Mofman said:

Hi All,

I am an electronics newbie trying to understand how to calculate power

dissipation/voltage drops/current for simple circuits involving

semiconductor components.

Calculating these values is easy if only "passive components" are involved.

However, as soon as you introduce diodes or transistors, I start having

trouble and I have reached the limits of my research (although not much).

So does everyone else, in certain cases. The nonlinear math can be a

bear. This is one of the reasons circuit simulators were created.

I have the following questions:

How do you calculate the voltage drop across a diode?

There is a logarithmic function that describes the typical current to

voltage relationship, but most diodes drop about 6. to 1 volt in their

normal operating range. To see how a particular diode will act, it is

a good idea to check the data sheet. This usually contains a graph of

voltage drop versus current with three curves for three different

temperatures, room temperature, very cold and very hot, to give you an

idea of the range.

How do you calculate the base-emitter voltage drop for a transistor? What

about the collector-emitter voltage drop?

The base to emitter voltage is a diode junction, so the generalities

above apply. Again, the data sheet often has a lot of detail on this,

including how it changes when the transistor saturates.

If you want to wade in to the math, here is a paper that compared two

models of the voltage to current relationship. Don't be put off by

the greek letters or the fact that they use U for voltage.

http://www.comppub.com/publications/MSM/2000/pdf/T54.04.pdf
The collector to emitter voltage is totally dependent on the base bias

current and the collector load, as well as the current gain and other

details of the particular transistor, so there can be no simple rule

to guess this voltage. You will have to learn a lot more about how

transistors actually work to make good estimates of this.

But the one rule that always works is that the power going into a

device in any given instant is the product of the current through it

times the voltage across it. This includes the case where the current

is going opposite the way the voltage is trying to push it, except,

there the power is negative (applies to batteries and other energy

storage components).