Electronics > Beginners
Driving LED with Solid State Relay?
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rcbuck:
Red and yellow LEDs forward voltage drop is around 1.5 to 2.2 volts. Green, blue, and white LEDs forward voltage drop is about 3.3 volts. So 1.5 volts "may" work for the first three without a dropping resistor. However, you need to measure the current flow through the output LED to make sure you are not exceeding its limit.

But 1.5 volts is not enough voltage for the second three. Also, 3 volts applied to the first three would result in too much current that would destroy the output LED. The MOSFET ouput of the optocoupler is rated at 1 amp so you would probably not destroy it.

The AQY211EHAX uses an ultraviolet LED to turn the MOSFET on. The UV LED requires about 3.3 volts to turn on. So a 1.5 voltage source will not be enough for the optocoupler to turn on. In other words the circuit will not work with a 1.5 volt supply.

You can use a 500 ohm pot in series with your DMM in current measurement mode to adjust for the desired current flow at 3 volts. You would do this for the optocoupler LED first. Measure the pot value and replace it with the nearest fixed resistor value. Then do the same thing for the output LED. Once you have the correct resistance values for 3 volts you are good to go.
ryanmills:

--- Quote from: Nerull on July 10, 2019, 01:01:48 am ---A LED is a diode. It is incapable of regulating its own current, and requires some form of current limiting. An ideal diode becomes a short circuit above its forward voltage. A real diode has some resistance, but this should never be relied upon.

That line is the current going off to infinity and the diode burning up after Vf is reached.

In your first post you are complaining that the diode seems to be shorting out the output. This is why - because you are shorting out the output.

--- End quote ---

Thank you, learning as I go there are holes in my education. I fully understand diodes but I could never intentionally short one like you would with an LED. It also just realized the cheap amazon LEDS we use in bulk have a current limiting resistor built in so I have never needed them. Just never occurred to me that an LED would need one.

Today "a dim bulb got just a little brighter" |O
Zero999:
I answered your question in the other thread. Can you please keep it to one topic per project/question.
https://www.eevblog.com/forum/beginners/do-ssr-require-current-limiting-in-some-fashion/msg2540685/#msg2540685
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