VOGONS


And then there was white smoke...

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First post, by pan069

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So, I connect this 3.5 floppy drive to my PSU, flick the switch and... white freaking smoke starts bellowing from behind the disk drive... WTF!!! Turn the power off as fast I can, yep, that smells burned... It's the red wire that's all bubbled up...

I just used this drive the other day without issue. I suspect that I misconnected the power connecter. Either upside down or 1 pin out. Could this have caused the white smoke?

The drive seems fine on visual inspection but my PSU cable is pretty burned up, from the floppy connector to the first molex up.

What do you guys think? Chuck the entire PSU? It's an AT power supply that I bought "new" late last year. Have been using it for the past few months without issues..

Thoughts welcome..

Reply 1 of 20, by retardware

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Photos would be helpful. Maybe also from the drive's PCB and connector, they might reveal hints.

Some inferior hardware (drives, PSUs, adapters) have omitted the plastic covers that prevent cables being inserted incorrectly, just to make things a bit cheaper.
It's good for the economy anyways when people have to purchase new stuff.

Stuff might work or might not work now, after cutting off the burnt cable extension and sealing off the loose ends using shrink tube.
It is very useful to have a test adapter with dummy loads on at least +12, halogen lamp, fan, resistor, you name it, just to test PSUs.
This way I examine power supplies of unknown functional state for safe outputs (voltage, ripple) without putting valuable electronics at danger.

Reply 2 of 20, by quicknick

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The two middle pins (ground) are tied together, so inserting the connector with 1 pin out will short one of the rails to ground. In this instance, it was the 5 volt line. I think the psu and drive will be most likely fine, after you repair the damaged wire.

Reply 3 of 20, by treeman

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I did this once, forced the power cable upside down, drive blew smoke. Opened it up and a chip on the circuit board was all burned, this will most likely be same in your case

Reply 5 of 20, by pan069

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Thanks guys

I hope my mobo is OK though... No bad sights or smells..

The PSU cable. This is the only part that has visible damage. Not sure about the condition of the PSU yet but it doesn't smell like the cable does (yuk!). the PSU just smells like.., electronics. Think of cutting that big molex off because the red wires seems to be fused. Will certainly do before I try the PSU again.

Pwvh8Vn.jpg

The PSU:
Lr7IqRe.jpg

Some residue on my floppy cable but I think she's alright:
4HHw8Yh.jpg

The drive itself. No visible damage but the thing smells realy bad. Unable to open it. It's a NEC and build pretty sturdy. I'll probably chuck it:
5H4JQNJ.jpg

UPDATE

Managed to get the drive open, and yes, it's burned. There two clear burn marks; one at the bottom to the right of the J16 and one to the left (more faint) of the Mitsumi NLC017:

fNhpn3m.jpg

Reply 8 of 20, by Vynix

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Toss also that Athena Power PSU as well... I heard they're quite "questionable" in terms of quality... Given how the cable melted..

Proud owner of a Shuttle HOT-555A 430VX motherboard and two wonderful retro laptops, namely a Compaq Armada 1700 [nonfunctional] and a HP Omnibook XE3-GC [fully working :p]

Reply 9 of 20, by SirNickity

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I did this once back in the 486 days. I was helping a friend build his first PC, and he got two 3.5" floppy drives from this ghetto local computer store. They definitely marketed their wares toward low-budget builders, so a lot of the stuff they sold was whatever they could get cheapest. OEM sound cards, bulk Taiwanese AT cases (which admittedly is part of why I have so much nostalgia for them now), etc. Anyway, the 3.5" drives had the flimsiest plastic on the power connector, such that it didn't even feel "wrong" when I plugged in the PSU lead, but it was definitely off by one pin. Shorted one of the rails to ground and got a little bit of smoke before we noticed and shut it down. Everything survived fine. I didn't even notice any damage to the cabling, though I'm sure there was some. I'm guessing there wasn't much over-current protection in the bargain-basement PSU that came with the cheap case. 😉

You've clearly got heat damage on the PCB of the floppy drive, but it may be salvageable if you're interested in doing so. Might be worth scraping the soldermask off and reinforcing with some bare wire and solder. There's a ton of solder on the power connector ground pins, so that looks to have survived intact. I can't say by the picture whether the area on the PCB by the ICs is heat or just leftover flux. There seems to be a lot of flux residue on that board, and I'm not sure why there would have been a significant current path up in that area of the board anyway.

If you have some old CD-ROMs or hard drives that you don't care about, you can use them as a dummy load to verify the PSU. You might need a couple. One HDD and a pair of old optical drives is my test load. It's just enough that the rails are more or less in regulation, and you can at least verify the supply is viable. If you can wire up a couple 5W to 20W automotive light bulbs on the +5V and +12V rails, you'll be better for it.

I would agree, though, that the Athena Power supply is probably not a great long-term solution. But then, the only other options are to maintain an old AT supply, or deal with adapters and potential -5V issues from using an ATX supply.

Reply 10 of 20, by pan069

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Vynix wrote:

Toss also that Athena Power PSU as well... I heard they're quite "questionable" in terms of quality... Given how the cable melted..

Ah OK. Are you saying that with a "quality" PSU this would not have happened? i.e the cable would not have melted?

You might find me foolish in thinking so but, I am actually quite surprised that this happened at all. It's quite dangerous.

Reply 12 of 20, by Vynix

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pan069 wrote:
Vynix wrote:

Toss also that Athena Power PSU as well... I heard they're quite "questionable" in terms of quality... Given how the cable melted..

Ah OK. Are you saying that with a "quality" PSU this would not have happened? i.e the cable would not have melted?

You might find me foolish in thinking so but, I am actually quite surprised that this happened at all. It's quite dangerous.

