Explanation of Open Collector

[Centroid_Tech]

The concept of an open collector output is that when the transistor is enabled, the collector of that transistor is pulled to the emitter of that transistor which in our circuit the emitter is connected to COM or the ground terminal of the power supply. When that transmitter is not active, the collector of that transistor is just floating. There is no voltage associated with that terminal. That is why you need to add a "pullup" component such as a resistor which will pull that floating point up to some voltage, +VDC. Some drives use +5VDC logic while others use +12VDC logic while others use +24VDC logic.

Don't think of the open collector output as "supplying voltage" as it does not do that. On the ACORN, all it does is tie the output to COM whenever it's active. When it's not active, the output is floating so there is no voltage on that terminal until you connect a "pullup" device such as a resistor which one side is connected to the output while the other side is connected to the required voltage that is needed. The "pullup" device is not a protection device for that output. On newer revisions of ACORN there is protection circuitry on board that will prevent the open collector outputs from exceeding +24VDC in order to minimize damage.

If that is not clear...Think of 'open collector' as a switch, just like any other switch the voltage the switch is switching is determined by what YOU feed it. The Acorn open collector step and direction output can be supplied with 5 volt DC, 12 volt DC and 24 volts DC this allows the Acorn to connect to a wide variety of drives that is most compatible with the drive for the best performance.

[RGSparber]

Although implied, let me add that an (NPN) open collector can only sink current. In other words, current can flow in from a voltage more positive than COM. Damage can occur to the transistor's drive circuit if current is pulled out to a voltage below COM.

Rick

[ScotY]

I am revisiting this topic which has been confusing to me. Just to clarify, for Acorn, we want to use only NPN sensors for inputs and NPN relay boards for outputs? Is this correct?

[martyscncgarage]

I am revisiting this topic which has been confusing to me. Just to clarify, for Acorn, we want to use only NPN sensors for inputs and NPN relay boards for outputs? Is this correct?

Yes on NPN sensors for inputs
Use the new relay board from Centroid for outputs