The right kind of fuse

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In theory, picking the right fuse should be easy, an appliance will have a current rating or it can easily be calculated using its power rating. However, in practice this is only a guide because switching on and off will often result in currents far greater than the rating. I well remember trying to set up a room full of computers for a course in a science lab where every time we switched on the 4^th computer the circuit breaker/current limiter tripped. We then had to go round the school looking for an older lab with less protection and then summoned my registration group to carry the equipment from one end of the school to the other. We are indebted to Dr. Peter Copley for the elegant demonstration of this principle.

A simple circuit with 12V bulb, ammeter sensor (channel 1 of Datadisc) and switch are all that is required. Choose the fast option from the Measure menu and set a duration of 1600 ms. Start the recording and immediately switch on and then off. Some practice will be needed to get as good a graph as is shown below. In this case the current rating of the bulb would appear to be just under 1A, yet the initial current is nearly 2A.

See the experiment with Ohm's law and a lamp for a further discussion of the issues raised.

Our thanks to Dr. Peter Copley for this experiment and data.

You can contact him at:
Newcastle-under-Lyme College
Liverpool Road
Newcastle-under-Lyme
Staffs
ST5 2DF
Phone 01782 715111
Fax 01782 717396