# Need to shed som eliught on transistor switches

Discussion in 'General Electronics Chat' started by FrenchConnection67, Mar 7, 2014.

1. ### FrenchConnection67 Thread Starter New Member

Sep 9, 2013
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hi,

I was wondering why one of the motion sensors (HR100 hotron) that we use on auto doors which is fitted with a transistor switch NPN can handle both switching the negative and positive side of a circuit. The theory says that a npn can only switch to ground and pnp to positive but why when we wire that sensor's npn transistor to switch to positive, it works too.

I even emailed hotron about this and they confirmed that it is a npn transistor (so as the manual) and it could interface with door controllers with 24vdc signals.

????? now I am confused. why would we have two types of transistors with two specifics functions if one can actually perform both.

is there anything I am missing here? is there any specific wiring to make those transistors more flexible
perhaps?

Last edited: Mar 7, 2014

Nov 7, 2013
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3. ### tindel Active Member

Sep 16, 2012
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I suspect the NPN is just completing the circular current flow of just the high-side or low-side of the circuit. Probably the low-side if there is only a single transistor. This is being used as a single pole - single throw switch.

I'm not aware of a single transistor being able to do a double pole - single throw switch.

Any answer is speculation without a schematic.

4. ### Wendy Moderator

Mar 24, 2008
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It is all about direction of current, nothing more. PNP and NPN bipolar transistors are just the inverse of the other.

5. ### ScottWang Moderator

Aug 23, 2012
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If the high side output using PNP then the Vout = Vcc-0.2V
If the high side output using one stage of NPN then the Vout = Vcc-0.9V
If the high side output using two stages of NPN then the Vout = Vcc-1.5V
The output voltage similar as above, if using two stages of NPN and want to increasing the voltage close to Vcc, then it can be adding two bjts as npn and pnp and then the Vout will be as the first line.

Sep 9, 2013
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7. ### FrenchConnection67 Thread Starter New Member

Sep 9, 2013
7
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https://onedrive.live.com/?cid=89277b39817fdfa2#cid=89277B39817FDFA2&id=89277B39817FDFA2!2291&v=3

let me know what you think

Apr 5, 2008
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9. ### FrenchConnection67 Thread Starter New Member

Sep 9, 2013
7
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so if I understood, we can wire a NPN transitor as switch on the high side of a circuit but the Vout will be decreased compared to a PNP transistor switch wired the same way. am I right???or I am still missing something

10. ### crutschow Expert

Mar 14, 2008
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3,362
If the output transistor does not have its common connected to the load circuit but just has two wires connected to the load, then it can switch either side of the load with equal efficiency. It effect it would be rather operating as an opto coupler would.