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Variable capacitor

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Ignoramus21085

Jan 1, 1970
0
I need to make a variable capacitor from 1000 pF to 19,000 pF
(roughly). Using something like a 11 position rotary switch is fine. I
can build something homegrown, but would like to know if there is
something already available, so that I am not missing anything
obvious. If I should build it, I will appreciate some design
tips so that I do not miss anything obvious. Thanks.

i
 
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martin griffith

Jan 1, 1970
0
I need to make a variable capacitor from 1000 pF to 19,000 pF
(roughly). Using something like a 11 position rotary switch is fine. I
can build something homegrown, but would like to know if there is
something already available, so that I am not missing anything
obvious. If I should build it, I will appreciate some design
tips so that I do not miss anything obvious. Thanks.

i
search for capacitor decade box, they are quite expensive, so a couple
of rotary switches will do what you want.


martin
 
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Ignoramus21085

Jan 1, 1970
0
search for capacitor decade box, they are quite expensive, so a couple
of rotary switches will do what you want.

Thanks. I need an assortment of caps, from 1000 pF to 10,000 pF. It
would be best if they were of axial type, makes them easy to parallel
and solder together. What kinds of caps would be best? Polyethylene
film or ceramic or what? I do not need much from them in terms of
accuracy or voltage.

i
 
T

Tam/WB2TT

Jan 1, 1970
0
Ignoramus21085 said:
I need to make a variable capacitor from 1000 pF to 19,000 pF
(roughly). Using something like a 11 position rotary switch is fine. I
can build something homegrown, but would like to know if there is
something already available, so that I am not missing anything
obvious. If I should build it, I will appreciate some design
tips so that I do not miss anything obvious. Thanks.

i
The best I have seen for a variable capacitor, and these would probably be
iompossible to find new, is a 3 x 365 PF. With 11 steps, your rotary switch
would have a resolution of (19000 - 1000)/11 = 1636 PF. If you only change
the setting infrequently, you could use 8 SPST switches, BCD coded, and you
would have a resolution of 19000/256 = 74 PF. You would need capacitors of
74PF, 148PF, 296PF, 592PF, 1184PF, 2368PF, 4736PF, and 9472PF. Since you
never need less than 1000PF, you could actually do a little better than this
by adding a non switched capacitor.

Tam
 
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Ignoramus21085

Jan 1, 1970
0
The best I have seen for a variable capacitor, and these would probably be
iompossible to find new, is a 3 x 365 PF. With 11 steps, your rotary switch
would have a resolution of (19000 - 1000)/11 = 1636 PF. If you only change
the setting infrequently, you could use 8 SPST switches, BCD coded, and you
would have a resolution of 19000/256 = 74 PF. You would need capacitors of
74PF, 148PF, 296PF, 592PF, 1184PF, 2368PF, 4736PF, and 9472PF. Since you
never need less than 1000PF, you could actually do a little better than this
by adding a non switched capacitor.

Yes. There will be another, parallel non-switched 1,000 pF cap. This
is for frequency regulation. With 1 megohm pot for duty cycle
adjustment, and permanent 1,000 pF cap, I would have the following 11
values for frequency:

pF Hz
1000 500
2000 333
3000 250
4000 200
5000 166
6000 142
9000 100
10000 90
12000 76
13000 71
16000 58
19000 50

(that's for a low frequency switching circuit, hence very low frequency).

The exact values do not matter terribly too much. What I want is to
find a cheap assortment of radial caps that would let me implement
these capacitances easily.

i
 
T

Tony Williams

Jan 1, 1970
0
Ignoramus21085 said:
Thanks. I need an assortment of caps, from 1000 pF to 10,000 pF.
It would be best if they were of axial type, makes them easy to
parallel and solder together. What kinds of caps would be best?
Polyethylene film or ceramic or what? I do not need much from
them in terms of accuracy or voltage.

Think "1-2-4-8-etc, binary" and a small array of
miniature toggle switches.... BTDT, very useful.
 
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Mike Harrison

Jan 1, 1970
0
I need to make a variable capacitor from 1000 pF to 19,000 pF
(roughly). Using something like a 11 position rotary switch is fine. I
can build something homegrown, but would like to know if there is
something already available, so that I am not missing anything
obvious. If I should build it, I will appreciate some design
tips so that I do not miss anything obvious. Thanks.

