[HN Gopher] My new PSU burns out - I fix it, and torture it by c...
___________________________________________________________________
My new PSU burns out - I fix it, and torture it by cracking water
Author : todsacerdoti
Score : 151 points
Date : 2024-05-27 09:55 UTC (13 hours ago)
(HTM) web link (tomscii.sig7.se)
(TXT) w3m dump (tomscii.sig7.se)
| cjk2 wrote:
| 2oz copper plus fat track, multiple via stitches, huge ground
| planes as well as isolation between the sampling and power
| delivery parts of the boards. There's a lot of design that goes
| into a simple power supply PCB. Check out an Agilent supply for
| detail :)
| Yossarrian22 wrote:
| I think you mean Keysight these days
| cjk2 wrote:
| No I mean Agilent. The stuff since keysight rebrand is mostly
| garbage.
| Yossarrian22 wrote:
| Too true
| ramenbytes wrote:
| Care to expand? I have some gripes with them myself, but I
| haven't demanded enough of their stuff to find technical
| issues yet. My problems are of the "oscilloscope is
| pestering me about windows updates" and "service manuals
| suck" variety.
| cjk2 wrote:
| Lack of service information, poor treatment of customers
| compared to HP/Agilent (try getting anything fixed
| without a service contract now), design shortcuts (U8002A
| is a buggy piece of shit), absolutely nightmare trying to
| order parts which at least in the UK means someone random
| calls you from Spain and asks for your credit card
| details and you may or may not get the parts. Oh and the
| whole fact that half the gear seems to turn into a brick
| fairly quickly compared to older models.
|
| 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.
| formerly_proven wrote:
| Lab power supplies have always been a staple project but most
| just aren't very good (which is fine). This one certainly has
| many of the staple ingredients: TL072 op amps, slow current
| limit, large output capacitance (look at C3||R22 x hFe(Q4) x
| hFe(Q5), there's probably additional capacitance directly on
| the outputs that I'm overlooking in the schematic), CC/CV
| cross-over saturates the other regulator and thermal design.
| Besides poor response, these are usually unstable with various
| loads, there's often a loss of regulation when switching on or
| off, they don't survive a dead short or a sustained load
| (especially the "not quite dead short" case is problematic for
| designs like this without current fold-back [1]). Most blow up
| when you do the file test.
|
| 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 =
| 25degC. Does a small heatsink keep Tcase at 25 degC while
| dissipating 125 W?
| cjk2 wrote:
| Yup. The killer on these things is usually actually feedback
| phase shift. Sometimes the output load can have a complex
| reactance which turns the supply into a convenient power
| amplified oscillator. Fat output capacitor solves most of
| these problems but sometimes that has its own problems (old
| HP supplies had nice barrier strips so you can deal with them
| yourself :). Obvious trade off is step response there etc but
| your load should be properly decoupled anyway.
|
| 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.
| tariksbl wrote:
| haven't made a pcb in a long time but when i did, used Altium &
| there were design rule checks for everything; but, none for
| creepage. Interesting, thanks for sharing.
| MrBuddyCasino wrote:
| Interesting. A quick search says there is now a creepage rule,
| but its not enabled by default:
| https://www.altium.com/documentation/altium-designer/pcb-ele...
|
| I wonder why that is.
| exmadscientist wrote:
| Because it doesn't work quite right. It's also extremely
| slow. Extremely. Even by Altium DRC standards.
|
| Source: tried to do HV design with it around two years back.
| tyingq wrote:
| Diagram of the bare circuit board. So you can see the overhead
| view of the run from VIN to Q5 that blew.
|
| https://tomscii.sig7.se/images/linear-dual-lab-psu/pcb-front...
|
| Edit: I'm not the writer of the article...
| tinco wrote:
| Cool article! Where did you learn PSU design? Are you
| "classically" trained or self taught?
|
| edit: The article on the design of the PSU is also great:
| https://tomscii.sig7.se/2024/04/300W-Linear-DualTracking-Lab...
| pjc50 wrote:
| Author is fairly early on in the self-taught stage from this
| sentence:
|
| "(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...
| camtarn wrote:
| This is excellent.
|
| "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."
| user_7832 wrote:
| Your comment is solid in terms of how deep any field
| goes... and is also a good motivation for me to not bother
| with DIY power supplies :)
|
| (Don't worry I anyway wasn't planning to mess with mains
| voltages, only 12-24V)
| exmadscientist wrote:
| Power supplies are really interesting in that regard,
| because they're both (1) one of the best ways to learn as
| a beginner or intermediate beginner and (2) one of the
| worst value/$ ways to learn. There are so many decent
| power supplies out there to buy that I think building
| your own is not really the right choice, unless you see
| yourself doing more offline power or even high-power
| (audio?) amplifiers in the future.
