+Fran Blanche Oh its Fran! Again. In the comment box. :D Good on you Fran!

+BloodySword Everyone loves Fran!

Was just thinking the same; and Dave said it when he replaced the faulty FET. They can't run the board through soldering with screws and heat sinks on. Another thing: What's the fuzz about the flux? Could be an issue in high impedance design, but the gate looks into the output of a low impedance op-amp (or the like). No worries from my point of view.

It is strange that they decided to use chinese caracters instead of something more universal (+ , -). I guess whatever works.

sorry to disturb the dead but, they use black and red labels because the underpaid Chinese assembly workers don't know anything about electronics. they simply just put the square peg in the square hole and the round peg in the round hole; likewise the red cable in the spade labeled "red"

To tell the assemblers how to assemble :D

Oh and my first PSU was a switching one of similar quality. I killed it by powering a DC-DC converter. Somehow. I'm pretty sure I managed to repair it by replacing a blown MOSFET, but then it died again and it had to go in the bin.

I have a "LONG WEI" 30V/5A supply I got for 45€. It had the fan that ran forever, so I put an 80°C thermal switch on the heatsink and a 55°C switch on the transformer (so if either of these get hot it will turn on the fan). I also put a 95°C switch on the heatsink so that if it ever gets THAT hot, it will blow the fuse (so that I am force to troubleshoot why it got so hot). I also partially covered the vents on the side leaving about 2cm open towards the front on both sides so it passes all the air around the transformer, and changed the JWCO caps with Elna 105°C ones. Best 45€ ever spent. This thing is EXTREMELY robust. I used it to charge a 250Ah/12V battery bank (with the current pot set to maximum!!!). Of course it took 4 days to do that, but during those 4 days it was on 24/7, running at 5A and 14.4V and it never blew the fuse! I have also abused it in several other ways, including using it to power self-oscillating relays, shorting the output accidentally too many times, using it to overrev DC motors, powering "dirty" circuits like flyback inverters etc. It is still going strong after 4 years of abuse. I usually also leave it on overnight to charge batteries at low currents (so the fan doesn't come on) without any worries. The only thing that happens is that when it gets hot the voltage display drifts a bit. The regulation is rock solid however, so I usually set it when it's cold and then leave it like this.

I'm guessing they did factory testing of a known issue. Sure, half of this batch was dodgy, we'll factory replace it if it fails the stress test. Oh, it failed? Replace it. Test it again. It passed? Ship it! Only now you have a component that is known to be borderline by nature of the batch, and has been stressed by the factory testing, and now Dave puts sparks on it. It's kinda no wonder why it failed.

Except that the only inductance would be in the leads between the relay contacts and the power supply. It was not the channel powering the relay coil but the relay contacts alone that blew, so no coils involved, just a few feet of wire for the alligator clips, so not much inductance there. Plus there is several hundred microfarads on the power supply output, so even if there were an inductive spike, that is a lot of capacitance to overcome before reaching harmful voltage levels. At this point I would be more suspicious of inductance in the power supply's own transformer, though, again, there is an awful lot of input filter capacitance to overcome, which ought to make that an unlikely source as well. It is very possible, even highly likely, that the MOSFET was previously damaged by ESD in manufacturing or assembly (prior to final incorporation into the PCB) and had already been heavily damaged and the repeated shorts just finished off what good silicon remained.

+Ethan Poole yep, and they didn't even bother to clean up the flux.

+Larry Bolan As Ethan said. It's the contact channel that blew, only lead indictance involved. It's most likely a control loop response time thing taking out a MOSFET that couldn't handle it.

+Ethan Poole +EEVBLOG Doh! Wrong side of the relay... I feel really stupid. Knowing that, I concur with dave that the control loop wasn't designed to handle such spikes. That's the trap with digital "advanced" DC supply design; if you just use a plain feedback 1-transistor (or FET) supply circuit, you have no short circuit protection and rely on the speed of your microcontroller to perform current limiting. There is a three-transistor circuit that I remember from school that does have short-circuit protection built in; however, in a case like this that may not be the desired functionality.

