Is there a way to find out if an LED has a built-in resistor or not? It seems like a simple question to me, but somehow I can't find an answer to it or I google for the wrong phrase.

@Rizier123 I'm pretty sure the resistor-included LED would show up rather differently on a curve tracer

@Shalvenay I only have a multimeter as a tool, no oscilloscope or anything fancy like that. So not sure if it is possible to find out just with a multimeter?

@Rizier123 it might be -- what color LED is it btw?

"normal" red LED

@Rizier123 try measuring the forward voltage across it with your DMM in diode test

no resistor will be 1.8-2.2V

resistor? probably a fair bit higher, depends on the current output your meter uses for testing though

Or 9 volt battery and 470 ohm resistor. Then measure volts across LED.

@Marla that works too

@Shalvenay Not sure what I do wrong, but I simply have a 5V power supply on my breadboard with just the test LED plugged in (Which I know has a resistor built-in) and with diode mode it shows me 0 V

@Rizier123 don't have the LED plugged in for the diode-mode test

Okay. I tested the LED unplugged now. And for both LED's with and without resistor both show .0L in diode mode. The only difference is that the one without built-in resistor lights up a tiny bit. But on the multimeter both show .0L

@Rizier123 strange. try the meter in ohms

they should both show OL in reverse, but the one with the resistor will have a higher reading when forward biased

It shows OL for both LEDs in reverse and forward bias

@Shalvenay I feel like I can't get any useful reading from the LED's at all

@Rizier123 is it a DED? :P

because it's either that, or your meter is total rubbish :P

No, both LEDs light up when I plug them in

Could it be the meter? Because it isn't a very good one I think.

@Rizier123 could be

Even good meters sometimes only go to 1.8V

Morning EE.SE! Question, a design calls for a 408v to 5v, 60Hz, 1VA transformer (100k load) for mains data recording purposes. These are not commercially available. However, an 80:1 ratio could be obtained by wiring a 20VA 480v/24v (20:1) to a 5VA 120v/30v (4:1). Question is, to what degree will this alter the resulting waveform? Never tried this, but have scoped the output of "unloaded" transformers and they seldom look sinusoidal for one, let alone two transformers. Thoughts?

What about a variac?

no idea either way though

The distortion of the waveform also happens in that one transformer case, if you want to keep the waveform, then what you need is one with a rather flat response, probably custom made

@Shalvenay I looked up the manual for my meter and if I read it correctly my DMM only supplies 1V with 1mA in diode mode, not sure if I look at the right thing, but this would explain why I can't test my LED correctly since they need a forward voltage of around 1.8V. So any other idea how I can test my LED if it has a built-in resistor or not?

@Marla I don't understand how you should then be able to tell if it has a built-in resistor or not?

Take one apart?

@Rizier123 . . . with 9 volt battery, 470 ohm resistor, and your standard red LED, you could measure about 1.5 volts across the LED. When using an LeD with built in resistor, the voltage across the LeD would be substantialy higher than 1.5 volts (depending upon value of built in resistor).

However, if the built in "resistor" was actually a constant current source, you might have to apply more than 9 volts for the experiment.

My meter supplies about 100uA and 2.5V but reads only until 1.8V

is there one?

Is it accurate to say that PIL and OpenCV-Python are independent?

dont know for sure, but I would think so, maybe both use the opencv c library, don't know... if you have a linux wiht both installed you could simply look at the package infos

@Marla @Shalvenay So simply measuring the voltage works. Because the LED with the built-in resistor just acts as a big resistor which drops the entire 5V, I measure 5V across the LED leads. While with the LED without built-in resistor I measure around 2V and the other 3V I of course get over the resistor.

@Rizier123 . . good to hear that. Happy hunting now :)

When I want to build a very low frequency antenna... which is better, a big coil horizontally, vertically or at approximatelx 45°? I can install a big one ina room here, have about 5m in one direction and then varying amounts for the other angles, smalles vertical, biggest 45

@PlasmaHH . . Keep in mind that when you are using a coil for an antenna that you are only using the magnetic field portion of the electromagnetic wave. This might help guide you for antenna orientation.