Electronic Design

Novel PC-Compatible AC Power Controller Uses Solid-State Relay

This design describes a simple, cost-effective solution for an ac power controller using PC-compatible conduction-angle control. A solid-state relay (SSR) is activated through a pulse-width-modulated (PWM) transistor-transistor-logic (TTL) timing pulse sequence (Fig. 1). The sequence is generated by a single-bit serial-pulse stream from the PC's LPT port. To make and break contact, the SSR requires "On" and "Off" pulses. SSRs are designed to accept a pulse width for the required power-on time. In this design, the synchronized PWM pulses will trigger SSR operation in synchronization with the ac cycle, yielding the proper firing sequence.

This particular design receives a PWM pulse corresponding to a PID error function. However, any other function required by the control program may be used: On/Off, PID, or linear heating, on-sweep, etc. The control program converts the PID function output to a triggering pulse with the appropriate timing for the 50-Hz (20-ms) cycle. Then it's sent through the PC's LPT port as serial single-bit PWM information. If the PID error value calls for "more," the PWM pulse will fire the triac in a short interval from the zero crossing of the ac cycle (i.e., near 0°) and deliver more power to the heater/load.

For lesser PID values, the triggering pulse gets generated near the 10th millisecond (near 180°) in the positive half cycle and near the 20th millisecond (near 360°) in the negative half cycle, delivering less power to the load. The rising edge of the ac cycle zero crossing is monitored through the PC's LPT1 status port via the diode-hex inverter 74HCT14 circuit for synchronization. Figure 2 shows PWM sequences of the ac power for three different PID function values.

The control program sends an appropriate single-bit data with proper delay for the PWM pulse through LPT port address 0x378 (LPT1), or 0x278 (LPT2). The control program can be written in any language, including C, C++, VB, VC, and even graphical languages like LabView, BridgeView, and HP VEE. The main advantages offered by an SSR are an internal zero-crossing circuit and snubber network, along with a rated heatsink. Also, the optical coupling in the front end of the SSR provides complete isolation of the PC's LPT port from the ac power line.

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