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Electronics => Electronics => Topic started by: airman00 on October 28, 2009, 05:48:08 PM

Title: NPN Transistor Switching higher voltage
Post by: airman00 on October 28, 2009, 05:48:08 PM
Assuming that the trigger voltage for the NPN was below 3.3V, and forgetting about resistors for now, what voltage would the LED receive? Would the LED receive 3.3V or 5V?
(http://i273.photobucket.com/albums/jj202/erobot/Misc%20Documentation/Picture1-4.jpg)
Thanks.
Title: Re: NPN Transistor Switching higher voltage
Post by: Soeren on October 28, 2009, 07:30:18 PM
Hi,

Assuming that the trigger voltage for the NPN was below 3.3V, and forgetting about resistors for now,
Never forget about resistors!

Well, I've said it over and over...
NPN goes on the low side and PNP goes on the high side.
Or to rephrase; you need the emitter to the rail, to be able to do what you think you're doing.


what voltage would the LED receive? Would the LED receive 3.3V or 5V?
Neither, but the actual voltage depends entirely on the V_f of the LED, as you have included it in the equation by putting it between emitter and the rail.
As it is, the base needs to be V_f_LED + ~0.65V above ground to start opening the transistor, so if V_f_LED > ~2.65V, the answer to your question is: less than what turns your LED on.

The emitter cannot be more than V_b - ~0.65V.

If you put the LED on the collector instead, the full voltage (here 5V) less the saturated voltage drop of the transistor will be available to kill your LED, since you don't like to include resistors and since the typical failure mode for an LED is shunted, the transistor likely follows it in the blink of an eye.

Use resistors!
Title: Re: NPN Transistor Switching higher voltage
Post by: airman00 on October 28, 2009, 07:41:13 PM
If you put the LED on the collector instead, the full voltage (here 5V) less the saturated voltage drop of the transistor will be available to kill your LED,
Thanks that answered my question.

Assuming that the trigger voltage for the NPN was below 3.3V, and forgetting about resistors for now,
Never forget about resistors!
I obviously would not build this circuit without resistors, but this circuit was just a quick schematic I drew up to ask a question on a concept, that a transistor can switch a higher voltage.

Well, I've said it over and over...
NPN goes on the low side and PNP goes on the high side.
Or to rephrase; you need the emitter to the rail, to be able to do what you think you're doing.
Like this : http://www.kpsec.freeuk.com/images/trancurr.gif (http://www.kpsec.freeuk.com/images/trancurr.gif)

Thanks again for the help!   :)
Title: Re: NPN Transistor Switching higher voltage
Post by: Soeren on October 28, 2009, 07:44:17 PM
Exactly :)