# Voltage regulator for a AC-DC PSU

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#### dennis@

##### New Member
Hi

We have to build a AC to DC PSU on friday, using the following schematics

The AC input is 15 V 50Hz, and the output must be: 9V 100mA

I need help with these problems:

1) Dimension of the Capacitor.
2) What voltage should the zenierdiode be on
3 How big should R1 be
4) Effekt lost in Q1

1) Now for the capacitor i found the 10% ripple is fine (Power Supplies)
But if i want 5% is this the right way to do it:
Vc=Vs*(1-e^(-t/RC))

15*5% = 15*(1-e^(-0.01sec/x) that gives x=0,00244 F The 0.01 sec is from the 50Hz AC to 100Hz DC.

2) Q1 needs 0.7 volts to work, so the zenier should be able to handle output + VQ1

Thats 9v + 0.7V = 9.7V

Is this right?

3) R1 = (15V - 9.7)/(Izenier + IQ1) Or is it 15V - (9.7 + 0.7V)/(Izenier + IQ1) 0.7V for the zenier?

4) Im lost on this one :S

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• PSU.JPG
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#### crutschow

##### Well-Known Member
The filter capacitor discharge is not an RC exponential. Because of the regulator, the load current (and thus the filter capacitor current) is a constant i = Vcc/Rload. Thus the capacitor ripple voltage is approximately V = (i*t)/C where t = 10ms.

The zener voltage should be the desired output voltage plus the transistor base emitter voltage of 0.7V or 9.7V as you stated.

R1 must supply the minimum desired zener current, plus the transistor base current required for the maximum output current with the minimum transistor gain (beta or Hfe) .

#### dennis@

##### New Member
Hi crutschow Thank you for you reply.

Iknew i messed up with the capacitor

To calculate R1 do i have to consider the 0.7 Volts the diode takes or is it just 15-9.7?

#### crutschow

##### Well-Known Member
To calculate R1 do i have to consider the 0.7 Volts the diode takes or is it just 15-9.7?
The DC voltage on the capacitor is more than 15V, which is the RMS AC voltage. The peak AC voltage at the capacitor is about 1.4*15 minus two bridge diode's forward drop. For worst-case you would want to use a voltage slightly below the minimum voltage at the bottom of the ripple as determined by the load current and the capacitor size.

Edit: If you simulate the circuit with simulator such as LTspice (free download) you can see how your circuit works before you build it.

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