Maximum input voltage of the 4N25 Optoisolator?

[3Deye]

Hi,

Can I use the 4N25 optoisolator with an input voltage up to 28V ? what is the maximum voltage?

If yes, then the current limiting resistor should be (28V - 1.5V) / 10mA = 2.65K , correct?

 

[Tahmid]

Yes, you can. Since the input is an LED, all you need is the current limiting resistor. Your calculation seems fine. So, it's okay. There's not much to worry about.

Hope this helps.
Tahmid.

 

[betwixt]

Your resistor calculation is correct.

There isn't really a maximum input voltage until you reach about 5KV when a flash-over becomes likely. The LED is a constant voltage device so no matter what current you pass through it, the voltage will remain fairly steady at about 1.5V. What does happen as the current increases is the power dissipation rises and this eventually damages the internal LED. You must be careful not to allow too much reverse voltage across it though. In reverse direction the LED doesn't conduct so no voltage is dropped in the resistor so all of it appears across the LED electrodes. The maximum rated for the 4N25 is 6V, if you exceed that, there is a risk of permanent damage in the LED structure.

Brian.

 

[3Deye]

Thanks a lot for quick replies.

I got the IC but I can't make it work, I'm trying to figure out what's the problem. Most probably I burnt the LED because of wrong choice of the limiting resistor from the beginning :/

 

[Tahmid]

The LED has a maximum continuous current rating of 60mA. What value resistor did you choose? What was the current? Don't you have another 4N25 you could test?

 

[3Deye]

What's the minimum current needed by the LED to operate? Can I limit the current to say 1mA or less ?

@Tahmid, Actually I forgot to connect the limiting resistor! but quickly disconnected the supply and tried again after adding it but no hope.

 

[Tahmid]

You might be able to make it work, but why do you need to go that low? The output current at the collector will be too low. You shouldn't go below say 5mA.

You should refer to the datasheet for details: www.fairchildsemi.com/ds/4N/4N35M.pdf

Hope this helps.
Tahmid.

 

[3Deye]

This is the circuit I'm designing:



The previous stage to the optoisolator is a 22V zener diode. Can you help me selecting the values of resistors? I'm making trials using ISIS simulator.

Thanks.

 

[alexan_e]

Is there a purpose for the zener circuit?
If you are doing this just for the OPTO led then you only need a resistor and an inverted diode in parallel with the opto led to protect it from the reverse voltage.

If you need help with the zener calculations use Zener Diode Calculator

For the led use

 

[3Deye]

@Alex, thanks for the calculators, very helpful.

The input voltage may vary from 28V to 40V according to my application, so I need a constant voltage, that's why I used a zener.

It seems i'll need to use high power resistors.

 

[alexan_e]

It all depends on the set led current, if for example you set the current to 20 or 30mA @28v then it will increase to 30 or 40mA @40v, it isn't a problem.

---------- Post added at 20:42 ---------- Previous post was at 20:41 ----------

By the way if you are going to use DC input then there is no need for the anti parallel diode I have described.

 

[betwixt]

Actually, given you have so much voltage overhead to work with, it might be simpler to use a constant current source than constant voltage. Even at 40V input it would only dispate about 0.4W and you could hold the current at a constant 10mA down to an input voltage of maybe 5 - 6 Volts.

Brian.

 

[Tahmid]

You can just set it for 20mA at 40V. That means, you use a resistor: (40-1.4)/0.02 = ~2k. You can use a 2.2k resistor. Then, you get around 17.5mA. Then, at 28V, you'll have 12mA. Still enough current. You'll have a maximum dissipation in the resistor of ~0.7W. You can use a 2W resistor. Then, you don't need to worry about much.

However, if efficiency is a concern, then you need to change the drive, to reduce the power dissipation.

You don't need the zener. If you're using DC input, then, no reverse voltage protection is needed either. You can have a diode in series with the opto-coupler/resistor if you need reverse voltage protection.

Hope this helps.
Tahmid.

 

[3Deye]

Thanks guys for cooperation!

@Alex and Tahmid, The datasheet does not specify a minimum current, how can I know that the current below the maximum specified (say 20mA) will be enough for triggering the transistor?

The idea of removing the zener is very good, but I should guarantee that in all cases, the current will be enough, and I don't know the lower limit as i mentioned above :/

@Betwixt, can you suggest a current source IC/Circuit ?

 

[alexan_e]

The Current Transfer Ratio (CTR) is 20% min to 50% typical.

This is the ratio of how much collector current in the output transistor you get given a certain amount of forward current in the input side LED, 50 % means 1mA input -> 0.5mA output

 

[betwixt]

The simplest constant current circuit, if you exclude special constant current diodes is this:

Use a 78L05 regulator.
Connect a 100nF capacitor between it's input and ground pins
Connect a 500 Ohm resistor (470 will do) between it's output and ground pins
Connect the supply to the input pin and opto-coupler to the GROUND (not output!) pin.

It will supply 10mA constant current with an input range of about 9V up to 35V. If you need to work with higher input voltage, add a Zener diode in series with the incoming supply. For example if you used a 12V diode, the supply range would be about 21V up to 47V.

Brian.

 

[Tahmid]

Thanks guys for cooperation!

@Alex and Tahmid, The datasheet does not specify a minimum current, how can I know that the current below the maximum specified (say 20mA) will be enough for triggering the transistor?

The idea of removing the zener is very good, but I should guarantee that in all cases, the current will be enough, and I don't know the lower limit as i mentioned above :/

@Betwixt, can you suggest a current source IC/Circuit ?

If it goes down to 5V, and you use the 2.2k resistor, you still get more than 1mA. If you have such a large variation, you may use the constant current regulator or you may have a low output current in the transistor and use another transistor (high gain) to amplify the current.

Hope this helps.
Tahmid.