# Variable capacitor circuits

Discussion in 'General Electronics Chat' started by walters, Jan 13, 2006.

1. ### waltersBanned

Joined:
Jun 4, 2005
Messages:
638
Likes:
0
just self taught with some books

do u know the formulas i need please

2. ### _3iMaJNew Member

Joined:
Jun 28, 2005
Messages:
182
Likes:
0
Using a formula isn't the answer. Understanding the "why" will answer the "how."

3. ### mstechcaNew Member

Joined:
Mar 28, 2003
Messages:
1,595
Likes:
1
Location:
Why do you care?
I think walters needs this question asked to him again:

What are you trying to achieve?

I mean why would you want one formula for input, RC time, and phase? To me it is senseless to try to combine everything into one formula.
I only look at the RC time. Input is just input. and if the input is a speaker, check it's resistance, and then treat it as a resistor.

Joined:
Jan 12, 1997
Messages:
-
Likes:
0

5. ### audioguruWell-Known MemberMost Helpful Member

Joined:
Mar 16, 2004
Messages:
32,595
Likes:
952
Location:
ONLINE

The low frequency resonance of a speaker causes it to be a lousy microphone.
Try making an allpass filter, a variable phase shift or a comb filter with just a speaker.

I know what you can do with the speaker: spin it! :lol:

6. ### mstechcaNew Member

Joined:
Mar 28, 2003
Messages:
1,595
Likes:
1
Location:
Why do you care?
In fact, connect a speaker to a circuit of some sorts, and to a motor, and see if you can power the motor from the techno music in the room! :lol: :lol:

7. ### waltersBanned

Joined:
Jun 4, 2005
Messages:
638
Likes:
0
Are Varactors a reversed bias Zenor diode or a regular diode?

8. ### waltersBanned

Joined:
Jun 4, 2005
Messages:
638
Likes:
0
varactor diodes the capacitances are in the picofarads

Can i use Varactors in "series" to increase the capacitances?

Or i would need a "Custom" Varactor diode to have the capacitance in the microfarads?

What would the Varactors doping process do different and how? the PN junction needs to be Bigger?

I need a Varactor is the Mircofarads? which ones is there please?

9. ### akgNew Member

Joined:
Sep 2, 2005
Messages:
1,404
Likes:
1
Location:
India
varactors a reverse biased diodes with specific dopping to act it as 'varactor' .
and u cannot (practicaly)get a uF varactor.
hava a look at
http://en.wikipedia.org/wiki/Varactor
http://www.vias.org/simulations/simusoft_varactor.html
http://www.play-hookey.com/semiconductors/specialized_diodes.html

10. ### waltersBanned

Joined:
Jun 4, 2005
Messages:
638
Likes:
0
thanks for the help

How can i get a a uF varactor?

I need uf Varactors?

How can i make one or put them in series or so to make a uf varactor?

11. ### akgNew Member

Joined:
Sep 2, 2005
Messages:
1,404
Likes:
1
Location:
India
u cannot (practicaly)get a uF varactor. afaik

Joined:
Jun 4, 2005
Messages:
638
Likes:
0
why not?

13. ### akgNew Member

Joined:
Sep 2, 2005
Messages:
1,404
Likes:
1
Location:
India
it requires a large depletion layer region (large area).
Also varacters are used in HF/UHF ckts for tuning , that reqs only pf variation
i hope u have visted the links i suggested.

14. ### waltersBanned

Joined:
Jun 4, 2005
Messages:
638
Likes:
0
So i have to get this Custom made to have a large depletion layer region (large area)?

Joined:
Sep 2, 2005
Messages:
1,404
Likes:
1
Location:
India
AFAIK

16. ### waltersBanned

Joined:
Jun 4, 2005
Messages:
638
Likes:
0
A large depletion layer region large area would give me more capacitance to be mircofarads?

Most Varactors are used:
tunable filters and oscillators , frequency multiplication and frequency mixing

17. ### akgNew Member

Joined:
Sep 2, 2005
Messages:
1,404
Likes:
1
Location:
India
walters
u have a 1000 1PF capacitors , how would u make a 1uF capacitor out of it?

18. ### waltersBanned

Joined:
Jun 4, 2005
Messages:
638
Likes:
0
put another one in series? or 3 or 5 in series? to ADD them?

19. ### akgNew Member

Joined:
Sep 2, 2005
Messages:
1,404
Likes:
1
Location:
India
if u have got some capacitor and a measurement eqp: , try it out and check what happens.

20. ### waltersBanned

Joined:
Jun 4, 2005
Messages:
638
Likes:
0
Yea but i still can't Vary the Varactors in Mircofarads i need to have a variable .01uf or .47 or 220uf Varactors i wish they made these

I need to use the .01uf Varactors in a phase shift network filter for audio not radio

21. ### akgNew Member

Joined:
Sep 2, 2005
Messages:
1,404
Likes:
1
Location:
India
audioguru wrote :
Repeat