Variable capacitor

[ecollen]

"So would they be the 30 pF ones Dorke mentioned above.?"

No.They are about 120p or higher. The mylar dielectric gave them the high capacitance.
On talkingelectronics.com under Crystal Set I show how to make a variable cap with cooking foil

Hi Colin
Thank you very much. That's worth a fortune. I'm defintely going to try that. BTW have you seen the other sites with instructions for making air variable capacitors?
E.g.. http://www.eham.net/articles/5217
I'm tempted to try that, but I think I'm better off with yours first.
Thanks

 

[ecollen]

You could always be-friend a good S African man I know called Martin J Lorton.
A very trusted guy in the electronics world.
You can contact him via youtube 'mjlorton' and ask if he can order for you and re-direct within S Africa?
You have nothing to lose!!

Martin

Hi Martin
What excellent advice! Many thanks. I'm going to see if I can do that.

 

[dorke]

o.k looks like you are using this circuit.

The tuned frequency as an L*C term in it.
So "naively",
if we change C from 365pF to 30pF (lowering by a factor of 12.17).
we need to use 12.17*L instead of the value used before
(i.e for the 220uH use 12.17*220uH=2.7mH etc.)
note,
that "naive" calculation is done ignoring all other capacitances in the circuit
and needs to be justified( i didn't...)

As far as using a varicap look at the circuit below:
A suitable diode can be the BB112

 

[Colin Mitchell]

You can make the whole thing with 2 sheets of cooking foil. You really need a couple of stages of amplification to get any sort of volume out of the project. This is all covered in the Crystal Set article.
You can even do away with and use a variometer.

 

[CDRIVE]

Hi Chris
Thanks. That's interesting. So would they be the 30 pF ones Dorke mentioned above.?

No, as Collin said, I believe they were =>120pF fully meshed.

Chris

 

[CDRIVE]

o.k looks like you are using this circuit.

The tuned frequency as an L*C term in it.
So "naively",
if we change C from 365pF to 30pF (lowering by a factor of 12.17).
we need to use 12.17*L instead of the value used before
(i.e for the 220uH use 12.17*220uH=2.7mH etc.)
note,
that "naive" calculation is done ignoring all other capacitances in the circuit
and needs to be justified( i didn't...)

As far as using a varicap look at the circuit below:
A suitable diode can be the BB112

View attachment 23535

Just an FYI and I know that Dorke knows this: The schematic indicates 0 - 365pF but "0pF" is never going to happen in this world.

Chris

 

[dorke]

Chris,
You are correct, absolute zero capacitance is never reached .
The BB112 capacitance range is 18-500pf for Vr in the range 1-8Volt.
That range covers much more than needed in this app. (85-365pF)

 

[ecollen]

Hi Dorke
Thank you very much. That's an immense help. You see I didn't know one can adjust L and C like that, but now I do and I'll never forget it. Good teacher. Thanks! That is going to simplify things a lot. Not only here, but in many other things I want to do which have looked impossible because of unavailable components.
And the same goes for the varicap circuit in place of the variable capacitor. It looks straightforward enough and opens up lots of new avenues.
Thanks again.

 

[ecollen]

You can make the whole thing with 2 sheets of cooking foil. You really need a couple of stages of amplification to get any sort of volume out of the project. This is all covered in the Crystal Set article.
You can even do away with and use a variometer.

Thanks, Colin. I'm definitely going to be trying that soon. I've seen the crystal set article, but will read that part of it again in more detail. Great help. Thanks mate.

 

[ecollen]

FYI, the small transistor radios of the 60's to...?? didn't use air dielectric variable caps. The tuning caps employed in those radios were about 1/4 the size of the typical air gap type of equal capacitance. They used a solid paper thin, flexible dielectric between the plates. This allowed for the stator and rotor plates to be spaced much closer than the air gap designs, without fear of the plates shorting. The stator and rotor plates were also very thin and very flexible.

Chris

Hi Chris,
Okay, thanks for the explanation. I had a look at a couple of old transistor radios I've got here and saw immediately what you're talking about. So those fiddly little things now do the job that those nice, man-sized air-spaced variable capacitors used to do. Amazing. Well, if they work that's okay I guess.

 

[ecollen]

I feel your pain. I have a few. If you lived closer I'd trade for different parts you might have. People have called me a hoarder ( and even worse) but after45+ years of collecting junk electronics, I have a pretty good size junk box of materials.

Difficult to find, but not impossible. Just Google air variables and you will find what you need. Expensive, but it's out there.

When I was in 7th grade I built a crystal diode pocket radio as my science project. (1971). I used a ferrite antennae rod from an old transistor radio in parallel with a Mica padder / trimmer capacitor off a larger variable. The padder capacitor had a smaller range but was enough that I could tune a couple stations in the broadcast band.

Seek out and find others with your interests. Trade and barter for parts.

Good Luck!

Hi Y2KEddie
Thanks for the reply, and thanks for the offer. That would be great - a pity about the intervening 12 000 miles! But I like what you say, and I'm a bit of a hoarder myself. Not very much electronic stuff though, however I do have a bit and I keep scratching around among it. I will keep looking for something suitable on ebay - but price and distance (and the postal service!) make things a bit awkward. However, I've since learned many alternative solutions from the good people who have replied to my query, and I'm happy now that I'll get the project done.
I like the way you built that crystal set at school out of old parts. It's very satisfying to be able to put a receiver together without buying expensive fancy components.

