"Industrial standard" push pull design

Thread Starter

Axel_sr

Joined Feb 28, 2022
22
Hi. I was going to add a very simple push pull circuit to a semi-commercial product. After a second thought... maybe this is not as simple as adding two transistors - as this is typically described in theory articles. This needs to be robust enough to be added to... well, kind of commercial product.
24V push-pull outputs are "everywhere" so maybe there is a cheap chip for this. All I found include opto-coupler - which makes sense since that is probably often the requirement in such applications.

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crutschow

Joined Mar 14, 2008
30,100
That circuit will add current gain to the signal, but no voltage gain.
It will also have crossover distortion.
Is that what you what you are looking for.
What load do you want to drive?
 

nsaspook

Joined Aug 27, 2009
9,985
Is this for a sensor output? That's about the only time I see 24V push-pull outputs on common industrial I/O. Need more information please.
 

Thread Starter

Axel_sr

Joined Feb 28, 2022
22
Right, right... this is not even the circuit I am looking for. This is current amplifier.
I need to convert 3V3 "TTL" to 24V "TTL". This is driving high impedance input.

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jiggermole

Joined Jul 29, 2016
56
The experience I've had with Allen Bradley input cards is they are current driven. You can put 24vdc to the input all day, but if it can't source 8ma the input won't turn on.
Keep in mind this wasn't for something that needs to be sold, this was just for my own curiosity.
What I ended up using was a rail to rail capable op-amp with an output resistor to limit the current to that 8ma, and that worked pretty decently. So in reality you could probably do it with a robust level shifter. I don't have the part numbers for the stuff I used, sorry its been a long while.

Edit* Also if you're looking for it to be in a commercial product you will most definitely need to put in "bad electrician" protection. It should be able to take 24vdc to any input, on the industrial side, without being damaged. I have done a bad before and if the device won't survive that, I don't buy that brand anymore. I work around operators that drop sharp parts all the time and no amount of protection will stop a determined idiot.
 

Thread Starter

Axel_sr

Joined Feb 28, 2022
22
Thank you for the suggestion. Probably OpAmp approach is the simplest and the most robust. In this case I do not need to save $0.50 or so.
And yes, totally agree what was commented about input protection.
 

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ericgibbs

Joined Jan 29, 2010
15,691
hi Axel,
What signal Gain are you expecting from that OPA circuit.?
E

Added clip. Note Supply limit
EG 1666.gif
 
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Thread Starter

Axel_sr

Joined Feb 28, 2022
22
Yes I'm aware of that. The exact opamp I am going to use is TLV272DR2G. The standard TLV272 does not have enough output current either.

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BobTPH

Joined Jun 5, 2013
5,227
If it is driving a high impedance input, you don’t need a push-pull output, a pull-up or pull down resistor is fine.

Bob
 

MrAl

Joined Jun 17, 2014
9,155
Hi. I was going to add a very simple push pull circuit to a semi-commercial product. After a second thought... maybe this is not as simple as adding two transistors - as this is typically described in theory articles. This needs to be robust enough to be added to... well, kind of commercial product.
24V push-pull outputs are "everywhere" so maybe there is a cheap chip for this. All I found include opto-coupler - which makes sense since that is probably often the requirement in such applications.

A
You need to add a high side driver to the input because you can not drive the upper NPN with a 3.3v signal. It has to be 24v or greater if you want the full 24v (or near that) output at the output of the emitters.

The base voltage has to be around 0.7v higher than the emitter voltage, so if the emitter is to be 23v then you need around 23.7v at the base and it has to be with enough current to drive the output load properly.
To get this another transistor on the input is often used.

You have to check these kinds of circuits out well so you dont blow out the transistors.
 

jiggermole

Joined Jul 29, 2016
56
I liked some other options that will also do the trick. Things to keep in mind. You don't need full 24v on all the input cards I've seen. I think the highest voltage ive seen for minimum detection was 19V. So you don't necessarily need rail to rail on the opamp. And again you don't need amps of current either. I can't stress enough to look at the datasheet for the input card.
 

Thread Starter

Axel_sr

Joined Feb 28, 2022
22
What I find a bit odd is that we are discussing about push-pull designs options ang the thread was moved to "PCB Layout etc" category.
jiggermole, yes and I agree. However, in this case I only have the specs.There are probably several input cards this design will drive eventually.

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