Hi all,

I design a circuit with a PC814 optocoupler to detect zero cross but I'm finding so trouble calculating the saturation current.

The PC814 output is connected to a digital input of an Arduino with pull up resistor. Assuming a pull up res value of 20k and a <2.5V for the arduino input to recognize a fallen edge input I may assume that I need a
(5-2.5)/20k = 0.125mA Ic current on the PC814's output, right? If so, what would be the trigger current on the PC's led (If)? How may I calculate that current?


Regards

 

Have you read the data sheet for the PC814?
The CTR (Current Transfer Ratio) gain is given on the first page as:

You want to saturate the opto output to give near 0V (2.5V is a logic-high minimum and is not a design goal) so you want the opto output to sink 0.25mA.
Thus the worst-case minimum If would be 0.25mA/50% = 0.5mA

To insure saturation of the output they use a factor of 1/10 input-to-output gain in the data sheet for the saturation measurement.

On that basis you would use an If value of 2.5mA for 0.25mA output
But that's probably overkill, so a value of 1mA would likely be sufficient for your application.

Is supplying that amount of input current a problem?

 

I'm sorry but I'm still learning about electronics.

If CTR is 50% to 600% at input current of 50mA, you assumed 50% because it's the worst case, right? But, if I happens to be 100%, to sink 0.25mA I need only 0.25mA at the input, am I correct?

You said that worst-case minimum if would be 0.5mA at 50% but then you said that on the basis of the 1/10 input-to-output gain factor, i should use an if value of 2.5mA for 0.25mA output... This made me confused.

I'm sorry but can you explain it to me better? I'm still learning and I want to understand this.

 

I'm sorry but I'm still learning about electronics.

Nothing to be sorry about.
But learning to read a data sheet is fundamental to learning electronics.

f CTR is 50% to 600% at input current of 50mA, you assumed 50% because it's the worst case, right? But, if I happens to be 100%, to sink 0.25mA I need only 0.25mA at the input, am I correct?

Yes.
But a good design always uses worst-case values, otherwise the circuit may not work properly if you happen to get a device with worst-case parameters.

You said that worst-case minimum if would be 0.5mA at 50% but then you said that on the basis of the 1/10 input-to-output gain factor, i should use an if value of 2.5mA for 0.25mA output... This made me confused.

Sorry for any confusion.

The CTR value really refers to using it as a linear amplifier with some bias voltage on the transistor collector (the data sheet uses 5V).

When using it as a switch, you normally use the values given for testing the device, which is 10mA input for a 1mA output.
Since you only have a 1/4 mA output, I had assumed that you could use a relatively lower value input current, but after another look at the data sheet I see that's not the case. My error.

So you should use a value of at least 10 times the collector current for If, or If = 2.5mA for a transistor switch current of 0.25mA.

That all make sense now?

 

Thank you for your answer. That all makes sense, yes.

As an input, I have a 230Vac with a series resistor of 23.5K (47k//47k). My circuit works but I was finding it hard to prove how it works and why I have such a zero cross synchronization (the 230V zero and the optocoupler output zero signal). In the worst case I can get 2,5mA in the input at 27,5 volts if I'm understanding this correctly.


I can't find that Vcesat info on anydatasheet for the PC814. Where did you get that? I only see this:http://www.mouser.com/ds/2/365/pc814x_e-181274.pdf

 

I'm sorry but I've been trying to study the datasheet and.. the more I think about this, the more confused I am.
From the datasheet:

If I need 0.25mA in the Ic, this should be done for about 0.6mA If, with a CTR of 40%.. right?

In the other hand... From the datasheet:

This also means that for a collector current of 0.25mA, with around 1mA I can get a Vce near zero..
Finally:

This is telling me that with a If = 1mA and Ic = 0.5mA I can also get Vce around 0V...


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In a circuit like this:

With
RL = 20k
Vcc = 5V
Input is an AC source.

What will be the trigger current that will saturate the transistor? I need that Ic = 0,25mA.. But so many graphics are really hard to understand for someone like me.

 

If the input is an AC source, and you are using an AC input optocoupler, what is your question about the posted circuit?

The circuit you posted has little to do with the operation of the IC. It is a circuit used to test the optocoupler's response time.

 

It's exaclty the same circuit but the input is AC. The question remains the same.. calculate the trigger current. I mean, if the transistor is at the same conditions, shouldn't the trigger current on the led be the same?