Use a BCD or binary coded switch ( or make one from multiple poles). Thay way you can get 16 values
from 4 capacitor values - see also :
http://www.electricstuff.co.uk/capbox.html
 
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Ignoramus21085

Jan 1, 1970
0
Think "1-2-4-8-etc, binary" and a small array of
miniature toggle switches.... BTDT, very useful.

That's interesting, but in my case, I want to select frequency with a
rotary switch, not with binary bit toggle switches. Maybe one day I
will sell this welding machine, and no one will understand binary bit
logic except for those literate in copmputer logic and such, so no one
will want to buy this welder.

Regarding selling it, it is not at all my current purpose. It would be
uneconomical to spend so much time to make a few hundred bucks. I want
to have a super nice modern welder for myself, made out of this cyber
tig, hence my current project.

That's my plan. Since I do not know what the future will hold, I have
to make sure that I end up with something sellable.

i
 
J

John Devereux

Jan 1, 1970
0
Ignoramus21085 said:
That's interesting, but in my case, I want to select frequency with a
rotary switch, not with binary bit toggle switches. Maybe one day I
will sell this welding machine, and no one will understand binary bit
logic except for those literate in copmputer logic and such, so no one
will want to buy this welder.

No problem, you can get rotary switches with a binary output. Google
"rotary hex switch".
 
D

Don Foreman

Jan 1, 1970
0
Yes. There will be another, parallel non-switched 1,000 pF cap. This
is for frequency regulation. With 1 megohm pot for duty cycle
adjustment, and permanent 1,000 pF cap, I would have the following 11
values for frequency:

pF Hz
1000 500
2000 333
3000 250
4000 200
5000 166
6000 142
9000 100
10000 90
12000 76
13000 71
16000 58
19000 50

(that's for a low frequency switching circuit, hence very low frequency).

The exact values do not matter terribly too much. What I want is to
find a cheap assortment of radial caps that would let me implement
these capacitances easily.

i

Given this low frequency, you could make a synthetic variable
capacitor using 1 cap, a few resistors, a pot and a dual opamp.
You'd need both + and - bias supplies for the opamp. Varying the
pot varies the effective capacitance to ground.
 
C

Chris Jones

Jan 1, 1970
0
Ignoramus21085 said:
I need to make a variable capacitor from 1000 pF to 19,000 pF
(roughly). Using something like a 11 position rotary switch is fine. I
can build something homegrown, but would like to know if there is
something already available, so that I am not missing anything
obvious. If I should build it, I will appreciate some design
tips so that I do not miss anything obvious. Thanks.

i

If this is for varying the frequency of your weld pulse thing, then I would
suggest going for caps in the ratio 1,2,5,10,20,50,100,200,500 etc. or even
just 1,3,10,30,100,300 etc. since that way you cover a large range in few
steps. I doubt you'll see much difference between 180Hz and 200Hz or
whatever it is in your welder, but it might be more interesting to explore
a wider range.

Also forget about selling it, I reckon. If someone buys it then it will be
more pain than it's worth, they'll keep calling you to fix it or they'll
break it and accuse you of causing everything that ever went wrong in their
life. Just build it to work the way you want, maybe you'll even feel like
keeping it.

The only concession I would make with regard to selling it, is to make the
modifications to the welder itself basically reversible so you can get back
to what you started with.

Chris
 
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Ignoramus21085

Jan 1, 1970
0
If this is for varying the frequency of your weld pulse thing, then I would
suggest going for caps in the ratio 1,2,5,10,20,50,100,200,500 etc. or even
just 1,3,10,30,100,300 etc. since that way you cover a large range in few
steps. I doubt you'll see much difference between 180Hz and 200Hz or
whatever it is in your welder, but it might be more interesting to explore
a wider range.

Yes, that's my plan indeed, it will be approximately exponential
scale, from 50 to 500 Hz.
Also forget about selling it, I reckon. If someone buys it then it
will be more pain than it's worth, they'll keep calling you to fix
it or they'll break it and accuse you of causing everything that
ever went wrong in their life. Just build it to work the way you
want, maybe you'll even feel like keeping it.

I do intend to keep it. I would not undertake this project to make a
few hundred bucks. In the time that I spend on this, I could have made
thousands of $$s by computer programming.


The only concession I would make with regard to selling it, is to make the
modifications to the welder itself basically reversible so you can get back
to what you started with.

