As you probably know, a transistor (usually) has three connections; emitter, base, collector. There is (again, usually) no connection to any of those three until the transistor is wired to a circuit. "Open Collector" means that when the transistor is mounted and wired to a circuit, nothing is wired to the collector except external terminals. A load (lamp, relay, motor, etc), is connected to the collector of a transistor through those terminals, without connecting the collector directly to a power source and load resistors. The load is then connected to the power supply as the source or return voltage for the transistor and is also the load for the transistor. The transistor can then be biased to "turn-on", "turn-off", or "vary" the load voltage. It is important to note that the load current also flows through the transistor.
Why are they used, you ask? Since load resistors and power supplies are not included, it is cheap.
Open collector outputs can drive a load connected to any supply voltage up to the rated voltage of the transistor. They can also be wire-ored together so that multiple outputs can drive a single load (thus the OR-ing action). They are sink outputs only in that they can only sink current and not source it.
Srinivas,
The significance, or meaning, of open collector is that the collector of the output transistor is not connected to anything but the output terminal, although sometimes there is a pull-up or pull-down resistor to the logic supply voltage.
There are several reasons OC outputs are useful:
1) When you need a single input from multiple sensors, the sensors can be paralleled for a "wire-AND" configuration. In this way, any one sensor that is made or "on" will activate the input.
2) When you are driving an input with a voltage that is different from the supply but has a common ground, the sensor can be used with a pull-up or pull-down resistor to the secondary supply voltage. For example, you can actuate a TTL level signal with a 24 Volt supply proximity switch; the output terminal (Open collector) can be pulled up to five volts, and will transition to near zero volts upon sensor actuation. You have to make sure there is no internal pull-up or pull-down in this case.
3) You can also use an OC output with an optoisolator to make either positive-true or negative-true logic, by either turning the optoisolator ON when the output is made (series with load) or by turning the optoisolator
OFF (load shunted).
Open-collector outputs, basides all of the
listed uses for driving devices, have the unique
ability to have THREE logic states : High/Low/Disconnected.
A normal logic device must either drive its
output High or Low. This is inconvenient for
busses (ie, in a computer), to which the outputs
of many different logic devices - if one tries to
drive it Low but another tries to drive it High,
who's in charge?
If open-collector devices were used, we could
set all of the devices, except the desired one, to Disconnected. Then the buss would match the
state of this one output.
Willy Smith wrote:
>2) When you are driving an input with a voltage that is different from the supply but has a common ground, the sensor can be used with a pull-up or pull-down resistor to the secondary supply voltage. For example, you can actuate a TTL level signal with a 24 Volt supply proximity switch; the output terminal (Open collector) can be pulled up to five volts, and will transition to near zero volts upon sensor actuation. You have to make sure there is no internal pull-up or pull-down in this case.<
hi ,willy , could u explain it more simply , i have a hard time to understand it.
(For these drawings to appear correctly, they should be in a non-proportional font (like Courier). You may have to cut and paste to a
word processor)
Here is an ASCIIart prox switch schematic with an INTERNAL pullup to the
power supply (uppercase Z's are for a resistor):
Prox Sw
+--------+
| <---+----24VDC---
| Z |
| Z |
| Z |
| |/+----Output---
| -| |
| |>-----Gnd------
+--------+
Note that you should NOT hook this output up to a five volt logic circuit! It's possible to do with a 4.7 volt zener and a series resistor, but then
the next guy will have to figure out what you did, which ability doesn't necessarily follow B^)
Here is an ASCIIart prox switch schematic with NO internal pullup, showing how to put a pullup to the logic supply:
Prox Sw
+--------+ ^ +5VDC
| <--------24VDC--- |
| | Z
| | Z 3.3K-10K (typical)
| | Z
| |/-----Output---------+-----> to logic
| -| |
| |>-----Gnd------
+--------+
Note that in this case, the logic minus supply (ground) must also be
connected (in ONE place to prevent ground loops) to the -24VDC supply (common).
