Need to find an old variable capacitor

[michael1978]

You can usually remove plates from an old variable capacitor to make a lower value capacitance.

Varicaps will work but the high capacitance needed for use on the BC band are now rare. Low voltage rectifier diodes, such as 1N4001, often met the specifications. The area of the diode is what makes the difference, so older designs have higher capacitance. The best I found were big diodes from old automotive alternators. These days I might investigate power mosfets as they have high capacitance between the gate and source. Tie the source and drain together.

One problem with varicaps is cancellation of the distortion in oscillators, or when big signals are present. Distortion is best reduced by using opposed matched pairs of diodes as shown here.
https://en.wikipedia.org/wiki/Varicap#Tuning_circuits

thak you, but strange i don't find tutorial about varicap.

 

[Baluncore]

Varicap = Varactor.

http://www.radio-electronics.com/info/data/semicond/varactor-varicap-diodes/basics-tutorial.php

http://www.futureelectronics.com/en/diodes/tuning-varactor.aspx

http://www.physics-and-radio-electr...tordiode-construction-definition-working.html

 

[michael1978]

Varicap = Varactor.

http://www.radio-electronics.com/info/data/semicond/varactor-varicap-diodes/basics-tutorial.php

http://www.futureelectronics.com/en/diodes/tuning-varactor.aspx

http://www.physics-and-radio-electr...tordiode-construction-definition-working.html

thank you i read all, but i still don't know how to measure varicap, i know how to bias, but can you show me one example how to measure maximum picoFarad
and i read in google resistor isulation, somewhere say 10K to 47K, some several mega ohms, i don't understand.

 

[sophiecentaur]

i still don't know how to measure varicap,

Measuring small values of Capacitance is quite difficult. You can use a "Capacitance Bridge", which is probably regarded as old fashioned. You can resonate with a known Inductance and the C can be deduced. But small Capacitances are dogged with other stray capacitances and it requires a fair bit of experimental skill to get a reliable value. Someone may well post another more modern method but I'd bet the equipment would cost you a bit!

 

[Baluncore]

and i read in google resistor isulation, somewhere say 10K to 47K, some several mega ohms, i don't understand.

No DC current flows, only leakage, so a high R can set the diode DC bias voltage while not over-loading the AC resonant circuit.

A varactor can be any diode that is slightly reverse biassed. All diodes have changing capacitance in that region of operation, so if that changing capacitance is used, it is a varactor in that circuit.

Best results are with small signals since the biggest change in capacitance occurs close to the point where the varactor will start to conduct and so rectify the signal and start to pump charge and set it's own bias.

To measure the capacitance you can make a tuned circuit with a low amplitude signal or oscillation and measure the frequency. You might start investigating with a simple oscillator and a pair of power diodes, using between 2 and 10 volts of reverse bias. The change will be small at such high reverse bias voltages but the signal will not affect the bias. Use smaller signals and lower reverse bias as you progress.

You might build a crystal set receiver with any variable mechanical capacitor, then replace the mechanical tuning cap with a pair of power diodes and a bias network controlled by a potentiometer.

 

[berkeman]

Someone may well post another more modern method but I'd bet the equipment would cost you a bit!

Yep. This is how we do it in our lab. Use the bias voltage feature of the old HP 4194, and be sure to calibrate it beforehand if you want to be measuring small capacitance values...

http://www.geocities.jp/etm_llc/pub/Z_report/pic2/4194A_16047D.jpg

 

[sophiecentaur]

Ha ha. Like I said - it would cost a bit. hp equipment is nice stuff. It always used to impress me that different pieces of hp kit would actually agree with each other. I always reckoned that there was some sub-ether comms between all hp instruments so that they could all get their stories to agree.
For home measurement, the best that most of us can hope for is a socket on a DMM for component measurement. Near enough for Jazz mainly.

 

[sophiecentaur]

@michael1978 I notice that you are double posting about this varicap measurement question. The Mods may complain about that. Best to keep things together or people can miss the conversation.
Also I think we are getting a bit astray from the original question. Your friendly man in the electronics shop was really not helping you when he suggested using a diode instead of an air spaced variable C. They are used for entirely different purposes.
You want to make an old fashioned AM receiver then make one. It is probably within your capabilities but trying to re-design the tuning stage to include a different technology is certainly not going to work for you. You need a result out of this and not just frustration. I know. I have been in the same position.

