Understanding Open Collector Relays
Open collectors are electrical components that control input signals to different devices. These outputs have a simple design but play an important role in applications spanning multiple industries. Here’s a look at what they are, how they work and what they do in electrical relays.
What Is an Open Collector?
Open collectors are outputs you can find on different electronics and most integrated circuit (IC) boards. These work with DC power and behave a lot like power switches or Solid State Relays (SSRs), allowing you to toggle the input signal to turn different circuitry on or off. An open collector is either wholly off or fully on — they do not also amplify or attenuate. An open collector diode is sometimes included in the series to protect against unexpected voltage shifts.
How Are Open Collectors Used?
Open-Collector Outputs are a specific type of BJT (bipolar junction) transistor configuration. In addition to the collector, an Open-Collector Output also has a base and an emitter. These transistors have a passive open collector output that either connects to an electrical ground or generates a floating voltage.
The component takes its input signal from the IC board and activates. During open collector transistor operation, the BJT transistor sends the output signal to a Negative-Positive-Negative (NPN) transistor. Unlike standard digital outputs, an open collector doesn’t supply any power to the load. Instead, these components flip the switch that connects an energized circuit to an electrical ground.
What Are the Applications for Open Collector Outputs?
As an integral part of many IC chips, Open-Collector Outputs are useful for countless applications, but they typically require pull-up resistors to work. Any time you use an open collector with a floating ground, you need to have a pull-up resistor between the VCC (Voltage Common Collector) and ground to stop your circuits and logic gates from self-biasing and sending false signals.
Using the pull-up resistor, you can change the open collector transistor operation to send signals in different voltages. This capability makes them useful for controlling external circuitry, like powering a 12-volt relay or interfacing with devices that require varying input voltage levels, like digital logic gates, amplifiers, sensors, servo motors and logarithmic converters.
Why Are Open Collectors Used in Relays?
Open collector transistors offer better flexibility for voltage and current than standard logic gates. The component design lets you connect multiple outputs to a single connected line. With open connectors, you can choose which devices you want to activate, without interference. Relays with open collectors also do not need to match the input voltage. You can use them on 5-volt logic circuits that drive components requiring voltages five, 10 or beyond 20 times higher.
Expert Open-Collector Output Testing and Repair
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