My new PSU burns out – I fix it, and torture it by cracking water
tomscii.sig7.se
tomscii.sig7.se
1. It's not bad! Really, it's not. You're obviously a thoughtful designer who knows at least a little bit. Unfortunately, you're kind of in the uncanny valley... this is better than rank amateur stuff but that gets it judged by professional standards, and it's not there by those measures.
2. Schematic pages are free. Really. They are. Use them. Put the power path on one sheet, the feedback on other, the digital on a third, the setpoint on a fourth, display on a fifth. Whatever. You get the idea. Do not just cram it all in on one. And put comments on each block saying what it's supposed to be, what else you could do (especially substitute parts), why you picked that approach, what happens if it goes wrong, et cetera. Most senior engineers don't do this. I do, and everyone always comments on how great it is when they review my work. You have some of this... (the range notes are nice!)... but go all in on this. You won't regret it.
3. Learn to decouple. 0.1uF 0805s are not the right way to do it, though they probably worked here. Look at https://www.eevblog.com/forum/projects/location-and-value-of... and similar posts.
4. Mark out mains explicitly on your schematics. It should be super easy to see what is mains and what is not. This schematic is pretty good here, but this is important enough that I have to state it explicitly.
5. You have high voltage MLCCs in this design. How much capacitance are they really good for?
Just to stress this, voltage derating is important as it can be significant. Your 10uF capacitor might effectively be just 3uF, or worse, just due to DC voltage across it.
Some resources:
https://www.digikey.com/Site/Global/Layouts/DownloadPdf.ashx...
https://fscdn.rohm.com/en/products/databook/applinote/ic/pow...
I don't exactly have the creepage tables memorized but that immediately jumped out at me! It's just too optimistic. Especially since it seemed not to have been covered in soldermask (although that may also have blown off?). There are all sorts of possibilities for bits of loose metal (solder whiskers, cut TH lead fragments etc) to turn up there and ruin your day.
Mains PSUs often have slots in critical locations to deal with creepage. Can't creep across an empty space.
Air is like 50V per mil. Cheap plastic is like double that?
If the voltage of a given supercapacitor isn't high, then air could work fine I suppose. At least on those two dimensions, I haven't considered anything else.
Air will have less creep than most (all?) materials, but worse voltage limits.
If you're arcing kilvolts, you want that higher dialectric constant, and probably don't care about small amounts of creep. If you're building a supercapacitor, that equation might be different because the voltages can be quite small.
I've been out of EE for a long time though, so maybe I'm misremembering things here.
This is a current-limited power-supply right? Test your fuse! Would not be the first time a fuse-shaped wire is sold[1]
If you want protection you can use electrical fuses/load switches that turn off in microseconds (which still might not save the fet but should at least save the pcb)
Even legit fuses can vary quite unpredictably: https://www.youtube.com/watch?v=WG11rVcMOnY
electricity is lazy it takes the easiest path
Now the fuse you want to have the utmost trust in, test it at at half the time it should fail for a given i^2t. The temperature, resistance, and power curve should match the ones you tested destructively. The temperature at operating current should also be similar. If so, you can expect it to fail the way your destructively-tested fuses did.
Now, if you trust the metal composition and fusing element shape, a simple resistance meter will tell you if two fuses will behave the same.
I have a lab. What do 99% of consumers do? This is why the grift works.
Another option is to experimentally send through various current profiles so that you - more intuitively - get a better understanding of whether your thinking of what you want to protect from, might actually happen.
Right. This. Even if you shove the standards in front of their faces, most engineers don't know what fuses really do (prevent fires) or what they don't (save circuit boards) or how fast it happens (not very). Asking your typical engineer to test a fuse lot is not going to give a useful result.
This is why there are so very many safety agency marks on fuses and why even the Chinese often skip the BS and just pay for name brand fuses.
I have a multiple-fuse assortment kit that was a great (too good?) deal from AliExpress. Now I'm thinking I need to do some of those tests myself to verify their rating.
And if you're building aircraft, why are you shopping on Aliexpress?
https://tomscii.sig7.se/images/linear-dual-lab-psu/pcb-front...
Edit: I'm not the writer of the article...
edit: The article on the design of the PSU is also great: https://tomscii.sig7.se/2024/04/300W-Linear-DualTracking-Lab...
"(I really need to start digging into chapter 5 of The Art of Electronics, which is about achieving precision in electronic circuits)."
The author is aware of their learning-by-doing process, which is pedagogically great, but does mean they have to slog through finding things out the hard way.
"I really need to ditch the TL074"
- yes, it's from 1979. You'd also benefit from moving away from complicated analogue arithmetic and just buying a better ADC; remember that "losing half the range" is only one bit! You can buy some more bits at the bit store! (well, up to about 24, but then things have already got hairy in the analogue front end at that point)
The word "bandwidth" does not appear in this article, which means the author has not yet encountered control theory and is therefore not aware of a whole range of possible ways for a PSU to suck.
