A fuse resistor is an ordinary resistor (or a special *fusible resistor*, as Mike mentioned above) that is used as a fuse when its electrical characteristics are violated, resulting to its destruction --thus breaking the circuit.

In order to protect a portion of a circuit against over-current of, say, 100mA, I will simply insert a *suitable* resistor in he current path, according to the formula below:

P = I^2*R = U^2/R <=>

R = P/I^2 = P*U^2

where P is Power in Watts, I is current in Amperes and U is voltage in Volts.

For example, to calculate a 100mA fuse-resistor I simply apply the formula above:

R = P/I^2 = 0.25W/(100mA)^2 = *25 Ohm / 1/4W*, or

R = P/I^2 = 0.125W/(100mA)^2 = *12.5 Ohm / 1/8W*, etc.

Of course, it is not that simple; but this is the principle of using fuse resistors.

-George