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Is it possible to use a transistor to act as an relay? What I mean by that is using a 3V battery to control the gate so that a separate 12V battery can let its current flow to other parts of the circuit. Or must a transistor use only a single power supply? Some examples would be great.

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  • \$\begingroup\$ Using a low voltage to switch or control a higher voltage is exactly what a transistor is there for. Though, a major property of a relay is that it provides galvanic insulation which a transistor does not. When switching 12V, galvanic insolation probably isn't required. \$\endgroup\$ – Sim Son Jul 5 at 19:27
  • \$\begingroup\$ Thank you. I was confused because the transistor only has 3 pins and a relay has 4. Still not 100% sure how and why that is but the main point is solved. Cheers! \$\endgroup\$ – Jumperz Ko Jul 5 at 19:49
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    \$\begingroup\$ A transistor only has three pins because the lower and higher voltage parts have the same ground reference. A relay consists of two seperate circuits, each having + and -. \$\endgroup\$ – Sim Son Jul 5 at 19:52
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schematic

simulate this circuit – Schematic created using CircuitLab

Your load is R1. You could also use a PNP transistor.

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  • \$\begingroup\$ Note: R2 is important, don't leave it out, unless you are using a MOSFET transistor which doesn't need it. \$\endgroup\$ – user253751 Jul 6 at 11:41
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Yes it is, but there are some differences.

  1. In a relay, generally the actuation coil and the switching contacts are electrically isolated from each other -- so (for example) the coil can be driven from a safe-to-touch circuit, while the contacts may be switching the mains voltage.
  2. In a transistor, you'll need the supplies to have a common point -- typically this would be the negative supply of each.
  3. Transistors are generally much more delicate than relays. Small overloads or overvoltage will destroy them, and they might fail short-circuited
  4. A MOSFET will be easier to drive than a bipolar (e.g. NPN) device. In a bipolar device you'll need provide sufficient base current to saturate the device. This will require knowing the maximum load current (and dividing by the worst-case beta). In a MOSFET, you can just apply sufficient gate voltage -- 3 V will usually work, but the FET will work better with a higher voltage.
  5. Relays work as well with AC loads as DC loads; transistors don't. A MOSFET has an internal body diode which will always conduct if the drain voltage is lower than the source voltage. A bipolar device will also conduct in this case (it's a little more complex and depends on the voltages involved).
  6. Relays may have normally-open and normally-closed contacts -- a transistor is closer to a normally-open switch.
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  • \$\begingroup\$ Thanks for clearing up some things for me \$\endgroup\$ – Jumperz Ko Jul 6 at 4:52

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