Re: Rotary switches, whut?
Posted by induction on Dec 03, 2020; 11:12pm
URL: http://guitar-fx-layouts.238.s1.nabble.com/Rotary-switches-whut-tp49494p49504.html
Hi Marbles,
I think it might be less confusing if you start by understanding the function of the switch (how throws and poles work) before you consider the usage (how to set up clipping options). I apologize in advance if I'm repeating anything you already know.
Multipole switches mostly work according to the same logic, whether they are toggles, rotaries, sliders, whatever. The number of poles is just the number of individual switches that can been operated by the same mechanism. The number of throws is the number of possible connections available to each pole. A simple on/off switch (like a light switch) is an SPST toggle. When it's toggled one way, the two lugs are connected. When it's toggled the other way, the lugs are not connected. You can use this to connect and disconnect a light bulb from one of the power rails turn turn it on and off.
Poles: Adding a pole (2PST) is just adding another pair of lugs to be controlled by the same mechanism. When toggled one way lugs 1 and 2 are connected to each other, and lugs 3 and 4 are connected to each other. When toggled the other way, none of the lugs are connected. Now you can make and break two different connections at once, so you could turn two lights on and off together, even if they don't share any power rails (say one light uses the mains and another uses a battery, for a contrived example). 3PST would mean three separate switches operated at once, etc. In all cases, none of the poles are connected to each other unless you jumper them.
Throws: Adding a throw means adding a lug to make an alternative connection. With our SPST light switch example, we could add another lug and make it and SPDT. One of the lugs now has a special status: the common lug is the one that is alternately connected to the other lugs (let's call them the 'switched' lugs). In position 1, switched lug 1 is connected to the common lug. In position 2, switched lug 2 is connected to the common lug. This means that you could turn one light on and another light off with the same switch if you connect the common to ground and each of the other bulbs to one of the thrown poles. (Note that the usual numbering for switches make the common lugs 2 and 5. The switched lugs for pole 1 are 1 and 3. The switched lugs for pole 2 are 4 and 6.)
Here is a page with some diagrams.
Some double-throw switches have a third position for 'off' where none of the switched lugs are connected to the common lug. Some DPDT's are on/on/on and the middle position connects the opposite pairs of lugs in each pole. They come in two types, and look like this:
Combining the poles and throws concepts, you can switch X circuits in Y different ways each with an XPYT switch. Since toggle switches with more than 3 positions are rarely practical, we tend to use rotary switches for this instead.
Usage: For the purposes of this discussion, I will assume hard clipping from a zero-centered signal (positive and negative voltage swings) via antiparallel diodes to ground. There are several ways to use switches for clipping selection, but the simplest is to use a single pole with as many throws as you want clipping options:
- To turn a single clipping option on and off, take a pair of antiparallel diodes and connect one side to the signal node, and the other side to lug 1 of an SPST. Connect lug 2 to ground.
- To choose between Y pairs of diode clippers, connect one side of each antiparallel diode pair to the signal node, connect the other side of each pair to the Y switched lugs. Connect the common lug to ground. In each position of the switch, one set of diodes will connect the signal node to ground and become active. The rest will float and have no impact on the signal.
So if you can do this with SP switches, why do people use DP switches instead? Two reasons:
1. DP switches are easier to find, and rarely cost any more than SP switches. You can always ignore any unused poles in a multipole switch.
2. DP switches save space on the board and generate less spaghetti. With an SP switch, you need to connect one side of each diode pair to the signal node, and run a separate wire from the other end of each pair to one of the switched lugs, and another wire from common to ground. That's Y+1 wires for Y clipping options, and all the diodes mounted on the board, which makes it bigger. With a DP switch you can run one wire from the signal node to the common lug of pole 1, another wire from the common lug of pole 2 to ground. Then mount the antiparallel diode pairs across the equivalent lugs on pole 1 and pole 2. That's 2 wires and a smaller board.
