Transistor selection on LED project, clever switch

For your pull downs I would suggest 10k that is a common value and won't draw much current away from the LEDs (1.26mA). Many small signal transistors have an hFe around 100 so if you want your E-C current to be about 60mA then you should select the resistor that will sink about 0.6mA from the base. To allow for some margin(and to make easy calculations). I'll design the resistor to sink 1mA.

In the lowest sink case the voltage at your emitter will be 12.6V, assuming a Vbe drop of 0.7V(fairly standard) the put the base voltage at 11.9V base resistor is pulled to ground we want it to sink only 1mA so using ohm's law 11.9V/1mA= 11.9k resistor.

The nearest standard value is 10k. If we use that in our calculations the base current at 12.6V supply is 1.19mA, with 14.5V it goes up to 1.38mA which is fine.

So I recommend you use 10k resistors all around.

For the transistor I recommend you use the ubiquious 2N3906 PNP Transistor, its cheap and available from a bunch of manufacturers, The currents your manipulating are well within the limits of the transistor.

Since the transistor is being used as a switch the power dissipated can be calculated by multiplying the current time the Vce saturation voltage. Punching the numbers comes out to 30mW of power dissipated which if multiply time the junction to ambient thermal resistance(200C/W) gives a temperature rise of 6C above ambient which is not a big deal.

So to recap use

10k 1/4W resistors all around

2N3906 PNP transistor

hth,

Jorge Garcia

Jorge,

Thank you for the quick and thorough reply!

Looking at my circuit, you can see that at pin 2, I will have to dissipate approximately 0.87W. (14.5mV * 60mA)  Or rather, I may have to handle that kind of power. I really do not know what is coming in exactly. I am thinking I though at least triple the power rating, and make sure the transistor can handle what it coming to it.

The 2N39062N3906 you recommend only can handle a pd of 0.625W.. So I worry.

Also, I want to drive this transistor into saturation, and make sure it is completely on or off. I don't want a dimly lit LED somewhere in the rest of the circuit.

Cabe

I am in fact using pull down resistors at 10K.

Also, the LEDs that pin 2 power draw the 14.5V @ 60mA.

So you know.

Cabe

I am in fact using pull down resistors at 10K.

Also, the LEDs that pin 2 power draw the 14.5V @ 60mA.

So you know.

Cabe