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5StarGN
3 posts
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08-10-2010 05:32
In the 3D LED Pyramid (Summer Circuits 2010 pg. 32)
Shouldn't T9,T10 & T11 be PNP transistors with the emitters on the 5v bus and the collectors driving the LED banks? I cant see how the BE junction would be forward biased in this configuration.
Thanks
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Colin55
4 posts
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08-10-2010 11:06
The three transistors in the 3D Pyramid Circuit on Page 30 of the Summer circuits issue are in EMITTER-FOLLOWER mode in which the emitter follows the base and is about 0.6v lower than the base.
This can be done when the transistor is connected to a supply that is the same voltage as the section that is delivering a voltage to the base - as is the case in this circuit.
This type of arrangement will also work where the two voltages are different but the emitter (in all these cases) will only rise to about 0.6v less than the lower of the two voltages.
In the emitter-follower arrangement, there is no phase-change between the output of the microcontroller and the line powering the LEDs.
If you use a different driver, such as a PNP transistor, the program in the micro may have to be changed.
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5StarGN
3 posts
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08-10-2010 11:40
I see. When using transistors as switches, I’ve always use NPN’s when turning the ground on/off and PNP’s when switching the +positive rail on/off to a load. The MCU code notwithstanding; the main difference I see is the voltage drop across the transistor. Using EMITTER-FOLLOWER mode the load will see ~VCC-.6 (Vbe); using a PNP the load would see ~VCC - .2(Vce sat). Is that right?
Thanks for your time-
Chuck
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Colin
1 post
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08-10-2010 21:32
Yes. You are correct.
The only other difference with the two arrangements is this:
With the common-emitter design, the load is started to be powered when the voltage on the base rises above about 0.6v and is fully powered possibly before the input voltage rises to full voltage.
The emitter follower design is not powered fully until the input voltage is as high as it can go.
This means the common emitter load gets powered faster than the rising waveform and that's why you can call it a switch.
The emitter-follower load gets powered at the rate of the rising input and this is important when the input waveform is very slow, as the transistor will heat up more with an emitter-follower design.
This is not a problem with the output of a micro-controller as the output rises and falls very fast.
In addition, the voltage across the collector-emitter of an emitter follower will be about 0.6v and the losses will be higher than a common-emitter configuration.
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