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Desing a regulated voltage source with a transistor and a zener

Annie Fernández

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Hi. I have a project for University, i need to desing a regulated voltaje source with a transistor and a zener diode. The only parameters I have es that the input voltage must be 120[Vrms] at 60Hz and an output voltage of 6[V]DC with a maximum current of 300[mA] 

Could you help me with the desing??

This is the circuit model:



WhatsApp Image 2021-02-27 at 22.03.43.jpeg

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Part one:

Starting on the right: the load resistor minimum would be Ro = Vout/Iout. As the transistor is used as an emitter follower the output will be near a voltage gain of one.

For the Zener diode and Vout of 6.0v you must allow for the voltage drop Vbe (Vb to Ve) of 0.6 to 0.7 volts.  So Vz = Vout + 0.7 volts or a Zener with a voltage at or above wanted Vz.

R1 supplies current to the base of the transistor and the Zener. The current input into the base of the transistor can be found using the hFE (the forward current transfer ratio) or current gain of the transistor. Ib * hFE  = Io. hFE is in the order of 100 or so. So Ib = Io / hFE; thus Ib adds little to the current through R1.

You can calculate the minimum value of R1 given the wattage of the Zener. Iz(max) = watts/Vz. A zener for this type of regulator could be 500 milliwatts. Then R1(min) = (Vin - Vz)/Iz the voltage drop across the resistor divide by the current through it.

But what is Vin?  Using my rule that the input voltage is 1.5 volts times the Vo or better. Picking a 8 volt transformer (for now). That gives a Vin of no more than 8 * 1.4 or 11.2 volts as the peak voltage.

What is there a maximum value for R1?

to be continued...

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New section:

For R1(max). Assuming a 8 volt transformer. Vin = 8.0  * 1.4 or 11.2

Taking the min and max to be plus or minus 10%. One would not need a voltage regulator if there where nothing to regulate.  

Vmin = (Vin - 10%*Vin) - (2 * voltage drop across the two diodes). Typically 0.7v each.

And  Vmax =  Vin + 10%Vin - (2 * voltage drop).

R1max = (Vmin - Vz)/(Izmin + Ib)  Izmin is found from the zener data sheet; typically 10 ma.

What happens at Vmax?  R1max = (Vmax - Vz)/(Izmax + Ib)   Where Izmax is the only unknown here. Izmax must be within  the maximum current for the Zener. If that works out okay use R1max.

For C1; using the equation from Scher & Monk's "Practical Electronics for Inventors" section 11.6.

Vripple(rms) = 0.0024s * Il /Cf  

Where Il is the output current and the current through Iz plus Ib.
Taking the Vripple  = 100 mv or 0.10vrms = 0.0024 * Il / Cf  Remember Cf is in farads and Il in amperes. You may expect Cf in the order of 1000uf.   Use whatever Vripple you wish. Digital ICs  need less than 0.25% * Vout but the zener does regulation also.

If you wish I can run whatever values you come up with through the circuit simulator if you do not access to one.


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