# Homework Help: Solving for the resistance in a Voltage Regulator

1. Mar 28, 2014

### azlm

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

Create a 15V regulated power supply capable of providing a maximum load current of 25 mA.

Circuit looks like this: http://i.imgur.com/fMtscbG.png?1

ILmax (maximum load resistance) = 25mA

Izt (test current) = 20mA
Izk (knee current) = 25mA
rz = 11 Ω
Vz (nominal zener voltage at Izt)= 15V
Vs = 39V$\pm$1V
RL = 1kΩ
R = ?

2. Relevant equations

Vz = Vz0 + rz*Iz

3. The attempt at a solution

Vz=14.78 V

To find R I need to determine the total current in the circuit. I'm not sure whether it's correct to assume that the current Iz=0 when IL=25 mA. Since the load voltage is 25V when IL is at its max I thought it meant that the zener diode would be disconnected.

Last edited: Mar 28, 2014
2. Mar 28, 2014

### Simon Bridge

Compare yours with this one:

What's wrong with:
$V_{out}=V_{in}-IR$ ... you know Vout and Vin during the constant voltage operation, and you don't want the current to exceed Imax ... solve for R.

3. Mar 28, 2014

### Staff: Mentor

Hi azlm, Welcome to Physics Forums.

You want to operate a zener at or beyond its knee current in order for it to regulate at the zener voltage. That puts the minimum zener current at Izk even when the load is at maximum draw.

It's odd that you've specified a fixed load resistance since that will pin the load current to a fixed value if the voltage is regulated to 15V, making the specification of a maximum load current unworkable. Perhaps the given RL is meant to be a typical load?

Usually one wants to design for the worst case scenario and choose the voltage dropping resistance (R1 in your diagram) accordingly. Then check the operating conditions at the other extreme to make sure that all is well there too ( a bit hard to do here because the maximum zener current and/or power dissipation is not given). Can you describe the worst case regulation scenario (voltages, currents)?

4. Mar 28, 2014

### azlm

Thanks for the information.

The maximum regulator current is 76mA, power dissipation is 1W.

You're right about RL not having a fixed value. I think I might have mixed up two different problems.

Would the worst case regulation scenario be when ΔVs = -1V and Iz = Izk?

5. Mar 28, 2014

### Staff: Mentor

The worst case scenario for regulation is usually some combination of the extremes of the external factors: supply voltage and load current.