Digital output (open collector)

Question

I have a controller with a digital output configured as open collector. The wiring diagram in the manual is shown in the image below. The manual indicates that the diagram is internal (if I'm understanding it correctly), so presumably the pull up resistor of 33k ohms is included in the controller?

When I cycle the digital out (without connecting to a relay), measuring the output voltage of the output relative to ground with a DMM, it will cycle between 0 and 14V (the supplied controller voltage is 24V - presumably Up). I just wanted to check if I am understanding the manual correctly. I am using the digital out to cycle a relay, where the relay's control voltage is 17V (SSR). So presumably I will then need to get a relay with a control voltage of well below 14V (say 12v) to be able to cycle it with this digital output. The motion controller's technical manual is @ https://www.faulhaber.com/en/products/series/mc3603s/ The relay's manual is @ https://www.phoenixcontact.com/en-za/products/single-solid-state-relay-opt-24dc-24dc-2-2966595

Thanks for your assistance

Digital Output Circuit Diagram

• Stefan Wyss: It would help a lot if you could provide a link to the datasheet/manual of this controller.
• CL.: What current does the relay need? (Or what is its resistance?) Can that current flow through the 33 kΩ resistor?
• DM010: The solid state relay requires 7mA. See phoenixcontact.com/en-za/products/…
• TonyM: A very warm welcome to the site. As well as the info already asked for, if you measure DigOut voltage with a DMM when nothing else is connected, do you get 24V? After that, it's the load current on DigOut causing a drop in the 33K resistor. That may be absolutely fine. Incidentally, ordinary relays don't normally have control voltages, coil voltages so I suspect the relay's more an SSR or something. Please edit your question (don't add new info in comments) and add a link to the relay datasheet.
• DM010: Thank you. When I measure with a DMM on the digital output (no relay connected) the voltage will cycle between 14V and 0V depending on whether I set the output, via the control software, to high or low.

Answer 1

The documentation (see page 56) does not say much about that digital output, but it does imply that the 33kΩ resistor is present inside the device. The most useful information would have been how much current that collector (actually a MOSFET drain, if the schematic is to be believed) can sink, something you really should ask the manufacturer to elaborate upon. The document is impressive - it takes serious effort to say so little, with so many words and pictures. It looks like it even got its own schematic wrong. Which "DigOut" is the actual output? It's not even clear if the MOSFET is really there, inside the unit. I'll assume it is, since the document explicitly states on page 20 that DigOut1 and DigOut2 do have open collectors. Perhaps they mean open drains, which doesn't change anything from the end-user's perspective, but some consistency would be nice.

I would be surprised if an internal 33kΩ resistor is present, since this would invalidate the claim that the collector (drain) is open. Whether it is there or not is somewhat moot in your case, since you need 7mA of relay input current, with which this resistance has nothing to do. I'll address that later, but in the meantime, here's a simple test you can perform, to find out. The first step is to connect your own 10kΩ resistor as follows:

schematic

^{simulate this circuit – Schematic created using CircuitLab}

The purpose here is to verify that you actually have control of OUT. With voltmeter VM1 (or digital multimeter) and resistor R2 connected as shown, when you explicitly set OUT high or low, you should see VM1 reading close to the full 24V (OUT high) or 0V (OUT low), regardless of the presence or absence of an internal 33kΩ resistor.

Once you have established that output OUT is behaving as expected in the above test, you can then test for the presence of R1, the internal 33kΩ resistance, as follows:

schematic

^{simulate this circuit}

In this arrangement, if you find that voltmeter VM1 reads near zero, in the case where OUT is set high, then R1 is not present. This is because there is nothing inside the unit to raise the potential of OUT, and your own 10kΩ resistor R1 is able to hold that output low instead, near zero volts.

If the 33kΩ resistor is present, and is indeed 33kΩ, then you will see a reading on VM1 in the vicinity of 5.6V when OUT is set high. That's because R1 and R2 from a resistor potential divider, where:

$$ V_{OUT} = U_P \frac{R_2}{R_1+R_2} $$

If when you set OUT high, you see 5.6V (or thereabouts) on VM1, then internal R1 is present, and has the value 33kΩ.

---

Once you have established the presence or absence of an internal R1, then you can proceed to connect your relay. Its requirement for 7mA of input current at 24V means that there's only one way you can directly and reliably connect this relay to your MC3603:

schematic

^{simulate this circuit}

If from your experiments above you have determined that R1 is present inside the MC3603 unit, then you do not need to include it externally here. However, adding 33kΩ across the relay input anyway will not harm anything, even with R1 already present.

This above setup will cause the relay to be closed (active) when OUT is low, which may or may not be what you need. To invert this behaviour, and have the relay activated when OUT is high, you will require some additional elements, which are fortunately not expensive or complicated:

schematic

^{simulate this circuit}

Once again, if R1 is present inside the controller unit, then you may omit it here, but including it externally anyway will not alter behaviour, or harm anything.

• DM010: Hi Simon, thank you for your detailed review of the manuals and for your suggestions. I performed all the tests you mentioned above and R1 is present in the controller. I then connected the SSR as suggested and it works perfectly. Thanks again for your feedback - I greatly appreciate it.