Protecting transistor output modules

Hello guys!

I have a project where a S7-1500 has transistor output modules. There are also several ET200SP and ET200MP, which also have transistor output modules connected to. I am just saying this to insist on the fact I have several types of transistor output modules.

I am about to control pneumatic valves which have coil powers 0,9W ; 2W and 5W, mainly the first two.

At some places, I am running out of space and can't install external relays for 600 outputs. According to the wiring diagram, there are zener diodes in the module inside which is some protection. Another parameter is the inductive shutdown voltage, which is set to -53V, if I am correct. Okay, let's assume the coil won't reach that value.

Okay, is it safe to directly control the valves with the transistor outputs? Should I put a diode in parallel? As I said, I am running out of space and at some places putting additional terminals would also be problematic. I am planning direct cables out of the module to additional distribution boxes, several meters away from the enclosure with the modules.

Attached is some reference information I found but I am not sure how reliable it is.

Another general question:

How do you generally decide if the load should be directly connected to the transistor output or an additional relay is to be put. I am asking about other factors, apart from the 0.5 A current limitation. What is your practical experience?

Hello mitis,

you can of course directly control your valves from transistor output, as long as you do not exceed electrical limitations for your output. I would at least put a standard diode (1N400x) in line to protect from reverse voltage.

You have to be aware of the limitations of voltage, power and current. Especially current needs a close look, as your outputs will have to provide not only normal operation current, but also initial switch on current, which is short, but several times higher than normal operation current.

When would you drive an output directly without a relais in between?

1.) If you need fast switching: a relais need typically 30 to 50ms to switch on or off. Some applications require faster switching and therefore are driven directly.

2.) If you have high switching frequency at an output: If an output needs to be switched on/off very often, a relais in between would soon exceed maximum switchings and thus fail in short intervalls.

3.) if the device driven is very low power and danger of getting overvoltage/reverse voltage is low: In this situation the additional price for relais, sockets and wiring would not be justified.

regards

For small pneumatic valves I never use additional relays, I always use transtor outpu module 0.5A. For large valves I do use transistor outputs 2A. I have had never issues regarding back EMF of coils with transistor ouputs, more problematic are relay outputs - you do need varistor or freewheeling diode.

I don't know what's inside transistor output module, but it is made to drive inductive loads, too. Normally you won't notice the performance, but in case you have a large coil, like electromagnetic brake then of course you would need a relay or ssr to drive such big inductive load. Now pay attention - if you use freewheeling diode the time constant is very high, because the current decays exponential :

i=i_steady*(1-exp(-t*L/R)), R is the resistance of coil + series resitance. Now using a freewheeling diode, the resistance of diode is small and the current is circulating for long time. Using a varistor which has a breakdown charactersitics it acts as high series resistance when coil is deenergised, thus decay time is very slow.

Therefore with couplings, brakes I do use relays and varistors, for small power solenoids like pneumatic valves I do use transistor output (with integrated freewheeling diode - I asume, otherwise it would burn instantly)

Thank you both for your comments. Since I don't have much space, maybe I won't put a diode as the output should be protected inside the module.

@Marco, I believe you should have burned many outputs so far so we assume protection is good for small loads (like mine - 0.9W coil).

If you know where your cables are laid and you dont have to fear any reverse voltage (like induction voltage) you can of course leave protection diodes away.

I just mentioned it, as normally when I do a startup I dont know that for sure.

regards