Indeed it is, normally the PSU would have shut itself down if something was shorting the power rails, but in your case, the power supply apparently didn't and the wires started melting down (imagine if that happened while the power supply did that when the computer was running with the case closed).

So yeah I wouldn't trust a PSU like that after such an incident with it...

Proud owner of a Shuttle HOT-555A 430VX motherboard and two wonderful retro laptops, namely a Compaq Armada 1700 [nonfunctional] and a HP Omnibook XE3-GC [fully working :p]

Reply 13 of 20, by mdog69

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canthearu wrote:

Good PSUs have short circuit protection and would have cut out in a shorting situation like this.

You have to detect the short circuit in the first place. In this case the circuit (PSU - red wire- floppy w/dead short - black wire - PSU) wasn't a dead short - the resistance of the wires, whilst minimal, became significant once a high current started to flow making the wires get hot. Then once the wire started getting hot, the resistance starts to further increase. This will act to limit the current flowing through the zero ohm dead short at the end of the cable.

Put another way, If you have a PSU capable of delivering +5v at 38A, a motherboard, HDD, CDROM pulling 3A@5v between them, and two feet of copper wire glowing cherry red consuming 10A@5v then not even the world's greatest gold plated PSU is going to shutdown.

Reply 14 of 20, by SirNickity

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That's absolutely true. Some of the mega-giant gazillion-watt PSUs that have single 12V rails at ungodly amperage would not, and should not, shut down by shorting any given lead. Those wires are only meant to carry a few amps each. It's the combined load of all the drive leads, the mobo lead, etc., that adds up to the total sum wattage.

OTOH, it's quite likely that a typical-for-the-time low-wattage AT supply could saturate a rail with a single cable. Not indefinitely (before the cable failed), but it ought to be enough to trigger the OCP. In that same vein, I have my doubts that this Athena Power supply is actually capable of 400W, or 38A on +5V. Maybe it really is. Who knows. But I bet my 145W Delta supply would've shut down safely, while having enough gusto to power any legitimate load I'm going to put on it with a retro build.

Reply 15 of 20, by realoldguy23

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mdog69 wrote:
canthearu wrote:

Good PSUs have short circuit protection and would have cut out in a shorting situation like this.

You have to detect the short circuit in the first place. In this case the circuit (PSU - red wire- floppy w/dead short - black wire - PSU) wasn't a dead short -

Putting the floppy connector off by 1 is a dead short on either the +5V or the +12V rail. I think the TO mentioned that this could have been what he did. And the red 5V wire has a molten insulator. There was clearly too much current flowing through that wire. This is a failure of (or lack of) the over current protection of this PSU. I wouldn't trust it.

Reply 16 of 20, by quicknick

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22AWG wire, commonly found on floppy power leads, supports a maximum of 7 amps. Molten insulation means a much higher current went through it, but probably it was less than the 38A required for the over-current protection to kick in. I'd say this is primarily a user-error, and not a fault of the PSU. There is indeed a problem and a fire hazard by having such thin wires on outputs so powerful - nothing guarantees a perfect load balancing - but this is common to many high-powered PSUs.

This thread brought back a distant memory, from the '90s... I did the same, after cleaning and reassembling my PC... Floppy connector off by 1 pin, luckily it refused to start. My PSU was just 200W.

Reply 17 of 20, by realoldguy23

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quicknick wrote:

This thread brought back a distant memory, from the '90s... I did the same, after cleaning and reassembling my PC... Floppy connector off by 1 pin, luckily it refused to start. My PSU was just 200W.

This is what I meant. I think, floppy connector off by 1 pin happened to most of us at some time. It sure did happen to me once in a while and it was always the case that the PC did not start then. And why did it not start? Because the PSU was going into over current protection mode. The one in question here didn't, and that's why I think it is crap.

BTW: Even if the lead is rated for 7 amps only, it can bear a lot more for a short period of time without glowing and insulator melting, which is enough for the PSU to sense the over current and shut down.

Reply 18 of 20, by mdog69

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realoldguy23 wrote:

BTW: Even if the lead is rated for 7 amps only, it can bear a lot more for a short period of time without glowing and insulator melting, which is enough for the PSU to sense the over current and shut down.

How can a 35A power supply differentiate between a normal 10A load and 10A flowing though a piece of glowing wire surrounded by smouldering insulation?

I've already given you the answer - It can't!

If you want to give me the details of the power technology that can differentiate between a "nice" load and a "naughty" load, let us know and I'll put the popcorn on.

Reply 19 of 20, by SirNickity

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I don't remember offhand (and too lazy to look it up), but wire ratings are something like the current that it can support with a 10C rise at 25C ambient, or something like that. 38A is over five times the rated capacity of that wire. It's something that maybe should be considered in the design of a better PSU -- perhaps splitting up high-current rails into multiple smaller ones, for e.g., or at least monitored separately. Or at least using a minimum of 18AWG cable everywhere just in case. (Although then maybe it's not the wire that fails, but something else -- so you've just moved the problem.)

But consider this: On power-up, much of the PC's electronics ARE a dead short. There are probably a few thousand uF of empty capacitors on every rail -- outside of those in the PSU itself. When empty, a capacitor acts like a short circuit. Stopped motors (fans, HDD, CD drives, etc.) also consume a significant amount of current to start. So there's a real balancing act on how sensitive you want the OCP to be.

There's a lot to be said about right-sizing your supply to its intended load. In most cases, 38A is gross overkill, and presents more of a danger than an aid. With power, responsibility and all that. If you choose a PSU that is capable of 125-150% of your maximum average load, and can handle peaks of 200%, then you're probably in a good place. If you need 400W, though, you have to be a lot more careful about how you handle it. That's enough to warm a small room!