Turning on an optocoupler with 1.7v?

I am trying to tap at the cathode and anode of an LED and it's voltage difference is 1.7v when it is on while 0.4v when it is off. Is there any way I could turn on an optocoupler like 4n30 with only 1.7v?

Sure, substitue the LED inside the optocoupler for the existing LED. Or wire the optocoupler LED in-series with the existing LED.

Or put a resistor in series with the existing LED, then put that LED+R in parallel with the optocoupler LED.

It depends on the comparative forward voltages of both LEDs, but with some messing around you sould be able to get both LEDs lighting up, although at a lower brightness than one LED.

MikeMl said:

Sure, substitue the LED inside the optocoupler for the existing LED. Or wire the optocoupler LED in-series with the existing LED.

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Sorry MikeMl, I don't get it. What do you mean substitute the LED inside the optocoupler? If I wire it in series will 4n30 function with 1.7v??

Mr RB said:

Or put a resistor in series with the existing LED, then put that LED+R in parallel with the optocoupler LED.

It depends on the comparative forward voltages of both LEDs, but with some messing around you sould be able to get both LEDs lighting up, although at a lower brightness than one LED.

Click to expand...

Mr RB, is there a saturation point for 4n30? I really don't know if it turns on fully the darlington transistors with a low voltage.

lloydi12345 said:

I am trying to tap at the cathode and anode of an LED and it's voltage difference is 1.7v when it is on while 0.4v when it is off. Is there any way I could turn on an optocoupler like 4n30 with only 1.7v?

Click to expand...

Hi,

The 4N30 d/s states 1.2Vfwd to 1.5Vfwd at 10mA.

Providing the 1.7V source can supply ~10mA , with a say a 47R series resistor, assuming the 4N30 has the Vfwd of 1.2V it might work, BUT I doubt due the drop across the resistor that drives the existing LED, it will not be able to source sufficient current to fully drive the LED and 4N30.

lloydi12345 said:

Sorry MikeMl, I don't get it. What do you mean substitute the LED inside the optocoupler? If I wire it in series will 4n30 function with 1.7v??

Click to expand...

You are measuring the voltage across the diode section while it is ON. Perhaps you might have used 5V DC to drive the diode section, using a series resistor. In such case the additional LED can well be wire in series to it. you might need to reduce the series resistor value to manage the needed current , say 10mA or so.

ericgibbs said:

Hi,

The 4N30 d/s states 1.2Vfwd to 1.5Vfwd at 10mA.

Providing the 1.7V source can supply ~10mA , with a say a 47R series resistor, assuming the 4N30 has the Vfwd of 1.2V it might work, BUT I doubt due the drop across the resistor that drives the existing LED, it will not be able to source sufficient current to fully drive the LED and 4N30.

Click to expand...

Hi ericgibbs, I tried using 47R and you are right! Voltage drops around ~1.2V. When I connect 4n30 the LED doesn't turn on but 4n30 works nicely. The LED couldn't handle the voltage drop. I am trying other values of resistor to see how everything would go.

mvs sarma said:

You are measuring the voltage across the diode section while it is ON. Perhaps you might have used 5V DC to drive the diode section, using a series resistor. In such case the additional LED can well be wire in series to it. you might need to reduce the series resistor value to manage the needed current , say 10mA or so.

Click to expand...

Yeah, 5v should be the ideal level to tap a voltage that could turn 4n30 ON but I am modifying a very large system and its PCBs are too much complicated and would need much more time to disassemble and assemble everything back. There are I think 7 PCBs or more sticked together with a lot of wires. So I am only left with the option of tapping at the LED since the resistor connected to it (in series) is from another PCB which is connected through a ribbon cable.

Hi everyone, thanks for the help! I changed the resistor to 120 ohms and now they are good to go. Both LED and Optocoupler now works.