TESTING FUSES, LEADS AND WIRES

All these components come under the heading TESTING for CONTINUITY. Turn off all

power to the equipment before testing for shorts and continuity. Use the low

resistance "Ohms Scale" or CONTINUITY range on your multimeter. All fuses, leads

and wires should have a low, very low or zero resistance. This proves they are

working.

A BLOWN FUSE

The appearance of a fuse after it has "blown" can tell you a lot about the fault in the

circuit.

If the inside of the glass tube (of the fuse) is totally blackened, the fuse has been

damaged very quickly. This indicates a very high current has passed through the fuse.

Depending on the rating of the fuse, (current rating) you will be able to look for

components that can pass a high current when damaged - such as high power

transistors, FETs, coils, electrolytics. Before re-connecting the supply, you should test

the "SUPPLY RAILS" for resistance. This is done by measuring them on a low OHMs

range in one direction then reverse the leads to see if the resistance is low in the other

direction.

A reading can be very low at the start because electrolytics need time to charge-up

and if the reading gradually increases, the power rail does not have a short. An

overload can occur when the supply voltage rises to nearly full voltage, so you

sometimes have to fit a fuse and see how long it takes to "blow."

If the fuse is just slightly damaged, you will need to read the next part of this eBook,

to see how and why this happens:

FAST AND SLOW BLOW FUSES

There are many different sizes, shapes and ratings of a fuse. They are all current

ratings as a fuse does not have a voltage rating. Some fuses are designed for cars as

they fit into the special fuse holders. A fuse can be designed for 50mA, 100mA,

250mA, 315mA, 500mA, 1Amp, 1.5amp, 2amp, 3amp, 3.15amp 5amp, 10amp,

15amp, 20amp, 25amp, 30amp, 35amp, 50amp and higher.

Some fuses are fast-blow and some are slow-blow.

A "normal" fuse consists of a length of thin wire. Or it may be a loop of wire that is thin

near the middle of the fuse. This is the section that will "burn-out."

A "normal" fuse is a fast-blow fuse. For instance, a 1amp fuse will remain intact when

up to 1.25 amp flows. When a circuit is turned on, it may take 2-3 amps for a very

short period of time and a normal 1 amp fuse will get very hot and the wire will stretch

but not "burn-out." You can see the wire move when the supply turns on.

If the current increases to 2amps, the fuse will still remain intact. It needs about 3

amp to heat up the wire to red-hot and burn out.

If the current increases to 5 amp, the wire VOLATILISES (burns-out) and deposits

carbon-black on the inside of the glass tube.

A slow-blow fuse uses a slightly thicker piece of wire and the fuse is made of two

pieces of wire joined in the middle with a dob of low-temperature solder. Sometimes

one of the pieces of wire is a spring and when the current rises to 2.5 amp, the heat

generated in the wire melts the solder and the two pieces of wire "spring apart."

A slow-blow fuse will allow a higher current-surge to pass through the fuse and the

wire will not heat up and sag.

Thus the fuse is not gradually being damaged and it will remain in a perfect state for a

long period of time.

A fuse does not protect electronic equipment from failing. It acts AFTER the equipment

has failed.

It will then protect a power supply from delivering a high current to a circuit that has

failed.

If a slow-blow fuse has melted the solder, it could be due to a slight overload, slight

weakening of the fuse over a period of time or the current-rating may be too low.

You can try another fuse to see what happens.

You can replace a fast-acting fuse (normal fuse) with a slow blow if the fast-acting fuse

has been replaced a few times due to deterioration when the equipment is turned on.

But you cannot replace a slow-blow fuse with a fast acting fuse as it will be damaged

slightly each time the equipment is turned on and eventually fail.