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Relationship between a transistor and a relay!? watch

    • Thread Starter

    Hello, I just wanted to know in simple terms not too complex what a transistor and relay do to eachother i dont understand very well, is it that the transistor receives a small voltage and the relay produces larger voltage?


    From John Wood

    I'm not sure what you mean. They're two different things. A transistor is a current amplifier - when you pass a current into the base and out of the emitter, it conducts current from the collector and out the emitter. The collector starts to conduct when there's 0.6V between the base and the emitter.

    A relay is just a coil of wire and a switch, totally separate from each other. If you pass a (smallish) current through the coil of wire, the magnetic field pulls the switch closed.

    It's if you have a power circuit that has a lot of current flowing, you'll want to use a relay because your control circuitry can only pass enough current to operate the coil not the big thing (imagine the headlights in a car - if the circuitry that turned them on were connected straight to the bulbs, it would all burn out. The circuitry operates the relay coil, which makes the relay switch close and completes a totally separate high-current circuit that lights the bulbs.)

    You might also need to use a relay if your two circuits operate at different voltages, eg. you need to turn a mains lightbulb on with the computer. You'd connect the computer to operate the relay coil, and that would make the relay switch close in the mains circuit - this way the mains voltage doesn't make your PC go bang.

    You can use the two things together though. They have vaguely similar uses - a tiny tiny current from a chip might operate the transistor, which passes a smallish current to the relay coil, and the relay switch allows a huge current to pass.


    silver has outlined it quite well.

    The main use for relays is to isolate one part of an electronic or electrical circuit from another. It provides a physical barrier that offers protection from high currents and/or high voltages that could damage relatively cheap components such as low or medium power transistors.

    silver gave a couple of examples. The car analogy is a good one. The car lights will draw a fair bit of current - imagine two 60W main beam headlights plus two 50W dipped headlights operating at the same time (driving on dipped headlights and then "flashing" the main beam). The car electrical system is 12V (in reality, closer to 14V) and the current required to operate them will therefore be in the region of 220W/12V = 18.3A (approx). Most small/medium power transistors would not be able to handle that sort of current flowing through them. Higher power transistors would need a substantial heatsink to keep them cool enough to operate reliably. The rationale behind using relays in cars is that they are a) cheap and b) easy to replace and the electronics can be relatively small and (again) cheap.

    If you look at a typical relay base you will see quite a few contacts. Normally two of those are connected to the relay coil. They would be the ones that could be connected to a transistor.

    The simplest relay operates a bit like a light switch. There will be one pair of contacts going into the body of the relay and attaching to what look like long thin strips with small bulges at the end. In the car headlight example, one of the strips would have the car elctrical supply connected to it and the other would be connected to one side of the headlight bulbs, with the other side of the bulbs being connected to the car chassis. When the headlights are "switched" on, what happens is that power is applied to the coil of the relay, the relay operates and physically pushes those strips together so that the bulges touch and the electrical circuit is then made from the battery, through the relay and then the bulbs to the car chassis and then back to the battery. Current flows and the bulbs light.

    Transistors are generally used to operate relays by supplying a relatively small current to the relay coil, that causes a mechanical movement in the relay that physically pushes the relay contacts together. Sometimes it's the opposite effect that's needed and the relay contacts are physically separated to break an electrical cicuit. The first type is known as "normally open" because the contacts are not closed when there is no power applied; the second type is known as "normally closed" because the contacts are closed (touching) when there is no power applied to the relay.

    There are other types of relay known as "latching" relays. Those are generally used in slightly more specialised applications where you would not be able, or maybe not want, to have power applied all the time.

    Probably way too much info there on relays and not enough on transistors .... if so, sorry about that.

    Hope it helps though.
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Updated: June 21, 2005
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