# Zener regulated supply formula help plse

Discussion in 'Homework Help' started by stinky111, Jun 28, 2011.

1. ### stinky111 Thread Starter Member

Apr 7, 2011
39
1
Hi all,

I have a zener regulated supply circuit supplied with 16.1v (Vin), the output & Zener voltage is 8.1v (Vout).
The output load will be 90mA.
Im trying to work out what size resistor is required in this circuit?

Anyone please help me with a formula to calculate this, havent had much luck searching for one,thanks in advance.

2. ### bertus Administrator

Apr 5, 2008
18,459
3,554
Hello,

Take a look at this page from our eBook:

Bertus

3. ### stinky111 Thread Starter Member

Apr 7, 2011
39
1
Thx, Great info, but unfortunately it does not include the formula im looking for..

4. ### bertus Administrator

Apr 5, 2008
18,459
3,554
Hello,

The current through the limiting resistor must ALWAYS be higher as the load current.
You state a load current of 90 mA.
You state Vin = 16.1 Volts AND Vout = 8.1 Volts.
The voltage accross the limiting resistor is (Vin - Vout) = 16.1 - 8.1 = 8 Volts.
Choose a current higher as 90 mA and do the further calculations using Ohms law.

Bertus

5. ### stinky111 Thread Starter Member

Apr 7, 2011
39
1
Thx,

So then..R=V/I
8v/90mA = 88.88Ω???

(i know the answer for the above equation, its 81Ω but im trying to work backwards showing my workings to achieve this)

Can you see where im going wrong? thx

6. ### bertus Administrator

Apr 5, 2008
18,459
3,554
Hello,

This resistor of 88.88 Ohms is the maximum value.
With this resistor NO current will go through the zener at 90 mA, so regulation is not garanteed.
With the 81 Ohms resistor the resistor current will be 8 / 81 = 98.76 mA, so there is still 8.76 mA going through the zener.

Bertus

7. ### stinky111 Thread Starter Member

Apr 7, 2011
39
1
Wow, thx Bertus..
Last question how did you arrive at the figure 8.76 mA?

8. ### bertus Administrator

Apr 5, 2008
18,459
3,554
Hello,

The current through the zener is the current through the limiting resistor minus the current through the load.
This is also explained on the page I gave you before.

Bertus

stinky111 likes this.
9. ### stinky111 Thread Starter Member

Apr 7, 2011
39
1
Appreciate your time Bertus, Thx