Designing a "variable" capacitor ?

[rjenkinsgb]

EDIT: Unfortunately, it seems that the DIP swtiches can only cop with 100mA or 150mA, and not enough voltage.
I need 500mA and 50V components, I'll search another solution, or adapted DIP switches.

You are not looking in the right places:-

This ITW / ERG one is rated 1A 100V (5A non switching), and 1pF maximum between open contacts:

ED57372 | ERG 8 Way Through Hole DIP Switch 8PST | RS

uk.rs-online.com

https://www.itwswitches.com/wp-content/uploads/datasheets/SpectraDIL-023-Series.pdf

 

[slenderglitch]

Could you buffer the dip switches with some kind of transitor; or does that violate the "no electronics" missive?

I think if I use only few transistors it will be okay, they just don't want me to do a complicated circuit, because this project is not "prioritary" (I have time to spend lol). It's also because transistors reduce (or remove?) the negative side of the signal I believe...

Thank you for your answer

 

[slenderglitch]

You are not looking in the right places:-

This ITW / ERG one is rated 1A 100V (5A non switching), and 1pF maximum between open contacts:

ED57372 | ERG 8 Way Through Hole DIP Switch 8PST | RS

uk.rs-online.com

https://www.itwswitches.com/wp-content/uploads/datasheets/SpectraDIL-023-Series.pdf

Perfect!
I was searching on Farnell website...
I keep this reference, thank you!

AJ

 

[atferrari]

I used this simple solution in two different circuits: a function generator and a switched capacitor filter. Worked OK.

Maybe it could spark an idea.

 

[slenderglitch]

I used this simple solution in two different circuits: a function generator and a switched capacitor filter. Worked OK.

Maybe it could spark an idea.

Hello atferrari, do you know if transistors will have (bad) effects on my signal because of low capacitance of it?
Thank you

 

[atferrari]

Honestly, I did not perceive anything in my circuits.

What about testing the idea with one fixed plus two or three small value caps?

I would test the different combinations using my idea against their respective counterparts all soldered.

 

[slenderglitch]

Honestly, I did not perceive anything in my circuits.

What about testing the idea with one fixed plus two or three small value caps?

I would test the different combinations using my idea against their respective counterparts all soldered.

Okay, I just wanted to know if there was an impact, but for sure I will try this solution! It would be really useful because capacitor is "activated" (linked to the circuit) by electronics, which is maybe less capacitive than mechanical switches.

Now I'm doing CAD for the first solution I'll try: three DIP switches (total 24 ways) to have 24 capacitors that are usable (12 values, 2 capacitors available per value). When I'm finishing this part I'll try every other solution.

Greetings,
AJ

 

[rjenkinsgb]

the first solution I'll try: three DIP switches (total 24 ways) to have 24 capacitors that are usable (12 values, 2 capacitors available per value).

That seems excessive? Using binary scaled values such as 1-2-4-8-16-32-64-128 pF or nearest available, you have 1 to 255pF with just eight switches!

1-2-2-5 sequence for each decade may be easier to "read" and set, but you can still get 100:1 with eight switches or 1000:1 with twelve.

 

[slenderglitch]

That seems excessive? Using binary scaled values such as 1-2-4-8-16-32-64-128 pF or nearest available, you have 1 to 255pF with just eight switches!

1-2-2-5 sequence for each decade may be easier to "read" and set, but you can still get 100:1 with eight switches or 1000:1 with twelve.

Yes it could seems excessive, but I need values from ~5pF to ~750pF so I need at least three 8-ways DIP switches, I believe? Because the total is equal to 255*3=765 which is enough pF for my project.
Thank's a lot
AJ

 

[Les Jones]

If you have the steps in units of 5pF (Instead of 1 pF) an 8 way switch will give up to 1275 pF. (255 x 5)

Les.

 

[rjenkinsgb]

~5pF to ~750pF so I need at least three 8-ways DIP switches, I believe?

Nope.
Ten switches & binary could give 1-1023 pF in 1pF steps.
Or,
Twelve switches as 1-2-2-5 and decades (10-20-20-50, 100-200-200-500) gives 1 to 1110pF in 1pF steps.

 

[slenderglitch]

Hello,
I thought that DIP switch were like normal switches but less capacitive and smaller.
My schematics with 1 8way DIP switch (in this example eight capacitors are 10pF):

If I active C1 swtich, 10pF will be added. If I active every switches, there will be only 10*8=80pF isn't it?
Or maybe there's a thing that I don't understand...
Thank you AJ

 

[Nigel Goodwin]

Hello,
I thought that DIP switch were like normal switches but less capacitive and smaller.
My schematics with 1 8way DIP switch (in this example eight capacitors are 10pF): View attachment 133167
If I active C1 swtich, 10pF will be added. If I active every switches, there will be only 10*8=80pF isn't it?
Or maybe there's a thing that I don't understand...
Thank you AJ

Binary!! - make C1 = 1pF, C2 = 2pF, C3 = 4pF and so on.

 

[rjenkinsgb]

If I actived C1 swtich, 10pF will be added. If I actived every switches, there will be only 10*8=80pF isn't it?
Or maybe there's a thing that I don't understand...

Using the same cap on each switch is the least effective way.

Use eg. 10, 20, 40, 80, 160 etc. for binary steps; by selecting the correct combination of switches, any value can be set in multiples of the smallest value, up to (double the largest - the smallest)

or, 10, 20, 20, 50, 100, 200, 200, 500 etc.

that translates to decimal digits rather than binary and again any value can be set in multiples of the smallest, to over double the largest.

 

[slenderglitch]

Oh okay!!
I understand now. I remember in studies I've used a DIP switch to control DMX adresses on a scene light, and it was like binary.
This solutions sounds even cooler!
Thank you I'll try like this I'll give you the news.


AJ