Optocoupler resistor value

[firewater]

Could someone please show how me how to hook up an optocoupler with a diode in reverse parallel for protection. I'am using a 4N25-6 **broken link removed**
The sheet shows the max forward voltage to be 1.5V at 10mA and I'm getting 1.7V peek so I need to increase the resistor value correct? I'am coming out of a basic stamp (5V) through a 1.2KOHM resistor to the anode/cathode side.
This graphic I made in Illustrator CS2 should be helpful.

 

[audioguru]

If the output voltage from the STAMP is 5.0V (it is probably about 4.5V) then the 1.2k resistor has 3.3V across it and therefore the current in it and in the LED of the opto-coupler is only 2.8mA. A fairly low current.

There is no reverse voltage so a protection diode is not needed.

The forward voltage of 1.7V is odd because the typical voltage at 3mA is only 1.1V for an American one. Maybe your Oriental one is different.

 

[ericgibbs]

Could someone please show how me how to hook up an optocoupler with a diode in reverse parallel for protection. I'am using a 4N25-6 **broken link removed**
The sheet shows the max forward voltage to be 1.5V at 10mA and I'm getting 1.7V peek so I need to increase the resistor value correct? I'am coming out of a basic stamp (5V) through a 1.2KOHM resistor to the anode/cathode side.
This graphic I made in Illustrator CS2 should be helpful.

hi firewater,
If your test meter can measure current, place it in series with 1K2 resistor and the opto diode, lets know how many milliamps are flowing in the opto LED.

I know the meter will effect the current value, but it will give a clue to whats happening,
also measure the +5V and the voltage across the 1K2.

Can you post a circuit showing how the opto is connected?

 

[Nigel Goodwin]

I know the meter will effect the current value, but it will give a clue to whats happening,

No it won't, the resistance of an ammeter is totally insignificant with a 1.2K in series with it - and in almost all cases, ammeters are designed that way, measuring the very low voltage across a very low shunt.

 

[ericgibbs]

No it won't, the resistance of an ammeter is totally insignificant with a 1.2K in series with it - and in almost all cases, ammeters are designed that way, measuring the very low voltage across a very low shunt.

I dont know what type of meter he is using, do you?

Also, I didnt say it would be significant, it was just to make him aware that when making a measurement, the act of taking the measurement can affect the result!

I have a suspicion that its not a 1K2, but much lower.

 

[aljamri]

Hi,

it depend on the meter he is using, my fluke 79 has a 0 to 20 ma range separate from A range, so he can tell if his meter will measure mA with no significant tolerance.

 

[Nigel Goodwin]

I dont know what type of meter he is using, do you?

To measure current you use an ammeter or a milli-ammeter, both meausre the tiny voltage drop across a low resistance shunt - so unless he's using something totally unsuitable (a barometer perhaps? ) then it's not going to cause any problem.

There are certain very specific instances where you need to consider the drop across the shunt, but they are really, VERY, VERY rare and unusual.

 

[ericgibbs]

so unless he's using something totally unsuitable (a barometer perhaps? ) then it's not going to cause any problem.

Nigel, like you I have witnessed engineers doing even stranger things than using a barometer to measure current.

Like an engineer working in our lab [years ago] was using a Megger to measure the input resistance of FET's.

I prefer to err on the side of caution and make a 'learner' aware of the possible
problems when carrying out measurements.