hello,
something bugs me which is the fact that while working i somehow mistakenly applied positive voltage to both the anode and cathode of a diode 1N4148 at thesame time but i was quick to stop when i realized my mistake but somehow the diode survived and i was wondering why since normally we apply positive voltage to the anode while the cathode is left alone so current flows as usual.
in my thinking the diode should be destroyed or something like that.
what could happen when this situation occurs?
Hi,
audioguru explained the diode action pretty well, i'll just add a little more of my own personal thoughts.
After reading your post i get the feeling that you might believe that the diode should only be biased one way and only one way. But actually the diode can be biased two different ways and that's when we see the main action of the diode.
If we make the diode anode more positive than the cathode (forward bias), we see the diode conduct a significant current. If we make the diode cathode more positive than the anode (reverse bias) then we see the diode conduct only a very very small current as long as we dont exceed the reverse voltage rating of the diode.
Because we have different reactions of the diode depending on the polarity at the two terminals, we see the basic diode action which means it conducts more in one direction than the other, and that is what gives us rectification. This is the most important quality of the diode, that it can rectify a bipolar signal into a unipolar signal and that's how we get raw DC from an AC source.
So the diode can be biased either way, but how much current that flows is dependent on which direction we bias it. With an AC signal it gets biased both ways in turn and only when it is forward biased do we see significant current flow.
Diodes like the 1N4148 have fairly low resistance when correctly forward biased but in reverse the resistance can be as high as 500 Megohms. Note you can measure the reverse resistance at low reverse voltage with a multimeter on the Ohms scale (if it goes up that high), but you may not be able to measure the forward resistance because that requires a certain minimum voltage level and varies quite a bit with the actual voltage. If you happen to have a 'diode check' function on your meter that may help measure the voltage drop at a particular test current that is put out by the meter.
If you've never worked with diodes before this might sound strange, but after doing just a few simple experiments it will be come very clear how amazing the simple diode is.
A simple experiment is to connect a 9v battery in series with a 1k resistor in series with the diode. Measure the current with the diode forward biased, then measure the current with the diode reverse biased. Note the huge difference in current levels.
[Note the current can be measured with a high input impedance meter by measuring the voltage across the 1k resistor. The conversion is 1v equals 1ma of current.]