Fusible resistor = flameproof resistor

I need two fusible resistors 0.68ohm 1/6W for a repair job (CD player).

A "fusible resistor" is the same as a "current sense resistor"?

So this one would do the job?
http://www.mouser.com/ProductDetail/KOA-Speer/CF1-4LR68J/?qs=sGAEpiMZZMtlleCFQhR%2fzRaR6SGe7doN1mbM93GBl6c%3d

Cheers.

Script said:

I need two fusible resistors 0.68ohm 1/6W for a repair job (CD player).

A "fusible resistor" is the same as a "current sense resistor"?

So this one would do the job?
http://www.mouser.com/ProductDetail/KOA-Speer/CF1-4LR68J/?qs=sGAEpiMZZMtlleCFQhR%2fzRaR6SGe7doN1mbM93GBl6c%3d

Cheers.

Click to expand...

No generally they are "flame proof" resistors so they fail open circuit from over current and don't start a fire.

Current sense resistors are for different applications...

JR

Thanks JR.

Thanks for reassuring me. In the Japanese service manual it says "(fusible resistor)" [in English] underneath the Japanese word for "fuse resistor". Anyway, I just came back from Electric Town in Tokyo, where I found a little shop where they sell "flameproof" resistors. The older lady behind the counter said that she had never seen values lower than 1R before. Will put too higher ones in parallel then just to be "fail"-safe.

Interesting, I just came across these in a Yamaha M1516 module.  One measured 10 ohms, the other was open.  The part is spec'd as 10 ohm 160ma.  It is labeled FN19100J and below that MICRON 39.  Google had no hits, neither did Mouser.  I replaced it with a 10 ohm 1/4 watt flameproof and replaced the power supply coupling caps - all good.  Just not sure the 1/4 watt will blow in time if there is ever a problem.

Script said:

Thanks JR.

Thanks for reassuring me. In the Japanese service manual it says "(fusible resistor)" [in English] underneath the Japanese word for "fuse resistor". Anyway, I just came back from Electric Town in Tokyo, where I found a little shop where they sell "flameproof" resistors. The older lady behind the counter said that she had never seen values lower than 1R before. Will put too higher ones in parallel then just to be "fail"-safe.

Click to expand...

Using ohms law, I= SQRT(P)/R where I, P and R are Watts and Ohms. I= sqrt( 0.1666) / .68 (see chart below) = 600mA.

Two rails burning 600mA is 1.2Amps, 2 rails blowing at once, indicate that it was using 1.2Amps!

If you can't find a replacement, up the resistance a little.  Use a 1 ohm resistor, 1/4watt.  Here is one at mouser (flameproof):

http://www.mouser.com/Passive-Components/Resistors/Film-Resistors/Metal-Film-Resistors-Through-Hole/_/N-7gz41?Keyword=flameproof&FS=True


Again ohms law, I=sqrt(.25) / 1 = 500mA, which is reasonable.

Here are the effects of doing this:  1) the voltage drop across them might be a bit more so if they are after the regulated PSU, you might undersupply the components a little, probably not an issue unless they were already marginal...but it would be a small difference probably no effect.  They might blow again if the CD player really does use that much power.

You could use a larger power rating if you want, but understand that resistors are not fuses, and unlike fuses they are not spec'd to blow at a particular current.  They are spec'd NOT to blow below a particular device dissipation (they will blow earlier in a hot case full of dust, and later in a clear case with good ventilation in an air conditioned room).

They are Fail Safe devices... they are designed to make sure that if a short occurs in the component or it malfunctions that a fire does not start, and that the components inside are protected (you don't want to blow hard to find and replace IC's because an electrolytic cap went short circuit).  But unlike real fuse, it is a blunt instrument.


Maybe your CD player uses 1.2 amps! (does it make toast?).  Or maybe you know why they blew (someone dropped coins in the vent holes?  Coca Cola?).  But if not, it would be worth looking closely at the board for burned components, leaky or swollen caps, etc.  And if it were me and I cared about the CD-Player, I would start with lower current ratings and move up.

It could be it was really close and it needed 1.2 amps and they blew.

What voltage are the rails these are on?

Thanks for the chart and calculations

I was thinking two 1/8W resistors in parallel per "rail" cos I found them. However, I can't find small enough ohms... The schematic quotes +12.9V and -13.9V on these "rails", so a few mAs only. 1R 1/4W (500mA) are available (thanks for the link) and is what I'll try. It's slightly lower in mA and thus will fail easier -- which is good.

These resistors are part of the laser "feed", "gain" and "tracking" adjust plus the "tray open/close" meachanism. There's also a bunch of transistors, some of which seem slightly "off", so I'll replace them. All caps "measure" fine in powered-off status, but I'll swap them anyway. These "rails" connect directly to a heatsinked power opamp with thermal shutdown and current limiter function (that IC is probably fried!).

Anyway, the case (and the entire CD player) is a real brick (10kg or so) and has no ventilation whatsoever(!). Sheer madness, cos it sure gets toaster-hot inside the case, especially in summer when you can easily get 98% of air humidity and 30°C inside your Tokyo home if you don't turn on the AirCon...

Why not just replace the resistors as a start, and see how it goes. 

If you think it got hot, then it could be it got hot, some silicon parts got to the avalanche point or whatever and the fuseable resistors worked, protecting everything.

If it was me, and there was no obvious blown part, I would replace the resistors and see what happened.

Finally had time to get back to this CD player and swapped the resistors for new ones.

It worked for a few seconds and then one of them blew again, just as I had expected.
However, that was enough time to form a theory and figure out what is actually wrong.

The current booster IC (with internal thermal shutdown) had one fried amp, so the "tray is fully inserted" stop signal would not be amplified enough to turn off the tray motor (transistor switches), until this overloaded the fusible resistors.

Got a new current booster IC and put a much bigger heatsink on it, as that chip's tiny heatsink got pretty hot within seconds (a classic case of "planned obsolescence", I'd say). Anyway, now it's music again

a choke coil with low DCR would limit transient response,

Script said:

Finally had time to get back to this CD player and swapped the resistors for new ones.

It worked for a few seconds and then one of them blew again, just as I had expected.
However, that was enough time to form a theory and figure out what is actually wrong.

The current booster IC (with internal thermal shutdown) had one fried amp, so the "tray is fully inserted" stop signal would not be amplified enough to turn off the tray motor (transistor switches), until this overloaded the fusible resistors.

Got a new current booster IC and put a much bigger heatsink on it, as that chip's tiny heatsink got pretty hot within seconds (a classic case of "planned obsolescence", I'd say). Anyway, now it's music again

Click to expand...

Congrats... it all makes sense after you fix it...  8)

Doesn't it feel good to bring old soldiers back to life?

JR

Yeah, it feels very good.



And after (re)adjusting the "tracking" and "focus gain" (they were a few mV off) it sounds even better. But now comes the worst part: finding all the case screws etc again -- I know I put them somewhere safe  :

Thanks to everyone in this and other threads for the help.