When to use open-collector and relay outputs

Open Collector Output

 

The Open-Collector Output is a transistor circuit configuration used in a wide variety of electronic designs including many integrated circuits (ICs). Three parts make up a transistor: the emitter, the base and the collector. These three parts form two distinct junctions, the emitter-base junction and the base-collector junction. These regions are either forward-biased or reversed biased depending on current flow through the transistor. Other components are added to control the rate of current flow.

 

In the open-collector circuit the current flow is either fully on or completely shut-off. The output acts as either an open circuit (no connection to anything) or a short circuit (to ground). The transistor's collector is typically connected to an external pull-up resistor, which sets a higher voltage to the output when the transistor is open. When any transistor connected to this resistor is turned on, the output is forced to 0 volts. Open-collector outputs are useful in many applications including summing, limiting and switching circuits.

 

For the switching circuit, instead of outputting a signal of a specific voltage or current, a control signal is applied to the base of an internal NPN transistor whose collector is externalized (open) on a pin of the IC. The emitter of the transistor is connected internally to the ground pin. The open collector provides a pull-up resistor that does not need to be connected to a voltage at the same potential as the chip supply (VCC). It is possible to use a lower or higher voltage. Therefore, open collector circuits are often used to interface different families of devices that have different voltage levels in their operating logic or to control external circuitry that requires a higher voltage level (for example a 12 V relay).