hymentoptera
Well-known member
I'm sourcing parts for a custom decade capacitance box.. I have rotary switches, banana jacks, etc, on the way, and it's time to start buying lots of precision capacitors so I can make this project happen.
I wanted to ask you fine folks if there is any particular types of capacitors I should be looking into (or avoiding?).
I need better than 5%, so my options for larger values are slim I think, and will require me to buy large quantities and hand select the ones closest to nominal value. I'd like at least 1% precision across the entire band, except for at the picofarad range where I'll likely trim them out with variable capacitors I think.
I'm thinking polystyrene for most of the nanofarad range, as they can be found in low tolerances, but most of them aren't rated for more than 63v (I'd like 100v to be my target minimum voltage rating here, no reason why really), but I'd have to buy much fewer of them as opposed to if I went with big foil types, since I'd have to sort through dozens just to find what I needed in 5% parts.
I've got a metric boat-tonne of micas coming with 1% and 2% tolerances, as I've read that they are pretty temperature stable, and stability is important for what I'm trying to do. Modern MLCC's have TERRIBLE voltage coefficients, and as for temperature, regular NP0/C0G ceramics are hard to find in 1% tolerances. Is there a certain type I should be looking for in any given value range which meet my needs?
I'm willing to order from Mouser, DigiKey, etc, or even ebay sellers if needed. Price is not really a concern. Tolerance and stability (volt and temp) are most important.
One thing to note: If I can scrap together enough capacitors (and if the rack chassis space allows!) I'm going to stack every single switch position with it's own bank of capacitors in parallel, as to avoid the common configuration in commerical decade cap boxes where only the E12 values are available, i.e. 10pF, 12pf, 15pf, 18pf, 22pf, 27pf, etc... instead I want a TRUE decade box, even if it's means 100+ capacitors in a 20lb box of madness with hundreds of solder connections. I realize I'll be limited to around 1uF max like this, but this is what I really want. I know this is crazy, but this is DIY, and this is how I'm going to do it. Many of my values (E48 values) are almost available as micas and styrenes, but only go up to a few tens of nanofarads and then become to bulky to consider.
So my question is, what types of capacitors should I be looking for for any given range? And also, what other considerations are there in building a decade cap box that I may have missed?
And one more question; I plan to have a switch on the front which shorts the jacks together through a resistor to drain off any built-up charges. What value resistor should I consider (I'm thinking 10Meg?), and any other advice on this section as well.
So I guess that's four or five questions really...
The planned specs I'm aiming for include:
*19" rack mount decade capacitance box
*1% or better tolerance
*ranges from single digit picofarads (i know this isn't easy!), up to single digit microfarads.
*high stability
*banana jacks and rotary switches, probably in a 19" rack enclosure
*switchable bleeder resistor(s) strapped across banana jacks
*???
I wanted to ask you fine folks if there is any particular types of capacitors I should be looking into (or avoiding?).
I need better than 5%, so my options for larger values are slim I think, and will require me to buy large quantities and hand select the ones closest to nominal value. I'd like at least 1% precision across the entire band, except for at the picofarad range where I'll likely trim them out with variable capacitors I think.
I'm thinking polystyrene for most of the nanofarad range, as they can be found in low tolerances, but most of them aren't rated for more than 63v (I'd like 100v to be my target minimum voltage rating here, no reason why really), but I'd have to buy much fewer of them as opposed to if I went with big foil types, since I'd have to sort through dozens just to find what I needed in 5% parts.
I've got a metric boat-tonne of micas coming with 1% and 2% tolerances, as I've read that they are pretty temperature stable, and stability is important for what I'm trying to do. Modern MLCC's have TERRIBLE voltage coefficients, and as for temperature, regular NP0/C0G ceramics are hard to find in 1% tolerances. Is there a certain type I should be looking for in any given value range which meet my needs?
I'm willing to order from Mouser, DigiKey, etc, or even ebay sellers if needed. Price is not really a concern. Tolerance and stability (volt and temp) are most important.
One thing to note: If I can scrap together enough capacitors (and if the rack chassis space allows!) I'm going to stack every single switch position with it's own bank of capacitors in parallel, as to avoid the common configuration in commerical decade cap boxes where only the E12 values are available, i.e. 10pF, 12pf, 15pf, 18pf, 22pf, 27pf, etc... instead I want a TRUE decade box, even if it's means 100+ capacitors in a 20lb box of madness with hundreds of solder connections. I realize I'll be limited to around 1uF max like this, but this is what I really want. I know this is crazy, but this is DIY, and this is how I'm going to do it. Many of my values (E48 values) are almost available as micas and styrenes, but only go up to a few tens of nanofarads and then become to bulky to consider.
So my question is, what types of capacitors should I be looking for for any given range? And also, what other considerations are there in building a decade cap box that I may have missed?
And one more question; I plan to have a switch on the front which shorts the jacks together through a resistor to drain off any built-up charges. What value resistor should I consider (I'm thinking 10Meg?), and any other advice on this section as well.
So I guess that's four or five questions really...
The planned specs I'm aiming for include:
*19" rack mount decade capacitance box
*1% or better tolerance
*ranges from single digit picofarads (i know this isn't easy!), up to single digit microfarads.
*high stability
*banana jacks and rotary switches, probably in a 19" rack enclosure
*switchable bleeder resistor(s) strapped across banana jacks
*???