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# zener diode voltage regulator

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#### fm101

##### Member level 5
I got the following voltage regulator circuit but the output does not look regulated voltage.

Pretty much everything here is wrong.

Maybe if you applied a DC voltage instead of an AC voltage your circuit would work. It's a good thing you didn't actually build this circuit because chances are it would burst into flames.

You're applying 120V to a transistor that has a maximum voltage of 40V.

You're dissipating 10W across that 1K resistor.

It's a voltage regulator, not a rectifier.

Hi,

What do you expect?

My opinion:
The test makes no sense.
* either you don't know how it is meant to work
* or you don't know how to test it

Before you do the same test on a real circuit, please call the fire department!
Tell us about your idea.

Klaus

Added: Barry was faster ;-)

Very funny - that regulator is for DC volts in to a lower DC voltage out

if you want to run on AC, preceed the regulator with a bridge rectifier and 1000uF

you're welcome ...

unregulated DC to regulated DC converter? how to make unregulated dc supply input signal to check this circuit, i need some example of unregulated dc supply. also how to calculate the capacitor value?

added:
1. this part of the circuit comes after the transformer, diode ring and capacitor?
2.unregulated dc means fluctuating dc signal?

Last edited:

Hi,

unregulated DC...
But your test setup is far from "unregulated DC". Instead it´s the opposite. It´s pure AC, no DC at all.

If you want DC input, then also test it with DC input.

*****
how to calculate the capacitor value?
There is only one capacitor and it already has a value: 10uF. What do you want to calculate?

*****
1. this part of the circuit comes after the transformer, diode ring and capacitor?
You ask us? We don´t know what your idea is, thus it´s rather on us to ask about what you want to achieve.

2.unregulated dc means fluctuating dc signal?
It means DC , but the voltage level is not stable.

Klaus

Hi,

But your test setup is far from "unregulated DC". Instead it´s the opposite. It´s pure AC, no DC at all.

If you want DC input, then also test it with DC input.

*****

There is only one capacitor and it already has a value: 10uF. What do you want to calculate?

*****

You ask us? We don´t know what your idea is, thus it´s rather on us to ask about what you want to achieve.

It means DC , but the voltage level is not stable.

Klaus
i am just learning things here, i found this circuit diagram and wanted to know what it does. that is i don't have any aim to build it for anything/projects like that just to know what is it. the capacitor value i just used randomly without knowing how to actually calculate it.

Hi,

O.K. I understand.

I personally don´tthink its a good way to learn things.

It´s like you find a screw in a box and want to find out what all you can do with it.

****
I recommend to learn electronics step by step.
What resistor does and hwo to calculate, diode, capacitor, BJT...

There are plenty of written tutorials as well as more than enough tutorial videos.

Klaus

Hi,

1) Yes, for your purpose here.

2) Unregulated DC would be e.g. a steady-ish 5V that goes from e.g. 4.9V to 5.1V, what you see and use as the input voltage to the voltage regulator after the bridge rectifier and smoothing capacitors you called diode ring, etc.

Read about how to calculate smoothing, or bulk, or filtering, capacitance in power supplies, it will help, and that in voltage regulator datasheets there is a useful to know parameter called line rejection - how much input ripple a regulator can handle and with regard to frequency before it can't keep up any more and regulate the output.

I'm sure you know that Zener + BJT regulators are fossilized circuits and (very) bad due to various weaknesses Zener and BJT each have, plus lack of real Error Amplifier feedback, but useful as introductory learning concepts and tools. Have fun!

Pretty much everything here is wrong.

Maybe if you applied a DC voltage instead of an AC voltage your circuit would work. It's a good thing you didn't actually build this circuit because chances are it would burst into flames.

You're applying 120V to a transistor that has a maximum voltage of 40V.

You're dissipating 10W across that 1K resistor.

It's a voltage regulator, not a rectifier.
correct me if i wrong but the 1N5242B has max zener voltage of 12.6V

Are you expecting that the zener diode reduces the transistor voltage? It doesn't, the transistor has to be still rated for the maximal input voltage. Also power dissipation of all components must be observed.
--- Updated ---

A strange detail of the post #1 circuit hasn't been addressed yet. How does it achieve -110V output across two forward biased diode junctions (transistor CB junction and Z-diode)? The simulated input current is probably several 100A.
--- Updated ---

The simulator can measure it without burning your face.

Last edited:
Are you expecting that the zener diode reduces the transistor voltage? It doesn't, the transistor has to be still rated for the maximal input voltage. Also power dissipation of all components must be observed.
--- Updated ---

A strange detail of the post #1 circuit hasn't been addressed yet. How does it achieve -110V output across two forward biased diode junctions (transistor CB junction and Z-diode)? The simulated input current is probably several 100A.
--- Updated ---

The simulator can measure it without burning your face.
i know from replies that the input signal was wrong, but i am asking whether the diode maximum voltage is 12.6V or not

i know from replies that the input signal was wrong, but i am asking whether the diode maximum voltage is 12.6V or not
do you know how to do an internet search?

- Tau = RC = 1k * 10uF = 10ms (*5 = 50ms)
- 1 second/60 Hz = 16ms OR 1 second/50 Hz = 20ms
- 1N5242B = response time? I guess it turns on fast enough. The datasheet I just looked at said nothing of capacitance or turn-on time, no graphs, nothing useful except for VZener nominal of 12V ranging from 11.2V to 12.8V.

I mentioned time because I think that an overvoltage at Vout may slip past the Zener whilst it is turning on, probably not, and doubt that matters in your application.

This might be useful to you, attached.

Regards, Dana.

#### Attachments

• 1991_Motorola_TVS_Zener_Device_Data.pdf
26.7 MB · Views: 101

Also, 2N3904 is a TUN, if Vbe max. is 6V, why the 12V Zener rather than a 5.6V Zener? Vcb is max. 60V, as well. Absolute maximum ratings and recommended operating conditions. Datasheets are very helpful. Normal is: idea > schematic > datasheets > simulation and/or prototype > final version of circuit. Once the headache of gradually understanding their hieroglyphics and strange terminology is gone, datasheets are as important as the component.

Also, 2N3904 is a TUN, if Vbe max. is 6V, why the 12V Zener rather than a 5.6V Zener? Vcb is max. 60V, as well. Absolute maximum ratings and recommended operating conditions. Datasheets are very helpful. Normal is: idea > schematic > datasheets > simulation and/or prototype > final version of circuit. Once the headache of gradually understanding their hieroglyphics and strange terminology is gone, datasheets are as important as the component.
TUN??
The 12V zener because the intended output voltage is 12V, not 5.6V. (-Vbe, approximately 0.6V).

TUN??
The 12V zener because the intended output voltage is 12V, not 5.6V. (-Vbe, approximately 0.6V).
Hi Barry,

Thanks, I'd realised that point. I just thought it might be helpful to fm101 to understand that - as FvM briefly said - every part, including a Transistor Universal NPN (TUN), has its maximum ratings, and 12V is double the 6V maximum Vbe for BJTs.

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