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How can I use transistors and other discrete components to make a circuit that functions like a DPDT (double pole double throw) switch, and can be extended for any number of poles required? For examples assume an input voltage of 12VDC, required current of 200mA per pole and use general transistors and standard values where possible.

Note: it must act as double throw; i.e. power goes to two separate outputs, not one output or floating. Please include some form of schematic.

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  • \$\begingroup\$ We need to know if you expect the NC (normally closed) state of the "switch" to work passing current when the power supply to the transistor circuit is turned off? If so then you may very well be better off implementing this with the electro mechanical type components. \$\endgroup\$ – Michael Karas Oct 7 '12 at 9:02
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    \$\begingroup\$ Do the channels need to be isolated from the control circuit and each other? Are you trying turn something off/on, run motors in forward/reverse, or something different entirely? There is no device you can make out of transistors that behaves exactly like a mechanical switch, so we need to know more about what you're trying to accomplish before we can recommend a circuit design. \$\endgroup\$ – Theran Oct 7 '12 at 9:04
  • \$\begingroup\$ I am actually going the route of two DPDT switches tied together, but I wanted to know how possible such a circuit would be using only transistors and simple components. Maybe such a circuit isn't possible without specialising more. Assume that no special treatment is required, just that all left-hand outputs pass the supply voltage through when the master switch is off, and all the right-hand outputs do so when it is on. They can be left floating or tied to ground when they are not on. Clear enough? \$\endgroup\$ – cortices Oct 7 '12 at 9:10
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You can implement this with semiconductors provided the power supply to the circuit stays on all the while you want to have the transistors conducting current. This problem is also much easier to solve if you design for the current flow through the switches to always be the same direction.

Let's say that you design for 12V coming into the C (common) terminal of each "switch" and the NC (normally closed) and NO (normally open) "connections" will always be used to gate power to a ground referenced load from the incoming 12V.

For each pole you will need two FETs driven from a pair of control signals that are logically inverted from each other. You drive one FET ON while the other is OFF and flip the states when the "switch" changes state. In this 12V design you may find it easiest to use a P-FET since turning on the FET works nicely by pulling the FET gate to GND with the control signal. (If you use N-FETs it is necessary to provide sufficient gate drive above the 12V level signals that you are trying to switch). The two FET drive signals can derive from a single control signal by adding an inverter within the path to one of the FET gate drives.

Example Schematic - Note that you have to select the appropriate P-FET per your requirements.

enter image description here

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    \$\begingroup\$ Specific example/schematic please? I understand but for memory and documentation purposes I'd love a diagram. \$\endgroup\$ – cortices Oct 7 '12 at 9:16
  • \$\begingroup\$ @iamtheguy - I added a diagram to add to your happiness \$\endgroup\$ – Michael Karas Oct 7 '12 at 9:50
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An alternative to using transistors while making the circuit modular and expandable is to replace each switch section with a SPDT relay. Then use a single SPST switch to control the coils of as many relays as you want.

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  • \$\begingroup\$ I know there are other ways to achieve the same thing, but the question specifically asks how you can do it with transistors and the like. \$\endgroup\$ – cortices Oct 7 '12 at 12:04

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