What does collector mean?

Answer 1

Open collector output is normally used in logic circuits and can be viewed as a common emitter configuration for a BJT transistor and normally of type NPN. A typical common emitter configuration requires the emitter to be connected to the ground and a resistor Rc from Vcc to the collector. When the transistor becomes forward biased, the collector will pulled the voltage across the resistor down, from approximately Vcc down to almost 0V, normally in the order of 0.1V at the collector.

We can see that the output at the collector will swing between Vcc and ground. In open collector outputs the collector resistor Rc and Vcc have been left out. This enables the engineer to use any voltage and any pull-up resistor that will meet the specifications of the output transistor.

Normally it is acceptable in logic circuits that multiple inputs may be connected to one output, but not multiple outputs to one input due to the nature of the active outputs. With open collectors however multiple outputs may be connected to one input without damaging the device.

Low state will result in a lower impedance than a high state since the high state is obtained by current flowing from Vcc through the pull-up resistor Rc.

Open collector can be used to convert 5V logic to 8V or 12V logic or down to 3.3V since the logic high will be determined by the external supply to the pull up resistor.

Open collector also prevents component failure during output short-circuit or over-current. If an open collector is short circuited to the ground, it will cause no harm to the device since all the current will be supplied via the Rc resistor. Short circuit to a +supply rail can however damage the device.

Open collector is not used that commonly anymore but many variations of the same concept can be found in modern electronics. For example: micro processor devices that employ a I2C (Inter-Integrated Circuit) communications interface make use of a open-drain lines.

It appears to work very much in the same fashion with pull-up resistors and all. Open drain is associated with MOSFET transistors instead of BJT's. In this example an I2C interface will fail if pull-up resistors are not used. Like open collector, open drain lines are also passive and does not provide any active high output of its own. The High output is supplied by Vcc via the Rc or Rp pull-up resistors. The open collector or open drain will pull the pull-up resistor voltage down to the ground to create a low impendence, low logic state.