"Lamp-Fuse" stops
230V 'Bang' during

During repair, there is one thought that makes the heart of every technician stand dead still for a few beats - "I hope it is safe to turn on!". Most times it is, but every so often it's not and the "bang" as the switch if flipped makes a search for a cup of tea of paramount importance!

This is especially true when repairing direct rectified power supplies such as the type found in PCs. The switch-mode PSU is not only confined to PCs, but is now used in almost every piece of electronic equipment that derives power from the mains. I clearly remember this same feeling when repairing 230V driven process control instruments.

This feeling is perfectly understood as the mains has a reasonable amount of power unleashed during a fault condition. It's not just amps that makes up power, but the volts too. With 230 of them, the most innocuous of faults can release many kW in a very short space of time. If this finds its way into the rest of what was working circuitry, well, the emphasis would be on the "was working" bit!

What is required is to somehow limit the amount of current flowing through the mains fuse on a piece of equipment during the first switch-on after repair. It is also convenient to have this during the soak-test time should the fault decide to reappear when one's back is turned! All this without tying up valuable pieces of test equipment.

A proven solution is to take a blown fuse and to carefully solder two wires on to the end caps and to bring these out to two 4mm sockets. This allows for connection to any form of instrumentation or, better still, to be put across a lamp. The idea is that should there still be a fault that is potentially going to draw a lot of current from the mains, the worst it can now draw is what the lamp will allow.

When it comes to testing the device just been repaired, then simply chose a lamp that will allow sufficient current through to the device to allow it to operate, but should there be a fault then not too much will flow to cause further damage. Experience has shown to have a range of lamps from 15W pygmy, to 250W (ah!, sorry, almost forgot, they must be incandescents!).

The lamp has two very convenient features. It tends to operate in a "constant current" manner. It's not perfect, but heck, it's close enough for our purposes. But, before it starts to regulate the current it too has a bit of inrush (the lamp's cold resistance is quite low by comparison). This allows any capacitors to be charged before the device itself starts to draw current. Almost a version of "kick starting"!

Just remember there are many shapes and sizes of fuses so be armed with as many as possible. A preference is the "ceramic safety fuses" as they are stronger and tolerate being inserted and pulled out many times. When having to constantly repair the same type of item, then having a fuse permanently soldered to the bottom of the suitable lamp is a neat trick.

There is nothing stopping one from building this into a box with a plug and socket. This does, however, have a limiting feature in that it only allows for testing of the device via the mains. I have often found that there is a need to test after the rectified portions, as commonly found in TVs.

There is one word of caution. I have found on many occasions that choosing a lamp of too low wattage can leave one believing there is still a fault - the lamps burns quite brightly. Sometimes there is a need to "go up a notch" as the device under test may have a high inrush or starting current which drops once the device is running.

How nice it is to not blow fuses, violently, during repairs any more!

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© 02.11.02