Probably this is all covered in Tim's link. I was still typing when he posted it. Apologies if it's redundant.
URL: http://guitar-fx-layouts.238.s1.nabble.com/Rotary-switches-whut-tp49494p49504.html
Hi Marbles,
I think it might be less confusing if you start by understanding the function of the switch (how throws and poles work) before you consider the usage (how to set up clipping options). I apologize in advance if I'm repeating anything you already know.
Multipole switches mostly work according to the same logic, whether they are toggles, rotaries, sliders, whatever. The number of poles is just the number of individual switches that can been operated by the same mechanism. The number of throws is the number of possible connections available to each pole. A simple on/off switch (like a light switch) is an SPST toggle. When it's toggled one way, the two lugs are connected. When it's toggled the other way, the lugs are not connected. You can use this to connect and disconnect a light bulb from one of the power rails turn turn it on and off.
Poles: Adding a pole (2PST) is just adding another pair of lugs to be controlled by the same mechanism. When toggled one way lugs 1 and 2 are connected to each other, and lugs 3 and 4 are connected to each other. When toggled the other way, none of the lugs are connected. Now you can make and break two different connections at once, so you could turn two lights on and off together, even if they don't share any power rails (say one light uses the mains and another uses a battery, for a contrived example). 3PST would mean three separate switches operated at once, etc. In all cases, none of the poles are connected to each other unless you jumper them.
Throws: Adding a throw means adding a lug to make an alternative connection. With our SPST light switch example, we could add another lug and make it and SPDT. One of the lugs now has a special status: the common lug is the one that is alternately connected to the other lugs (let's call them the 'switched' lugs). In position 1, switched lug 1 is connected to the common lug. In position 2, switched lug 2 is connected to the common lug. This means that you could turn one light on and another light off with the same switch if you connect the common to ground and each of the other bulbs to one of the thrown poles. (Note that the usual numbering for switches make the common lugs 2 and 5. The switched lugs for pole 1 are 1 and 3. The switched lugs for pole 2 are 4 and 6.)
Here is a page with some diagrams.
Some double-throw switches have a third position for 'off' where none of the switched lugs are connected to the common lug. Some DPDT's are on/on/on and the middle position connects the opposite pairs of lugs in each pole. They come in two types, and look like this:
Combining the poles and throws concepts, you can switch X circuits in Y different ways each with an XPYT switch. Since toggle switches with more than 3 positions are rarely practical, we tend to use rotary switches for this instead.
Usage: For the purposes of this discussion, I will assume hard clipping from a zero-centered signal (positive and negative voltage swings) via antiparallel diodes to ground. There are several ways to use switches for clipping selection, but the simplest is to use a single pole with as many throws as you want clipping options:
- To turn a single clipping option on and off, take a pair of antiparallel diodes and connect one side to the signal node, and the other side to lug 1 of an SPST. Connect lug 2 to ground.
- To choose between Y pairs of diode clippers, connect one side of each antiparallel diode pair to the signal node, connect the other side of each pair to the Y switched lugs. Connect the common lug to ground. In each position of the switch, one set of diodes will connect the signal node to ground and become active. The rest will float and have no impact on the signal.
So if you can do this with SP switches, why do people use DP switches instead? Two reasons:
1. DP switches are easier to find, and rarely cost any more than SP switches. You can always ignore any unused poles in a multipole switch.
2. DP switches save space on the board and generate less spaghetti. With an SP switch, you need to connect one side of each diode pair to the signal node, and run a separate wire from the other end of each pair to one of the switched lugs, and another wire from common to ground. That's Y+1 wires for Y clipping options, and all the diodes mounted on the board, which makes it bigger. With a DP switch you can run one wire from the signal node to the common lug of pole 1, another wire from the common lug of pole 2 to ground. Then mount the antiparallel diode pairs across the equivalent lugs on pole 1 and pole 2. That's 2 wires and a smaller board.
Probably this is all covered in Tim's link. I was still typing when he posted it. Apologies if it's redundant.
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