A guide to fuse replacement

Many moons ago I had a guy bring in a stereo receiver for repair. The cove r was off so he could show me where the fuses went.. He also had a 5 pack o f Radio Shack fuses and just one remained. He told me he changed the fuse four times and it blew immediately each time.

I asked him why he stopped at 4..

Is it like keeping one bullet to avoid being captured alive?

Mike.

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I recently keep coming across brand new unsold equipment which has been supplied with a fast blow mains fuse that has blown at turn on.

This is from more than one manufacturer.

I have no idea what that is all about.

Gareth.

Second customer returns after a returned "repair" done with the wrong fuse?

If that's true, you would do well to investigate who is actually doing this. What class of equipment?

--
Adrian C

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** A friend who did Fender warranty work was getting a lot "failed on the s howroom floor" amps. Always the same, an F1A or F1.6A supply fuse popped. F itted a T fuse instead and no more problem. Of course, the fuses concerned were buried inside on the main PCB !!

T fuses were not always easy to buy, so it was common to use an oversize F fuse instead - labelling often encouraged you to do so.

Whatever the combination of dumb ideas, it is not hard to do a number of on

-off cycles while keeping a close eye on an F fuse. If it bends severely a t switch on it's gonna fail soon.

.... Phil

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** A friend who did Fender warranty work was getting a lot "failed on the showroom floor" amps. Always the same, an F1A or F1.6A supply fuse popped. Fitted a T fuse instead and no more problem. Of course, the fuses concerned were buried inside on the main PCB !!

T fuses were not always easy to buy, so it was common to use an oversize F fuse instead - labelling often encouraged you to do so.

Whatever the combination of dumb ideas, it is not hard to do a number of on-off cycles while keeping a close eye on an F fuse. If it bends severely at switch on it's gonna fail soon.

.... Phil

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Fender is one culprit.

Most others are Active studio monitors of various brands.

Gareth.

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** I assume the fuses supplied ARE as marked on the schems and PCBs ?

The Fender HR Deluxe, 40W all tube amp, comes supplied with F1.6A fuses fitted for 230/240VAC use.

Despite the NTC device, the fuse value is right on the limit for tolerating worst case, inrush surges.

With such amps, the power transformer surges hard when the AC supply is switched on near a supply voltage zero crossing - plus tube heaters draw significant extra current when first turned on ( about 6 times the rated amount ) and lastly the filter electros caps surge if the supply is switched near a supply voltage peak.

A last straw step happens when the operator flicks the AC switch on then off and quickly on again - as could easily happen playing about with an unfamiliar amp in a store. Surging the delicate F fuse wire while it is still hot.

** Double surging an F fuse would be likely here too, whether the PSU is transformer or SM.

.... Phil

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** Factory techs working in places like Mexico and China, sometimes elsewhere. ** Anything that has a significant inrush surge. Toroidal transformer based PSUs are major culprits.

.... Phil

Universal self-selecting SMPS , with fuses supplied for 110V countries?

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Gareth Magennis wrote:

** IME, fuse current ratings for 120VAC use are exactly double what one needs for 240VAC use. See that a lot.

The problem is "inrush surge" which nearly every common appliance exhibits.

About the only types of appliance that have no switch on surge are resistance heaters for food, water and air - plus the common soldering tool.

FYI:

peak and RMS current values measured for some 240VAC items.

4A > .25A 60W incandescent lamp

12A > 7A 700W microwave

20A > .1A 15W CFL

52A > .55A colour CRT monitor

55A > 1.6A 1980s 100W Marshall

.... Phil

I had a solid state amplifier for my ham gear that used a 20 or 25 amp

3AG type fuse. While the fuse was rated fine for the current ( about 5 amps more than the current drain), after about a month it would do what I call a 'sag out'. The element would just be bent to the bottom of the glass and quit making connection.

I guess that the contacts for the plug in with a twist lock cap was not making contact well enough to keep the heat down.

The current would cycle from almost no current to full current about 20 or more times a day on the average.

I'm not sure if this is about power line fuses or 12Volt, or something in the circuit. If this is a for a 120VAC line, you are possibly already overloading the house fuse or breaker.

As far as that sag in the fuses, I'd suggest using a SLOW-BLOW fuse. They are made to handle high starting current. Or switch to a breaker. Of course there could be a defect in the amplifier too, such as a failing power supply capacitor.

Knowing more about the application for the fuse would make it easier to suggest what could be wrong.

Oh yes please, we're all waiting on pins and needles for your suggestions.

The application was for an amplifier for a 2 meter repeater. The amp is a solid state and operates at a nominal 12 volts DC. I think it had a 25 amp fuse and operated at 20 amps. The holder was a panel mounted with the cap on it that you give about a half turn to remove.

The amp draws almost no current when in standby and the full 20 amps when activated . The amp; did not have any devective capacitors and the ones in it were only a few MFD at the most and they would only have about 12 volts on them. The amp was fed with a seperate 12 volt supply that was rated for over

30 amps.

Coming very late into this....

Fuses are very simple beasts made complicated by a fundamental lack of unde rstanding.

a) The nature of the load. A purely resistance load is entirely different f rom a motor load. Both of which are different from a filament load. b) How inrush current works. A purely resistance load has a massive inrush until the resistance element heats up - whereupon the load drops in proport ion to the temperature. An AA5, for instance, is essentially a dead-short a t turn-on. As are most tube loads. c) The differences between motor loads, filament loads and electronic loads . d) Rated Operating Voltage. Fuses rated below 240V are to be taken with muc h salt. e) The differences between a standard fuse, a fast fuse, a slow-blow fused and a dual-element fuse. READ THE OPERATION TABLES!

And, unless you have no investment in the connected load or its fate whatso ever, eliminate all slow-blow AGC fuses from consideration - full stop (the one with the wire element wound around a ceramic rod)!!

From here, do your own research. For the power-supply in question, I would suggest starting with a 20A DE Fuse rated at 250V. NOTE: Many AGC fuses are rated at only 32V. Avoid them. If this blows consistently, only then would I suggest moving to a 25A DE Fuse.

Peter Wieck Melrose Park, PA

indeed

filaments are practically speaking purely resistive - as long as you're not driving them at rf, when their tiny L can become relevant.

some are like that, many aren't at all

not so

I wouldn't recommend salt really. 32v fuses are fine on 12v.

and other types.

Then there's breaking capacity, a rather important thing especially with mains fuses.

sometimes very slow fuses are just the job. That's why they're made.

NT

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** Fuses only seem simple ...... Many blown fuses look perfectly OK, needing a continuity test to prove the point. Ceramic case fuses for one.

Blown fuses deserve a careful inspection, to make sure the right amp rating and type ( F or T) was installed. Don't ya hate folk who toss them away, sometimes along with the fuse cap.

.... Phil