Basic fuse question

Google doesn't seem to have any trouble finding it. First hit. Assume you fixed what blew it.

"Arfa Daily"

** Fuses cost money and I don't like to see them blow at switch on.

So my standard practice is to fit a new fuse and then connect the AC lead to a Variac via a current meter ( true rms) and slowly wind up the voltage. With a SMPS, the current reading should be small until it suddenly strikes and runs at some voltage between 80V and 160V as shown on the Variac's dial.

If a current near to the fuse's rating appears at a low setting on the Variac, then its game over.

In cases where the PCB has to be exposed to get at the fuse, visual inspection for damage is the first step and then an old fashioned moving coil multimeter on the low ohms range to check PSU diodes and the switching transistor for shorts.

BTW:

Guitar amps regularly arrive with either no fuse or a glaringly wrong size or type fuse - ie fast fuses in the AC.

However, the ones I really hate arrive with NO fuse cap in the holder.

Grrrrrr.........

.... Phil

"mike"

** Got any idea what the letter "T" stands for ?

Hint - it ain't "time".

... Phil

"mike"

** Got any idea what the letter "T" stands for ?

Hint - it ain't "time".

... Phil

How about T for tiny... like your dick!

Better yet if you can find one cheap, a Sencore PR570.

It has the capability to automatically trip and shut down it's AC output if the current exceeds whatever setting you dial in. It doubles as an isolation transformer and variable AC power supply as well. Got one of these for $150 a year ago on Ebay, I can't count how many times I've used it.

A Dim Bulb current limiter will protect better than a Variac. fyi.

"dave"

** A Variac alone provides no protection.

But the method I outlined is vastly superior to the old light bulb trick.

Variac + true rms current meter + fuse (which can be a fast blow type).

There are far too many anomalies when a series lamp is fitted to the AC supply.

... Phil

"dave"

** A Variac alone provides no protection.

But the method I outlined is vastly superior to the old light bulb trick.

Variac + true rms current meter + fuse (which can be a fast blow type).

There are far too many anomalies when a series lamp is fitted to the AC supply.

I find the old lightbulb trick quick and easy, providing it is used in appropriate circumstances.

Case in point being the bog standard small combo with the blown mains fuse I used it on today, where it is a binary Yes there is a problem, or Not - the fuse just blew. And no harm done in the process, being as maximum mains current draw is 500 mA or so. The amp will still run and make sound with the bulb in place.

I understand you can't use this techinique reliably on high power things like Power Amps, cos they don't like undervoltage and will trip and misbehave and do all sorts of weird stuff. Here, I do use the variac in series with the bulb, just to see visually what current is being drawn and how, and when. And it is also sometimes useful to use the bulb as a soft start, then bypass it, once you are happy the amp isn't going to explode.

Likewise I use the variac and bulb on SMPS's, again to visually and audibly get an idea of whats happening and when. You can hear when it kicks in, or shuts down, and that auditory information is backed up by the behaviour of the bulb.

Horses for courses.

Gareth.

"Gareth Magennis"

** A bulb that passes 0.5A rms when lit will pass 8A peak at switch on.

That is no soft start.

** Then you have to remove it and all protection is lost. ** A valve guitar amp of over 100 watts will not warm up and run with a 0.5A bulb in the AC. ** A"soft start" is always provided when using a Variac. ** You can stick with what you like and are used to.

.... Phil

I routinely repair 100 Watt "valve" Marshalls, Fenders, Peaveys, Ampegs (SLM), etc. and always use a 150 Watt bulb in series (and my 8 Ohm 100W dummy load) until its time to check the bias. The one I use is clear (at Home Depot fyi) and the orange glow on the filament is as sensitive an indicator of current change as a panel meter. On very low idling only part of the filament glows; play a power chord and the room lights up.

For a solid state amp up to about 75 Watts (no load) I use a clear 40 Watt oven bulb. Even on the bulb Marshall DSP works fine, the voltage drop is low if the filament is cool.

Mine is a ceramic lamp socket, crudely spliced into a 15 Amp molded extension cord, with a ground lift to accept 3 prongs, a switch, total expenditure about $4.20. Oh yeah, the bulbs cost another tree fitty.

"Arfa Daily"

** OK, my test set up is used with ALL items for repair.

The AC current meter uses a Hall effect transducer ( totally isolates the mains supply) feeding a true rms converter IC and then a 3.5 digit LED display. There are two ranges, 2A and 20A with 1mA and 10mA resolution respectively.

A standard 3 pin outlet is fitted to the front and all jobs get plugged in there.

