Tech Forum

Fuses are specified according to the circuits they are designed to protect. Unless you are certain that the original design specified a fuse of inadequate capacity or excessively rapid response, you should never attempt to substitute a higher current or slower acting fuse. If your power supply is blowing fuses, you need to determine why this is happening. Are you overloading the power supply? Does it have an internal fault? Do the fuses you are using match the original type specified by the power supply?

Properly designed and operated equipment that is in good working order should not ever blow fuses.

It’s not a good idea to over-fuse your power supply (PSU). The fuse was designed to protect the supply from damage (and fire) if it becomes defective internally or is operated beyond its design limits. You are probably exceeding its capabilities in some way, such as overcurrent due to too heavy load. If you’re not exceeding its specifications, there might be something wrong inside the PSU, such as bad filter capacitor(s) or a defective power transformer.

Have you checked the output under load with a scope? That will tell you if the PSU has high ripple under load, an indication of poor filtering inside the PSU. I suggest that you do a little investigation to determine whether the blown fuses are due to trying to operate it beyond its capabilities or bad component(s) in the PSU.

Your multimeter is a good tool to help do this. Watch the needle or display and see what the output voltage does right before the fuse blows. Analog meters are better in this situation. The first thing that comes to mind; does your load have a large capacitor that needs to be charged by the PSU? Large capacitors need high values of surge current from the supply until they acquire a full charge. If that’s the case, you might lower the value of the capacitance at the load. Use your multimeter as an ammeter to watch the current to the load. Is it at or beyond the specificied rating of the PSU? Again, an analog meter is best.

If you’re operating the PSU at its limits, the internal circuitry could be overheating. Mount a fan or blower so that it directs air over the heat-producing components (heat sink, power transistors, power transformer). If you absolutely need to run the PSU at its limits, you might consider getting a more robust PSU. It will be more likely to survive.

Cheap PSUs are sometimes over-specced, meaning that they meet specs only under very controlled conditions. Also, is the PSU rated for full output continuously? It might be overheating if it’s not rated for continuous operation.

The last thing I can suggest is to check your mains voltage to the PSU. Is it at or near the high limits of the PSU? If so, you might use a Variac or bucking transformet to lower the mains voltage to the supply. Hope that gives you some ideas that will help determine why the fuses are blowing so frequently.

The first thing you need to discover is why you blow fuses!

1) Are you trying to directly measure “current” by putting your meter leads directly across the supply’s output terminals? (a mistake always made by first week electronics students)

2) Does the device you want to power demand more current than the supply can deliver (i.e., 1A supply trying to power a 10A device)?

3) Is there a short circuit in the device you’re trying to power? Use your meter to measure the resistance between the device’s power terminals: a reading less than 10 ohms indicates an unwanted short circuit.

Find out what’s causing the supply fuse(s) to blow before you try substituting higher-value/slo-blo types. The power supply will be much happier overall and you’ll minimize the chance of having the supply blow up altogether!

Do not use a fuse with a higher amp rating. Try the same value fuse in a slow-blow fuse. A fast-blow fuse with a higher rating could lead to blown components, melted wires, and possibly turn your power supply into a smoke generator.