| creer wrote:
| Depends on the objective. In some cases, slapping
| together a few components is fine and straightforward. In
| particular if you pay attention to how other people are
| using them - perhaps pay attention to the provided
| standard schematic and layout in the spec or application
| note - and if you are well within the expected use for
| these components. Notice magazine articles. Use several
| sources. Something might still go wrong, but then you
| might also buy the wrong widget from the wrong seller...
|
| 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.
| marcosdumay wrote:
| > You can buy some more bits at the bit store! (well, up to
| about 24...
|
| 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.
| dfox wrote:
| Note that the PSU is not really a tracking one as it can
| providde different positive and negative voltages in CC mode.
| True tracking bipolar PSUs do not do that (which is the point
| why they exist, otherwise you could just connect two PSUs in
| series).
| willis936 wrote:
| Here's a good list of resources for calculating clearances. I
| usually go by IPC-2221 Table 6.1. I find that all of these are
| rather conservative compared to what I get from hi-pot testing,
| but YMMV.
|
| https://www.smps.us/pcbtracespacing.html
| amelius wrote:
| The ground plane is copper-poured in that area.
| ThrowawayTestr wrote:
| PCB design is a science unto itself. Nice write-up.
| naikrovek wrote:
| Enameled wire _completely_ submerged in water makes for a
| fantastic electrical load.
|
| 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.)
| mtsr wrote:
| Sunlight puts in a serious amount of energy. Roughly 1000 W/m2
| at sea level as a global average according to
| https://en.m.wikipedia.org/wiki/Solar_irradiance
| Filligree wrote:
| Less at high latitudes, due to the angle. But if you build a
| solar power plant there (I have), you might be surprised to
| learn the electrical output is nearly as large as at the
| equator.
|
| Of course that assumes angled panels, so the space taken is
| larger.
| naikrovek wrote:
| Yes and CO2 keeps that energy here, and the ocean absorbs a
| lot of that heat.
|
| 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).
| fragmede wrote:
| when they say the ocean temperature rises 1 degree, they should
| really put it in terms of joules it would take to do that,
| which is a really big number, rather than 1 degree, because
| people read more into bigger numbers.
| NikkiA wrote:
| And if you really want to scare people, convert it into Mt of
| TNT.
| naikrovek wrote:
| No one knows what a Joule is though. "1 Watt Second" is not a
| familiar term for most people.
| fragmede wrote:
| convert it into Hiroshima's then.
| 8note wrote:
| You could go with enough energy to drive an f150 around the
| equator 69720537774 times at 24 miles per gallon
| creer wrote:
| Surplus power resistors are great for this. Less likely to open
| immediately in a cooling glitch. (And yes, submerged too). It's
| really unfortunate electronics surplus stores are extinct
| around here.
| aredox wrote:
| ... Did he really crack water in that box, letting gaseous oxygen
| and hydrogen mix exactly in the ratio of 1:2 in a closed
| container?
|
| I mean, it's good he choose a less caustic electrolyte, but the
| potential explosion in itself is not fun.
| Filligree wrote:
| He seems to have an unhealthy tolerance for disasters. I'm sure
| he could convince himself that was perfectly fine given zero
| sparks, but wow.
| adrianN wrote:
| It looks like the gas collects in the pet bottle and the
| reaction stops when the bottle is full.
| Filligree wrote:
| And then you have a bottle of what is colloquially termed
| "bang-gas", which is happy to release all the gathered energy
| in a joyous millisecond detonation.
| adrianN wrote:
| It looks like you have a bottle of oxygen and one of
| hydrogen.
| thfuran wrote:
| I'm not sure pressure would ever stop electrolysis (short of
| fusion interrupting things), but I do know that it's
| sometimes performed industrially in excess of 10,000 psi and
| there's no way that container is getting anywhere near there
| before opening one way or another.
| adrianN wrote:
| The electrodes lose contact with the solution if enough gas
| is collected.
| jcalvinowens wrote:
| The pressure vessel is a tupperware... the lid would harmlessly
| blow off. As long as you're more than a foot away when that
| happens, you're fine.
| alright2565 wrote:
| > 2.5A primary side fuse (integrated into the AC inlet) did not
| melt
|
| This is a current-limited power-supply right? Test your fuse!
| Would not be the first time a fuse-shaped wire is sold[1]
|
| [1]: https://youtu.be/B90_SNNbcoU?si=HwDCRvz7E-aCR1rQ&t=431
| stouset wrote:
| There's a special place in hell for people who sell fuse-shaped
| wires.
