resistor question

[wejos]

hi guys,

forgive me but why's there a 50-ohm resistor in the cathode part of the optoisolator?

i'm guessing this semiconductor had a triac inside and not some darlington.



**broken link removed**


thanks in advance


source: http://www.glacialwanderer.com/hobbyrobotics/?p=10

 

[Hero999]

It's to limit the current through the LED.

Have you read the datasheet?
https://www.electro-tech-online.com/custompdfs/2010/04/PN10447EJ03V0DS.pdf

What's the Arduino?

 

[wejos]

i have no idea i'm thinking to use different PIC - my wild guess it's a PIC.

thanks hero the other optos must have confused me since cathode is usually pin 1

and it's not triac it's darlington i wonder if this will really trigger

i'm sorry if this was a stupid question

 

[Hero999]

Well I don't know much about the flash you're using but normally the trigger is a small pulse applied to a small high voltage transformer which creates a few kV to ignite the tube.

The trigger pulse is normally a couple of hundred volts but low current, which is why a high voltage transistor is needed. It's also often delivered by a capacitor so a TRIAC might work.

 

[wejos]

thanks hero for the info.

i remember now since it's LED the supply should be reversed also.

i'll buy a opto with builtin triac tomorrow

 

[Reloadron]

The circuit you posted should trigger the flash just fine the way it is. The Diode portion of the optocoupler has a forward voltage of 1.1 volts at about 80 mA. Therefore if you want to trigger using a 5 volt pulse you have 5 volts (supply) minus 1.1 volts (forward voltage of the diode) gives you 3.9 volts. Therefore 3.9 / .080 = 48.75 and thus your 50 Ohm current limiting resistor. Connected just like the image the flash should fire when you apply 5 volts to your opto coupler with that 50 Ohms in there to limit the current.

Ron

 

[Hero999]

Sorry if I mislead you, there's no need to buy a optp-TRIAC, if you already have a photo-transistor, however if you can't get hold of a photo-transistor go with the opto-TRIAC.