Transistor Power Switch

One inexpensive and practical method of reducing dissipation in a digital circuit is to power the active components only as they perform a function, as for instance when operating briefly during a low duty cycle. Power is saved by clocking on the power for the IC's only when necessary, so that the IC's draw no energy during periods of inactivity.

A solid-state switch/low-pass pifilter has been designed to source these components. The filter employs a storage capacitor that is trickle-charged to present a constant load to the main power supply. Other filter components are used to smooth transients as the IC's are switched on. The circuit may be useful in battery-powered equipments.

A schematic of the switch/filter is shown above. Power is supplied to the switching transistors Q₁ and Q₂ via the filter network comprised of C₁, C₂, R₁, and R₂. As Q₁ and Q2 turn on, current is drawn from C₁, a capacitor that is charged to a voltage equal to the IC supply voltage V_CC (plus the collector-emitter drop across Q₂).

The V+ supply therefore is slightly greater than 5 volts for TTL devices but is less than 5.5 volts (the design maximum specified for these devices). The values of R₁ and R₂ are chosen to permit C₁ to charge to the required voltage during the off cycle. Components C₂ and R₂ are optional; they provide additional circuit filtering.