I don't have enough 1R resistors to begin with if not never since I used lots of them previously so I doubt I still have any left. For 10-turn pot, should I put a wire from its outer body to ground? or from one of the pins to ground? Also, if plastic case is used can't we get away with the same method of connecting the body of pot to ground? The case should be connected to ground? that means i wire from the ground pin or pad to the case itself. I don't know if I could solder it or so. |
| Ian.M:
Assuming the pot has a metal mounting bush, if the case is grounded, its body will be as well. If you are testing it loose on the bench, wrap a wire round the mounting bush and clamp it with the nut to ground it. To ground the case, the easiest way is via the PCB mountings if you are using screws for that. For one of the mountings, simply use an internally threaded brass spacer with a serrated spring washer between it and the case, and a Belville spring washer on top of the PCB under the screw to keep it in firm contact with the brass spacer. Use a plated through hole in the PCB with a bare tin or ENIG plated pad both sides larger than the spacer and washer diameters, and within the pad surround the hole with a ring of vias to tie the pads together even if the hole plating gets damaged. For an example look at any PC motherboard. It gets a bit harder if you are sliding the PCB into guides. You'll either need a leaf spring soldered to the PCB, or better, a ground wire with a ring terminal at the case end, bolted down with a serrated spring washer between it and the case. For testing you can improvise the resistor with a long loop of insulated thin copper wire. e.g. 2.4m of 24AWG wire should give you about 0.2 ohms. Fold it in half, and wind it on a cotton reel or similar then tape it in place to make it compact enough to work with. By folding it in half before you wind it, the winding is non-inductive. It will have a horrible temperature coefficient and won't be very accurate, but will certainly do to test the circuit. If you've got thinner wire, you'll need less length. If you know its actual diameter its easy, we can calculate it, but if not, simply rig a LM317 + a 15 ohm resistor in parallel with a 100 ohm preset as a current source, adjust to 100mA, put a long length of the wire in series and measure the voltage drop across the wire so you can calculate its resistance per meter. |
| VEGETA:
I will slide the PCB inside the case but for grounding I can connect a cable from PCB ground pad to the inside bottom of the case at a place where I scratch it with a screw driver to remove the anodizing\painting then solder the wire directly. This could be even better than putting a washer with bolt. This is my 10-turn POT: https://www.banggood.com/3Pcs-3590S-2-103L-10K-Ohm-BOURNS-Rotary-Wirewound-Precision-Potentiometer-Pot-10-Turn-p-1061331.html So I screw a small wire to the metal part of it and connect it to circuit ground? that is easy. However, what if we didn't do any of that? what could happen? I mean will it introduce some error in current like not zeroing out? or will it introduce huge amounts like 10s of mA? |
| Ian.M:
Soldering to Aluminum alloys is very difficult without special techniques. A bolted connection is usually much easier. If you need a flat surface on the outside of the case, use a countersunk screw, which is usually not too obtrusive. If you don't ground the pot bush, the current will probably jump about when you bring your hand near it to adjust it. 1mV of noise will cause a 20mA fluctuation of the current in the final four MOSFET design or 5mA for a single MOSFET so it would be very difficult to adjust accurately. |
| VEGETA:
As you see, the case has no internal screw holes but only front panel ones, so where would I bolt the ground wire? Here we have no choice but soldering unless the back panel screws can do the job without the wire being exposed outside. Actually, back panel should have a cut for the DC jack input which in turns have a ground pin. So I could wire it to the back panel (or even bottom part) the way I told you. I could also try to put a wire from ground pin of DC jack to one of the 4 screws if it works fine. I don't think this anodization can conduct current, does it? as for the pot, I think I can put a wire from the place shown in picture to ground pin... Soldered from both ways since it is easy. I will try without grounding first to see how stuff works, just in case. As for grounding the case, is it necessary? also, it won't be mains earth referenced but only to ground of the circuit itself. So what if we didn't ground it? |
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