Zener Dode question

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Zener Dode question

Postby spoontex » 08 Oct 2017, 09:10

I want to build this cirucit:

http://www.r-type.org/articles/art-010g.htm

But, I don't understand how he gets the values of R2, because I can't get matched values.

If I require an output voltaje of 150V, I get five 30vf zener.

Vin=240
Vout=150
mA= 36
R2=x

R2= (240 - 150 ) /36 = 2,7K

Prs= (250 - 150 ) x 36 = 3,6mW


What I'm doing wrong?
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Re: Zener Dode question

Postby deltafred » 08 Oct 2017, 11:45

250 is the AC voltage, after you full wave rectify and smooth it that will give a DC voltage of about 250 x 1.414 = 350VDC.

The way to view this schematic is that the zener stack is there to prevent the output voltage from rising above 150V not to apply a load of 36mA to the transformer/rectifier/capacitor at all times.

First step is calculate the voltage dropped across the R2 (4.7K) resistor when supplying a current of 36mA without the zener stack in circuit.

E = IR = .036 X 4700 = 169V

So without the zeners the output voltage would be about 181VDC. They need to conduct enough current to drop a further 31 volts across the R2.

I = E/R = 31 / 4700 = .006

So when circuit is supplying 36mA to the load the zener stack will be conducting 6mA

This assumes a perfect transformer where the output voltage does not vary with load. In practice the no load voltage will be higher 5 to 10% higher than the loaded value.

Disclaimer - It is almost 50 years since I did this at college so I may have overlooked something.

If you want to get deeper into it look up Kirchhoff's Laws.

R1 is there for your safety. It discharges the capacitors once the mains is switched off. If not fitted then build yourself a 'Jesus stick' capacitor discharge tool.
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Re: Zener Dode question

Postby spoontex » 09 Oct 2017, 07:39

Great, many thanks!!

I'm building a power supplt for a tube phono preamp. This shematic is to old. Do you know other method to regulate 350vdc and get 250 dc 30mA? I was searching high voltaje regulators, but I don't find anything.
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Re: Zener Dode question

Postby deltafred » 09 Oct 2017, 10:10

Tube power supplies are not normally regulated, instead there is usually 2 or 3 series dropper resistors each with a filter capacitor to 0V after it.

If you look at a tube power amp schematic there are several dropper stages, the input stage has the most filtering because any mains hum introduced here will be amplified by each subsequent stage.

Each resistor drops DC voltage dependant upon the current flowing through it and having a capacitor to ground after it this forms a first order (6db/octave) low pass filter.

Each of these filters will roll off at 6db/octave below the corner frequency (f= 1/(2*pi*C*R) (C in farads and R in ohms See http://www.electronics-tutorials.ws/fil ... ter_2.html) which means that mains hum will be attenuated. Putting several stages in series increases the roll off by 6db/octave per stage.

This method does not waste any current but is not a regulator as the output voltage varies with the current (and mains supply voltage). Fine for preamps where the current drawn is virtually constant.

If you know the current that your preamp will draw you can calculate the total value of dropper resistor required and divide that by the number of filter stages you decide to have (2 or 3).

Or find it empirically by starting with a single high value resistor and reduce it until you have the correct supply voltage on your preamp then divide this value by 2 or 3 and replace with series resistors and add the capacitors.

Tube stages are not usually critical on supply voltage so as long as you are within a few percent you should be fine. Your HT will vary with mains supply voltage so don't try and be too precise.

The zener method is a shunt regulator, the voltage is controlled by shunting current to 0v causing the series resistor drop the excess voltage.
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