I agree. There will be some drilled holes in the front panel, but not
many. I could cover them with a picture of a naked woman or some such.

i
 
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Ignoramus21085

Jan 1, 1970
0
Given this low frequency, you could make a synthetic variable
capacitor using 1 cap, a few resistors, a pot and a dual opamp.
You'd need both + and - bias supplies for the opamp. Varying the
pot varies the effective capacitance to ground.

that's interesting, can you elaborate?

i
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C

Clifford Heath

Jan 1, 1970
0
John said:
No problem, you can get rotary switches with a binary output. Google
"rotary hex switch".

What he said, but look also for thumbwheel switches with BCD output.
 
T

Tam/WB2TT

Jan 1, 1970
0
Ignoramus21085 said:
Yes. There will be another, parallel non-switched 1,000 pF cap. This
is for frequency regulation. With 1 megohm pot for duty cycle
adjustment, and permanent 1,000 pF cap, I would have the following 11
values for frequency:

pF Hz
1000 500
2000 333
3000 250
4000 200
5000 166
6000 142
9000 100
10000 90
12000 76
13000 71
16000 58
19000 50

(that's for a low frequency switching circuit, hence very low frequency).

The exact values do not matter terribly too much. What I want is to
find a cheap assortment of radial caps that would let me implement
these capacitances easily.

i
Since this is not a sine wave oscillator, why not tune it with a variable
resistor; like a nice 5 or 10 turn pot? You would only need 2 or 3 capacitor
values to cover the range. A 74HC4046 should work well.

Another approach that I would look at is to cover the top octave, and use
some binary counters to get the lower frequencies. Use the rotary switch to
pick the octave.

There are also a number of ways that you could do it with a single tuning
knob, and no band switch.

Tam

Tam
 
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Ignoramus21085

Jan 1, 1970
0
Since this is not a sine wave oscillator, why not tune it with a variable
resistor; like a nice 5 or 10 turn pot? You would only need 2 or 3 capacitor
values to cover the range. A 74HC4046 should work well.

Check out datasheet for XR2206:

http://igor.chudov.com/projects/Homemade-TIG-DC-to-AC-Inverter/Schematic/XR2206.pdf

the pot is used for duty cycle adjustment
Another approach that I would look at is to cover the top octave, and use
some binary counters to get the lower frequencies. Use the rotary switch to
pick the octave.

There are also a number of ways that you could do it with a single tuning
knob, and no band switch.

I am not quite up to date on all the terms that you use, sorry. I am
new to electronics.

i
 
K

Ken Smith

Jan 1, 1970
0
On Tue, 25 Oct 2005 15:18:59 +0100, Tony Williams


That's interesting, but in my case, I want to select frequency with a
rotary switch, not with binary bit toggle switches. Maybe one day I

If you can't get a binary encoded switch, you can use one with multiple
decks to do the same job. Fir that matter, you can get the same range of
values with less capacitors if you want.
 
M

Mike Young

Jan 1, 1970
0
Yes. There will be another, parallel non-switched 1,000 pF cap. This
is for frequency regulation. With 1 megohm pot for duty cycle
adjustment, and permanent 1,000 pF cap, I would have the following 11
values for frequency:

pF Hz
1000 500
2000 333
3000 250
4000 200
5000 166
6000 142
9000 100
10000 90
12000 76
13000 71
16000 58
19000 50

(that's for a low frequency switching circuit, hence very low frequency).

The exact values do not matter terribly too much. What I want is to
find a cheap assortment of radial caps that would let me implement
these capacitances easily.

I don't suppose I could interest you in a single chip solution, no weird
stuff to glue together into a messy bundle? 14 pins, only Vcc, gnd, and all
12 tick-tocks running concurrently on their own pins. (Trade ya one or a
dozen for that messy 'zuki motor in your garage.)
 
M

Mike Young

Jan 1, 1970
0
Mike Young said:
I don't suppose I could interest you in a single chip solution, no weird
stuff to glue together into a messy bundle? 14 pins, only Vcc, gnd, and
all 12 tick-tocks running concurrently on their own pins. (Trade ya one or
a dozen for that messy 'zuki motor in your garage.)

Better still: single chip, 8 pins; 1024 frequency steps between 50 and 500
Hz; duty cycle variable in 0.10% steps.
 
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Ignoramus21085

Jan 1, 1970
0
Better still: single chip, 8 pins; 1024 frequency steps between 50 and 500
Hz; duty cycle variable in 0.10% steps.

Yea, I am interested... Let me know what the solution is. Would it be
controlled by a pot? I feel that I am indeed missing something clever.

i
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