### Author Topic: NPN Transistor Switching higher voltage  (Read 4516 times)

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#### airman00

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##### NPN Transistor Switching higher voltage
« 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?

Thanks.
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#### Soeren

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##### Re: NPN Transistor Switching higher voltage
« Reply #1 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,

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!
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

#### airman00

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##### Re: NPN Transistor Switching higher voltage
« Reply #2 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,

Assuming that the trigger voltage for the NPN was below 3.3V, and forgetting about resistors for now,
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

Thanks again for the help!
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#### Soeren

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##### Re: NPN Transistor Switching higher voltage
« Reply #3 on: October 28, 2009, 07:44:17 PM »
Exactly
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?