| mozman wrote:
| it's likely the track was weaker than the fuse - and you have
| long blow/slow blow fuses
|
| electricity is lazy it takes the easiest path
| whiw wrote:
| The transformer secondary was maxing out at 4A, 36V. The
| primary current will be much lower, probably about
| (40V/240V)*4A, ie about 0.67A.
| stavros wrote:
| How do you test a fuse?
| charlieo88 wrote:
| Terminally.
| stavros wrote:
| I admire your optimism that all the fuses will behave
| similarly.
| alright2565 wrote:
| For a hobby project like this, I don't think you need
| utmost certainty.
|
| And if you're building aircraft, why are you shopping on
| Aliexpress?
| stavros wrote:
| Not utmost, but if I put one in my car, it matters if one
| burns at 2A and the other at 4A.
| jabbany wrote:
| destructively
| bitwrangler wrote:
| I would use a variable power supply, amp meter and some
| sacrificial fuses. Slowly turn up the voltage while watching
| amps on the circuit until it blows? Or simply apply the rated
| amps (with current controlled power supply) to verify if/when
| the fuse blows.
|
| 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.
| coryrc wrote:
| Get a set of visually-similar fuses that claim to be the same
| rating. (Visually because you need the shape to be the same).
| When testing a few to destruction, record the temperature of
| the fusing element and resistance and power over time. Also
| measure the temperature at operating current.
|
| 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.
| creer wrote:
| You can destructively test samples out of a box of them. But
| even then the proper current profile that a fuse is supposed
| to accept or block is far from obvious or intuitive. Reading
| the spec helps. And then you still have the problem of
| supplying an acceptable sequence to get a correct result as
| you desired.
|
| 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.
| exmadscientist wrote:
| > But even then the proper current profile that a fuse is
| supposed to accept or block is far from obvious or
| intuitive. Reading the spec helps.
|
| 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.
| Terr_ wrote:
| You could... whoah, wait a second... isn't this one of the
| few places where that interview question of "determine the
| durability of N objects by dropping them from different
| floors in a building in a minimum of attempts" may actually
| be relevant?
| exar0815 wrote:
| Some weeks ago I also relied on a 1A Quick-Blow fuse to
| safeguard my project from accidentally blowing up. However,
| while tuning the parameters for the 10 Amp 5-60V Buck Boost
| converter (which was only loaded output-wise for 5W at the
| moment) I got a thermal runaway at one of the mosfets, which
| instantly vaporized the FET and the FR4 below it. While my
| error was that the Input PSU was still set to 250W, that fuse
| was completely okay afterwards. Don't rely on fuses alone. They
| suck.
| pjc50 wrote:
| Fuses are just not that great. Quickblow fuses cannot keep up
| with quickblow MOSFETs. All they can do is limit the size of
| the fire in your enclosure.
| creer wrote:
| Specifically, fuses (I mean actual fuses) have specific
| specs and MOSFETS have different specific specs. There is
| nothing wrong with using fuses if you understand and pay
| attention to that spec. Indeed a "quickblow" fuse will not
| protect you from everything you can think of - just some
| things - just because it's "quickblow".
| pjc50 wrote:
| > Apparently, the 8-mil (0.2 mm) clearance between VIN+ and
| ground was simply not enough to keep the 40-or-so volts apart.
|
| 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.
| ForOldHack wrote:
| "Cant creep across empty space " No, but it can arc, which now
| explains perfectly why my motherboard failed, and threw an arc
| across the battery and sent out a jet of flame. I kept it
| around to show people what could never happen - does. Now long
| lost.
| amelius wrote:
| Wait, how small are the distances inside a supercapacitor?
| dmoy wrote:
| Small
|
| 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.
| paulgerhardt wrote:
| Glass immediately comes to mind as having less creep. If
| you go back to the Faraday, Hertz, Tesla et al days when
| they were arcing kilovolts across their labs you'll often
| see them using glass insulators. Some ceramics and Teflon
| too.
| helsinkiandrew wrote:
| The damage to the top of the wire terminal blocks suggest a lot
| of force was applied with a screwdriver to tighten them -
| straight onto the PCB near the breakage. I wonder if physical
| stress was also a factor.
| geocrasher wrote:
| I was disappointed that there wasn't a picture of the repair, but
| an insulated wire would have been plenty indeed.
|
| 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.
| exmadscientist wrote:
| Some advice to the designer of this PSU:
|
| 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?
| mometsi wrote:
| Please don't use stainless steel or other chromium-containing
| alloys as an anode for DIY electrochemistry!
|
| It will contaminate your solution with hexavalent chromium as it
| corrodes.
___________________________________________________________________
(page generated 2024-05-27 23:01 UTC)