+Doug McArtin the coil was on channel 2, which didn't fail

+Кирилл Рагузин I agree and have seen this happen to me.

I've seen the same thing happen. In fact, the flux conducted enough to start burning the circuitboard, creating a carbon track! O.o

+Benjamin “Ozias” Esposti Gee, carbon tracks are pretty dangerous once we are talking about mains. I hope it was not under mains.

+Krisztián Szirtes Nope, it was 12V!

+Benjamin “Ozias” Esposti Well that's a lucky case then. If it becomes graphite it can do some nasty little ( and lethal) things.

This is just the meter refresh rate

It's probably the sense/negative feedback causing the voltage to leak out onto the output when it's output transistor was blown. Why it isn't smooth, god knows. It could be the multimeter lagging as well.

I am also studying for a KZfaq degree in electronics by watching these videos. This topic is so interesting!

+Francis Rodgers You still need in-field experience - that in itself is the best kind of experience. Some colleges literally teach you the math, then send you out with a degree, without even getting you familiar with the difference (appearance-wise) between a resistor and a capacitor!

+Benjamin “Ozias” Esposti - As a software engineer, I understand and agree fully with what you are saying. This is why I will remain a lifetime member of KZfaq University.

+electronicsNmore LOL. that would have affected the 12 volt source! NOT the 30 v.

+electronicsNmore No, he was apply 30V 3A to contacts side - not coil side so no magnetic field - spikes happen. I would not go for polyfuses in that case, he have 5A fuse so it's way up 3A limited. Only scenario when this fuse will will go open is hardware fail - not done by user - so the best scenario is cut power. Poly will turn power again after getting cold. They are good for protecting output when You can accidentally short leads without current limiting.

Dave Blane Well he mentions in the video "possibly" back EMF, talks about a blocking diode in the PSU, so clearly it was not back EMF if he did not connect the relay coil to the Rigol PSU. The PSU mosfet simply did not like the 30V/5A shorted output. Maybe he connected up the relay coil off camera to quickly test it, then later discovered the problem. Who knows.

+Electronic Noob Blog Incorrect. The wire to the contacts is still an inductor, and it can create very fast, high frequency transients because of the low inductance.

I read about this recently...it has something to do with the collapsing field and the voltage/current generated within the inductor and sends it backwards?

2N3055 was the go-to transistor in many instrument amps then. And in some hifi gear too... The famous J. Linsley Hood class A amp, and the NAD 3020 (3055/2955). Their limit is really the Safe operating area, so above appr. 50 W output, the 2N3442 was a better choice. Kind of miss the days when an amp was 5-6 transistors, an output stage and a huge power supply...

+Bo Helsted Yes I know exactly what you are talking about. My Pioneer SPEC 2 power amp is nothing but huge caps, a massive torridal transformer, TO-3 (6 per side) output transistors and massive heat sinks. 60+ pounds! I bought it while overseas in the PI in 1975 and it is still going strong. Probably be an item contested in my will when I pass on. And to think that we have Bob Carver and the Phase Linear 400 power amp that started it all.

your right, but I think the point here is that it should be fast enough.

Low Quality Mosfet ?

+Bruce Miller Yep. Need to summon the spirit of Bob Pease to do battle with the dark forces.

I used two TO-247 MOSFET for my 30V5A/15V10A 150W-max bench power supply to get better thermal performance. With multi-tapping transformer, the max dropout power was < 50W.

I can dissipate 250W in such a package, though its back was soldered to piece of copper before going onto the heatsink.

+Mike James Maybe the power plug was wiggling. I once killed the MOV and the auxiliary rail switcher IC of a non isolated SMPS that way.

True, I have seen arcing primary side dry joints due to a loose live connection in plug.

I killed many power transistor to learn about this concept :)

Eh, any component can fail at any time but pass all tests before shipping. It's why there's warranties!

+Zadster Never had another item in the lab blow a mains fuse just doing nothing.

Ah, I get it now. When you said it's on the other side, you meant it's *before* the sense resistor. And it's not in series with the load as it looks in the video, it's across the two rails (which you said, but which I didn't appreciate at first). Got it.