Who Has Used Resistors as Fuses

Been there, done it! On 48v DC supplied from glass cells, each the size of a small fridge, the disengaging fuse acts as well working welder, and the enclosures explode happily, Fortunate are people working in AC where the welding conditions stay on till the sine crosses the zero volt line and does not reignite on the small opening in fuse. Some people are lucky, others have to learn.

Stanislaw

On a sunny day (Sun, 24 Jun 2007 17:12:29 -0700) it happened snipped-for-privacy@ieee.org wrote in :

Not my experience, and anyone can get a couple of 10 Ohm 1/4 watt carbon resistors and verify that on a LAB supply.

Actually you can also test you strands of wire from a flatcable that way, to see how many you need for say 3A (burn out).

You are probably referring to composite carbon resistors, like 'Vitrohm' used to make, they would open split apart, and perhaps occasionaly short.

Normal carbon resistors these days have ceramic body with a spiral carbon track around it. The track width and number of turns sets resistance, and thickness. In case of overload the track just opens.

Irrelevant, even if TC was 10% per degree C, it is going to blow up so who cares.

Sure, that is normal practice, and a metal film mounted like that burned a hole though my peeseebee.

The majority of tramways still run on 600 - 750v DC. To protect against lightning strike, each tramcar has an air-cored choke in series with the incoming supply, with a spark-gap on the 'hot' side of it. Once intitiated, the discharge across the gap would continue until the 600v supply was interrupted.

On older tramcars, the interrupter was mounted under the roof canopy just above the driver's head. It quenched its own arc with a magnetic blow-out coil, which stretched the arc down a fireproof chute and out through the side of the breaker. This made a very loud bang or sometimes even a shriek or a whistle.

Another characteristic of older tramcars was that the cupboard under the downstairs seats, which housed the spark gap, was rarely swept-out and accumulated a lot of flammable dust.

The sequence during a thunderstorm was:

1) Lightning strikes...

2) Spark Gap operates...

3) Dust explosion blows front off cupboard to expose flaring arc...

4) Breaker just over driver's head operates with flash and loud bang.

The instructions were that the driver should then re-set the breaker and continue driving normally, as quickly as possible. Brave men!

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

[... resistor fuse and a real fuse ...]

The resistor can only work as a fuse in the test department ie: before the real fuse is in the circuit.

Yes once the thing was assembled.

No because the PCB isn't in the chassis at that point.

This thread reminds me of something in my TV servicing days.

At one point the market was flooded by a certain brand of far eastern 14" portables, a stock fault with these was dried up electrolytics in the PSU which resulted in regulation failure, core saturation and destruction of the chopper transistor - the inrush limiting resistor always protected the fuse!

The resistor in question was a 4.7 Ohm 5W WW - one of those white ceramic 'box' with a spiral wound element cemented in.

I watched while my boss set up the situation and measured currents and voltage myself once he'd got the resistor into the stae he wanted. This was back in 1975, and the guy was an active member of the group controlling the "intrinsic safety" rules for electronic equipment to be used in areas wherre the was a risk of igniting inflammable gases or liquids. I've forgotten exactly how he got the current to concentrate itself into a red-hot filament in the carbon film - he had a procedure that worked reliably, probably picked up at an instrinsic safety committee meeting.

They can probably replicate your experience more easily than mine, but the two situations aren't mutually exclusive.

Most metals have a postive temperature coefficient of resistance, so this is irrelevant.

No, it was a good quality Philips carbon fim resistor.

Mostly. Raise the power dissipation at the right rate and you can create and sustain a low reistiance hot channel through the carbon film.

cares.

An incorrect and potentially dangerous misconception.

It wasn't normal practice anywhere I've worked - axial lead resistors were supposed to contact the printed circuit board at two points. If they were mounted out of contact with the board, they became susceptible to vibration - with body of the resistor acting as a mass on the end of the notionally springy metal leads - and people fussed about eventual fatigue fractures in the leads.

-- Bill Sloman, Nijmegen

On a sunny day (Mon, 25 Jun 2007 06:50:40 -0700) it happened snipped-for-privacy@ieee.org wrote in :

dinosaurs ruled the world back then. I predate these.

Maybe just like the greens today.

