School circuit

[JohnBM]

Hello

I'm stuck with a circuit i just can't seem to put into math... It's this power supply:

**broken link removed**

It should supply a VAB of 7V (as constant as possible) and have a I at R2 of 15mA.

Vi = 220cos(100PI*t)

I'm supposed to come up with optimized values to the other components.

Can anyone tell me how i should do it or where i should begin?

Thank you

 

[RODALCO]

Do your calcs for C1 first and allowe for a current flow of about 25 mA into the circuit.
R1 is to limit the inrush current into C1 and blowing up the zener downstream.

The zener DZ takes about 10 mA's to work properly so 15 mA is left over for the load R2.
It's nearest value will be 6V8 or 7V1 (6V8 + 0.3)

C2 will reduce the ripple at the output.

 

[DigiTan]

Well, here's one idea. Diodes D1-D4 create a full-wave rectifier, meaning for the most part--only two diodes are conducting ever at any given moment. IE: When D1 and D4 are on, D2 and D3 are off and can be ignored.

If you take that, plus the fact that the rectifier's output is roughly ABS( 220cos(2π*50*t)), you could redraw the circuit this way:

**broken link removed**

Looking a the largest circuit mesh:
Vi = Vd4 + Vr1 + Vc1 + Vdz + Vd1

RODALCO can help you set R1 and C1.

There are still 2 more circuit loops that need to be accounted for (between DZ, R2 and C2), but since the Zener diode is basically being used as a voltage regulator, we could assume that Vdz never exceeds DZ's zener voltage (Vzener). So if we ignore C2 briefly, the voltage on the zener will resemble a clipped sinusoid...

if: Vi - Vd4 - Vr1 - Vc1 - Vd1 < Vzener (condition A)
then: Vdz = Vi - Vd4 - Vr1 - Vc1
otherwise: Vdz = Vzener

To choose the value of C2 that works with R2, you have to a choose an RC time constant (R2*C2 = X seconds) that exceeds the amount of time "condition A" above is satified. Looking at it that way, I don't think there's any upper limit to a good C2 value, just the lower limit that's imposed by condition A.

That's just one possibility.