Capacitance substitution boxes

[watchmaker]

I am at the point where I want to vary capacitance in ckts so I am looking at substitution boxes.

I want one using 5% caps and decadal.  What are the "practical" choices for an amateur?  I see everything from 50 year old rotary dial boxes to $600 lab grade boxes.

Given I have the ST42, I can always measure the capacitance I think I selected.  Since it is used by an amateur, it does not need high life cycle switching (lab grade?).

THANKS

Dewey

[tggzzz]

I am at the point where I want to vary capacitance in ckts so I am looking at substitution boxes.

I want one using 5% caps and decadal.  What are the "practical" choices for an amateur?  I see everything from 50 year old rotary dial boxes to $600 lab grade boxes.

Given I have the ST42, I can always measure the capacitance I think I selected.  Since it is used by an amateur, it does not need high life cycle switching (lab grade?).

What is the required capacitance range and voltage and stability?

What is the allowable inductance? (Wires are ~1nH/mm)

[watchmaker]

Thanks.  As you may know, I am an "innocent" when it comes to this.

I cannot see needing more than 30/50 volts.  I am thinking a range of 100pF to 1 uF should suffice.  At this point in my studies, I am playing with bypass and coupling in transistor circuits, basic filters and ICs like the 385 and timers like the 555/556.

I am following Boylestad/Malvino/Real analog and so basically trying to replicate a college curriculum.  If I outgrow the box, not a big loss; but that is unlikely.

Ease of use and a small footprint would be helpful.  I have come to appreciate the importance of capacitance in just about any circuit.

For L, I do not have enough experience to know how to respond to your request.  I am not doing much over a few 100 kHz and 30 VDC. Inductance is my next sequence.

I am jumping around a little with some active circuits, but nothing yet has had much to do with inductance other than transformers which I "understand" on a middle school level.  And I do understand EMF and electromagnetism, rt hand rule, etc. But that is about it.  Nothing with real analysis.

Regards,

Dewey

[Phil1977]

Just to mention the cheapest solution that seems possible:

https://de.aliexpress.com/item/1005006235932003.html

[Kean]

Models with rotary switches tend to be quite expensive, or they are old and require contact cleaning/repair.

I bought one with slide switches from Element14 (available from Newark in the USA), but it does require you to do a little math.  I also have the resistance version.  Fine for low frequency applications.

https://www.newark.com/tenma/72-7265/capacitor-decade-box-100pf-11/dp/22H6398
It can also possibly be found on Amazon or elsewhere by searching for TENMA 72-7265

Specs are 100pF to 11.111uF in 100pF steps, 50V max, 5% tolerance (+50pF max internal)

[tggzzz]

For L, I do not have enough experience to know how to respond to your request.  I am not doing much over a few 100 kHz and 30 VDC. Inductance is my next sequence.

OK.

Start by understanding LCR circuits, especially resonance and how impedance varies with frequency. Initially ignore the effects of R, but do understand the different behaviour of LC series and parallel circuits.

Estimate the wire lengths you will have with the capacitance box, and use that to determine the resonant frequency for differing settings. Compare with the frequencies of interest.

(Realise that for decoupling digital circuits the clock frequency is irrelevant; transition time is the only important parameter)

[themadhippy]

If you go DIY the ole bcd push up down or thumbwheel switches (this sorta thing)


is another  way to go,yea youll need more caps per band,but the wiring is much simpler and you get a nice display included.

[watchmaker]

All,

Thank you.

TGZZZ, you get what I am trying to accomplish.  No idea why other than curiosity.  Not like I am going to apply for a job.

I did jump ahead of myself and decided to use the Elenco AM/FM radio kit as a learning platform.  It has its "ThaiKit" flaws, but that is not bad for my purposes.  I replaced the supplied LM385 with a real one for example (could not control the oscillation, which is one reason for the cap sub box).  Spent a LOT of time learning how to use my Sig 1064 and SDS 824.  Still got a long way to go, but...

