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 Post subject: Can a variable condenser be accurately measured???
PostPosted: Mar Sun 18, 2012 9:13 pm 
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I am working on a One tube reflex project from the mid 20's and the call is for .0005 mfd variable condensers. I have a couple kicking that I used but don't know if they are the correct value. Can I accurately attach my Cap meter and read their result?

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 Post subject: Re: Can a variable condenser be accurately measured???
PostPosted: Mar Sun 18, 2012 9:45 pm 
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amyotte wrote:
I am working on a One tube reflex project from the mid 20's and the call is for .0005 mfd variable condensers. I have a couple kicking that I used but don't know if they are the correct value. Can I accurately attach my Cap meter and read their result?


Sure it can be measured.
I use a home-brew L-C meter that I built - it'll show the thing vary as you turn the shaft...

And .0005 mfd - that's basically 500 pf, which is not much more than the usual variable capacitor that you can still finde (Midnight Science: http://midnightscience.com/catalog5.html ). Perhaps a bit bigger....

Mike Y
KM5Z


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 Post subject: Re: Can a variable condenser be accurately measured???
PostPosted: Mar Sun 18, 2012 9:56 pm 
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Location: Madison, WI
.0005 mfd condensers were typically 23 plate, +- a few.


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 Post subject: Re: Can a variable condenser be accurately measured???
PostPosted: Mar Sun 18, 2012 10:49 pm 
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Jon the Grimm wrote:
.0005 mfd condensers were typically 23 plate, +- a few.


When you say plates do you count all the plates? Just the Stator or just the Rotor?

All plates counted I am using would be 9 and 11.

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 Post subject: Re: Can a variable condenser be accurately measured???
PostPosted: Mar Sun 18, 2012 11:05 pm 
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Count just one set of plates... either stator or rotor.

Your count would indicate a considerably lower capacitance than 500 pF, unless your plates are huge.

Rich


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 Post subject: Re: Can a variable condenser be accurately measured???
PostPosted: Mar Mon 19, 2012 12:53 am 
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Rich, W3HWJ wrote:
Count just one set of plates... either stator or rotor.

Your count would indicate a considerably lower capacitance than 500 pF, unless your plates are huge.

Rich


The plates are different sizes and shapes...

Image

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 Post subject: Re: Can a variable condenser be accurately measured???
PostPosted: Mar Mon 19, 2012 3:02 am 
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Those are less than 500 pf.


Last edited by Jon the Grimm on Apr Sat 14, 2012 3:03 am, edited 1 time in total.

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 Post subject: Re: Can a variable condenser be accurately measured???
PostPosted: Mar Mon 19, 2012 1:15 pm 
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amyotte wrote:
Can I accurately attach my Cap meter and read their result?

If it's a dedicated LCR meter you can.
If it's the capacitance range on your multimeter - not likely, since these rarely measures any capacitance under 1000pF accurately.
If you do not have a dedicated LCR meter and want a circuit for a simple capacitance bridge, let me know so I can post some pictures.

The capacitors seems to be something in between 100 - 150pF.


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 Post subject: Re: Can a variable condenser be accurately measured???
PostPosted: Mar Mon 19, 2012 4:21 pm 
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Rune wrote:
amyotte wrote:
Can I accurately attach my Cap meter and read their result?

If it's a dedicated LCR meter you can.
If it's the capacitance range on your multimeter - not likely, since these rarely measures any capacitance under 1000pF accurately.
If you do not have a dedicated LCR meter and want a circuit for a simple capacitance bridge, let me know so I can post some pictures.

The capacitors seems to be something in between 100 - 150pF.


Please post the info on the capacitance bridge.

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 Post subject: Re: Can a variable condenser be accurately measured???
PostPosted: Mar Mon 19, 2012 4:33 pm 
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I was surprised that a cheap DMM, the Harbor Freight 37772, reads within 5% on a 100 pF silver mica capacitor.

http://www.antiqueradios.com/forums/vie ... p?t=153210

These are usually on sale for about $20-25 and will be good enough to give you are working value for your variable cap. Just mesh the plates fully closed and connect it to the meter in its lowest capacitance range.

My meter has about 30 pF of "stray" capacitance due to calibration and the long leads, so I just subtract that from whatever reading I get. When you turn on the meter with nothing connected, the value on the display is the "error" and needs to be subtracted from readings of actual parts. It doesn't make any difference if you are measuring a 1 microfarad cap.

