Open Collector and Open Drain Circuit

The Open Collector output is term for BJT transistor circuit. The Open Collector output is the unconnected Collector of an NPN transistor, make available to the external circuitry, which switches to ground when active. This type of output would no supply voltage to output device, unless pull-up resistor connected to the Collector

The Open Drain output is term for FET type transistor circuit. The Open Drain output is the unconnected drain of an N-type FET, make available to the external circuitry, which switches to ground when active. This type of output would no supply voltage to output device, unless pull-up resistor connected to the drain.

Example Open Collector / Open Drain Application

1. Normally, this circuit operate as switch and it can have difference voltage between control path and Load path. As connection show in figure at below, the control voltage can be 3.3V or 5V (lower voltage). It output pin (collector) is applied with 12V. The L1 is load. The Diode D1 is protection diode (see note).

2. Circuit shown in figure at below, this circuit is working but, you will seldom see it because this circuit always have power loss (power consume).When control off (Q1 turn off), the current flow to the load L1. When control on (Q1 turn on), the current flow to Q1. This circuit will always have current flow.
This consider as bad design, because it always consume power.
The circuit will invert the signal output from control signal.
The R1 act as current limiter in this circuit, but it also have other name call pull up resistor.

Advantage of Open Collector / Open Drain circuit

1. The control line and output (collector/drain) can use difference voltage level.
2. Increase the output pin power.

Example Open Collector IC

The ULN2003 is most common open collector IC. Its output allow sink high current and bias with high voltage (500mA and 50V) and its input is normal TTL signal. The typical ic output voltage is Vcc voltage, and source current is from 100mA ~ 150mA.

Key to Remember

1. The Open Collector / Open Drain circuit will not supply voltage/current. It only sink current.