 

[berkeman]

Ha ha. Like I said - it would cost a bit. hp equipment is nice stuff.

Well if you or the OP ever make it across the Pond and visit Silicon Valley, ping me and you'll get a tour of the HW Lab and you can measure anything you want with our equipment.

 

[sophiecentaur]

Well if you or the OP ever make it across the Pond and visit Silicon Valley, ping me and you'll get a tour of the HW Lab and you can measure anything you want with our equipment.

That would be nice. I will be visiting my son in Queens (NY) this year some time but it's still a bit of a way to go to California. I will need to present a convincing case to my dear lady wife.

 

[michael1978]

Yep. This is how we do it in our lab. Use the bias voltage feature of the old HP 4194, and be sure to calibrate it beforehand if you want to be measuring small capacitance values...

http://www.geocities.jp/etm_llc/pub/Z_report/pic2/4194A_16047D.jpg
View attachment 220573

thnx, i want just to know the maximum capacitance, where to read of to take information

 

[michael1978]

@michael1978 I notice that you are double posting about this varicap measurement question. The Mods may complain about that. Best to keep things together or people can miss the conversation.
Also I think we are getting a bit astray from the original question. Your friendly man in the electronics shop was really not helping you when he suggested using a diode instead of an air spaced variable C. They are used for entirely different purposes.
You want to make an old fashioned AM receiver then make one. It is probably within your capabilities but trying to re-design the tuning stage to include a different technology is certainly not going to work for you. You need a result out of this and not just frustration. I know. I have been in the same position.

I don't know what to think about him, he look very good man

 

[michael1978]

Well if you or the OP ever make it across the Pond and visit Silicon Valley, ping me and you'll get a tour of the HW Lab and you can measure anything you want with our equipment.

i think he cost to much of not?

 

[sophiecentaur]

I don't know what to think about him, he look very good man

He may be a good man but he runs a shop and wants to sell things.
Also, he may not be used to teaching. Rule number one about helping people is not to present them with an ever widening field of knowledge when it's not necessary. You now have extra worry about the details of varactor diodes, You do not need this worry - you need an old fashioned air-spaced mechanical capacitor. They are available from various sources and you just need to keep looking on the 'fringe market'. The actual value and range may not be very important and you do not need exactly the same capacitor as the one on the diagram you are using. Take some time with your search. Good luck.

 

[michael1978]

He may be a good man but he runs a shop and wants to sell things.
Also, he may not be used to teaching. Rule number one about helping people is not to present them with an ever widening field of knowledge when it's not necessary. You now have extra worry about the details of varactor diodes, You do not need this worry - you need an old fashioned air-spaced mechanical capacitor. They are available from various sources and you just need to keep looking on the 'fringe market'. The actual value and range may not be very important and you do not need exactly the same capacitor as the one on the diagram you are using. Take some time with your search. Good luck.

hey man he told me that they are not in use al long, and he don't know where to find it, now only varicap they use in place van variable capacitor

 

[sophiecentaur]

hey man he told me that they are not in use al long, and he don't know where to find it, now only varicap they use in place van variable capacitor

He's right when he says they are not in commercial use and he couldn't sell you one but he was wrong to say they cannot be obtained. He did not recognise your need to live a bit of history.
If you want to build a modern receiver then that would be possible, with Integrated circuits and semiconductor tuning capacitors but they are complex and you would not be getting the 'full experience'. Also, if it didn't work first time, you may not be able to sort out the problem. A very simple receiver could be much more practical for you to make - once you have located all the right components. I think we are all rooting for you about this!

 

[michael1978]

He's right when he says they are not in commercial use and he couldn't sell you one but he was wrong to say they cannot be obtained. He did not recognise your need to live a bit of history.
If you want to build a modern receiver then that would be possible, with Integrated circuits and semiconductor tuning capacitors but they are complex and you would not be getting the 'full experience'. Also, if it didn't work first time, you may not be able to sort out the problem. A very simple receiver could be much more practical for you to make - once you have located all the right components. I think we are all rooting for you about this!

thank you man i think i solved