# Low voltage switches

Discussion in 'General Electronics Discussion' started by Joshbartlett, Nov 4, 2015.

1. ### Joshbartlett

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Nov 4, 2015
Hey guys and gals..
Building Maintenance Tech here.
Im always learning new and things and growing. Been' wondering lately how the infamous transistor acts as a low voltage switch without essentialy messing up the whole curcuit when the switch is on allowing the larger current to pass through the collecter and emitter if they both share the same ground. Ha! Im stumped. Been fixing to many thermostats this week I geuss.

2. ### donkey

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Feb 26, 2011
I am a newbie at electronics but this is how I see it
there are 2 circuits(its one I know but try to follow me here). electricity follows the path of least resistance. so even though they share a common leg the high voltage will only ever go to its ground and the lower voltage will go straight for its ground

3. ### Joshbartlett

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Nov 4, 2015
I've been considering that explination myself. I myself am a newbie as well when it comes to this kind of tech but I am slowly getting pulled down the rabbit hole. If they share a leg wouldn't that essentially make it one complete circuit? Which you already said but how does a low voltage current return to the the controller completing the low voltage signal path and the high return to the highs source? But your saying it happens instantaneously and they dont interfere with each other. Correct?

4. ### donkey

1,293
56
Feb 26, 2011
there may be interference if you overload the transistor current or voltage specs but otherwise I am pretty sure its all good
the simplest way to look at DC voltage is path of least resistance. unless the transistor has a reservoir to store extra power it won't "spill out" onto another path

5. ### Joshbartlett

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Nov 4, 2015
I guess my question is this. If it takes .06 volts to activate a transistor then 5 volts pass through the switch, doesn't that become 5.06 volts passing through the emitter all at once? Maybe a resistor in the return path only allows .06 V tocomplete the controller circuit.

Last edited: Nov 4, 2015
6. ### duke37

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Jan 9, 2011
A silicon transistor needs about 0.6V between base and emitter to turn it on, NOT 0.06V.

Taking the emitter as the reference, current is passed into the base at about 0.6V to turn it on.
This allows current to pass between collector and emitter. If turned hard on, the voltage drop across the transistor can be as low as 0.2V i.e. less than the base voltage. I do not know the physics of this and I have never needed to know.

A good switch will have very little voltage across it when turned on, you will know this if you do building maintenance.

7. ### Joshbartlett

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Nov 4, 2015
I have a wide assortment of understanding in many topics and I put that experience to work everyday. There are many things I have to learn. But your post was helpful. Thx. I hadn't considered that the switch doesn't remain open.