two fun facts about these capacitors, I just read about these last night. 1) you notice that the shaft is off center (in the 3 last larger caps) and there is a larger area of spline on once side, this allows for the change shaft angle to change linearly but the capacitance change to be non-linear, I believe this is related to the resonany frequence of the lc tank being f = 1/(2*pi*sqrt(LC)) so to achive linear frequency change for linear shaft angular change you need to off centered shaft and some non cylindrical plate geometry. and (2) IMSAI guy has connected the sections in parallel but these sections would normally be used as 2 or 3 different capacitors each part of a tuned LC tank circuit with amps in between so again a single shaft angular change would tune 2 or 3 individual tanks that are in a serial (ampliified chain) all at once.
Interesting, 365pf used to be a very common value. You should do a video about the rotation angle vs capacitance, I am sure there is some interesting math around the offset shaft and plate area change vs rotation.
There was few types, i.e. linear frequency v.s. linear capacitance, maybe linear wavelength too. ( reference to radio signals being processed in circuits containing given capacitor )
I like those tunable air capacitors very much. I am always afraid that this kind of component will be no more available in modern times so I bought some of them. From old AM Radios I learned that one stack of plates has something around 500pF, mostly a little less. The voltage limit is around 500V and when testing it the sparks are evenly distributed telling that they are well aligned. The plate at one end has cuts to allow some fine tuning.
I got some that look like that big one except longer. Multi section salvaged from old HP audio generators. Four sections and I forget but they are at least 450 per section, maybe more. Guess I could build a monster antenna tuner.
I remeber as a kid, schematics in magazine and books often specced a 365pf air variable capacitor. Then I would go look up the price and they were so crazy expensive!
[snark] Wow it's cold there. -90°. C or F that's cold. More capacitance, lower frequency. Typically each "gang" was some part of a different circuit. In the two gang slicers one set would be for the tuning and the other set would be for the IF mixer frequency (+455Khz). If there are three the third is going to be for a preselector/antenna tuner or a second IF.
I picked up a WW2 era (Air Ministry surplus) one that had nominal max capacitance of 400pF. It was very unstable when I measured with a good quality bench meter. Next step is to clean it in the ultrasonic cleaner...
Called 'air-variable' caps. I guessed 15 pF on the first one. It's kind of cheating on the rest because I just glance in my air-variable cap bins to compare. I have quite a few. But what I don't have is a big butterfly cap for building a high Q mag-loop. Sigh. They aren't cheap. And like an idiot I gave away a good vacuum-variable cap to an experimenter. I'm a sucker for other experimenters. The 987 pF cap has plates that are very close together, raising the capacitance. It's a receive cap, (low voltage, no arcing,) with more precision construction. To make them still smaller they slide a dielectric between each of the plates, allowing them to be closer together without rubbing or arcing, and the dielectric also increases the capacitance.
The bigger ones were harder to guess, but I got pretty close on the smaller ones. I had guessed 20pF, 50pF, 70pF. Then I blew the next one with a guess of 250pF, and it only went downhill from there. :)
I don't remember the equation from Physics class, but it would be interesting to take some physical measurements of these variable caps and calculate the capacitance value. Imagine the size a capacitor in the micro-Farad range!
The formula is a useful thing for people who do radio frequency electronics work to memorize. Ignoring the fringing effects at the edges, the capacitance in picofarads for a parallel plate capacitor is about 0.225 times the total area between the pairs of plates divided by the plate-to-plate spacing, with the area expressed in square inches and the spacing expressed in inches. If a dielectric other than air or vacuum is used, then the result is multiplied by the relative dielectric constant of the material between the plates. If the dimensions are expressed in square meters and meters, then the constant, 8.854, replaces the 0.225. If I have done the math right, a square parallel plate one-microfarad capacitor with air dielectric and a plate spacing of one hundredth of an inch would require two facing square plates 17.6 feet on a side.
Those are the high-quality units. Ever see one of the all-plastic cheapo units from a pocket radio ? Stupid me had a precision variable capacitor with high-quality ball bearings and and a vernier dial.....and I threw it out decades ago. Hey...time for a new battery.
I think the most amusing part for me is, I just stumbled upon this, while journey-ing down the sidewalk! ha..ha.. love the steam punk! I'll just grab a little byte:🌭🌭🌭🍌🍦🧃🥧👍😬
More fun than guessing the number of jelly beans in the jar! Something like this should be held at every hamfest for a prize.
two fun facts about these capacitors, I just read about these last night. 1) you notice that the shaft is off center (in the 3 last larger caps) and there is a larger area of spline on once side, this allows for the change shaft angle to change linearly but the capacitance change to be non-linear, I believe this is related to the resonany frequence of the lc tank being f = 1/(2*pi*sqrt(LC)) so to achive linear frequency change for linear shaft angular change you need to off centered shaft and some non cylindrical plate geometry. and (2) IMSAI guy has connected the sections in parallel but these sections would normally be used as 2 or 3 different capacitors each part of a tuned LC tank circuit with amps in between so again a single shaft angular change would tune 2 or 3 individual tanks that are in a serial (ampliified chain) all at once.
Interesting, 365pf used to be a very common value. You should do a video about the rotation angle vs capacitance, I am sure there is some interesting math around the offset shaft and plate area change vs rotation.
