Pcb fuse help

[carbon dude oxide]

Hello, i am currently taking apart a broken sound bar and i have found a fuse that is blown (i belive) it is a pcb fuse shaped like a resistor with 3 colour bands which are: red, black and brown or gold. Unsure but i think its more brown. Can anyone help on this please?

[Paul Price]

It seems to be color coded as a resistor, a 200 ohm, I have seen some small inductors that look almost exactly like a resistor using the same color code scheme.
In that case it could be a burnt 200uH inductor used to create a higher voltage in a boost convertor.

[carbon dude oxide]

It imediatly comes from a transformer before going to the rest of the circuit.

[Paul Price]

If it is a resistor, that would make sense...hmmm, but if it is really some fuse...what measurement do you get with  your MM?
What does the other side of the resistor/fuse connect to?  Can you draw a simple circuit?

[carbon dude oxide]

There is no continuity, resistance shows 0M (auto ranging)

It goes into a bridge rectifyer after the fuse/resistor thing

[Paul Price]

Ok, if it is a fuse, check the circuit for shorted components, and put another fuse in that is a slow reacting type.
 

[carbon dude oxide]

What value?

[MrsR]

It sounds like a small Choke/Inductor if it's gone open circuit you shouldn't get a voltage reading after it.
I had one on a Amp that had me stuck for awhile.
Rachael


Have fun

[carbon dude oxide]

This is for a sound bar ipod dock

[carbon dude oxide]

The silk screen under it does also say fuse ^.^ i dont know why i missed that out

[Paul Price]

In that case it could be a burnt 200mA fuse.

[carbon dude oxide]

I dont think i have a 200mA fuse :/ il have a look threw my draws

[carbon dude oxide]

Closest i could find is a 500ma quick blow :/

[carbon dude oxide]

Quick question how can you tell its 200mA?

[Paul Price]

that's what the color code says, but if the third color(brown) was actually red that turned brown after an overload condition, then it might be a 2000mA fuse.

I sometimes try bridging the fuse with a very small strand of copper wire to  act as a fuse.  If you turn it on and noting overheats or smokes, you can measure the current though the fuse with you MM and put in a fuse that is about 150% higher.

[carbon dude oxide]

200mA fuse did the trick. The sound system is sounding good and i did a full cycle of music from min volume to max and did clasical to drum and bass and no problems

Thanks for the help

[carbon dude oxide]

ok my friend (who owns the sound bar) has just texted me that it has broken again with the exact same symptoms as before, i have not had a look yet but it is most probably going to be the 200mA time delay fuse in it. what could be causing it to blow? would it likely be a fault with the board itself? or that the initial start-up current was too much for too long?

[SeanB]

Probably capacitor charging. Add a PTC inrush limiter to the circuit, or add a series 1R to 5R 5W resistor as well to limit inrush current during startup. What value are the capacitors after the bridge rectifier fed by the fuse?

[carbon dude oxide]

The fuse is right next to the beefy power transformer and you can see the filament bowing outwards when you plug the sound br in. Could that be doing something to the fuse? (Its about 10 mm away from the transformer)

[lemmegraphdat]

Sounds like it's warm and getting close to melting.

[carbon dude oxide]

i was planning on bringing the fuse out so that it can be accessed on the side panel, this would also allow me to add the series resistor in line aswell to limet the current. i shall ahve the capacitor values when i open it back up again later tonight

[carbon dude oxide]

ok after the fuse come the bridge rectifier and after that are the capacitors which are 200uF and a 1000uf both of which are rated for 25V (those are the main ones anyway).

[David_AVD]

Got a picture of the original fuse?  I may have been a delay (anti-surge) type.  Using a fast blow type in place could result in it fatigue and failure after a few cycles.

[carbon dude oxide]

I am unable to find the original fuse. It was a blue resistor shaped fuse with 3 colour bands which are red, black and (brown/gold)

If that is any help

[David_AVD]

You can definitely buy axial style fuses in the anti-surge variety.  Maybe check Digikey, Mouser, Element14 or RS Components.

[carbon dude oxide]

Somethibg like this? : http://uk.rs-online.com/mobile/p/cartridge-fuses/0563362/

[David_AVD]

Somethibg like this? : http://uk.rs-online.com/mobile/p/cartridge-fuses/0563362/

Yes, if you're going to use an M205 panel fuse holder.

[carbon dude oxide]

I did use a 200mA time delay glass 20mm fuse from maplin.

Im guessing that is not sufficiant?

[David_AVD]

Maybe 1) the OEM value chosen is borderline, 2) the time delay characteristic is different, or 3) something in the electronics has changed to draw a little more current.