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MOSFET low side switching, Opto-Isolated Driver, isolated Amp Votage sensing !
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castingflame:

--- Quote ---You are messing with the datasheets; you don't have the parameters of the VO3150 but of the UCC27424.
--- End quote ---
    :palm:  Stupid mistake is a reflection of the state of my brain at the moment   |O   What a tool. There is no hope for me!

Anyway, i'll take a look at the correct datasheet.




--- Quote ---You're right; the FET is voltage controlled, since its gate is isolated from the conducting channel, it acts like a capacitor; to build up a voltage in a cap, there has to flow a current - the higher this current is, the faster you reach a certain voltage.

So, if you drive a FET directly from a uC which sinks or sources maybe 25mA, it can take a considerable time to get the gate of high power FET charged and conducting - in other words - when using a FET driver, the parameter of sink- and sourceable current tells you, how fast you can switch the FET on and off - via I = dQ/dt
--- End quote ---


I will spend some time looking at your coulombs over time equation, to try to get a better general understanding of it, as it's another fundamental concept I need to understand in the future and add it to my various personal documentation folders.




--- Quote ---0,5A for a driver is not such a great parameter; there are drivers, which can sink and source several Amps, but it's surely better than your setup with a common opto or directly driven by a I/O-pin of a uC
--- End quote ---

Indeed and as I said, PWM fast switching is not a Must, so any improvement in this area for my own education I am happy with at this point and I really was more concerned with the isolation so I am happy that this is as you said "surely better than your setup with a common opto or directly driven by a I/O-pin of a uC".  :-+


I have come back to electronics a year or so ago after many years away and I have really only been dealing with the digital stuff for my own amusement. More analog base circuits and the many fundamentals, principles, laws, algebra etc can not be all learned in a week.

I recently purchased The Art of Electronics  by Thomas Hayes book which has been a pretty great book so far and I am also following a Udemy course by Andre LaMothe  Crash Course Electronics and PCB Design which is 88hrs long and has also been very useful. At the same time I am trying to build this project which is quite a bit above my current ability level for my Father. With this in mind I appreciate all the time and advice that has been given to this 48 year old student  :clap:  :-+  I am trying hard, but also a bit 'trying' at times too :)


Paul
Zero999:

--- Quote from: castingflame on August 30, 2018, 08:32:25 pm ---Initially the microcontroller did not sense the voltage correctly until I shared the GND both sides of the Opto-Isolator, which makes sense(so to speak!). The microcontroller measures the voltage okay when the MOSFET is switched on but when I switch it off, the microcontroller is showing random(ish) values instead of 0. This was the end of the day and my wife was calling me so I did not have to to check this with the meter.
--- End quote ---
It think others have answered the other questions adequately, but unless I've missed something, no one seems to have dealt with this.

When the MOSFET is on, the voltage on the ADC input will be V = VIN*R6/(R5+R6) =  24*10/(100+10) = 24*10/110 = 22/11V

When the MOSFET is off, the voltage on the ADC input will theoretically be 24V, because the MOSFET will disconnect the lower side of the potential divider R5 & R6, leaving only R5 in series with 24V. In reality, the ADC input's ESD protection diode will start conducting, so the voltage on the ADC input, will one diode drop above the power supply voltage. Because the voltage would be outside the acceptable range for the ADC pin, the reading will be unreliable, hence the random values.

If you don't need any isolation, then the voltage across the load can be roughly measured by connecting the potential divider across the MOSFET. When the MOSFET is off, the ADC input will read 22/11V and when it's on, it'll read a very low voltage, probably around zero, as it will be under one ADC count. For a more accurate measurement, monitor the 24V, using a potential divider and the voltage across the MOSFET using a single current limiting resistor.
Wimberleytech:

--- Quote ---
When the MOSFET is off, the voltage on the ADC input will theoretically be 24V, because the MOSFET will disconnect the lower side of the potential divider R5 & R6, leaving only R5 in series with 24V. In reality, the ADC input's ESD protection diode will start conducting, so the voltage on the ADC input, will one diode drop above the power supply voltage. Because the voltage would be outside the acceptable range for the ADC pin, the reading will be unreliable, hence the random values.


--- End quote ---

Good job Hero.  I have been monitoring this thread, and shaking my head.  While I wanted to answer, I took the Socratic approach but flame refused to respond to my opening salvo. 
castingflame:

--- Quote ---Good job Hero.  I have been monitoring this thread, and shaking my head.  While I wanted to answer, I took the Socratic approach but flame refused to respond to my opening salvo.
--- End quote ---


I think this is a bit unfair. I have at no point pretended to be anything else but trying to learn and appreciate the help that was given. I have quoted that this is somewhat above my level and I am trying to help my father out. I am also spending a lot of time studying and I am very aware of my lack of knowledge. That is why I have asked for help.


"refused to respond to my opening salvo"  as you quote is not true either. I have refused nothing. In actual fact, I needed some time for my fried brain to relax a bit again for a day or so to process exactly what you meant. When I did understand I was not able to get back to the bench to measure it until today.



--- Quote --- I have been monitoring this thread, and shaking my head.
--- End quote ---
I help out in other areas when people request help. I would never mock or belittle them especially if they are trying hard and putting effort in to learn.





--- Quote ---It think others have answered the other questions adequately, but unless I've missed something, no one seems to have dealt with this.

When the MOSFET is on, the voltage on the ADC input will be V = VIN*R6/(R5+R6) =  24*10/(100+10) = 24*10/110 = 22/11V

When the MOSFET is off, the voltage on the ADC input will theoretically be 24V, because the MOSFET will disconnect the lower side of the potential divider R5 & R6, leaving only R5 in series with 24V. In reality, the ADC input's ESD protection diode will start conducting, so the voltage on the ADC input, will one diode drop above the power supply voltage. Because the voltage would be outside the acceptable range for the ADC pin, the reading will be unreliable, hence the random values.

If you don't need any isolation, then the voltage across the load can be roughly measured by connecting the potential divider across the MOSFET. When the MOSFET is off, the ADC input will read 22/11V and when it's on, it'll read a very low voltage, probably around zero, as it will be under one ADC count. For a more accurate measurement, monitor the 24V, using a potential divider and the voltage across the MOSFET using a single current limiting resistor.
--- End quote ---


Thank you for such a detailed review of what was going on. I had a very rough idea where the problem was happening but not with any amount of detail, equations, sound proof. Just a guess really. I'll spend time this evening when things are little quieter and go though your details. Thank you , I appreciate it.  :-+


For that part of the monitoring, I have actually moved to an i2c current/voltage sensor so I will be 'playing' with that over the next few days (life allowing). I have just spent today documenting everything I have learned so far along the way.

Wimberleytech:

I asked one question: "What are you reading?"
You did not answer. 

No harm no foul.  Hope your circuit is working now.

Sorry if I hurt your feelings.

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