12VDC to 3.3VDC LED Metered after resistor

[Stevelin9]

Hello.

I have a simple circuit consisting of a 12VDC supply and an led listed as follows:

Blue 1500mcd 3.5V 20mA Blue

The circuit is currently open at the - terminal of the LED.

If I meter where the 2 arrows indicate, i am expecting to see ~3.5V, however I see 12. The resistor meters at the expected ~510 Ohms.

Can anyone shed any light on this for me?

Thanks!

 

[Chemelec]

If the Circuit is OPEN, There is NO Voltage Drop.
Connect the LED, than measure the Voltage Across the LED

 

[Stevelin9]

Thank you for the help Chemelec.

I completed the circuit and metered across the LED. Reading .45V and LED is not emitting. Any advice from here?

 

[brevor]

Leave the circuit completed and measure across the battery and the resistor (separatly) and post your readings.

 

[Herschel Peeler]

Thank you for the help Chemelec.

I completed the circuit and metered across the LED. Reading .45V and LED is not emitting. Any advice from here?

Is your 510 ohm resistor510K?

 

[Chemelec]

Possibly your Battery is Bad.
Or you connected the LED up BACKWARDS

 

[Stevelin9]

Leave the circuit completed and measure across the battery and the resistor (separatly) and post your readings.

It's not actually a battery. It's coming off a GPIO from another device.

 

[Stevelin9]

Is your 510 ohm resistor510K?

510 Ohms. Green Brown Brown

 

[Chemelec]

LED Probably in Backwards.

 

[Stevelin9]

Connected LED in opposite direction with same result.

Again, thanks everyone for your suggestions.

 

[Chemelec]

With the Resistor and LED Connected, Does your 9 Volts REALLY READ 9 VOLTS?

If YES, Than I Suspect a BAD LED.

 

[Stevelin9]

With the Resistor and LED Connected, Does your 9 Volts REALLY READ 9 VOLTS?

If YES, Than I Suspect a BAD LED.

I assume you mean 12V, and the answer is 11.42V

 

[Chemelec]

Sorry, Yes 12 Volts.

The Circuit is TOO SIMPLE to be anything else, Other than a BAD LED.

 

[(*steve*)]

If the voltage across the led is 12v with it one way around and 0.45 the other way then it is likely that the gpio can't supply enough current.

When it is connected the right way around, measure the voltage across the resistor and then across the LED.

Chemelec is giving you bad advice. Ignore him.

Note that connecting the LED backward can destroy it, so if the LED has failed... Well, you know who suggested that.

 

[Chemelec]

STEVE: I Never Suggested to connect it Backwards.
I Suggested that he "May have Put it in Backwards", so try reversing it.
And According to his Reply, he was getting the Same reading, Both ways.
Just sounds like a Bad LED.

 

[(*steve*)]

No, he was getting a low voltage across it. This is NOT indicative of it being reversed.

What it I indicative of is that the gpio pin is an input with a weak pull up.

 

[Sunnysky]

LEDs are rated at -5V ( reverse bias ) at 10uA If you apply -12V in reverse, you will damage the LED.
Replace it and dont do that again.

 

[Chemelec]

Sunnysky: Just for curiosity, I hooked up a Blue, 20 mA LED with a 510 Ohm resistor and a 12 Volt supply.
Forward biased, it lit up as expected.
When I Reverse biased it, obviously it didn't light.
Than I Increased the supply to 25 volts and left it connected for a full minute.
(Reverse Current measured at 25 Volts is 54 uA.)
NO Damage to the LED, It still worked perfectly when connected Correctly.

But the real Question is: WHY DOESN"T HIS LED LIGHT UP?

 

[Sunnysky]

I guess Not all LEDs have the same failure threshold , but most will fail or be damaged with lower efficacy.

 

[BobK]

At 54uA? I doubt it. It is basically just a zener diode.

Bob