EDN -- 05.09.96 Circuit converts solid-state relay to timer

Design Ideas: May 9, 1996

Circuit converts solid-state relay to timer

Henno Normet,
Tavares, FL

Finding a 480V-ac solid-state timer having zero-crossover switching may prove to be an impossible task. Most solid-state timers are rated at only 1A and have voltage ratings no higher than 240V ac. Solid-state relays (SSRs), in contrast, have much higher ratings and are readily available. The universal timing circuit in

Figure 1 converts almost any SSR into a solid-state timer. The only requirement is a dc control-signal input (usually 3 to 32V dc) for the SSR. The minimum signal input must be slightly higher (8V dc) for the timer IC to operate reliably.

The programmable timer, IC₂, has a built-in, low-power RC oscillator. R₁, R₂, and C₃ determine the oscillator frequency. A four-position DIP switch provides control over the amount of time delay and the modes of operation. With S₁ open and S₂ closed, the time delay ranges from 0.3 to 15 sec. Other S₁-S₂ combinations (Figure 2a), multiply this basic range by 4, 32, or 256, for a maximum total delay of 64 minutes. S₃ and S₄ select the time-delay mode (Figure 2b). Four modes are possible: delay-on-make, interval, recycling (starting with off), and recycling (starting with on). In the recycling modes, the duty cycle is 50%. You apply the dc input at t=0 and remove it at reset. (DI #1863)