Burning carbon quickly oxidises with air, this was exactly why Swan (UK) and Edison (US) had to use vacuum in their light bulbs. If no vacuum the thing went out in seconds. But maybe the guy was from before electric light.

We were talking about fuses so it _is_ relevant.

mmm, Philips made very extensive use of these 1/4 W carbons as fusible, mounted away from the board, K6, K8 TV chassis IIRC. In fact I got the idea from them.

Edison would have loved you so much. Years he tried, and failed. Until he did read about some professor who called him an idiot, because air would burn his carbon wire.... Edison then bought a vacuum pump... And Menlo park had electric light. Or were you designing for space?

Oh well.

It is normal practice if you use these things as _fusible_. I dunno were you worked, but it cannot have been industry.

A look at some Japanese transistor radios showed them using resistors vertical, with one end folded back, the folded back wire painted, so it would be isolated and not short against an other one one millimetre away. You could drop these, and all resistors would point the other way, but it would still work 20 years later.

El Pante

Well.. I know a design for electronic current limiting of 120VAC with an adjustment range of 50mA to 15 amps. I guess I should build this project someday as it would be useful for years.. But it'll take 2 days.. It's quicker and easier to just blow up the occasional fuse or burn resistors acting as temporary fuses for prototype debugging.

Also..If I did make the electronic AC current limiter, I might need to imitate fuse behavior. Call it a fuse emulator? Fuse simulator? D from BC

Never worked in electric trains and such but it seems that those problems pop up in every branch of electric/electronic activity. To each his own. BTW I still remember from the late 40's an air gap on the radio antenna wire.

Stanislaw.

Nothing to do with environmentalists - it was a serious industry-based group, George Kent sold a whole lot of "intrisicly safe" electronics for use in oil refineries and the like.

Edison (US)

The resistor I saw did not burn out in seconds, and the "red hot" channel only showed as a discoloured line along the surface of the resistor.

I was talking about why your should not use carbon resistors as fuses, and this point is thorougly irrelevant to that discussion.

Philips also made properly specified fusible metal oxide resistors, which looked pretty much identical to their 0.25W carbon and 0.8W metal film resistors. Are you sure you sure that the reisistors Philips used as fuses really were carbon film parts?

Funny that Philips went to the trouble of making proper fusible metal oxide resistors if their carbon film resistors would do the same job.

Edison wanted the surface of his carbon resistor to glow for months and years. A hot conductive filament buried inside a carbon resistor doesn't have to last for very long to make it useless as a fuse, and is nowhere near as acessible to atmospheric oxygen as as the surface.

I wasn't designing these devices, just acting as a captive audience while being instructed why carbon film resistors were not a good idea inside intrinsicly safe equipment, since they couldn't be relied on to fail open circuit.

Kent Instruments in Luton was industry. Cambridge Instruments in Cambridge made electron miscropes at the rate of several hundred a year, and looked very like industry to everybody working there, though they were a bit slap-dash in comparison to Kent Instruments and ITT- Creed in Brighton, where stuff was produced at rates going up to a couple of thousand units a year.

would still

But not for much longer than twenty years - mine stopped working after about twenty years, mainly because a few of the vertically mounted resistors had broken up their printed circuit tracks. I fixed it a couple of times but eventually bought something better.

Consumer products don't have to be as reliable as industrial units, and don't usually have to cope with as much vibration.

-- Bill Sloman, Nijmegen

Carbon film resistors can form a carbon arc if you hit them with a bit overload at sufficient voltage. Try putting a 1K 1/4W resistor across

120VAC.

You end up with a glowing ceramic rod and a lot of current flowing. SMT resistors, which are not carbon, are probably more reliable as fuses, but you can also buy SMT fuses all ready made like.

BTW, IIRC 10 ohm "carbon film" resistors are generally not actually carbon film, but some kind of metal film (nickel alloy or something like that). They transition between technologies at some resistance level, which I don't recall (probably manufacturer-dependent), but it is below 100 ohms. Philips used to list the characteristics in their detailed data-- the tempco was typically quite a bit better for low-ohm 5% leaded resistors than for higher resistances.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
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On a sunny day (Mon, 25 Jun 2007 19:01:54 -0500) it happened Spehro Pefhany wrote in :

Yes, that is why I wrote in my first reply on this topic that 'the distance must be big enough so the electrons take no shortcut'.

