Re: Open Collector
I think IanP is refering to an open-collector buffer/driver, for example 7406 with maximum VOH up to 30V and maximum IOL up to 40mA, thus the output load (for example an incandescent lamp or car signalling indicator used in automobile of 24V, 25mA.
VOH = Logic High or 1 Output Voltage
IOL = Logic Low or 0 Output Current (refers to sink current of the transistor when it is turned on, resulting logic low or 0)
In general, open-collector is used in the followings:
1. It can be used with a pull-up resistor Rp of about 1k to 10k for Wired-AND operation, whereby several open-collector outputs can be tied together to the same pull-up resistor Rp for driving logic 1 and 0.
However, in consideration of the load capacitance in the interconnects and transistors, Rp cannot be too large, else the switching speed is limited due to RC delay.
Rp cannot be too small because the this will increase the sink current from supply rail down through Rp and the transistor (that is turned on), thus too large current can kill the transistor.
So Rp should be carefully chosen so that IOL (max) is not exceeded and Rp is not too large that will make RC delay too long.
2. In some special applications whereby logic 1 or High is not needed, Rp is not used, thus high impedance output or logic 0 is required. This is used for example in the above incandescent lamp, which itself presents a load resistance, thus serving the purpose of Rp in the application. Thus the application does not require to add a pull-up resistor Rp.