# [HELP]Optocoupler, Level Shifter, and CT

Discussion in 'The Projects Forum' started by salz, Nov 25, 2013.

1. ### salz Thread Starter New Member

Nov 15, 2013
1
0
Hi,

I'm totally new here, read a lot, but never posted any. I've got a few questions for the project I'm working on. We're building a H-Bridge inverter, and I'm responsible in making the protections for both input (under V) and output (over V and over I)

First question, I'm really struggling with my optocoupler, am I wiring this right ?
So, what I am trying to build is a under voltage input protection.
It will always limit the voltage flowing into the output end to the microcontroller to be 3.3V, but when the input goes below 380V, it will give value less than 3.3V to the microcontroller, telling it there's something wrong.

In my optocoupler, I don't know why, the output only follows the activation voltage, and doesn't behave as I would like it to be. Please help!

http://imageshack.us/a/img853/3585/qghd.png
http://imageshack.us/a/img833/7858/t7cg.png

The second question is for my output over voltage protection. To be completely honest, I don't understand exactly how the circuit works, as in, yes I somewhat know what it does, but I don't know in detail why each number is there. I played with some circuits I found online. Please kindly explain to me how it actually works and how I can optimize the circuit ?

http://imageshack.us/a/img20/4135/o8zc.png
http://imageshack.us/a/img802/7404/iwfu.png
http://imageshack.us/a/img822/2425/bivx.png

So, our device outputs 240V AC, when it actually does, using the circuit, it will scale down the voltage to 2V, and shift it up from +-1V, to 0-2V for microcontroller to read. The problem is, when it goes up above 240V, the output will give like, chopped off wave (shown on the third pic, 500V AC). I don't know how it should behave, I don't think it can be shifted upwards to fit any other voltage above 240 V AC (so the output won't be chopped off) since we only have one reference voltage for 0-2V.

Third question is for output over current protection. I was advised to use current transducer. But I don't know at all how the device works, which component I should use, and how to hook it up. Please enlighten me ?

Thanks so much !

2. ### DickCappels Moderator

Aug 21, 2008
2,664
634
Not sure what "follows the reference voltage" means, but I think it means the transistor is not conducting (correct?). I would not expect it to conduct because the LED's threshold voltage is 6 volts, but your Zener diode is only letting the voltage get to 3.3 volts. A threshold voltage of 1.6 volts is more typical.

I will leave the other two questions for people who are more experienced than I.

3. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,813
1,105
Dealing with high voltage such as 380V or 240V is dangerous. It can KILL you. You MUST be aware of and take all due precautions.

Re Q1, if you want the micro input limited to 3.3V then the zener should be on the output side of the opto.
Replace the input-side zener with a reverse-biased silicon diode.

Re Q2, why bother level-shifting the voltage with the summing opamp? Just ignore the negative-going halves of the cycle. You can do your overvoltage detection using just the positive half cycles. For safety the 240V needs to be mains-isolated.

Re Q3, google 'current transformer'.

Last edited: Nov 26, 2013