I figured I'd start a new thread for this very probably foolish question. I did search, but am even more confused now.
What I'm doing is making a high voltage, high amperage mosfet-based relay, and thanks to replies on another thread , that part is working fine.
Now, I want to add an indicator LED into the control circuit, and am having a devil of a time figuring the value of the R1 resistor. The reason for my confusion is that, from the spec sheets, I see the FDA217 optocoupled mosfet driver has a voltage drop of 1.26 volts across the inputs, and the green LED lists a forward voltage of 3.2 volts. At first glance, I thought FV was the same as voltage drop, but I think that is incorrect.
Is the voltage drop of the LED @20mA equal to: Applied voltage (5v) - forward voltage (3.2v) = 1.8 volts? If so, LED voltage drop (1.8v) + FDA217 voltage drop (1.26v) = 3.06v; meaning the resistor value would be 3.06/.02mA = 153 ohms.
Something else I learned in the other thread was that the Arduino has an impedance of 40 ohms on the output pins, meaning the actual value of the R1 resistor would be 113 ohms.
Are my calculations even close to correct? Thank you!
The LED forward voltage is the voltage drop across it, at the specified current.
Post a link to your green LED, as 3.2 V seems too high. Also post a proposed circuit diagram for the opto input -- you cannot connect LEDs in parallel.
jremington:
The LED forward voltage is the voltage drop across it, at the specified current.
Post a link to your green LED, as 3.2 V seems too high. Also post a proposed circuit diagram for the opto input -- you cannot connect LEDs in parallel.
It would appear that Amazon's specs are not accurate. A spec sheet for another green LED shows the forward voltage at 1.8 ~ 2.2 volts. I don't know where the 3.2 FV figure came from on the Amazon page, unless that is the led's minimum operating voltage?
It should be OK to assume Vf = 2.0 V for a green LED. The chip top view is not useful to us, but if you are wiring the optocoupler input LED in series with a green one,
you can assume total forward drop = 4 V at 20 mA.
For 5V power, the series resistor should be about 50 Ohms, to drop 1 V.
The gate Zener diodes are backwards in your diagram.
jremington:
Post a link to your green LED, as 3.2 V seems too high.
All GaN LEDs, be they green, blue, violet or white are 3.0--3.2V or so, due to the 3.4V bandgap for GaN.
Other III-V semiconductors are used for green LEDs with different forward voltages. GaN is commonly used for the modern ultra-bright heterojunction quantum-well devices from green upwards.
Since you don't have a data sheet, and don't know the manufacturing process, just measure the forward voltage.
A ~1 K resistor in series with a 9V block cell will limit the current to a few mA. Your multimeter can then be used to determine the forward voltage for any of the LEDs.
jremington:
The gate Zener diodes are backwards in your diagram.
LOL. I suspect another learning moment is in store for me, but I don't understand; I have them hooked up with the striped end facing the positive side. I had them the other way at first, and was getting nothing to the mosfet gate. As I understood it, they are exactly like regular diodes, except that when the voltage exceeds the breakdown voltage, they conduct the other way....
jremington:
Sorry, my mistake. The Zener diode orientation is correct as shown in the schematic. The images is a bit blurry unless you download and expand it.
The "12V" refers to the (calibrated) reverse breakdown voltage. In the forward current direction, the Zener does act like an ordinary diode.
Thanks. You about gave me a panic attack. LOL I'll do a better drawing, with the FDA chip's internals, and discard the extraneous cruft to make it a bit easier to read.
It's the least I can do in return for you guys valuable help.
jremington:
Since you don't have a data sheet, and don't know the manufacturing process, just measure the forward voltage.
A ~1 K resistor in series with a 9V block cell will limit the current to a few mA. Your multimeter can then be used to determine the forward voltage for any of the LEDs.
Green LEDs will glow for any reason. I have hand soldered SMT green LEDs to a pcb with no power and merely touching the iron tip to the device creates a faint green glow!
jremington:
Since you don't have a data sheet, and don't know the manufacturing process, just measure the forward voltage.
A ~1 K resistor in series with a 9V block cell will limit the current to a few mA. Your multimeter can then be used to determine the forward voltage for any of the LEDs.
Much better schematic, thanks! As pointed out above, the current limiting resistor can have a very wide range of values and the LEDs will still light, as long as specs are not exceeded.
However, you must make sure that the optocoupler LED has sufficient current for solid switching, so it would probably be better to have a separate current limiting resistor for each LED, and wire the combinations in parallel, rather than wire the LEDs in series.
jremington:
Much better schematic, thanks! As pointed out above, the current limiting resistor can have a very wide range of values and the LEDs will still light, as long as specs are not exceeded.
However, you must make sure that the optocoupler LED has sufficient current for solid switching, so it would be better to have a separate current limiting resistor for each LED, and wire the combinations in parallel, rather than wire the LEDs in series.
All things considered, I think I will do just that. I measured the Vf on the green led, and if I measured it correctly, the Vf is 2.65 volts, although I have no idea how accurate my old Sears DMM is. It also shows 12.2 volts across the mosfet gate and source, even though there is a 1% tolerance 12v zener diode hooked across it.
krupski:
Green LEDs will glow for any reason. I have hand soldered SMT green LEDs to a pcb with no power and merely touching the iron tip to the device creates a faint green glow!
Then your soldering iron is not grounded and will destroy delicate components if its leaking enough mains to make an LED glow.
