# Homework Help: Power dissipated in R (zener diode circuit)

1. Aug 17, 2010

### TsAmE

1. The problem statement, all variables and given/known data

A zener diode runs off 10V and supplies 5V to a load that draws 100mA. The minumum zener current = 5mA. R = 47Ω What is the dissipation in the resistor if the output is short-circuited?

2. Relevant equations

None.

3. The attempt at a solution

Using R = V - Vz / Izmin + Iload:

I said Iload = 0 (since the output is short-circuited) then:

R = V - Vz / Izmin + 0
= 10 - 5 / 0.005
= 1000Ω

Power of R = (V - Vz)^2 / R
= (10 - 5)^2 / 1000
= 0.025W

But the correct answer was 2.13W. Could someone please tell me what I did wrong?

2. Aug 17, 2010

### skeptic2

I see two mistakes offhand. The first is you are confusing load with resistance. A short circuit may have 0 ohms of resistance which is an infinite load (theoretically).

The second mistake is that if the load voltage drops below 5 V, the zener stops conducting and is out of the picture.

The problem is much simpler than it seems. Draw it out and see if that helps.