I have never used SMT fusables... no comment.

You may well be right, maybe I am a bit too old school, I remember in my early electronic days metal film existed, but nobody used it, no shop had it... They first started appearing in consumer stuff like audio because of lower noise then carbon.

I dunno what Philips put in the resistors, I have scraped a few of, and measured some that had the paint burned but still the carbon (or whatever) spiral exposed, sometimes to see where it was open. Some crack is all it takes, it would sputter an burn the carbon at that spot. My recent experience with metal film was that it did _not_ open, and worked as heater element :-)

Maybe you remember those very high value high voltage carbon resistors too, a paper tube with a carbon spiral on it..... these are several centimeters long. For high voltage use longer resistor. And those would open too... Not used as 'fusible' though.

Maybe better forget fusible resistros, uses some real fuses, repair with wire.

If you do it right, you can end up with a lot of current flowing at a relatively low voltage drop with very little dissipation and no glow. This can be a bit disconcerting when you realise that you are seeing several amperes flowing through an ostensibly normal looking 10k carbon film resistor.

-- Bill Sloman, Nijmegen

I would say that was doing it wrong. What *I* think happens has nothing to do with carbon or metal film. I think if you slowly heat some ceramics then the *wrong* ones will become good conductors. This is very old knowledge indeed:

So your GreenPeace aspirant just selected an energy level that he should not have, heated up the ceramic rod, it became conductive, and voila (=French for 'see' ) the situation you describe. I did set out saying you need enough energy to 'evaporate' the stuff.

Now that makes sense.

Yes, picofuses are pricey and a fusible resistor is cheaper and works about the same. PTC is the fancy way to go, but you can also use... a light bulb. A simple 200W@120V light bulb will limit the current to 2A when your board shorts out, and the cold resistance is relatively low (about 120 ohms) so will permit normal operation at low currents.

It's a property of pure metals that they are ALL positive-tempco resistors. Tungsten filaments included.

peeseebee.

This is true, but irrelevant.

This is your theory, based on a thoroughly wrong-headed appreciation of what was going on.

The body of the resistor I observed was not noticeably hot - certainly not hot enough to conduct. The claim was that the current was being carried through a hot channel in the carbon film. The core of the channel was supposed to be very narrow, so it didn't take much power dissipation to keep it hot enough to sustain almost metallic conductivity. The resistor looked almost entirely normal after the current was turned off.

And you were wrong.

It might make sense to you, but it doesn't have anything to do with what I observed.

-- Bill Sloman, Nijmegen

On a sunny day (Tue, 26 Jun 2007 13:56:20 -0700) it happened snipped-for-privacy@ieee.org wrote in :

Observation is very subjective. I am sure you have some reference with measurements?

I was thinking this way:

1) manufacturer name: MuchProfit 2) Designer name: NoKnowledge

MuchProfit: For a 1/4 W 4.7 Ohm resistor the max voltage will be U^2 / 4.7 = 1/4 say 1V. For a 1% error the ceramic resistance at max temp can be 4700 Ohm, I will use cheapest ceramic from (China?).

NoKnowledge: I will use this 4.7 Ohm as fusible in a 325V DC circuit after the rectifier, it will limit surge too, it will carry about 210 mA, so drop about a volt, Within resistor power rating.

Event: Some time on, resistor temp 150 degrees C, a short happens, resistor carbon coating evaporates, now we have 300 V in 4700 Ohm = 90000 / 4700 W = 19 W dissipated in the ceramic, it get hotter fast, starts to glow, room lights up, resistance gets lower, more light

A NEW STAR IS BORN.

Sorry, could not resist.

Al persons are purely real, all theory need not be>

I wrote this. < end claimer>

.

Don't be silly. This was some thirty years ago. But observation isn't all that subjective.

The object of the demonstration was to make a point, and the point was made clearly enough that I remember it still - carbon film resistors can't be relied on to fail open circuit, and can (under appropriate circumstances) fail to a state that looks very like a short circuit.

Don't flatter yourself.

-- Bill Sloman, Nijmegen

On a sunny day (Tue, 26 Jun 2007 16:52:15 -0700) it happened snipped-for-privacy@ieee.org wrote in :

mmm, keep dreaming ;-)