They would benefit from reading App Note 47. In fact, anyone working with op-amps would benefit from reading App Note 47, from the late great Jim Williams. https://www.analog.com/media/en/technical-documentation/appl...
"Like all engineering endeavors, high speed circuits can only work if negotiated compromises with nature are arranged. Ignorance of, or contempt for, physical law is a direct route to frustration. Mother Nature laughs at dilettantism and crushes arrogance without even knowing she did it."
(Don't worry I anyway wasn't planning to mess with mains voltages, only 12-24V)
Coincidentally I am interested in that field in the future. Though I'm fairly sure that there are good quality existing designs on github/diyaudio.com, so I likely don't need to design one myself. Would you have any other suggestions for this topic/field?
Acquire a scope as soon as possible. Doesn't have to be good, expensive, or even digital, but without one you literally cannot see what you are doing. You can emulate this with PC interfaces, but it's so much more convenient as a separate bench instrument.
Decide how much of a traditionalist you are. Class D has overwhelmingly won in terms of sound quality, convenience and efficiency, but if you want to build a valve amp and warm your hands around the glow, that's a very pleasant hobby.
If you are deliberately striving to push the limits one way or the other, then yeah, you will run into the exact reasons nobody else did it.
Most of the world really can not. It can only get up to 16, and that's with shady suppliers thanks to China ignoring international law.
(ok, "32 bits" and "126db" aren't quite the same, but those who need to know understand the difference and why the extra bits are still useful)
There are some US export controls on DACs that have high bit-rates and high output rates (mostly over 3500 MSPS) which is far in excess of what you'd need for most things. And US export controls aren't 'international law'...
It will contaminate your solution with hexavalent chromium as it corrodes.
Those are all good things though. You're going to find these issues when using the PSU and try to fix them. A really good lab PSU is, like you say, a surprisingly tricky thing to engineer. Some major compromises as well. There's a reason why a great many of them have used the circuit invented at HP some time in the late 60s (which in itself makes a number of compromises).
[1] f.e. the suggested TIP35C: Ptot = 125 W, which is already less than the >150 W you need to dissipate when the supply is shorted. But also heed the conditions: Ptot is at Tcase = 25°C. Does a small heatsink keep Tcase at 25 °C while dissipating 125 W?
Lots of problems on power dissipation there as well as you state. The HP/Agilent designs tend to use an SCR pre-regulator which reduces Pd on the pass transistor considerably. But of course the principal cost in these things now is shipping and profit margin so it works out cheaper to cost cut even more and shift a small heatsink with a loud ass fan on it that does your ears in. Grr. (this is one of my many reasons for disliking Keysight)
The old HP designs are very robust. I've owned a few. Almost impossible to blow up, even the big ones. I actually had a Harrison one built in 1967 that was still working unrepaired and unmodified until I sold it recently.
Bob Pease did an interesting "zero output capacitance" supply article a couple of decades back. That was surprisingly stable.
I will always look elsewhere now. If you have to throw something away every 2 years, might as well buy some Chinese junk instead (Siglent / Rigol etc). Aim-TTi are still good though - the last bastion of stuff that isn't shite.
I’ve sunk 40 amps at 30VDC into a plastic tub filled with water for hours.
When the water gets hot, put fresh water in. For long term testing, trickle cold water in and let the hot water rise to the top and spill out.
DO NOT let the enamel on the wire burn off, or you will put some very nasty stuff into the air. Keep it completely submerged.
It is amazing how much energy it takes to heat up a volume of water. (This is also why it scares the shit out of me when I read about ocean temperatures rising and I think about how much water is in the oceans.)
If you’re trying to say that the sun is responsible for rising ocean temperatures, I’d like you to consider how long the sun and the oceans have been around, and I ask you why they haven’t boiled away, yet.
The sun is not responsible for the recent dramatic increase in ocean temperatures. That’s on us (humanity).
Of course that assumes angled panels, so the space taken is larger.
According to NOAA, there's 1.335×10^9 km^3 of water in the oceans. This amounts to 1.335×10^21 liters, and with the density of sea water of 1.025kg/L, 1.368×10^21 kg.
The heat capacity of water is 4182 J/(kg×K) around room temperature. This means that when the temperature of oceans raise one degree (Celsius/Kelvin), the energy needed is (1.368×10^21 × 4182) J = 5.722544×10^24 J.
One gigaton of TNT equivalent releases 4.184×10^18 J of energy, and dividing the above result with this, you get the amount of gigatons required.
that sounds a helluva lot worse than 1 degree
I mean, it's good he choose a less caustic electrolyte, but the potential explosion in itself is not fun.
I wonder why that is.
Source: tried to do HV design with it around two years back.
I've let the magic smoke out of several MOSFET devices in a QRP SSB Transceiver kit, and had jumper wires all over the place. The electrons are too dumb to know if it's an insulated wire, or a PCB trace, as long as the layout is good.
One thing this does enlighten me about is why physical air gaps are built into so many PSU PCB's between the high and low voltage sides, with often only a transformer crossing the gap.