reverse-biased diodes (zeners, etc.)

[julieCabazon]

hi EMDIY-

i wanted to ask a bit about reverse-biasing diodes for noise generators. i know that in most cases, there will need to be additional amplification..but i'm also running into some difficulty getting diodes to produce noise in the first place.

if you google "reverse-biased diode noise", you get things like this:
http://www.next.gr/uploads/135-9365.png

or this:
http://i.stack.imgur.com/sJuYO.png

i'm finding that these designs are not working for me. i've had no trouble getting transistors to produce noise, but for some reason, diodes are elusive to me.

i'm using a 12v bipolar power supply, which is different than many of the 9v single supply schematics...perhaps the components need to be adjusted for different values.

essentially, what i'm asking is this: is there a simple circuit to test different types of diodes for noise characteristics?

[blue hell]

hi julieCabazon



it is likely that 12V is still too low for the first schematic you linked at.

To get noise from a diode, or from a transistor (that is wired as a diode), you need some voltage higher than the reverse breakdown voltage. As a transistor used in this mode usually has worse specs than a modern diode would have it will work better for a transistor than for a diode. For the usual circuits the reverse breakdown voltage is a value one wants to be high - not so for a noise circuit though.

As to the second circuit, this should work wit a zener diode with a voltage a bit lower than the supply voltage. The 3.3V mentioned there seems too low, especially as low voltage zeners usually are made from regular diodes put in series, and so it would not be operated in reverse at all.

As for a testing circuit .. the actual noise circuit would be best for that I think ... you may want a power supply for which you can set the output voltage, to see at what voltage the noise starts happening.

Another thing is that the noise output from a single diode will not be very high, that' s why usually a transistor or an opamp will be added to level things up a bit.

[julieCabazon]

I'm not sure if I understand everything correctly - so please feel free to correct me:

Alright, so all diodes have a reverse "breakdown" voltage. These voltages are specified in data sheets with some degree of variation. This breakdown has to do with when a diode no longer behaves normally and allows voltage to travel in both directions.

How much beyond a "breakdown voltage" does one need to go, in order to generate noise? How far can one go before noise stops being generated or the diode explodes? What if one was merely using a 1n4148? Would it be possible to drive that into noise?

With zeners, the "breakdown" is specified in advance - usually at a specific, lower value than a general purpose diode. In this case, though, the ideal is that the breakdown is close to the supply voltage. I'm not exactly sure why that is. Is it possible to use a 5.6v zener with 12 volts brought down to approximately 9v (via voltage divider), for instance?

[blue hell]

Hmm .. it should not matter too much when the voltage gets higher I think .. but my semiconductor physics knowledge is getting a bit rusty

[blue hell]

http://www.diyaudio.com/forums/parts/35821-some-noise-measurements-leds-zener-diodes.html

has a lot of experiments.

http://users.cosylab.com/~msekoranja/tmp/04236738.pdf is interesting for zeners.

[julieCabazon]

Looks like either way, something is "breaking down."