Also, plugged all my switching PS devices into one switched power cord.  Takes time to lean without a TA looking over your shoulder.

They have the builder measure some voltages at bases and emitter resistors without explaining WHY they should be certain values.  The cool part was I decided to use my network theory to calculate the voltages using the measure resistances. and estimated junction voltage drops.  The theory WORKS!!

Having said all that, I get that in practice I will need to include L and that is my next sequence.  In the meantime, does this look useful?  Apologies if the URL appears naked and it is long.  I could not truncate it.

https://www.amazon.com/EDFM-6-DECADE-CAPACITANCE-BOX/dp/B071V4WX25/ref=sr_1_35?c=ts&dib=eyJ2IjoiMSJ9.sWrqcW9p9UOku5eFrcQQPeOBdaUYsMHAbpz_KQ7GhkbLzjqIqEYQdcTG1tEyqW_UVZC-4MI1f6CzxyJnUhPXl7dA2HteReQC5FMaRZJnAahnIs2y1Tdu9H3PZtm9UGbd-htsIEvowEJc3atggxFfjtcMNyaM7R4xmbpapuPVy7j_3_bUuoasYQwyNm6gRxE1x8iXHNhwDorcwzIxFQ-m6JqBFZM_XZyNZircE9ZcVzhlrVwmhzVFqzeK_40OSv_RwB2ys8CZKK5kH__QMtZEY9xqKnirtbynenYkUdUmgbs.S82vdetslYfNCri40-u83AFwoxIRECj1qD0R8u0xUm4&dib_tag=se&keywords=Decade+Boxes&qid=1726234959&s=industrial&sr=1-35&ts_id=1265103011

Regards,

Dewey

[watchmaker]

If you go DIY the ole bcd push up down or thumbwheel switches (this sorta thing)


is another  way to go,yea youll need more caps per band,but the wiring is much simpler and you get a nice display included.

That looks like what the Amazon Cap box uses!  THANKS.

[tggzzz]

They have the builder measure some voltages at bases and emitter resistors without explaining WHY they should be certain values.  The cool part was I decided to use my network theory to calculate the voltages using the measure resistances. and estimated junction voltage drops.  The theory WORKS!!

Theory and practice are both essential; they support and enlighten each other. (You do see some people thinking theory is relatively unimportant )

Practice without theory is blind fumbling.

Theory without practice is mental masturbation.

Go to it - and continue to have fun

[kripton2035]

another diy approach : http://kripton2035.free.fr/LabTools/lbt-capacitordec.html

[watchmaker]

I bought two substitution boxes.  One with the switches recommended by themadhippy and the other a less expensive type off Temu.

Both have small footprints and are accurate to 10% at the terminals as measured with my ST42.  Not sure the exact value much matters as you need to include the capacitance of your hookup wire as pointed out by Tgzzz.  In both, the steps are consistent.  I would simply measure the total capacitance from the end of the hookup wire and find a cap to match after I dialed in a "perfect" capacitance.

The Temu s about 1/6 th the price of the eBay one.  But I wonder about the Dip switches.  As they fail I would replace with headers and shorting blocks.

 https://www.temu.com/1nf-step-4---programmable-capacitor-board-polypropylene-film-capacitor-dc100v-ac70v-g-601099646299921.html?

The eBay one is certainly more expensive  https://www.ebay.com/itm/323759946271  .  But it is easier to use, and I would expect the switches to hold up.  The seller does take offers.

However, these are not like scopes or meters that get used every day.  SO switch robustness may not be an issue.  And for $10 the Temu is a good deal.

Both of these are a better value than the used Heathkit/Elenco/Eico boxes you find on eBay.

[tggzzz]

Not sure the exact value much matters as you need to include the capacitance of your hookup wire as pointed out by Tgzzz. 

Whether or not the exact value is important depends on what you do after you have finished twiddling the knobs

Depending on frequency, wire inductance is more likely to be an issue. Of course capacitors also have inductance, some more than others. There's no substitute for understanding

Have fun, realise the limitations of whatever equipment you are using.