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(hobbyist here, go easy :P)

Due to a loose wire hitting something, I busted my 12v 3.2amps 38 watts PSU and the magic smoke came out. Apparently a fuse popped and I want to replace it. I don't know what it is and after some research on Google, I can't find it. I always come across 4 or 5 bands color charts. Never 3.

I have a second working unit exactly the same. Without any load I measured the resistance at 0.9 ohms (my DMM only goes down to 200 ohm resolution so it may be not super precise).

When powered on, and with a small load of a couple of milliamps, there is no AC current drop and no DC current drop across it.

Here are some pictures:

busted one

where it once was

working unit

Thank you very much!

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    \$\begingroup\$ Red White Black gold 210 x0.1? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 27 '18 at 22:26
  • \$\begingroup\$ @TonyStewart.EEsince'75 Well the case is blueish and what looks like a band between the red and the black is actually a bump and a light effect. Weird. Thanks for looking! \$\endgroup\$ – Mike Gleason jr Couturier Jan 27 '18 at 22:33
  • \$\begingroup\$ I can't answer, but my guess is a fuse something around 1A or 0.5A if it's on the AC side. (if it's on the DC side, it should be 4A or more but I think it's on the AC side. Please check the trace to make sure). So IMO any fuse between 0.5 and 1A would be ok as replacement. BUT... you will be lucky if nothing else was destroyed. Maybe the whole PSU is kaput. At your own risk! \$\endgroup\$ – Fredled Jan 27 '18 at 22:33
  • \$\begingroup\$ You need to thru-hole penny. \$\endgroup\$ – Olin Lathrop Jan 27 '18 at 22:35
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    \$\begingroup\$ @KingDuken No I'm a software engineer and I play with arduinos from time to time. I am starting to get irritated by this kind of attitude here with newcomers. I tried to provide the maximum infos and tried to figure out myself before asking. \$\endgroup\$ – Mike Gleason jr Couturier Jan 27 '18 at 22:37
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Look around on distributor sites for PC mount fuses. Look particularly at the Panasonic line, since they have a series of power resistors that look almost exactly like that.

You could take the one out of the working unit, connect it to a lab power supply and slowly ramp up the current to see where it breaks. Now you need two fuses, but at least you know the current rating.

Since this is a one-off, you don't need to find one in exactly that package. You have a lot of room, so you can rig up most thru hole fuses to fit in there.

Another clue is the power rating of the power supply. It is rated for 38 W out. Let's be pessimistic and say 80% efficiency, so 48 W in. With 110 V AC input, that would only mean 430 mA. There will be some startup inrush, but that is short-lived and most simple fuses can ride it out. 1/2 A is cutting it close, but 1 A should be more then sufficient.

It's not that critical. Even if the original was 1/2 A and you replace it with a 1 A fuse, nothing serious is going to happen in this one-off situation. The most likely failure that blows a fuse is a dead short, or close to it. Whether the fuse trips at 1/2 A or 1 A won't matter for that.

That all said, there is a reasonable chance that replacing the fuse won't solve the problem. It may well be that the stray wire caused something to get fried, which then caused too much input current to be drawn, which blew the fuse. In that case, a new fuse will just blow again quickly.

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  • \$\begingroup\$ Thank you very much sir for your answer. I am not the downvoter BTW. I think I will go on the safe side and replace the whole unit. If the answer was more straightforward and more evident, maybe I would repair it. I'll fork the 10 bucks! \$\endgroup\$ – Mike Gleason jr Couturier Jan 27 '18 at 23:01
  • \$\begingroup\$ Whoever downvoted this, please explain what you think is wrong. \$\endgroup\$ – Olin Lathrop Jan 28 '18 at 13:33
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These units both look like they have been overstressed.

Speculation

The faulty R colors read as 290 x0.1 ( red white black gold) where the 0 is multiplier and gold is a divider. ( the white is missing perhaps due to the same background colour but the spacing suggests there should be a band between Red&Blk )

enter image description here

best guess

2.9 Ohm 1/4W 10% 1/4W =I²R thus est. I(ac)= 0.09 @ 240V = 22 VA which looks about right.

Its purpose to the protect the smaller e-caps from ripple and surge current and also act as a fuse but with much higher resistance than a 1A fuse. Note on the good unit one black cap lid is slightly bulged from heat gas pressure.

Using a fuse may accelerate primary cap failures due to random turn on surge that may occur at peak voltage.

I hope you did not do this. If so , that would explain the failures and the bulged smaller primary cap on the good unit.

If you need more power to drive LED's I suggest get a Banggood LED Constant Current PS rated adjustable for what you need.

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  • \$\begingroup\$ Hi thanks for the answer. No I did not attempt to parallel them. I powered a couple of milliamps (arduinos) through a step down converter. They are brand new. I am currently in Morocco so I don't know how long they have been on the shelf here or where they come from :) EDIT: oh I see the capacitor you are talking about. That's a shame I have never really use this one really. \$\endgroup\$ – Mike Gleason jr Couturier Jan 27 '18 at 23:07
  • \$\begingroup\$ 1: I don't see a white stripe. Looks like red,black,gold to me. 2: Even it it were red,wht,blk,gld, the digits values would be 290. There is no way to get 21... from that. 3: I don't see any evidence of abuse in the picture of the second power supply. \$\endgroup\$ – Olin Lathrop Jan 28 '18 at 13:39
  • \$\begingroup\$ "I don't see any evidence of abuse..." Thanks. I will still change the bulged cap. I ended up tearing apart the faulty one and salvaged all the components. \$\endgroup\$ – Mike Gleason jr Couturier Jan 29 '18 at 12:32
  • \$\begingroup\$ @OlinLathrop can you see white stripe now? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 29 '18 at 15:56
  • \$\begingroup\$ Maybe, if I tilt my head just right. It still looks more like a anomaly in the photograph, though. \$\endgroup\$ – Olin Lathrop Jan 29 '18 at 17:22

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