# Need help on a linear regulator

#### BlackMelon

Joined Mar 19, 2015
170
Hello,

I am trying to derive an equation for this regulator, so I can put them on the excel file. Please refer to the attached image.
On the case that D1 is on (zener breakdown mode), and Q1 is forward-active, I derived the equation of iBB and found it strange.

From the equation, as RL goes higher, iBB becomes lower. I have simulated this circuit on LTSpice and got the result the other way around.
However, I checked the equation I derived step-by-step, and found nothing wrong.

Best Regards,
BlackMelon

#### Attachments

• 156.8 KB Views: 25
• 362.9 KB Views: 25
• 144.4 KB Views: 22

#### crutschow

Joined Mar 14, 2008
34,841
Where is causing Rin?

#### BlackMelon

Joined Mar 19, 2015
170
Rin is in series with Vin. Please see the circuit.

#### ericgibbs

Joined Jan 29, 2010
19,102
hi BM,
E

#### BlackMelon

Joined Mar 19, 2015
170
hi BM,
E
Here is the .asc file.

#### Attachments

• 1.1 KB Views: 2

#### ericgibbs

Joined Jan 29, 2010
19,102
hi BM,
First question would be why have you chosen such high values for the resistors.?
The Zener current is in the pA range and the Load current uA,

Rin serves no purpose

E
Update:
You are driving a 13V zener with a 12V source voltage, how is that going to work.?

#### ericgibbs

Joined Jan 29, 2010
19,102
hi BM,
Run this version of your circuit.
E

#### Attachments

• 986 bytes Views: 0

#### BlackMelon

Joined Mar 19, 2015
170
Oh Dear!.... This is quite embarrassing but I think I know it. I put +12V at the input voltage, while I use 13V zener diode.
Ps. I have just also simulated at R1 = 1k, Rin = 10 ohm, RL = 10 to 30 ohms at get the same relationship as the equation (and your curve). Thank you very very much!!!! ^_^

#### BlackMelon

Joined Mar 19, 2015
170
And next time I really have to check the biases current. I mean current through the diodes and the bases, see if they match the curves on the datasheet and be able to turn the device on. May be last night I was too drunk. = ='

#### ericgibbs

Joined Jan 29, 2010
19,102
hi.
OK, so derive your regulator equations, see how they match with your simulation, let's know the result.
E

#### crutschow

Joined Mar 14, 2008
34,841
Rin is in series with Vin. Please see the circuit.
I see Rin.
My question was, what is its purpose, why is it there?

#### Irving

Joined Jan 30, 2016
4,049
Rin serves no purpose
Actually I think it does. It represents the real world output impedance of the supply; eg the internal resistance of a battery or the windings of a generator, or even the dynamic impedance of a power supply (ΔV/ΔI). Though 10ohm is much higher than reality - .01 - 0.1ohm is more realistic.

#### ericgibbs

Joined Jan 29, 2010
19,102
hi guys,
In the TS's own words...
May be last night I was too drunk. = ='

E