# optocoupler simulation problem

#### kmlprdn

Joined Feb 1, 2020
10
I've been trying to use the MCT2E optocoupler in my project but I am not getting any output response from it. I've converted in a simple circuit just to make it easier to solve the problem. Can anyone tell me what is the problem in this circuit?

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#### ericgibbs

Joined Jan 29, 2010
10,806
hi k,
Welcome to AAC
What is the drive voltage on the 1k.?
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#### Dodgydave

Joined Jun 22, 2012
9,459
Put R1 to 5V to put the led on...

#### MisterBill2

Joined Jan 23, 2018
7,020
The simulation needs a math expression to relate the resistance between the pins 4 and 5 to the current flowing from pin 1 to pin 2. The current/voltage relationship is like a diode forward biased, so the relationship is very non-linear. And the current to resistance relationship is an inverse one, where the resistance drops as the current increases.

#### kmlprdn

Joined Feb 1, 2020
10
hi k,
Welcome to AAC
What is the drive voltage on the 1k.?
E
drive voltage on 1k is 5 volts

#### kmlprdn

Joined Feb 1, 2020
10
Put R1 to 5V to put the led on...
it is 5V i dont understand why its not working

#### kmlprdn

Joined Feb 1, 2020
10
The simulation needs a math expression to relate the resistance between the pins 4 and 5 to the current flowing from pin 1 to pin 2. The current/voltage relationship is like a diode forward biased, so the relationship is very non-linear. And the current to resistance relationship is an inverse one, where the resistance drops as the current increases.
im not geting your point...where and how can i give a math expression in the simulation?

#### ericgibbs

Joined Jan 29, 2010
10,806
hi k,
The datasheet for the MCT2 states that 20mA emitter diode current [ thats the input side] the diode voltage drop is approx 1.1v.
So for a 5V driving source to the emitter 5Vin - 1.1Vdiode = 3.9V/0.02A = 195R resistor,, use a standard 220R for R1.

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Update:
When you use a 1K for R1, the diode current is approx 4mA.
Depending upon your application you could try 10mA emitter diode current. ie: a 390R

Last edited:

#### Alec_t

Joined Sep 17, 2013
11,644
The datasheet is telling you that if the input diode current is 10mA you may get only 2mA from the output, but you could be lucky and get more. Of course, a diode current less than10mA would give a correspondingly smaller output current. What output response are you expecting?

#### kmlprdn

Joined Feb 1, 2020
10
hi k,
The datasheet for the MCT2 states that 20mA emitter diode current [ thats the input side] the diode voltage drop is approx 1.1v.
So for a 5V driving source to the emitter 5Vin - 1.1Vdiode = 3.9V/0.02A = 195R resistor,, use a standard 220R for R1.

E
Update:
When you use a 1K for R1, the diode current is approx 4mA.
Depending upon your application you could try 10mA emitter diode current. ie: a 390R
View attachment 198118
i tried putting 220R resistor in R1 but also it is not working. perhaps the problem is with my proteus. i found a working circuit in the internet and tried to simulate that exact circuit in my proteus but it didn't work. i've attached the circuit which i tested simulating.

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#### kmlprdn

Joined Feb 1, 2020
10
The datasheet is telling you that if the input diode current is 10mA you may get only 2mA from the output, but you could be lucky and get more. Of course, a diode current less than10mA would give a correspondingly smaller output current. What output response are you expecting?
im expecting that the optocoupler may conduct in its output but im seeing the no response in the output. even in the input side there is no current flow

#### ericgibbs

Joined Jan 29, 2010
10,806
hi k,
I will do a compare sim in LTSpice.
You do know that the MOC is a Triac
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#### kmlprdn

Joined Feb 1, 2020
10
hi k,
I will do a compare sim in LTSpice.
You do know that the MOC is a Triac
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yes i know eric. i just tried to test whether the MOC will work or not but seems it won't work as well.

#### ericgibbs

Joined Jan 29, 2010
10,806
hi,
This is LTSpice.
Looks OK to me.
Use a similar OPTO
E

Update:
Does your simulator require a Common Node connection./?

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#### kmlprdn

Joined Feb 1, 2020
10
Can u simulate a simple circuit which has mct2e or moc3201 in it in proteus and send it to me? if it worked for u, it should definitely work for me right? and if it doesn't, i'll know my proteus is a trash.

hi,
This is LTSpice.
Looks OK to me.
Use a similar OPTO
E

Update:
Does your simulator require a Common Node connection./?
i dont have idea about common node connection. do you use proteus?

#### ericgibbs

Joined Jan 29, 2010
10,806
hi,
Sorry, I don't use Proteus.
Try this modification.
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#### kmlprdn

Joined Feb 1, 2020
10
hi,
Sorry, I don't use Proteus.
Try this modification.
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hey eric, thankfully the grounding idea worked. but its not about moc3201, its about mct2e...how can i make it work?

#### ericgibbs

Joined Jan 29, 2010
10,806
hi k,
Is it an actual circuit that is not working.?
If yes, can you post a clear photo shot of the project, we can check the wiring.
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#### kmlprdn

Joined Feb 1, 2020
10
hi k,
Is it an actual circuit that is not working.?
If yes, can you post a clear photo shot of the project, we can check the wiring.
E
no i've not done the actual circuit. the one in which moc3201 was used u said u dont use proteus and suggested using grounds, that circuit only worked but my actual concern is making mct2e to work

#### MisterBill2

Joined Jan 23, 2018
7,020
im not geting your point...where and how can i give a math expression in the simulation?
The model of a component describes the relation between voltage and current at the input connection of a device, and it also describes the relation of the output of the device to the input. That is fundamental to how a simulator works.
Hopefully you understand how your simulator functions.
If you have an ACCURATE model of the opto isolator for your simulator it will already have that math expression as a major part of the model. The simulator already has models for resistors and transistors, at least I presume it has models. I do not use simulators because I have no use for them.