I understand what diodes are for, but why the difference between them? Some have higher volts and amps, but why. Could I just buy the biggest diode, and use it for arduinos 5 volts? Any help would be appreciated as I intend to buy some soon. Thanks.
P.s. I put a multi meter to the diode I have, and it gave me a number, 658, what does that mean? Is that resistance or something?
In the ideal sense, diodes don't have resistance. (In the real world, they do of course, but that's not what your meter is showing you.) The metered value is the "forward voltage." Basically, a diode does not conduct until the voltage on the anode is higher than the cathode, by the amount in the forward voltage spec. Typical silicon diodes have a Vf of about 0.7v -- give or take some. This voltage is effectively lost, so if you ever need to drop 0.7v, a diode is one way to do it!
Once the voltage difference is above this point, it becomes essentially a short circuit. This is exactly how LEDs work, and why you always need to use a current-limiting resistor. Once the voltage is high enough to turn it on, it'll allow through as much current as will flow.
When the voltage drops below this forward voltage, the diode stops conducting. This is handy in rectifiers to turn AC into positive (or negative, depending on polarity of the diode) pulses. You then place a cap in there to store voltage during the low points, and voila you have a DC power supply. Works great at 50/60Hz, but in reality, the diode does not start and stop conducting immediately -- there's some slight delay. If your AC is in the hundreds of kilohertz range, you need a fast diode. They make those.
If you can't stand to drop 0.7v, there are diodes with a lower Vf. If you need to pass 100A of current, they make those too.
All diodes have a voltage point where, if the cathode is higher than anode by too much, it'll break down and start conducting in reverse. Usually this destroys the diode. (This is why there are voltage ratings.) But, then there are special classes of diodes called Zener diodes that are designed to withstand breakdown. They work like normal diodes in forward mode, but start conducting in reverse at a specific voltage -- without damage. This is really handy for a few different things -- you can use it as a reference voltage (by placing a resistor between the Zener and a voltage source, then reading the voltage at the resistor/Zener connection point), or as an input protector (by shunting excess voltage to ground or Vcc).
Basically there are two things with a diode, voltage and current.
The voltage is the maximum reverse voltage they can block without breaking, in a flyback this needs to be greater than the voltage you are using to drive the coil.
The current is the maximum current it can take before breaking. For a flyback diode make sure this is greater than the current you are driving the coil with.
Appreciate the info gentlemen. Thank you.
One more question concerning diodes. How do I read them? They are very small, and usually have numbers on them.
Bstanko6:
I understand what diodes are for, but why the difference between them? Some have higher volts and amps, but why. Could I just buy the biggest diode, and use it for arduinos 5 volts?
Are you confusing Zener diodes with regular Diodes?
We have Power diodes, fast switching diodes, Zener Diodes, low voltage drop signal diodes, infact there's quite a lot of diodes which all have different jobs.
So i don't believe you do understand what they're for..
Regular Diodes allow Voltage to travel one way.
Zener's clamp voltage (via it's reverse breakdown)
Signal Diodes (Low voltage drop for radio projects)
If you wanted to power an Arduino, you'd probably need a 1watt 5.1v zener diode and a resistor with a higher enough rating (wattage) to power your arduino but there's many many more and better ways to do it...
Typical silicon diodes have a Vf of about 0.7v....
Once the voltage difference is above this point, it becomes essentially a short circuit. This is exactly how LEDs work, and why you always need to use a current-limiting resistor.
Worthwhile remembering though, that Vf for an LED is 3-4 times that, somewhere up in the 2-3V range depending on the colour, ymmv.
Do you guys know how to read them? I have a couple of diodes, but do not know what volt they work with.
just read whatever it says and look up the part.
I Personally have given up trying to read zener's without a magnifier (and even then it's tricky) , I simply use a 1k resistor and connect it up and read the spit out voltage, much easier than reading and looking those things up.