A BNC socket on the back of the unit provides a waveform for the scope that is either 1V or 100mV per amp of AC current. Signal bandwidth is DC to 100 kHz and max output is +/- 10V, so current surges up to 100 amps peak can be seen on the scope.

The current meter's electronics has its own AC lead so the Variac does not affect it.

The set up easily caters for the smallest plug pak (wall wart) right up to

2.5kW/ch stage lighting dimmers and the largest power amps. Been in continuous use now since 1996.

.... Phil

I see Australian musicians have discovered the "universal replacement" fuse, too - it's the fuse in a particular physical size with the biggest number preceding the "A" printed on the box. It is guaranteed not to blow - something else will usually pop before the fuse does.

I used to work on trucks where AGC 10 A fuses were routinely replaced with 30 A fuses in an attempt to stop the fuse from blowing. Usually the actual problem was that an added wire had been run through a hole with no grommet, sleeving, etc and was grounding out.

For a while in the 70s, some US cars had "SFE" fuses that were all 0.25" diameter but different lengths - lower ratings were physically shorter - to help prevent overfusing. An SFE 4 was a little over 0.5", and an SFE 20 was the same length (1.25") as an AGC fuse. I think it was a good idea, but it went away with the changeover to the flat blade fuses.

At a previous employer, one of our products was in a 6U 19" rackmount chassis with two holders for 5 x 20 mm fuses on the back. We shipped this in its own cardboard box, and it always showed up with everything intact. Our customer then bolted that chassis into their cabinet, along with other equipment, and shipped the whole shebang on a flatbed truck to the site. When it got to site, there was a pretty good chance that the fuse holder caps would be missing. We told them a) don't ship it that way and b) put a piece of tape over the caps before shipping. They ignored a), did b), and asked us for a case lot of replacement caps, which we gave them.

In the previous generation of that product, the fuses were in the power inlet module - one where you have to remove the main power connector to open the little flap that lets you change the fuses. I don't know whether that inlet module was eliminated from the design for cost or customer reasons. (It wasn't quite an IEC inlet module, because the incoming power was DC with a unique connector, but it was the same idea.)

Matt Roberds

** I have a box full of 20A, 25A 30A and 35A automotive 3AG fuses pulled from guitar and power amps.

Not to mention all the blown ones wrapped in foil paper from a cigarette pack.

** In my example, the fuse caps were removed by the owners and thrown away !!!

They all wrongly imagine techs can easily get spare ones to suit.

With many USA built guitar amps, it means drilling or punching a new hole in the chassis to fit a new holder.

** That level sophistication of stops most of them in their tracks.

.... Phil

Em quinta-feira, 17 de outubro de 2013 20h55min39s UTC-3, Dan escreveu:

Hi,

The Ceramic Fuse can be found in a specialized electronic store. But this f use is a specific component. I have the same problem. The Fuse 3,15a/H250v. You need to replace the same component because there is a reason about the form and type in this device. Be carefull. I try another fuse and got a ex plosion. Yeah.

.15

Hey, I know there is some joking around going on here (and some animosity, too apparently) but the "T" DOES actually stand for: "T = Time. Time Dela y or Slow Blow.(Typically open between 100 and 300 ms @ ten times the rated current)"

Correct? Or am I missing something? BTW that comes from this google cached page:

l=us

I'm just trying to learn, I've got to replace that same damn fuse, and I'm frankly having a bitch of a time figuring out what's causing it to blow (it 's the psu from a 32' flatscreen tv if that matters) Replaced a bunch of b ad caps, and it powers up for about 3 seconds with a nice picture, and then bam blown fuse. Does rogaine help for hair regrowth that you pulled out yo urself lol?

T3.15

X

, too apparently) but the "T" DOES actually stand for: "T = Time. Time De lay or Slow Blow.(Typically open between 100 and 300 ms @ ten times the rat ed current)"

d page:

&gl=us

m frankly having a bitch of a time figuring out what's causing it to blow ( it's the psu from a 32' flatscreen tv if that matters) Replaced a bunch of bad caps, and it powers up for about 3 seconds with a nice picture, and th en bam blown fuse. Does rogaine help for hair regrowth that you pulled out yourself lol?

Oh and another thing that has me curious is the fuse was wrapped in rubber shrinkwrap, even though it wasn't anywhere near anything metallic that coul d short it out, any ideas why?

That's easier to guess: to protect anyone who happens to be looking at the fuse when it blows and the glass casing shatters?

Mike.

** Yes - the word with that T as the initial letter. It's the German word "Trage" meaning "dragged out" or leaden.

All the letter codes on common fuses are for German words.

F = " Flink " for quickly or rapid.

Amaze your friends...

... Phil