Decade Capacitance Box

The classic demonstration transformer apparatus is added with some additional safety features. Rather than the traditional mains (220-230V AC) coil we have designed a dedicated Demonstration Transformer Power Supply with fixed (25V AC) voltage and high current output to which 100-turn or 150-turn mains coil substitutes connect. The advantage is that no lethal mains voltage is involved, instead the voltages to these mains coil replacements is 25V AC and so safe. Additional AC and DC outputs are provided up to 4V for other experiments.

Investigate the working principle of circuit breaker type fuse, with this demonstration model. The apparatus is built on a robust plastic base and includes resistive loads to simulate external wiring circuits. Applications of the thermal response of bimetal strips and also electromagnetism (movement from a solenoid) are clearly demonstrated in this model.
When a 12VDC supply is connected (not supplied) the LED lights to show that current is flowing in the external circuit and the bi-metal strip can be seen to bend as its heater coil raises the temperature. After a few minutes the bending is sufficient to trip the mechanism and the current path is broken. At any current selection a short circuit situation can be initiated by pressing the red button and this produces an immediate response from the solenoid to disconnect the supply.