Electronics > Projects, Designs, and Technical Stuff
Just another DC Load
henmill:
To quote Dave: "Hi!"
I've been working on a DIY DC Load for m'lab. It started as most DIY DC Loads start I imagine: because you need a frickin adjustable load to test something! In my case, a silly mod to 15V power supply.
The goal was to first and foremost use stuff I have on hand or could have for cheap/free. So I ended up up-cycling a couple heatsinks and using a small 2U server PC case I have. It has been sitting unused for at least 3 years so I figured why not. I also happened to have an old ATX power supply that was hacked up and used briefly as my bench 5V and 12V supply. [Light bulbs coming on everywhere]
Originally I wanted to put it in a spiffy extruded case like one of those automotive inverters, but couldn't find a free one fast enough. But the PC case is pretty cool I think, albeit big. With all the space, it might eventually morph into a horribly hacked semblance of an SMU one day....
The performance goal was to be able to sink at least 50W at up to 24V. The 2nd proto (probably temporary final version) is able to do that pretty easily.
I went through an early proto stage with some random IRF FETs I had access to just to prove out the control board. Then I weighed my options between buying a few cheapish power FETs, without SOA for DC, maybe designing a PCB, or just doing it right by starting with an IXYS Linear FET. So I paid a little under 9 American Dollarbucks for a IXTH80N075L2, just so I could go full overkill and feel safe about not blowing this thing up (easily).
So here's the rundown of the design, and I'd love any input on what I should add, such as protections, improvements etc.
DCin --> Power Resistor (2 or 4 ohms) --> IXYS Linear Fet --> 100mR Sense Res -- GND
Quad op amp to buffer control input from 10-turn Pot, scale it down by 10, drive powerFEt, provide scaled Vout to panel meter
Up-cycled PC heatsinks for the FET and power resistors
Amazonian banana binding posts and power resistors. Have 5x 4R 100W resistors. Considering 2S-2P configuration if I can find the right heatsink
You can see all the pictures here: https://imgur.com/a/dc-load-linear-power-mod-xqmjoMd
Having trouble attaching more than one pic due to file size? Gotta get to bed but will direct upload more pics later.
Cheers!
drksy:
-why are you buffering Vpot twice?
-u4 needs compensation, otherwise this is just an oscillator. You need some 100p - 10n ceramic cap from u4_in_- to u4_output, remove the wire from u4_in_- to r_sense_+, and add a resistor instead (~10k - 100k) between u4_in_- and r_sense_+. With LMC648 probably 10k and 1n are sensible.
-what are those r_power resistors above the mosfet doing?
-you need some bulk decoupling for vcc, ~100u electrolytic in addition to some other ceramic caps in parallel, 1u + 100n + 10n, 10n as close to u4_vcc as possible.
-that 3d printed plastic clamp might melt and stink when you heat up the mosfet with a significant load. I would use some metal holder/screws instead.
Give this video a watch https://www.youtube.com/watch?v=vd5IBFFjnOc, shows how it's done (see pic for schematic)
overall: jank/10 :-+ But keep improving
MarkF:
Some links to other EEVBlog DIY electronic loads:
https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/
https://www.eevblog.com/forum/beginners/a-load-off-my-mnd/msg2151265/#msg2151265
henmill:
Hi drksy, thanks for the feedback. I'll address your comments point-by-point:
- buffering Vpot twice: Well no good reason really, I think I did it because I was changing up the configuration and it was convenient, and I thought this was a good way to use one of the unused opamps instead of terminating it. I realize this multiplies any offset error but in this case it doesn't matter too much as it is just the reference voltage. (sorry for providing a not 100% accurate schematic of what is implemented on the protoboard)
- u4 compensation: I actually do have a cap across the output to -input. I think I used a 47pF.. You might be able to point it out if you look at the close up of the PCB in the attached image.
And thanks for the recommendation on the connection to the sense node. I thought maybe no series resistance was necessary given the very high input impedance of the op amp. Is the Res/cap combo mostly to act as low pass filter?
- power resistors: They are there to share the overall power dissipation. Probably not necessary I guess with my awesome Linear FET. Just wanted some kind of overall current limiting in case I do something stupid (high probability)
- bulk decoupling: I have a 10uF on the board where 12V comes in, and you can't see it but there is a 100nF underneath the board as close to VCC pin as possible.
- 3d printed clamp: yes I realize this is a sh*t solution for mounting. I have some PET material I may use to make a new holder that can withstand a bit higher temp. The hope is my heatsinking will be so good that the plastic will not get very warm. I tried and failed to tap the copper heatsink.
I will keep looking for a more appropriate heatsink so I can mount the FET vertically and solder into a PCB. But for now I'm pretty pleased with the performance!
Thanks again for the comments! :)
EDIT: I need to correct my statement about u4 compensation. I had a feedback cap for the amp driving the scaled current monitor amp, NOT the amp driving the mosfet. I am still working out the value for feedback cap as of 1/27, but am learning a lot and having fun
Kleinstein:
The series resistor in the feedback is needed to make the capacitor work. It is the RC combination that matter. The shunt resistor is pretty low to start with. So the extra resistor is really needed to get good stability.
Another small point would be to have some RC combination across the FET from drain to source (e.g. some 100 nF and 100 ohms - I don't remember to best values). This would help against oscillation / ringing with a difficult load with a lot of low loss inductance.
The extra resistors are OK as a last line of defense (e.g. also with reversed polarity be acident). This is actually a good idea to have it.
Another way in this direction could be a fuse.
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