Blown fuse

Forum dedicated to helping people debug and troubleshoot non-functional pedals or builds. Please use an clear and informative title, indicating circuit and basic problem. Don't forget to mark the issue as fixed if this is the case.
Post Reply
User avatar
boblob
Breadboard Brother
Information
Posts: 85
Joined: 27 May 2010, 20:53
Completed builds: I've built clones of these:
Si and Ge Fuzz Face
Ampeg Scrambler
Vox Repeat Percussion
Big Muff triangle
Mosrite Fuzz Rite
Maestro Super Fuzz
MXR Distortion +
Rebote Digital Delay
Shin-Ei Companion
Has thanked: 5 times
Been thanked: 10 times

Post by boblob »

Hi,

I've been using little 62mA axial fuses in some of my builds (better a blown fuse than a knackered power supply should it develop a short). They work well and the few ohms resistance they add helps filter power supply noise. However one has blown in a super fuzz and I can't see the reason. If I short the blown fuse the pedal powers up and works fine. I've been using a decent regulated 9V supply and battery.

The SF only draws a 2-4mA in normal operation, so the only thing I can think of is switch on transients (which I don't know an awful lot about) as everything looks fine. Is that likely?

User avatar
PokeyPete
Resistor Ronker
Information
Posts: 371
Joined: 31 Aug 2008, 01:45
Has thanked: 11 times
Been thanked: 43 times

Post by PokeyPete »

I believe your assessment is correct. Assuming that your build is correct,
I would venture to guess that the cause is the filtering cap. I looked up a
schematic and it showed a 100uF capacitor. This capacitor is essentially a
dead short upon power up. It charges quickly, but still causes a large
momentary surge of current when first powered up. It discharges through
the load resistance quickly after being powered down, and will surge each
time it is powered up. The larger the cap, the longer time it takes to fully
charge. The resistance between the cap and the power supply will also
lengthen the charge time. This is why in many circuits you will see "Slo-blo"
fuses being used....they can better survive the initial surge.
“No man is so foolish but he may sometimes give another
good counsel, and no man so wise that he may not easily err
if he takes no other counsel than his own. He that is taught
only by himself has a fool for a master.”
–Hunter S. Thompson

User avatar
boblob
Breadboard Brother
Information
Posts: 85
Joined: 27 May 2010, 20:53
Completed builds: I've built clones of these:
Si and Ge Fuzz Face
Ampeg Scrambler
Vox Repeat Percussion
Big Muff triangle
Mosrite Fuzz Rite
Maestro Super Fuzz
MXR Distortion +
Rebote Digital Delay
Shin-Ei Companion
Has thanked: 5 times
Been thanked: 10 times

Post by boblob »

Thanks man. Always good to get a second opinion.

I think little axial leaded slo-blo fuses are difficult to get - might use a small value fusible resistor instead. That should provide decent short circuit protection and easily handle the inrush.

Post Reply