Improved phase control circuit

The following diagram shows a circuit using a neon lamp as a breakdown device. This gives smoother control and improved performance. The neon triggers when the voltage across the two 0.1μ capacitors reaches the breakdown voltage of the lamp (60-90V). Control extends from 95% to full off. The neon lamp phase controlled circuit shown below combines the low cost of the simple RC circuit shown before but gives improved performance. The circuit below gives half wave control from 95% on to full off. Full power can be easily obtained by putting a switch across the SCR. The circuit uses a neon. This gives the following improvements:

A higher impedance circuit can be used for control.

As a result, the control element (which is a 100k pot in the circuit below) can be replaced by a high impedance device such as a thermistor or light dependent resistor, for heating or light control applications.