Open Collector Output Explained by Three People


This has always confused me a bit, and it is not discussed in many text books. So I asked and got these replies presented here.

For an NPN transistor:

The 'open collector' output is the unconnected Collector of an NPN transistor, made available to the external circuitry that switches to ground when active. The Emitter of the transistor is typically connected to ground (or 0v or -Ve supply) and the Base is driven by a positive control voltage (WRT the Emitter) via a resistor. Thus the transistor 'translates' the low current low voltage output of a logic device (or linear op-amp/comparator device etc.) to provide an output that can switch a higher current and voltage to the specified limits of the transistor. 

This type of output can be used with a pull-up resistor to translate a logic level from one voltage to another, or to drive lamps, relays etc., that have a positive voltage on one terminal and require a switch to ground (or -Ve or 0v) to activate.


For a PNP transistor:

The 'open collector' output, is the unconnected Collector of an PNP transistor, made available to the external circuitry, that switches to the positive supply when active. The Emitter of the transistor is typically connected to positive supply (or Vcc or +Ve) and the Base is driven by negative control voltage (WRT the emitter) via a resistor. Thus the transistor 'translates' the low current low voltage output of a logic device (or linear op-amp/comparator device etc.) to provide an output that can switch a higher current and voltage to the specified limits of the transistor. This type of output can be used with a pull-down resistor to translate a logic level from one voltage to another, or to drive lamps, relays etc, that have a ground (or -Ve or 0v) voltage on one terminal and require a switch to positive (or +Ve or Vcc) to activate.

And of course, there are the equivalent open drain and open source configs for fets/mosfets.


2. From Dave:

Open Collector output.
This kind of output consists of the collector of an internal output transistor. It is is used when the output is required to sink considerable current from a load. It will not source current. i.e. current goes into the device but does not come out of it. Typical applications would be to turn on a relay or drive a LED. It is often used for current driving of long cables that have a pull upload resistor on the other end. As there is no pull-up resistor internal to the device an unconnected O/C output will give indeterminate voltage levels if viewed with a CRO or meter. O/C outputs can be connected together to provide greater current sinking ability or as a current driven OR gate.


3. From Stek:

The following applies to NPN type transistor with emitter tied to ground - which is the most common open collector output is simply where a transistor either presents a current path to ground, or not. It is not limited to any particular voltage level.

It can be used to translate from one logic voltage level to another. If the current rating is sufficient, open collector outputs can also directly control relays (with back e.m.f. diode in place) and LEDs (ULN2003 is good example of open collector drivers). This type of output is converted to voltage level logic by 'pulling up' the output with a resistor to the +Ve logic voltage level. Thus, when the transistor is turned off the output will be 'pulled up' to the +ve voltage. The two logic states are transistor turned on (current path / 0V) - transistor turned off (no current path / +ve voltage level).


4. Frank added:
I like to think of it as an electronic 'switch'.

In the case of an NPN transistor, one side of the switch is connected to ground so that when the switch is 'on' it is switching ground to a load.

In the case of a PNP transistor, one side of the switch is connected to a positive voltage source. When the switch is 'on' it is switching positive to a load.

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