DIODE ? help

[raphaelriv]

what is the meaning of Junction capacitance among diodes ? if one has 60pF and another one has 120pF when can this fact be crucial and for what applications?

this is what i found:

4.5.2 The junction capacitance
The junction capacitance is calculated using the expression for the parallel plate capacitance. This might at first seem unexpected since the charge is distributed throughout the depletion layer. However, when applying small voltage variations one finds that charge is only added and removed at the edge of the depletion region so that the capacitance simply depends on the dielectric constant, the area and the depletion layer width, yielding:

(pn14)
where the depletion layer width, w, was obtained from the electrostatic analysis.

 

[stevez]

Is this homework?

 

[Dr.EM]

It can also change depending on the current through it, I seem to remember a certain ex-member here trying to tune a radio with one . Special diodes exist which utilise this property for variable capacitance applications such as tuning radios.

 

[audioguru]

Hee, hee.
"The certain ex-member" has a slightly changed name and posted his still non-working super-regen "radio" recently on another electronics chat forum. It changes its tuning all by itself.

A varactor tuning diode is reverse-biased and its capacitance changes with voltage change. The junctions of a transistor are also variable capacitors with voltage tuning. That's how simple FM bugs modulate with FM, and why their frequency changes as the battery runs down.

 

[andy257]

I remeber reading a post on here or maybe electronics lab where a guy had used lots of individual LEDs to make an LED matrix. He connected this matrix to a computer where he programed some software to output some 3d graphics on the computer screen. Moving his hand over the LEDs or even touching them would indicate a response on the screen. I beleive he used the junction capacitance to control the effect he desired.

I will try and find the link if possible, others may remeber such a project.

Andy

ok i am totally wrong now ive found the old post. never mind. Its a cool diode project anyways.

**broken link removed**

 

[raphaelriv]

re:

naa is not homework, i basically have a board that is connected to a battery pack and i want to replace the diode with another one i just don't have the exact replacement, except i had one that matched almost everything but i saw that junction capacitance difference and i wonder what role it played if any?..

 

[Dr.EM]

So long as the board isn't a radio tuning board, juction capacitance is likely to be irrelevant. There are other factors such as speed and VF drop though. If you can tell us what the board is and where the diode is located, we may be able to suggest what is suitable. It may be a polarity protection diode which is of non-critical value.