Controlling a 12V relay directly from a 5V MCU using a Zener diode and a PNP transistor

Welcome,

Andrzej_Tomaszewski wrote:

....
The uC pin controls several components in series and there is a +12Vdc potential on the other side. In my opinion this is not correct.

here I agree, if the GPIO pin is in the H state without any 'pull' e.g. OC or OD output, then the resistance of this output to ground will be equal to infinity, so the anode of the Zener diode hangs in 'air' and we have a potential of 12V here, i.e. we have 12V on GPIO, which is strongly unhealthy for most circuits.
Of course, in real-world conditions it doesn't have to be like this, well, and the components have some finite resistance after all, but this is not a good approach in my opinion, balancing on the limit.
Regards

Andrzej_Tomaszewski wrote:

pawlik118 wrote:

the approach is unusual

agreement

pawlik118 wrote:

it may arouse resistance to the unknown

And what is the unknown here? Unless the arrangement is unknown because it is unused because it is incorrect.

pawlik118 wrote:

from an electronic point of view it is correct

The uC pin controls several components in series and there is a +12Vdc potential on the other side. In my opinion this is incorrect.

Let's assume we have a (relay) supply of 12V and a zener diode at 8.2V. The voltage the processor "sees" is 12-8.2=3.8V. This is just right, as the processor is powered by 5V.
The fact that "somewhere" further down the line there is 12V is irrelevant.

From the calculations yes and in the actual system how much will it be?

pawlik118 wrote:

Assume we have a (relay) supply of 12V and a zener diode at 8.2V. The voltage the processor "sees" is 12-8.2=3.8V. This is just right, as the processor is powered by 5V.
The fact that "somewhere" further down the line there is 12V is irrelevant.

Your calculations for ideal components, mainly assuming zero current of the Zener diode below the threshold are ok. But in this circuit, disconnect the uC leg and measure with a voltmeter the voltage at the anode of the Zener diode relative to ground.

>>21893971 The voltage drop across the diode depends on the current flowing through it. The current in turn must not exceed Ue-b / Re-b. This can be counted, measured. It is not a contact operation, and zener diodes do not have such a large spread of parameters. I deal with this on a daily basis.

The circuit looks like a dig at the early years of MCU production. Still from the days where the processor port was an open manifold type with a small pull-up. Often an external pull-up was done. The trouble was that it was hard to put out a current-efficient high state that reliably quenched the NPN transistor. Added to this was the behaviour in the reset state. Such a processor in reset mode had a high state on its leads that would drive the relays with the NPN transistor.

Someone mentioned the optotransistor, this is the best and simplest solution.
It eliminates all the disadvantages and you can control the low level (as in the figure) or the high level by changing the diode control.



Added after 1 [minute]:

The optotransistor in the darlington has sufficient current for most relays and even large ones.

And this for this AT

petrykcom wrote:

When the microcontroller (the one I just needed and many others) is switched on, there is a logical 1 at its output, so when the power is switched on for a short while until the IO is initialised, the relay controlled by the NPN transistor would be switched on

HD-VIDEO wrote:

As for this AT

But what, because I don't understand.

An optocoupler is used for galvanic isolation and not for amplification or signal inversion. That is what a triode, or transistor, is used for. Just because something is "suitable" for something does not yet mean that it should be used. Lame solutions should not be promoted, especially in an educational context.
In general, discussing this idea is akin to trying to grill a cockroach impaled on a stick with such reverence as if it were at least a decent pork chop. These arrangements are not worthy of attention or discussion, only of pointing out why they are weak.

Hello,

elukam wrote:

Transistor is used for galvanic isolation and not for amplification or signal inversion. That is what a triode, or transistor, is for. Just because something is "suitable" for something does not yet mean that it should be used. Lame solutions should not be promoted, especially in an educational context.
In general, discussing this idea is akin to trying to grill a cockroach impaled on a stick with such reverence as if it were at least a decent pork chop. This arrangement is not worthy of attention or discussion but only of pointing out why it is poor.

Perhaps a colleague should study the principle of the optocoupler and its applications, especially those not requiring isolation at first glance.
I agree with colleague @Janusz_kk that using it to control a relay is a good choice and even "even more right", as thanks to the isolation we can separate the power supply and avoid possible overvoltages, for example. Contrary to appearances, this is quite common in industrial equipment.
In this case, education is not only about showing weaknesses, but also about showing potential advantages, possible improvements, analysis of the circuit, etc., as just pointing out the flaws will not teach anything if someone does not know why it is wrong.
Regards

Regards

elukam wrote:

The optocoupler is used for galvanic isolation and not for amplification or signal inversion.

