Using Digital Output Lines to Switch Power Relays

Using your digital I/O board to switch AC or DC power relays gives your PC the power to handle control applications. The simple combination of relay modules such as those from CyberResearch, Opto 22, or Gordos, used with a digital I/O board, gives your PC On/Off control of any type of equipment. The relay acts as a light switch, and is no more difficult to program than writing “1s” and “Øs” to an address register.

Opto-Isolated Solid-State Relays

The diagram at right illustrates how an opto-isolated solid-state relay works. Notice that your digital I/O board functions as a current sink. When the voltage level on your digital I/O line goes low, it allows current to flow from the 5V power supply to ground on your digital I/O board. This is called "Negative True Logic" because the relay circuit is closed (turned on) when the voltage level of your digital I/O channel goes low (turns off). The light-emitting diode (LED) in series between the power supply and the current sink on your digital I/O board closes the circuit as the light it emits is picked up by a light-receptive transistor. This closes the contact and turns on power to your AC or DC power line. The greater the current sink rating of your digital I/O board, the more current will be able to flow from the power source through the LED, ensuring that enough light is emitted to close the circuit.

8255 Insufficient for Power Control

Many digital I/O boards, including several offered by CyberResearch, are designed around the 8255 PPI chip. This very popular chip uses technology capable of sinking only 1 TTL load (approx. 2.5mA). 2.5 milliamps of current flowing through the LED is not enough to close the circuit. There are two methods you can use to get around this problem:

1. Use a high current-sink board like the CyDIO 24H.

2. When using an 8255-based DIO board such as our CyDIO 24, 48, 96, or 192, use our CySSR 24 solid-state relay mounting rack with onboard buffers to compensate for the lack of current sink.

Of course, 8255-based DIO lines work just fine when used with Opto-22 sensing (input) relays. Sensing modules monitor the status of your power lines and therefore require no current sink.