500pf variable cap, how can i get it?

Sayer · Post by **Sayer** » Thu Jun 21, 2007 8:16 pm

I am working with a circuit that calls for a 500pf variable cap and I cannot find one ANYWHERE.

Tried the usual suspects mouser, digikey, jameco, allied. Any ideas would be very appreciated.

thanks as always,

sayer

Milkmansound · Post by **Milkmansound** » Thu Jun 21, 2007 8:29 pm

check surplus sales of nebraska, they usually have the arco variable caps.

otherwise, theres always the option of using a 12 pole rotary switch with several caps.

is this for an LA2A?

Sayer · Post by **Sayer** » Thu Jun 21, 2007 8:36 pm

is this for an LA2A?

I wish, my dad wants me to show him how to build his own am radio for fun.
I guess air america doesn't come in as well as he'd like in northern minnesota.

thanks man

sayer

ps. I don't think the rotary will work because it won't be sensitive enough???

mu_amps · Post by **mu_amps** » Thu Jun 21, 2007 9:24 pm

Leeds Radio definitely has them. it's the air gapped type used for tuning. Don't go by the website. it's only a fraction of what they have in there

www.leedselect.com

brianroth · Post by **brianroth** » Thu Jun 21, 2007 11:06 pm

365 pF was a very common unit "back in the day", and there are still some kicking around. Some were duals, so you could parallel two sections to get 730 pF.

I guess the crystal radio set builders would be an option, such as:

http://www.midnightscience.com/catalog5.html

I also found this:

http://www.oselectronics.com/ose_p96.htm

(hint...Google search for '365 pF variable capacitor')

What circuit are you looking at that requires a 500 pF variable cap?

Bri

Sayer · Post by **Sayer** » Fri Jun 22, 2007 6:28 am

thanks for the ideas people.

I guess part of the problem is that I don't know how precise the cap has to be to function correctly as a tuner. It is couple with a 200uh inductor. I also know that there are variable inductors but I am not sure what effect that would have propotionally.

heres a look at the circuit. this one seems to be floating around the web. There are a couple more that use ganged ICs but i thought i would rather keep the build more simple...

http://www.electronics-lab.com/projects ... index.html

Sayer · Post by **Sayer** » Fri Jun 22, 2007 6:45 am

brian, do you know much about gas or vacuum variable capacitors???

thanks

sayer

Sayer · Post by **Sayer** » Fri Jun 22, 2007 7:24 am

just since this IS the diy forum I thought I 'd post this page I saw about making your own variable capacitors.

http://www.eham.net/articles/5217

mu_amps · Post by **mu_amps** » Fri Jun 22, 2007 7:39 am

Sayer wrote:just since this IS the diy forum I thought I 'd post this page I saw about making your own variable capacitors.

http://www.eham.net/articles/5217

That seems like a lot of trouble to go through for something you could easily order.

Check out this guy's radio sets...

http://www.schmarder.com/

Sayer · Post by **Sayer** » Fri Jun 22, 2007 8:03 am

wow, thats pretty over the top. i like how his coils are wound into spider web like structures.

Ok guys if I use a 365pf variable cap, how would that effect the value of the inductor which is 200uh for a 500 pf cap.

P.S. I can't believe you can't get the 500pf variable cap as a ceramic?!

sayer

mu_amps · Post by **mu_amps** » Fri Jun 22, 2007 8:34 am

Sayer wrote:wow, thats pretty over the top. i like how his coils are wound into spider web like structures.

Ok guys if I use a 365pf variable cap, how would that effect the value of the inductor which is 200uh for a 500 pf cap.

P.S. I can't believe you can't get the 500pf variable cap as a ceramic?!

sayer

Here is a 2 section 270pf variable cap from leeds radio

http://www.leedselect.com/images-caps/c ... x270pf.jpg

2x270 will give you 540pf

I'm telling you this guy has everything under the sun! I but most of my parts there.

Sayer · Post by **Sayer** » Fri Jun 22, 2007 8:39 am

thanks man

540's gotta be close enough you might get a little more q that you want but it will function.

sayer

mu_amps · Post by **mu_amps** » Fri Jun 22, 2007 9:07 am

Anytime.

You might want to ask if he has 500pf just for the hell of it. But i think 540 will work well.

Sayer · Post by **Sayer** » Fri Jun 22, 2007 12:41 pm

Ok,

so now that I have the actual dilemma solved I'm still interested in some of the technology involved. Do any of you have any experience with varicap aka varactor diode (diode controlled capacitance)???

I don't understand how these could work?!

sayer

philbo · Post by **philbo** » Fri Jun 22, 2007 10:01 pm

A varactor is just a diode.

Here's a little light reading on it:

http://www.americanmicrosemi.com/tutorials/varactor.htm

http://en.wikipedia.org/wiki/Varicap
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500pf variable cap, how can i get it?

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