Here's what I want to do. I want to switch between three different input capacitors using a toggle switch instead of a rotary switch because of space concerns using the 1590B box.
Does anyone know of a toggle switch or a toggle switch wiring option that will allow the selection of one of three different capacitors?
TIA
Phillip
You could set it up with a center-off DPDT, aternating between two different caps (at the outer lugs) in parallel with the main cap (between the middle lugs).
It could work for you, depending on the cap sizes you want to choose from.
If you wanted .01, .022, and .047uF choices, put a .01 in the middle, a .01 at one end, and a .033 at the other.
Make sense?
Also, an ON-ON-ON DP3T toggle would work, I believe. The problem, however, is that they cost around 15$. B Tremblay's idea will work much better.
-Colin
I like the idea of using two capacitors, then puttng them together in parallel to increase the value.
I was looking for a SP3T toggle switch, and I found one in the Mouser catalog, but they're insanely expensive!
Phillip
Look at the bottom schematic here for how I do it:
http://freewebs.com/qrp/snippits.html
Pretty much the same thing as B Tremblay's solution, just from a slightly different angle.
Take care,
-Peter
A couple times...
I chose the 'thin' cap first...then piggybakked [paralelled] a cap or different cap ... SPDT...
Looked for a SPDT/nothrow ...one side/the other side/ no side....nofind
Hi Phillip,
Here is how I did a "Fat mod" on my RangeMaster using a cheap on-off-on SPDT switch. Center is off and just the original cap, each side switches in a different cap in parallel. Worked great!
(http://britboy.mysitespace.com/FAT_mod.gif)
The caps could also be directly wired to the switch but I designed them into the circuit.
Hope this helps,
Bill
All you need is a centre-off SPDT toggle to do it.
There are two ways to implement it, of equal simplicity but of different difficulty in calculations.
The simplest is to start with the smallest cap value you want, (centre position on switch), and use the two side positions of the switch to add another cap in parallel. In parallel, cap values are additive.
The problem with this method is that it leaves one end of the to-be-added caps free, which can result in audible popping when you switch. The way to get past the popping is the use the method John Hollis employs for the Zombie chorus, and that is to leave several caps in circuit and in *series*, and selectively shunt them.
In series, capacitance is calculated the same way you calculate the resistance of multiple resistors in parallel:
1/Ca + 1/Cb + ....1/Cn = 1/C
In the Zombie a pair of .001 caps are laid end to end. when both are used, the effective capacitance is 500pf. Shunt either one of them and you're left with .001uf.
To switch between 3 cap values, you need to calculate what the smallest sized cap would be, based on the impact of the other two caps on effective capacitance. It will always be lower than the smallest cap value.
So, if you had a .1uf cap, a .05, and a .02 cap in series, having all of them in circuit would get you a series capacitance of .0125uf. Shunt the .02 cap and the remaining pair give you a series capacitance of .033. Shunt just the .05 cap and you get .016uf. And so on.
How do you implement this, switch-wise? Several alternatives. First, you select your common for the centre lug of the switch. That could be either a junction between two caps (2 choice there), or one of the end points. If you pick an end point, then side position switch shunts cap 1, and side position 2 shunts caps 1 + 2, leaving only cap 3 nonbypassed. Cap 3 should be the maximum capacitance value you want.
If your common is a junction between caps, then you can shunt, say cap 1 or cap 2, cap 2 or cap 3, or mimic what an end-point common does by shunting one or two caps. None of these is any better than the other. The choice is made based on what sort of range of capacitance values you want to cover, and what in your parts bin allows you to do what you want after the calculations are completed.
Alternatively, you can use a different approach and provide continuously variable bypass or panning between two capacitance values. Joe Gagan uses this elegantly in his various Fuzz Face derivatives. Since the terrain is a 1590B, obviously the difference in real estate taken up by a mini-toggle vs a pot is not to be taken lightly.