Hi guys
I'm busy at the moment here completing lab reports. Fun times... The one I'm doing at the moment concerns the 1n4001 diode.

These are 2 of the oscilloscope readings



It's been a while since I did these - have to write them up for coursework. First ones easy enough. Forward bias. Yellow reading VS. Blue the diode. No current is passing through the potential barrier on the neg half cycle.

Now the second is in reverse bias. I expected no current at all here. Now this is probably going to sound like the most stupid comment you ever hear in your life but - can I say the negative voltage is passing through the barrier in reverse bias? Is that whats happening here?

 

No current is passing through the potential barrier on the neg half cycle.

1. You are applying an AC voltage - that is it goes positive and negative.
2. In your first diagram as you say "No current is passing through the potential barrier on the neg half cycle." and conversely current is passing through the diode on the positive half cycle.
3. In your second diagram, with the diode the other way round, you can write phrases as in point 2 above, but modified slightly, for the new situation.

 

I have never been good at remembering anode or cathode, so I just use the arrow of the schematic symbol to help me.

Anytime the diode "arrow" points to a voltage more negative than it points away from, it will conduct due to forward bias.

The absolute values mean nothing. -100 volts and -80 volts. The diode will conduct if the arrow points to -100 volts.

100 volts and 80 volts. The diode will conduct if the arrow points to the 80 volts.

 

Sweet... that's what I was thinking didn't know how to phrase it properly. Thank you.

I believe I need to rethink the way I think about voltage. I had this idea in my mind that voltage is like current, in that it flows in a certain direction. This output on the oscilloscope, that's the current right? Or maybe not... AC confuses me a little. Maybe its the voltage across the diode. In fact now I think about it, that feels correct. Hmm I'm not bad with complicated things. The basics I'm a disaster.

 

I had this idea in my mind that voltage is like current, in that it flows in a certain direction. This output on the oscilloscope, that's the current right?

No.
Current is the flow of carriers through a conductor; voltage is the EMF force that drives the current.
You see voltage, not current on an oscilloscope display.

Note that voltage is always measured between two points, it has no meaning otherwise.
Conversely, current is measured at one point.

You can have voltage with no current, such as from a battery.
The only way to have current without a voltage (drop) is through a superconducting wire.

 

Hello,

You should show the circuit also. That's because we need to see if the circuit is loaded.

Keep in mind that the oscope probe impedance may be 1 Megohm.

Of course you cant go over the reverse rating of the diode either.

 

Thanks Mr AI. No I don't think the circuit is loaded. I've uploaded a pic. We were just testing this for rectification. Forgive me for saying so but this wave isn't looking so rectified, if you ask me. I've a seen a DC voltage on this oscilloscope and it just looks like a straight line. funny story about the man that found the first superconducting wire. For him it was quite the shock.

 

The voltage across a diode is the voltage at the anode minus the voltage at the cathode.

In both of your cases, the diode is conducting current when it is forward biased (by more than a diode drop) and not conducting when it is reverse biased.

 

So is that why it's measuring a signal wave. I see said the blind man. Ok lets say I stuck a wire in the breadboard after the diode and measured it against the ground. Would that then show me a DC signal? Or hang on - that might be zero... or...

 

I have no idea what "after the diode means" -- you haven't even shown a schematic at all, let alone indicated which side of the diode is "after" the other.

Sketch a schematic and indicate where this wire would go. THEN we can discuss it.

 

Ok this is probably the worst schematic ever but here goes...

Don't beat me

 

this wave isn't looking so rectified, if you ask me. I've a seen a DC voltage on this oscilloscope and it just looks like a straight line.

Have a look at these two:
https://en.wikipedia.org/wiki/Rectifier#Half-wave_rectification_.28M1U.29
https://en.wikipedia.org/wiki/Rectifier#Rectifier_output_smoothing

 

Place the resistor on the other side of the diode (cathode side) in series to the bottom lead (lets call that common or ground).
Then if you measure the voltage from the resistor-diode junction to common you will see the rectified voltage.

 

Thanks guys. I understand this a lot better now. Those links really helped Albert ta v much. Right then last one now. Op amps yeh.