Understanding Open Collector Outputs: A Comprehensive Guide

Open-collector output is a concept that has become increasingly common in digital chip design, operational amplifiers, and micro-controller (Arduino) type applications. It is used for interfacing with other circuits or driving high-current loads, such as indicator lamps and relays that may be incompatible with the electrical characteristics of the control circuit. In this article, we will provide a comprehensive overview of open-collector output, its operation, applications, and advantages.

What is Open-Collector Output?

Open-collector output refers to an output that is connected to the collector of an NPN transistor. The NPN transistor allows the sinking of current to common. It acts like a switch that allows the circuit, after the load, to be connected to common. This means that a source is required for the output to work. A supply through a load must be connected to the output; otherwise, the NPN transistor is creating a path to common, i.e. a dry contact. Therefore, if you were to measure the voltage at the output of an open collector that is not hooked up to some supply, you would not see a change in voltage. The voltage should be measured across the output load to determine if the open collector is working correctly.

Working Principle of Open-Collector Output

The working principle of an open-collector output is straightforward. The NPN transistor has three terminals: the emitter, base, and collector. When the base-emitter junction is forward-biased, the transistor turns on, allowing a current to flow through the collector-emitter junction. In an open-collector output, the output device is attached to the open collector of the transistor, which is unattached to anything.

In order for an open-collector output device to work, the open collector has to receive sufficient power. In order to understand this, you have to understand NPN transistors. In order for an NPN transistor to work, the base and the collector both need sufficient power. The base turns the transistor on. A much greater current flows from collector to emitter, but only if the collector has sufficient positive voltage.

Common Emitter Configuration

The most common transistor configuration when used for amplification or switching is the common emitter configuration. This configuration provides gain, which is the ratio of the output current to the input current. In a common emitter configuration, the input is applied to the base, and the output is taken from the collector. The transistor is connected in series with a load resistor, and the emitter is connected to ground.

Applications of Open-Collector Output

Open-collector output has several applications, including interfacing with other circuits and driving high-current loads. It is commonly used in voltage comparators, where it functions as an output collector output. Pretty much all voltage comparator chips, such as the LM311, the LM393, and the LM339, all function as output collector outputs.

Open collector outputs are used in a wide range of applications, from controlling motors and solenoids to triggering alarms and sensors. They are particularly useful for controlling high-power devices that require a current sink, as well as for interfacing with other circuits.

One common application of open collector outputs is in voltage comparators, which are used to compare two input voltages and output a signal based on which voltage is higher. Voltage comparators typically use open collector outputs to connect the output pin to ground when the voltage is below a certain threshold.

Another application of open-collector output is when the transistor switch is in its off state, the load is disconnected. One advantage of this is that multiple open-collector connections can be tied together, and any one can turn on the load independent of the others, operating like an OR gate. If we add a pull-up resistor instead of a load, we get a logic circuit. Each transistor functions as a tri-state inverter, together making up a (in this case) 3-input NOR gate. If A, B, and C are all low, none of the transistors are conducting (i.e., on), which means the output is high due to the pull-up resistor. If any of the inputs are high, the corresponding transistor is switched on, connecting the output to ground.

Advantages of Open-Collector Output

The advantages of open-collector output are numerous. One of the most significant advantages is that it allows a circuit to be operated with a different voltage level than the output. This means that a high-voltage circuit can be controlled by a low-voltage circuit, without the need for level shifters.

Another advantage of open-collector output is that it protects the output device from overvoltage damage. The output device is connected to the open collector, which is unattached to anything. This means that the output device is protected from overvoltage damage, as any voltage that exceeds the supply voltage will be clamped by the internal diode of the transistor.

Conclusion

Open-collector output is a concept that has become increasingly common in digital chip design, operational amplifiers, and micro-controller type applications. It is used for interfacing with other circuits or driving high-current loads, such as indicator lamps and relays that may be incompatible with the electrical characteristics of the control circuit. The NPN transistor allows the sinking of current to common, acting like a switch that allows the circuit, after the load, to be connected to common. It has several advantages, including allowing a circuit to be operated with a different voltage level than the output and protecting the output device from overvoltage damage.

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