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Optocoupler & mcu

HellasTechn

Apr 14, 2013
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Hello everyone !

My latest project involves a micro controller and a solenoid that works with 220VAC. Once the solenoid gets the 220VAC on its coil, it opens for a few seconds. My goal is to count how many times the solenoid opens. I Think that i can use a bridge rectifier and an optocoupler to interface with the MCU.
Googling around, i found this picture which cant quite understand. What is the purpose of D2 ?

The way i see it, just the bridge with a small value cap in parallel to its + and - outputs would be okay to be fed directly to the optocoupler's A and K respectively. that would give me 0 on the mcu's input pin for as long as the solenoid is engaged. i guess there are optocouplers that can handle 300V right ? Or maybe adding a large resistor value in series with the optocoupler's anode pin.

Suggestions ?
 

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bertus

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Hello,

You do not need D2. It also would be in the wring direction.

Bertus
 

davenn

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Hello everyone !

The way i see it, just the bridge with a small value cap in parallel to its + and - outputs would be okay to be fed directly to the optocoupler's A and K respectively.

Suggestions ?

But your cap isnt in paeallel to the + and - of the bridge, it's in series with the +, so the LED isnt going to see a DC voltage
 

HellasTechn

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Yes you are both correct. Looks like polarities are messed up and the cap is no good placed like that. So the diagram i found online is wrong.

Could you suggest me some optocoupler and resistor value for it's Anode pin ?
 

HellasTechn

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Do you think that a et1103 would be suitable with a 150K resistor in the anode of the IR led ?
 

Harald Kapp

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Got the spec (datasheet) for the ET1103?
Here you go.

A simple circuit that should work with almost any photocoupler is here (unfortunately the text is in German):

You need to ensure the photocoupler is safe for operation across a >= 230 V differential between LED and photodiode/phototransistor and you also need to ensure proper creepage and clearance in your construction. The TCLT1103 has a pin spacing of 7-62 mm and is rated for 600 V (RMS) isolation voltage and thus is o.k. for this application.
 

HellasTechn

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Actually the IR led's forward voltage is not mentioned. Only the reverse voltage. The datasheet only states the forward current which is 60ma for the ET1103 so i suppose any reasonable voltage on the LED with a suitable series resistor to limit the current to no more than 60ma should be okay.
 

Harald Kapp

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i suppose any reasonable voltage on the LED with a suitable series resistor to limit the current to no more than 60ma should be okay.
Quite so. Assume a typical forward voltage of 1.25 V for your calculations.
This is what the datasheet for the TCLT1103 shows:
1706076051924.png
The IR LEDs in this kind of photocoupler are very similar in that respect.
 

HellasTechn

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Okay, got it.
In my case im thinking to use some opto with low forward bias current led so that i will not need a diode of large wattage.
The circuit of the above post is interesting indeed though i think that the zener could be skipped by increasing the value of r3 and replacing C2 with a 4.7uf 400V capacitor and D1 with a bridge rectifier.
 
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