# Inability to forward bias diode in a simple circuit

I am struggling with forward biasing a diode in the circuit in the attached.

All the wires are attached properly, as there is continuity. The diode drops only 0.400V with a 9 V battery connected, the resistor being 330 ohms. I cannot understand why the diode would not drop the amount required to forward bias itself and as a result allow current through the LED. At the moment most of the voltage is being dropped across the resistor and LED. Am I making some fundamental mistake I cannot see? Of course I tried connecting both the diode and LED in reversed polarity with no change.

Apologies for not posting more pictures. I have RSI and need to limit how much I use the computer.

• Please use the embedded schematic editor and post a circuit schematic. Jan 25 at 14:40
• I have RSI Making a clear schematic drawing on a piece of paper and including a photo of that is also perfectly acceptable! Jan 25 at 14:44
• BTW, looking at the background of the photo I can see some kind of fury rug. This is asking for ESD troubles. Jan 25 at 14:47
• Are you sure that Resistor is 330R? it's hard to see in the picture, but doesn't look like 2 orange stripes to me Jan 25 at 14:47
• The diode is forward biassed. I'm guessing the LED is wrong way round in which case there's > 8V across it and very little across the resistor. Jan 25 at 14:50

Diode is not the problem. You can remove it and replace it with a piece of wire to see that for yourself. Make a simpler circuit work first. Then when you add the diode it'll keep on working (as long as there's enough voltage driving the circuit).

You assumed that the diode is a problem, whereas before such assumption you should ask yourself: do you know enough to know that the diode is a problem? Remember that the voltage drop across a diode is ~logarithm of the current, so all you're seeing here is a diode acting like a rather sensitive current meter and telling you that microamps are flowing through it. You can replace the diode with a microammeter and see what the current is. I'd expect anywhere between 0.1uA and 10uA.

Once you get the circuit working, you should play with that diode - put it in series with a resistor, measure voltage across the resistor to determine the current, then measure the voltage across the diode. Use various resistors to "sweep" the current across the measurement range. Plot the diode voltage vs. current over at least 4 order of magnitude - say between 0.1mA and 100mA, and see whether the response is logarithmic (after all, I can be talking nonsense - you really should see it for yourself since it's easy and thus no reason at all to blindly trust me).

• This place has been my saviour so many times. Apologies for being dumb, you pointed me towards solving the problem. At first the problem was the wrong resistor value- 330k (can't believe I missed the K when reading it with a multimeter). After correcting that, for some reason the LED was fried so I needed to change it. I will performed experiments suggest as well! Thank you again. Jan 25 at 16:37

If "most of the voltage is being dropped across the resistor and LED" then the diode is forward biased. A significant current must be flowing through the diode. A voltage of 0.4V across the diode under forward bias and low current is not unreasonable.

• Thank you very much for posting a response in helping out. It was indeed a stupid mistake of choosing the wrong resistor value leading to low current. Jan 25 at 16:38

Diode looks fine. The long lead of the usb should be on the Resistor, (aka on the higher voltage) and the shorter lead of the led to GND (aka to the lower voltage). Make sure you have this correct.

Note: there is a chance your led died/fried/not working. Same goes for your diode. As @Kuba said, try replacing the diode or led with a wire.

Your resistor might also be of a higher value. This led might be a 9V led (you never know, check its datasheet).

• Thank you for your help. Jan 25 at 16:38