Open collector output to push-pull output converter

Thread Starter

PZUFIC

Joined Jan 7, 2012
44
Hello!

I have a question about the open collector output to push-pull output converter that I put together. In the attachment you can find the schematic.

In my case inputs and outputs Vcc are both 24 V DC. Outputs are meant to be connected to a PLC input. The circuit actually works fine, but at 24 V supply voltage I get approximately 21,2 V at the output, which is more than enough to activate the PLC input. But because of that there is about 2,8 V drop on the push-pull. I have measured the current and PLC input sinks about 8 mA, so the power dissipation on the push-pull ic should be fine.

I used push pull because I had troubles in the past using pull up resistor, because PLC was sinking to much current. When I was doing that I was using counter module, which probably draws even more current.

I wonder if there is something that I should be concerned about?

Thank you very much in advance.
 

Attachments

LesJones

Joined Jan 8, 2017
2,665
The 47k and 4.7K resistors form a potential divider so the input voltage to the output drivers will be less than 24 volts. It will be 24 x 47/(47 +4.7) volts = 24 x 47/51.7 = 21.82 volts You will then have the base emitter forward volt drop of about 0.7 volts to subtract from the 21.82 volts which gives 21.12 volts which is close to your measured value. If you wanted the output to be closer to 24 volts you could move the 47K resistors to the other end of the 4.7K resistors. As you say that 21.2 volts meets the input spec for the PLC and the dissipation is within the spec of the PMD3001D then I do not see any problem.

Les.
 

Thread Starter

PZUFIC

Joined Jan 7, 2012
44
Hello, I'm bumping this old thread because I encountered a very similar situation again.

Because of higher frequencies (different task) the chosen optocoupler was not up to the task so I replaced it with TLP2704 and also modified the circuit.

I'm not that much in the electronics, so I searched the web, but I was unable to find a good site according to this simple push-pull amplifier and how to use it. Can someone please comment the attached schematic? Is the current limiting resistor when switching to ground (R4) necessary at all?

Please refer to the attached schematic (R4 = 4k7, R3 = 47k).

Thank you very much in advance.

Primož
 

Attachments

cmrincon

Joined Oct 25, 2017
40
In case that you remove R4..how can you limit the current flowing through the push-pull?
If there is a low level in the open colector output, there is no resistance from vcc to ground because there is only a pn junction (pnp transistor) between vcc and ground.
To avoid this you need R4.
Regards
 

crutschow

Joined Mar 14, 2008
25,117
Is the current limiting resistor when switching to ground (R4) necessary at all?
No, R4 is not needed.
The push-pull module is configured as an emitter-follower so does not need an input current-limit resistor.
The push-pull input current will be just the output load current divided by the transistor current gain (Hfe or Beta).

Have you considered an optocoupler with a high-voltage push-pull output, such as this?
 

Thread Starter

PZUFIC

Joined Jan 7, 2012
44
No, R4 is not needed.
The push-pull module is configured as an emitter-follower so does not need an input current-limit resistor.
The push-pull input current will be just the output load current divided by the transistor current gain (Hfe or Beta).
So what will you recommend, without resistor or with it and which value? If I read the datasheet of both the optocoupler and the push-pull IC right and I want to cover output currents to about 200 mA, the current through the optocoupler transistor will be lower than the maximum value without current limiting resistor.

Have you considered an optocoupler with a high-voltage push-pull output, such as this?
I actually did some research but obviously not enough. I already ordered the parts so I will stick to the separate IC's but for the future projects I will definitely use something like suggested.

Thank you very much.
 

Thread Starter

PZUFIC

Joined Jan 7, 2012
44
The maximum frequency will be about 1 kHz. The converter will be used for a rpm measurement so the rise time should be adequate for that.
 

Thread Starter

PZUFIC

Joined Jan 7, 2012
44
For the current task "0" < 5 V max. 2 mA, "1" > 11 V typ. 9 mA. I know that for this purpose I could use the pull-up combined npn transistor, but I want to design it in a way that I can use it even if the load has a higher current consumption regardless sourcing or sinking. I had a case before where the pull-up was not sufficient or a pretty low value was needed.

If I use the suggested optocoupler with FET push-pull output is there anything that i need to pay special attention to? Of course I will limit the LED forward current and add a blocking capacitor to Vcc, but can I connect the output directly to the digital input to some device or let's say some load with current inside specs?
 

crutschow

Joined Mar 14, 2008
25,117
For the current task "0" < 5 V max. 2 mA, "1" > 11 V typ. 9 mA. I know that for this purpose I could use the pull-up combined npn transistor, but I want to design it in a way that I can use it even if the load has a higher current consumption regardless sourcing or sinking.
So for that you can just leave the 4.7kΩ pullup resistor and eliminate the resistor to ground.
But note that the output push-pull circuit has a current gain of about 300 so for a 200mA source output, its input current will be about 0.7mA, which must be provided by the pull-up resistor.
That would drop about 3.3V through the 4.7kΩ resistor.
If I use the suggested optocoupler with FET push-pull output is there anything that i need to pay special attention to? Of course I will limit the LED forward current and add a blocking capacitor to Vcc, but can I connect the output directly to the digital input to some device or let's say some load with current inside specs?
You can connect the output directly to the digital input load as long as you don't exceed any of the device ratings for voltage or current output.
 
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