There was few types, i.e. linear frequency v.s. linear capacitance, maybe linear wavelength too. ( reference to radio signals being processed in circuits containing given capacitor )
about 5pF to 500pF, the spacing is more important if they are turning an RF amp as the smaller ones will flash over easer.
I like those tunable air capacitors very much. I am always afraid that this kind of component will be no more available in modern times so I bought some of them.
From old AM Radios I learned that one stack of plates has something around 500pF, mostly a little less. The voltage limit is around 500V and when testing it the sparks are evenly distributed telling that they are well aligned. The plate at one end has cuts to allow some fine tuning.
Next installment will be: Guess the capacitance of these varactor diodes!
I was fairly close on all of them. Even more challenging these days is "guess the price of good air-spaced variables". The answers are hair-raising!
Lets play global thermo nuclear war.
How about a nice game of chess?
I got some that look like that big one except longer. Multi section salvaged from old HP audio generators. Four sections and I forget but they are at least 450 per section, maybe more. Guess I could build a monster antenna tuner.
I remeber as a kid, schematics in magazine and books often specced a 365pf air variable capacitor. Then I would go look up the price and they were so crazy expensive!
[snark] Wow it's cold there. -90°. C or F that's cold.
More capacitance, lower frequency. Typically each "gang" was some part of a different circuit. In the two gang slicers one set would be for the tuning and the other set would be for the IF mixer frequency (+455Khz). If there are three the third is going to be for a preselector/antenna tuner or a second IF.
I picked up a WW2 era (Air Ministry surplus) one that had nominal max capacitance of 400pF. It was very unstable when I measured with a good quality bench meter. Next step is to clean it in the ultrasonic cleaner...
Do you know the voltage ratings on all of these?
Called 'air-variable' caps. I guessed 15 pF on the first one. It's kind of cheating on the rest because I just glance in my air-variable cap bins to compare. I have quite a few. But what I don't have is a big butterfly cap for building a high Q mag-loop. Sigh. They aren't cheap. And like an idiot I gave away a good vacuum-variable cap to an experimenter. I'm a sucker for other experimenters.
The 987 pF cap has plates that are very close together, raising the capacitance. It's a receive cap, (low voltage, no arcing,) with more precision construction. To make them still smaller they slide a dielectric between each of the plates, allowing them to be closer together without rubbing or arcing, and the dielectric also increases the capacitance.
Thank you for another wonderful video
would be interesting, how much the difference in rated voltage for those spacings is
Some really nice tuning capacitors here, you can make a good superhet radio with them.
Greater air gap = higher voltage Ought to drag it out and measure but the air gaps in my old 2kW antenna tuner caps is substantial.
The bigger ones were harder to guess, but I got pretty close on the smaller ones. I had guessed 20pF, 50pF, 70pF. Then I blew the next one with a guess of 250pF, and it only went downhill from there. :)
That was fun :) I spend more time than I should guessing to myself what value they are when I'm at the junk store. hehe
As self appointed test monitor and testee, my score has something to do with "booger."
10
50
70
180
Now guess max voltage in air .....
I was doing good and hitting within 10% until that 987pf one 😂 I thought it was about half that.
What is the minimum for each?
It's generally 1/20 to 1/50 of the maximum
I was 100% correct on each cap, plus or minus, one micro-farad. LOL
I don't remember the equation from Physics class, but it would be interesting to take some physical measurements of these variable caps and calculate the capacitance value. Imagine the size a capacitor in the micro-Farad range!
The formula is a useful thing for people who do radio frequency electronics work to memorize. Ignoring the fringing effects at the edges, the capacitance in picofarads for a parallel plate capacitor is about 0.225 times the total area between the pairs of plates divided by the plate-to-plate spacing, with the area expressed in square inches and the spacing expressed in inches. If a dielectric other than air or vacuum is used, then the result is multiplied by the relative dielectric constant of the material between the plates. If the dimensions are expressed in square meters and meters, then the constant, 8.854, replaces the 0.225.
If I have done the math right, a square parallel plate one-microfarad capacitor with air dielectric and a plate spacing of one hundredth of an inch would require two facing square plates 17.6 feet on a side.
@@analog_guy , thank you! That would be a huge variable capacitor!
I had no clue and guessed 20pF for the first ! then I started to use my brain and was miles out.
Those are the high-quality units. Ever see one of the all-plastic cheapo units from a pocket radio ?
Stupid me had a precision variable capacitor with high-quality ball bearings and and a vernier dial.....and I threw it out decades ago.
Hey...time for a new battery.
yes, I use one in this video: ua-cam.com/video/a4pfbDJIoQA/v-deo.htmlsi=6Sf7I5A_CAUSH7Be
“Shall we play a game…”
I got the middle ones about right. The second too last one threw me off.
I aced the first one but got every subsequent one totally wrong. I was expecting larger values
"tiny one > pretty small > bigger > bigger > bigger > bigger > biggest"
Last one is incorrect.
My guesses:
1. 14,5 pF
2. 20 pF
3. 70 pF
4. 110 pF
5. 2 nF
6. 1800 pF
7. 700 pF
I was very off
Way off on all besides the 3rd one
within 1% on all,...
I think the most amusing part for me is, I just stumbled upon this, while journey-ing down the sidewalk! ha..ha..
love the steam punk! I'll just grab a little byte:🌭🌭🌭🍌🍦🧃🥧👍😬