multigang variable cap - Page 1
Forum section: electronics / rf-microwave

author: p.larner
author_rank: Frequent Contributor
timestamp: November 29, 2023, 07:47:12 am
content: i have a 3 gang air variable its 500pf,if i wire the gangs in series will it handle a higher voltage obviosly at the expense of max capacitance,thinking of tx flashover in a tuner/atu.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: November 29, 2023, 06:13:28 pm
content: It should capacitively divide if they are the same. Assuming the center point impedance is high to ground.

author: A.Z.
author_rank: Super Contributor
timestamp: November 29, 2023, 07:12:23 pm
content: Quote from: p.larner on November 29, 2023, 07:47:12 amhttps://pa0fri.home.xs4all.nl/ATU/FRImatch/FRImatcheng.htmcheck it

author: p.larner
author_rank: Frequent Contributor
timestamp: November 30, 2023, 06:05:54 am
content: i wanted to know if having all gangs in series will allow more tx power before flashover if used in an atu.?.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: November 30, 2023, 06:59:26 am
content: Quote from: p.larner on November 30, 2023, 06:05:54 amYes it will.Though it shouldn't make a difference whether it's 2 or 3 gang because if the latter one gang will be unused.The main difficulty with this arrangement is ensuring that the frame is electrically isolated as it won't be connected to anything.So you'll need some form of insulating material eg plastic to screw it to to hold it in place.The characteristics of the antenna load are a another factor. If you still get flashover then you could make changes to bring it to a more match-friendly / less reactive impedance that doesn't cause this.

author: p.larner
author_rank: Frequent Contributor
timestamp: November 30, 2023, 08:01:13 am
content: finaly a decent answer,shame it took about 5 replies to get an answer that made sense to what i actualy asked tho,cheers +73

author: Hamelec
author_rank: Regular Contributor
timestamp: November 30, 2023, 08:43:23 am
content: it's a shame what you're asking of the forum, please stay away and go to chatgpt. 55 + 99

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: November 30, 2023, 04:28:32 pm
content: I do not know if all the ganged variables are the same. Do they have the same spacing in the plates?Once you get a flashover the plates flashing are likely then damaged and will flash again at lower voltage.Besides in ganged caps the one plate is common and is physically designed that way.

author: TimFox
author_rank: Super Contributor
timestamp: November 30, 2023, 04:43:17 pm
content: Note that almost all 3-gang variable capacitors have a common connection to the rotor blades (connected to the rotating shaft), and individual connections to the three stators.In that case, the rotor is connected to the frame, which is usually grounded.If you put two sections in series, the frame and the shaft must be insulated from ground.A good reason to connect two sections in series (with isolated shaft and frame) is that there is then no current through the bearing and brushes between shaft and frame.This is often called "split stator" in the literature.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: November 30, 2023, 04:49:22 pm
content: Quote from: p.larner on November 30, 2023, 08:01:13 amYou get what you pay for. Remind me to ignore you.

author: p.larner
author_rank: Frequent Contributor
timestamp: November 30, 2023, 05:49:09 pm
content: NOTE: This message has been deleted by the forum moderator Simon for being against the forum rules and/or at the discretion of the moderator as being in the best interests of the forum community and the nature of the thread.If you believe this to be in error, please contact the moderator involved.An optional additional explanation is:

author: vk4ffab
author_rank: Frequent Contributor
timestamp: November 30, 2023, 09:29:20 pm
content: Quote from: p.larner on November 30, 2023, 08:01:13 amI am pretty sure that the flash over point of an air variable capacitor is a function of the distance between the plates. Putting multi gangs in series does not change the plate gaps just the total capacitance value and the flash over point will remain a function of the distance between the plates with the narrowest gap in the gang. (I could be wrong) But even if they add, you still need to calculate the flash over point for a single gang to have an idea of what the cap can handle.All this can be worked out mathematically, you can work out the highest voltage the cap will see based on the transmitter power and impedance miss match, you can take measurements of the cap and work out its flash over point.If memory serves me, 2000v AC will have a minimum flash over of 0.5mm, a 400w transmitter (UK Legal Limit) has an AC voltage of 141v and the voltage at the point of transform will be a function of frequency and impedance miss match. But for argument sake, at 80m with a 10:1 mismatch you are looking at 7000v AC being seen by the output cap in a L match. The voltages at extreme mismatches drop off with frequency, but esr power dissipation increases and peak currents are all over the place and depend on frequency and capacitance.So at 80m and your 500pF total the cap might be perfectly fine and be the extreme scenario but at 6m 70pF might be the point where it all goes caddywompus. And while I have spent a lot of time doing the math and making spreadsheets to design my own remote tuner for worst case scenarios, you are going to have to do this all on your own for the materials you have at hand.Quote from: p.larner on November 30, 2023, 05:49:09 pmThere comes a point in every hams life where they need to start reading books, doing math and understanding things all by themselves. You have been offered some pretty good advice from people in many threads and not even a single thank you has been offered to any of them in return. He is right though, you get what you pay. Books and math in the hand are worth 2 comments on a forumAnd while you did not understand the first two comments from Joe and Ah, they gave you real food for thought. Those two comments show how to think about the problem because the answer is not a simple yes or no, the answer is it depends. When you do not understand something that has been offered to you, just say so and ask for an explanation. In doing so, you just might learn something.

author: Halcyon
author_rank: Unknown
timestamp: November 30, 2023, 10:03:12 pm
content: Quote from: p.larner on November 30, 2023, 05:49:09 pmIs this really appropriate?

author: Andy Chee
author_rank: Super Contributor
timestamp: December 01, 2023, 12:47:37 am
content: Here's more information about ATU flashovershttps://owenduffy.net/transmissionline/aev/index.htm