On a sunny day (Sun, 24 Jun 2007 20:18:22 GMT) it happened D from BC wrote in :
Just do not use metal film, I did, and it burned a hole in the peeseebee. Carbon is OK. But the energy dissipated in it when a sort circuit or overload happens must be high enough to evaporate the carbon. And the size must be enough so the electrons do not take a shortcut and flash over. Old tube TVs used wirewound reistors in ceramic tubes that heated a soldering joint holding a spring breaking the contact.
There are also microfuses that are re-usable for a variety of volatges and currents.
I have also split apart the strands of flatcable and soldered that over the fuseholder clips... when no fuse at hand.
Digikey only has stock of a 100mA 230V version of the above link.
An LVR200S with 2A 240V looked good...but no Digi stock.
Digikey doesn't always list the voltage rating for PTC thermistors.
I'd like >170Vpk. So when the PTC trips it doesn't internally arc, burn or decompose or whyever it's got a voltage rating. I"ve notice on Digi that prices are about $1.00 each.. Maybe shoe-in resistors are much cheaper and easier to get and therefore easily expendable.. D from BC
I've seen the wire used in glass fuses.. I've been tempted to source that wire to make custom fuses or temporary fuses. It might be just a matter of selecting a wire length or winding for different fuse currents. D from BC
I have used a resistor as a fuse in a PCB that went into production equipment. Once the unit makes it through test and is installed, the system fuse protects the fuse resistor. The fuse resistor is only the protect against shorts during testing. It worked out quite well once I explained for the 15th time "No, we don't need a higher wattage resistor there".
Fuses have the advantage that they are clearly labeled as fuses.
Polyswitches are a good way to go but you may have to still have a fuse to protect the PTC device. Don't trust the ratings you see on the first page of the data sheet.
Later in the data sheet you will often find the real limit on voltage is much less than the first page says. The first page's rating assumes that you gently take the current up until it trips and then apply a short pulse of the voltage. You can't take a 600V PTC and hook it to a 600V supply and have it survive.
??? The system fuse protects the fuse resistor?? Does this mean the system fuse is more sensitive and the fuse resistor can never blow..??
Was that fuse resistor in series with the true fuse? But wouldn't a dummy fuse be needed during testing? The dummy fuse is a short circuit. Assuming a removable fuse in clips. The dummy fuse is replaced with a real fuse at the end..
I'll be looking carefully at those PTC voltage specs.. thanks.. D from BC
Carbon is not okay. It has a negative temperature coefficient of resistance and if things go wrong, you can find your 10k carbon film resistor carrying about an ampere at a voltage drop of a few volts or less - all the current is flowing down a narrow, red-hot channel through the carbon film
Metal fim and metal oxide resistors do at least have a positive temperature coefficient of resistance, They are designed to run hot when dissipated their rated load - somewhere around 250C - so if yu do want to use them as fuses, bend the leads so the resistor body sits a couple of millimetres above the printed circuit board.
From my experience, this will only work for a gross overload. A 1/4 W resistor might cook for a long time at 1W before anything happens. On the other hand, put 12V across a 1/4 w 5.6 Ohm carbon resistor, and you will get instant gratification. No smoke, no discoloration; it will instantly break in half. Make some tests with your resistors. The Resistor Police say you should wear goggles.
Those are supposed to blow if a mouse sneezes within two meter radius.
What so holy about fuses? Why resistors when in any workplace there are various wires, copper, aluminium, steel that can be used. And if you have a normal worktable pull out few nails, they parade as fuses, no complains whatsoever.
Well, they're characterized, and guaranteed to have certain characteristics. They may use uncommon materials. They are generally safety-agency approved so they should be less likely to start fires or allow fires to start.
I guess the 5lb spool of AWG39 copper wire I have is a thousand-lifetime supply of ~2A fuses.
Metal film precision resistors fuse okay, but you'd have to experiment to find the characteristics.
Best regards, Spehro Pefhany
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Joking aside, the choice of the correct fuse is not always as obvious as just finding the right bit of wire.
I designed some equipment which I protected on the incoming mains side with a 2A slow-blow 20mm glass fuse in an enclosed plastic holder. It wasn't rated to withstand a dead short, but there was a 3A HRC plugtop fuse in series, which would do that. Unknown to me, there was an earthing strap built into one of the equipment's terminal blocks; and it was on a terminal which I had designated as the live. It put a dead short across the mains.
The first time I switched on, there was an almighty flash and bang - and a tinkling noise from behind me. The fuse holder had been blown apart and pieces of it had just missed my face and hit a metal venetian blind, the tinkling noise was the debris tumbling down the slats. At the point of impact, there was a dent in one of the metal blind slats.
Further investigation showed that the plugtop fuse had a very slow characteristic and had sustained the current long enough to dissipate a lot of energy in the glass fuse.
A 1W 0.25 ohm resistor would happily carry 2A and would gently overheat and blow if the current went up to 5A - but under a short-circuit on
240v mains, it would dissipate over 200kW. You wouldn't want that to continue for very long, so check the time rating of the mains fuse.
~ Adrian Tuddenham ~
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