Protecting the Fat Man

In my utillities closet (where the meters, for gas, electricity and water live) I have this big fat transformer (2 windings of 9 Volts @ 81 VA each) that I use to power many small devices all over the house. At present I use it to run the doorbell, the router for the second floor, a small battery charger in the workshop and many gadgest in my dungeon, among which are the MWS webserver and the Tiny8515 AVR kit.

From the closet, two cables run through the house, with a fusebox at a central point. The most important fusebox is near my cable modem and associated WAN/LAN router. This article is about this fusebox.


To the right, you see the drawing for the fusebox. It relies heavily on the socalled self resetting polymet fuse. This is a PTC that was learned a new set of tricks. One is, to keep the high resistance even when the voltage drops drastically. You can read more about the 'PolyFuse' on the site of the manufacturer: .

The red LED on the far left signals if there is power coming in from the Fat Man. The five green LED's which monitor each of the protected outputs light if the fuse is not tripped. When a green LED is not lighting, the fuse blocks power for this peripheral.

The green LED's get their power AFTER the fuse, so if the fuse blocks, the LED goes dark.

Fusebox in action

To the left, you see the FuseBox in operation. The power enters the box on thge right side (green banana plugs) and at present two sets of cables accept power from it's outputs (blue and yellow banana plugs). One green LED appears to be out, but it is simply blocked by a cable.

Page created on 2 October 2006 and