Rich


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 Post subject: Re: Can a variable condenser be accurately measured???
PostPosted: Mar Mon 19, 2012 7:17 pm 
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I'll try to do this as short as I can, so if I have forgotten something; ask.

Image

Hint: The 470p capacitor needs to be 5 percent or better. A ceramic NP0, mica or styroflex will work well.

Yes,I know it looks totally ridiculous, but the circuit works surprisingly good, and as is with the values shown it will measure from about 30pF up to 800 or so with 2 percent accuracy. You put the values of R1 and R2, in kilo-Ohms, directly into the formula. The same goes for C1 in pF.

Image


Last edited by Rune on Mar Mon 19, 2012 7:25 pm, edited 1 time in total.

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 Post subject: Re: Can a variable condenser be accurately measured???
PostPosted: Mar Mon 19, 2012 7:20 pm 
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Location: Akershus, Norway
Rich, W3HWJ wrote:
I was surprised that a cheap DMM, the Harbor Freight 37772, reads within 5% on a 100 pF silver mica capacitor.

Thats quite unusual. Even my 500 dollar DMM doesn't do that.


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 Post subject: Re: Can a variable condenser be accurately measured???
PostPosted: Mar Tue 20, 2012 5:44 pm 
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Rune wrote:
Image


Quick-n-dirty, I love it ! Thank you. :mrgreen:

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 Post subject: Re: Can a variable condenser be accurately measured???
PostPosted: Mar Tue 20, 2012 10:46 pm 
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Rune didn't give the instructions for operating the bridge, which may not be obvious for those who've never used a bridge type measuring circuit:

Once the unknown capacitor Cx is connected, and the audio source connected, you adjust resistance R1 until you get a null (minimum audio) in the phones. Then, the value of the unknown capacitance is given by:

Cx=(R1 x C1)/R2

(The formula is shown in Rune's photo, but it's a bit fuzzy and may not be easy to read.)


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 Post subject: Re: Can a variable condenser be accurately measured???
PostPosted: Mar Wed 21, 2012 10:02 am 
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Location: Ennismore, Ontario
BobWeaver wrote:
Rune didn't give the instructions for operating the bridge, which may not be obvious for those who've never used a bridge type measuring circuit:

Once the unknown capacitor Cx is connected, and the audio source connected, you adjust resistance R1 until you get a null (minimum audio) in the phones. Then, the value of the unknown capacitance is given by:

Cx=(R1 x C1)/R2

(The formula is shown in Rune's photo, but it's a bit fuzzy and may not be easy to read.)



Thanks Bob.

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 Post subject: Re: Can a variable condenser be accurately measured???
PostPosted: Mar Wed 21, 2012 8:45 pm 
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Sorry about that. I was a little bit too quick there.
I'm sure some expressed a ''No you can't'' in front of their monitors when they saw my ''professional'' measurement device yesterday. :lol:
If I remember correctly, Bob, Oerjan and I had a little discussion over at The Radioboard on this little bridge, and I think Bob can confirm it is working very well.
Yes, it is meant as a little device someone can lash up in a hurry, to measure a variable capacitor for example, if a more convenient instrument isn't available, or if you absolutely don't want to spend money on one.


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 Post subject: Re: Can a variable condenser be accurately measured???
PostPosted: Mar Thu 22, 2012 12:10 am 
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Location: Saskatoon
Yes, I can confirm that these work extremely well. Better than most people would expect.

In fact, you can even substitute a pair of ear bud type phones for the piezo earphone, although it becomes a bit less sensitive.


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 Post subject: Re: Can a variable condenser be accurately measured???
PostPosted: Mar Thu 22, 2012 1:12 am 
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That's neat!!!.... Might be nice to sub in an AF oscillator and skip the radio???

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 Post subject: Re: Can a variable condenser be accurately measured???
PostPosted: Mar Thu 22, 2012 2:42 am 
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Yes, if you're planning to build a permanent tester. The reason behind using a radio or other audio source is that it saves a bit of time if you're in a hurry to throw something together.


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 Post subject: Re: Can a variable condenser be accurately measured???
PostPosted: Mar Mon 26, 2012 11:55 pm 
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Quote:
Can I accurately attach my Cap meter and read their result?


Brian, since everybody seems to be telling you everything except what you asked permit me....what capacitor tester are you using? Any decent tester in alignment can measure that variable cap, even one from the 30's.
I often use one from 1946 that I overhauled and its almost as accurate as a digital HP LCR meter originally costing thousands.

Carl


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