Mate

elukam wrote:

Transoptor is for galvanic isolation and not for amplification or signal inversion. That is what a triode, or transistor, is for. Just because something is "suitable" for something does not yet mean that it should be used.

Mate, sorry but you're babbling nonsense, an optocoupler is a full-fledged transistor only the gate is controlled slightly differently than usual.
So your whole thesis is gone.

elukam wrote:

In general, discussing this idea is akin to trying to grill a cockroach impaled on a stick with such aesthesia

The discussion is needed because the author's solution is, from the point of view of electronics and operational reliability, hopeless. Such a thing can be done by an amateur but not by an electronics technician who has the ambition to be a professional.
And this is written to you by an electronics engineer with 50 years of experience still repairing industrial electronics, which means that I have seen many different solutions, but not this one.

No one here is raising the need for power or interference isolation. In the general case, optocouplers are not used to control relay coils and only special optoisolators are actually suitable for this. Such a solution is more expensive, takes up more space, and is insecure due to the uncertainty of the optoisolator's transmittance. Finally, it puts quite a heavy load on the digital port unnecessarily. Of course, you can special, better etc etc but here the idea was to have a simple, cheap and reliable circuit.
The use of an opto-isolator does not guarantee safety from interference. Safety from interference is guaranteed by the designer's knowledge.
Why this circuit is weak has already been written, and what it should be replaced with has also already been written.
Any improvement of any circuit inevitably involves complicating it. This is why real professional equipment is so packed with "meat".
A circuit should be complicated only when necessary and only to the extent necessary. You should start with the optimal solution and not with a poor one and then improve (or rather repair) it, if at all possible.
A good design is one that works correctly with as few harsh boundary conditions as possible. Ideally, under all conditions. A great many amateur circuits work only because the parameters of the components used and the operating conditions have happily "aligned". These are bad solutions, even though they work.

Janusz_kk wrote:

the transistor is a full-fledged transistor only the gate is controlled a little differently than usual.

As you can see, servicing automation even for a lifetime does not make a person an electronics engineer. So does a badge on a suit or a position held. And not to be surprised, I know that this is rather the rule. It's just a pity that such 'revelations' become part of the online knowledge resource that students try to draw on. And then we are surprised that everything goes downhill.

elukam wrote:

As you can see servicing automation even for a lifetime does not make a person an electronics engineer.

I think you mean yourself, I don't comment any more because I don't want to get banned by you. i can only recommend learning and reading pdfs because even this is going poorly for you, compare the parameters of a darlington optocoupler with a digital output from a prck as proposed by the author or a regular npn transistor.

>>21897404

The previous speaker was right about the main application. You are confusing the optocoupler with the phototransistor itself. An optocoupler is not a single transistor, but a complete circuit (transmitting diode + detector) enclosed in a single housing, designed precisely to provide galvanic isolation. Yes, there is usually a transistor at the output, but due to the huge non-linearity of optical transmission, using an optocoupler as a classical signal amplifier is not worthwhile.

Congratulations on 50 years in industrial automation service, that's an impressive result. Regardless of seniority, the physics of the components remains the same - and confusing a complete optoisolator circuit with a single transistor is simply a factual error.

filipcichowskidev wrote:

The previous speaker was right about the main application.

I see you are not letting go, you must have the final say.

filipcichowskidev wrote:

You are confusing the optocoupler with the phototransistor itself.

I'm not confusing anything, it's you who doesn't understand electronics.

filipcichowskidev wrote:

Of course, there is usually a transistor at the output, but due to the huge non-linearity of the optical transmission, using an optocoupler as a classical signal amplifier misses the point.

And that's why you're babbling nonsense, here non-linearity has no meaning, but you don't get it.

filipcichowskidev wrote:

Regardless of seniority, the physics of the components remains the same - and confusing a complete optoisolator circuit with a single transistor is simply a factual error.

and you plunge even further with such 'wisdom'. Go into philosophy rather than electronics, you'll do better.
For my part EOT, because I don't want to get banned by a philosopher like you.

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>>21905755

Incredibly rude response considering I responded in a factual manner. I advise against treating people this way, it doesn't get you anywhere.

You know very little about me. Well I design electronics for satellites on a daily basis, several of my developments are already flying in orbit, so think before you start insulting me.

You are recommending that I practice philosophy, so I assume that you have experience in this matter yourself.

Contemplate and think why the arrangement you cite with a relay in the emitter will work, but is not optimal.

The correct answer: the transistor in this configuration will not be able to enter deep saturation and you will have unnecessary power loss on it.

A little humility won't hurt and may help.

And you can only get banned for your rudeness, I don't tell you to treat people like pigs in a pigsty.