Electronics > Beginners
DC dummy load circuit calibration
VEGETA:
I have made a new schematic in KiCAD and here it is as .pdf in attachment.
Key points to be taken:
1- Made maximum current set resistor 750K since it is a standard value which now gives 2.11A as max current, no POT needed.
2- Adjusted LM317 circuitry to be 240 Ohms || 1.5K which gives 9.06V but the 1.5K is 1K in series with 1K || 1K.
3- Added ICL7660 negative rail generator with our agreed diode + resistor reference which feeds 10K POT and 10R to ground. 10K wiper is V_bias while other pin is not connected since it is in variable resistor configuration.
4- Power On diode is now available which takes its current from LM317.
5- Panel meter shunt resistor is now connected as it should be.
6- Added some caps here and there.
7- Labelled power rails correctly. Vdd = DC input, Vcc = 9v, Vee = -9v, V_bias = negative bias from zeroing POT.
8- Arranged Op-amps + MOSFETs circuitry in a civilized way (or so I thought) for easier reading.
Looking forward to your remarks on the schematic, so I can dig into PCB right away.
Ian.M:
I haven't spotted any obvious mistakes, but I haven't put hours into checking it.
IMHO the modern trend in amateur schematics to make heavy use of net labels instead of drawing the actual interconnections is reprehensible and should be strongly discouraged. At it worst you get schematics that are nothing more than a pinout for every chip with a net label on each pin which are absolutely horrible to trace signals on except in the original schematic editor by hiliting the whole net. Fortunately yours is much better than that.
I would break it down into far fewer sections. e.g:
Draw straight through from the pot + full zero bias adjustment circuit, then the the current sink section (OPAMP, MOSFET, sense resistors and feedback loop), then paste three more copies of the current sink section below it with I_set as their input. The negative rail generator, positive regulator, DC in and power LED all belong together, which just leaves you with one block for the D.U.T. PSU and meter connections.
I don't know if KiCAD has usable hierarchical blocks without having to set up a hierarchical sheet. If it has, it would be preferable to use them rather than having four clones of the current sink section.
VEGETA:
I will try to re-arrange it if I have the time tonight. In terms of good schematics, can you mention few points to follow?
I would rather prefer if you have any last thing or note on the schematic to say it now before digging into PCB. Probably I will wait for another day to be able to work on it.
In KiCAD there are global labels and local labels. I've used global since they appear nicer and there is no other schematic embedded within this one. Instead of doing hierarchical schematics, it is better to have just another page right? I still don't know how to do that yet in KiCAD. I don't mean another .sch file but just another page within the same one.
VEGETA:
V_load v0.2
it is updated schematic as we agreed above, kindly check it out.
now it is less-newbie with wiring instead of labels... more organized blocks too.
I will consider it final, to begin in PCB soon. Hopefully before Ramadan ends (~ 2 weeks) since I won't do a thing during feast.
I still need to know how to define board shape in KiCAD as well as how to know the actual board size which depends on the case itself. I received the case but it is not with me now, I will get back home and then I may be able to measure it using my digital caliper.
Aside from that, wouldn't putting some thermal vias help (even a bit) in thermal dissipation? just saying.
EDIT: Fixed the PDF.
ledtester:
You might be interested in this kit:
https://www.aliexpress.com/item/DIY-Kits-150W-10A-battery-capacity-tester-adjustable-constant-current-electronic-load-discharge-Test/32870007246.html
The schematic looks very similar to what you are doing (see attachment).
Another forum member turned this kit into a completed project. You may be interested in what he and other forum members had to say about it:
https://www.eevblog.com/forum/projects/upgraded-lm324-based-150w-72v-10a-electronic-load/msg1506610/
Note, also, that the PCB has footprints for both TO-220 and TO-247 MOSFETS - this makes it easy to experiment with different load transistors.
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