Zener Voltage Regulator

Discussion in 'General Electronics Chat' started by Sharpiedeluxe, Feb 8, 2010.

1. SharpiedeluxeNew Member

Joined:
Feb 8, 2010
Messages:
1
Likes:
0
Hi, I'm really new to diodes, and I'm trying to solve a circuit problem creating a voltage regulator that can operate given four extreme conditions. As far as I have calculated, not all of the conditions can be met at the same time, but I'm not quite sure so that's why I'm asking! I am using a 1N4735 zener diode, with a Zener Voltage of 6.2V, the Pz is 1.0W and I need to find a resistance value given that Vin(min) is 10V and Vin(max) is 14V. the min Load amperage is zero, and the maximum is 70mA.

http://img8.imageshack.us/img8/1698/82118367.png

If you could tell me if I'm doing this right that'd be great.

I calculated the max Amperage based off of the Pz to be
Pz = Vz*Iz , so 1.0W = (6.2V)*Iz , Iz = 161mA

If I was to find the worst case scenario to deal with Power dissipation and stay below 1.0W,
I believe that having 14Vin and 0mA load would require 7.8V drop over a 48ohm resistor
in order to keep the zener voltage. This I assume would be the maximum that R could be.

However in order to keep the Zener Voltage, then the scenario in which there is Vin = 10V and
70mA current on the load means, that 91mA goes through the diode and the V drop over the resistor
would be 3.8V meaning the smallest R could be is 42ohms.

Therefore, I think if I design a circuit in which the Resistance is between 42 and 48 ohms I should be
able to meet the conditions required.

Does this information seem correct or am I missing something critical? Any help would be appreciated.
Thanks so much!

2. Diver300Well-Known Member

Joined:
Oct 19, 2007
Messages:
3,092
Likes:
102
Location:
England
Where Vin = 10 V and there is a 70 mA load, you want basically zero zener current. That gives your maximum resistance as (10 - 6.2) / 0.07 = 54 Ohms. Any more resistance and there isn't enough current at 10 V to supply the load.

The 91 mA is just wrong.

You calculated 48 Ohms correctly, but that is the minimum resistance. Any less and there is too much heat dissipated in the zener at 14V and no load.

So you need between 48 and 54 Ohm.

In practice, that is cutting things a bit fine. You have little spare current at 10 V and you are running the zener at the limit of it rating.

Joined:
Mar 17, 2009
Messages:
11,139
Likes:
566
Location:
AZ 86334
Is this for a practical circuit or is it homework?

Joined:
Jan 12, 1997
Messages:
-
Likes:
0

5. GrosselMember

Joined:
Sep 16, 2008
Messages:
925
Likes:
22
Location:
Norway

I would recomend you to search for "constant current source" on the web. You'll get lower power dissipation and a lot more stable system. Also do a search for "voltage follower" on the web becayse you'll need that.