# Voltage reference sources: Minimum value of series resistor

#### Christiaan

Sep 29, 2016
2
Hello I cannot find any similar question to this one of mine with this little ammount of variables in it to solve the answer and I was wondering if someone could please help me: A zender diode reference source is supplied by a 15V +- 1V DC Supply. If a Maximum Load current of 100mA must be delivered to a 120 ohm load, calculate the minimum value of the series resistor?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,728
This seems to be homework or another task you've been given. I therefore moved the thread to the appropriate forum section. You will not be given a complete solution, we'll show you how to arrive at your own solution - and we'll happy to check your solution for correctness:
1. What is the output voltage (voltage across the load) calculated from teh given parameters?
What zender diode voltage will you therefore use?
2. What is the total current through the series resistor?
You'll have to make assumptions as there is no indication given in the task description:
- ideal zener diode (0 % tolearance)
- no zener current at minimum input voltage (not a practically reasonable assumption as you'll want to have a minimum zener current in any case to have the zender diode operate in the zener region).
3. Under which condition will you require the min series resistor (Vin = max or Vin = min)? Consider the voltage drop across the series resistor in both cases.
4. From the conditions you found out so far, what resistance do you calculate?

Last edited:

#### Christiaan

Sep 29, 2016
2
This seems to be homework or another task you've been given. I therefore moved the thread to the appropriate forum section. You will not be given a complete solution, we'll show you how to arrive at your own solution - and we'll happy to check your solution for correctness:
1. What is the output voltage (voltage across the load) calculated from teh given parameters?
What zender diode voltage will you therefore use?
2. What is the total current through the series resistor?
You'll have to make assumptions as there is no indication given in the task description:
- ideal zener diode (0 % tolearance)
- no zener current at minimum input voltage (not a practically reasonable assumption as you'll want to have a minimum zener current in any case to have the zender diode operate in the zener region).
3. Under which condition will you require the min series resistor (Vin = max or Vin = min)? Consider the voltage drop across the series resistor in both cases.
4. From the conditions you found out so far, what resistance do you calculate?
Thank you for your response. I have no problem calculating Vrs and Vz. Where I hit a road block is at calculating Iz. All the examples I found in my resources give the zender power and voltage making it much easier. Thank you for your help

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,728
Assume Iz = 0 for minimum input voltage - you have been given (or you haven't shown us) other information to deduce Iz in this case.
This is the case where all current flows into the load.
For max. input voltage assume an ideal zener diode (Vz = 12 V) which keeps load current constant (since the load is fixed). From the change in input voltage at constant output voltage you get the change in current through the series resistor and therefrom Iz for this case.

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