[AMRadio] High Voltage Fuse Idea

Jim Candela jcandela at prodigy.net
Thu May 29 07:39:59 EDT 2008

I once had the task to find a suitable HV fuse for use on a 400 watt solid state lighting ballast that needed to isolate a bad ballast while holding off 360 volts DC. The ballast used a little over 1 ampere when working, and when a fault ocurred (shorted transistors) the current could bring down the main buss and possibly result in a fire. 

The fuse ultimately chosen was a 3 amp pico fuse. These are about the size of a 1/4 watt resistor. The key to making this work was the layout on the PCB. The approach was to mount the fuse upright with one side against the board (the incoming 360V), and the other lead folded back and soldered to the board about 3/4" away. When a fault ocurred on the 360 volts DC, the fuse would vaporize and the body would break apart and swing away such that the longer lead (Load side of Fuse) would bend the other way as the mini explosion took place. This would make a gap such that the current flow was interrupted. I still recall having a blast (pun intended) using a big 360v 10A power supply, a knife switch, and playing with board mounting arrangements with various fuse types.

This approach is only valid when we are protecting against a fault that becomes a short circuit. It is not suitable for isolating a circuit where the circuit must be interrupted at a value that is not large with respect to normal current flow.


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