Newbie transistor help!

Doesn't exist in a plain old bipolar transistor. The base-emitter junction flows current exponentially with voltage, for a regular old bipolar you'll only drop about 0.6V, maybe 0.7 when you're saturated. You cope with this by using a resistor in series with the base to limit the current.

Neither. V_ebo means that the maximum voltage you can put on the (reverse-biased) emitter-base junction with the collector open is 6V -- higher than that and you void the warranty. Review: the 'V' means voltage, the 'e' means emitter, the 'b' means base and the 'o' means open. V_ceo means pretty much the same thing, but it's going through the reverse-biased collector-base junction and the forward-biased base-emitter junction.

You want two things:

Your circuit should look like this:

to LED cathode | | | 0/5V ___ |/ ----|___|--o---| Q1 R1 | |>

.-. | R2 | | | | | | '-' | | | | | === === GND GND (created by AACircuit v1.28.6 beta 04/19/05

R1 keeps the base-emitter junction from burning up. R2 is optional; it turns the transistor off quicker (and keeps it off when temperatures are high). Q1 is what you want to do the work. You need to size R1 and R2 to provide enough current to the base so that when your LED is delivering 80mA the transistor is turned on hard -- you'll need to see what your transistor's h_FE is _at 80mA_ to see what base current you need to get that collector current (h_FE is collector current / base current).

The second thing you need is to follow the OP's suggestion and get a good book on basic transistor theory. I can't really recommend anything beyond the ARRL handbook; I couldn't tell you how detailed they get.

This is some sort of microprocessor project ??

Per this link:

"....on-chip are a BCD code-B decoder, multiplex scan circuitry, segment and digit drivers...."

So you are trying to "write a program" to do these functions ??

donald

Hi all,

I have a basic transistor question.

--Setup--

I am multiplexing 8 7-segment led's (common cathode) and will be hooking up the common to my transistors as Emitter +5v / Base 2.2k res Pic / Collector +5v LED Digit.

My LED's draw approx 10mA per LED (80mA total)

pre-Question:

I'm alittle lost on all the specs for the transistors. I want a transistor type TO-92 that has an input of +5v and an output of +5v

--Question--

If the spec says "V-ebo 6v, Vcbo 60v, vceo 50v" max ratings, does that mean if I send

Kam

You need to find a book on the basic operation of transistor

The volatages you referance are the maximum voltage that can be applied without damaging the transistor.

Neither.

What comes out depends on whether the transistor is switched on ( circuit operation ).

Those ratings are simply the maximum voltages the transistor can withstand.

Have you worked out if you need npn or pnp transistors btw ?

You need more than that to multiplex the displays incidentally.

Graham

Graham,

I'll be using a NPN since I'm using a Common Cathode 7-seg LED's. I'm writing a program (just like a max7219) that multiplexes the lines to do the updating...

Thanks again for the reply

~Kam (^8*

In which case the emitter will connect to GND not +5V !

Maybe you should post your schematic ( or part of ) on a.b.s.e ?

Graham

V-ebo 6v means that the maximum allowable voltage between emitter and base with the collector open circuit is 6V. This is the reverse bias polarity of the base-emitter diode, i.e., base negative with respect to the emitter for an NPN. Opposite polarity for a PNP. In normal operation, the base-emitter junction will be forward biased for a linear cvircuit. ~ Vcbo 60V means that 60 volts is the maximum allowable voltage you can apply to the Collector-Base junction with the emitter open circuit. For an NPN transistor this means the collector is + with respect to the base. Opposite polarity for a PNP. ~ VCeo 80V means that 50 volts is the maximum allowable voltage that you can apply to the collector-emitter junction with the base open circuit. For an NPN transistor this means the collector is + with respect to the emitter. Opposite polarity for a PNP. ~ Vceo is always lower than Vcbo.