 

[ecollen]

o.k looks like you are using this circuit.

The tuned frequency as an L*C term in it.
So "naively",
if we change C from 365pF to 30pF (lowering by a factor of 12.17).
we need to use 12.17*L instead of the value used before
(i.e for the 220uH use 12.17*220uH=2.7mH etc.)
note,
that "naive" calculation is done ignoring all other capacitances in the circuit
and needs to be justified( i didn't...)

As far as using a varicap look at the circuit below:
A suitable diode can be the BB112

View attachment 23535

Hi again Dorke,

Just two questions, please. What size of resistor should I use here? And as for the tuning pot, Would this one be okay?
POTENTIOMETER, SINGLE TURN, LINEAR, 100K METAL SHAFT, FLAT
Oh, and also, will this varicap work?
Infineon BB639E7904 Varactor Diode, 36pF min, 13.5:1 Tuning Ratio, 30V, 2-pin SOD-323

Many thanks,
Errol

 

[dorke]

The BB639 diode will not work in your circuit:
1.It needs much higher "tuning voltage" up to 28V
2.in the range 1-9V(that you have) you will get about 10-40pF while you need 85-365pF
Use the BB112 ,you can get here .
The 100k pot is o.k,but better use a log one not linear to give you more "hand fine tuning"
you can use R=1M (not critical)

 

[CDRIVE]

You can make the whole thing with 2 sheets of cooking foil. You really need a couple of stages of amplification to get any sort of volume out of the project. This is all covered in the Crystal Set article.
You can even do away with and use a variometer.

Colin, do away with what? As for VARIOMETER ...please elaborate.

Chris

 

[ecollen]

The BB639 diode will not work in your circuit:
1.It needs much higher "tuning voltage" up to 28V
2.in the range 1-9V(that you have) you will get about 10-40pF while you need 85-365pF
Use the BB112 ,you can get here .
The 100k pot is o.k,but better use a log one not linear to give you more "hand fine tuning"
you can use R=1M (not critical)

Hi Dorke,
Many thanks for your reply. I have ordered some BB112s, but since they come from China, they will take forever to get here, if in fact they ever do. Our post from abroad is very poor. Within the country it's not so bad at the moment, so I've also been looking at the local suppliers to see if I can get something similar to keep me going in the mean time. RS Components in South Africa seems to have the biggest selection of varicaps, though none of them has the same design as those BB112s from China. RS Components is where I found that BB639. They have other varicaps in that same box-like design, but no BB112. How do you work out the "tuning voltage"? If I knew that, perhaps I could find something at RS.
As for the pot, I've found this one
POTENTIOMETER, SINGLE TURN, 16MM, DUAL, LOGARITHMIC, 1K OHMS, 6MM
It should do then I reckon?
Okay, R=1M, thanks

 

[ecollen]

Colin Mitchell said: ↑
You can make the whole thing with 2 sheets of cooking foil. You really need a couple of stages of amplification to get any sort of volume out of the project. This is all covered in the Crystal Set article.
You can even do away with and use a variometer.

Colin, do away with what? As for VARIOMETER ...please elaborate.

Chris

Hi Chris,
It's meant to be "do away with the variable capacitor and use a variometer instead", I guess.
http://www.talkingelectronics.com/projects/CrystalSetRadio/CrystalSet.html
Colin has got a wonderful encyclpaedia of information on that site!

 

[dorke]

That is not complicated at all,here is the Cap/Vr graph from the BB112 datasheet .
you can see the points you need (typical value).
Since Vr/Cap is a logarithmic relation in Vr, it is better to use a logarithmic pot. to obtain the Vr needed.
You can usea 1K ohm pot and 1M,resulting in a higher current consumption from the 9V battery.

 

[James Louis]

Hey,
No doubt variable capacitors are very hard to find. There are some dealers providing variable capacitors from USA and here is one variable Gen Rad1422: http://www.used-line.com/capacitors/capacitor-air/gen-rad-1422/item-10591756

May be it will help a bit.

 

[CDRIVE]

Colin Mitchell said: ↑
You can make the whole thing with 2 sheets of cooking foil. You really need a couple of stages of amplification to get any sort of volume out of the project. This is all covered in the Crystal Set article.
You can even do away with and use a variometer.

Hi Chris,
It's meant to be "do away with the variable capacitor and use a variometer instead", I guess.

OK, I see your better at decryption of Colin's posts than I am but I still don't get his reference to a Variometer. Did you click the link I provided that describes a Variometer? Maybe he had a brain fart and meant Varicap.

BTW: His website beats the hell out of his posts!

Chris

 

[CDRIVE]

Hey,
No doubt variable capacitors are very hard to find. There are some dealers providing variable capacitors from USA and here is one variable Gen Rad1422: http://www.used-line.com/capacitors/capacitor-air/gen-rad-1422/item-10591756

May be it will help a bit.

Not really. General Radio was never known for consumer electronics. They were known world wide for top of the line (expensive) electronics instrumentation. Your link points to a capacitor lab standard.

Chris