Designing a power supply with a center tapped-transformer and a zener diode.

[kaichie]

Homework Statement

Design a full-wave regulated power supply using a 5:1 center-tapped transformer and a 7.5 V, 1 W Zener diode. The power supply must provide a constant 7.5 V to a load varying from 120 to 450 . The input voltage is 120 V(rms), 60HZ.

Homework Equations

The Attempt at a Solution

http://s1233.photobucket.com/albums/ff382/chie8/?action=view&current=asd.png
The oscilloscope shows no voltage drop across the 7.5V zener diode.

 

[gneill]

I think you'll want to rectify the secondary current -- a diode in each of the two outer legs of the transformer secondary to provide full wave rectification. Also, you should stick a load resistance on the circuit output where you have your 'scope' in order to simulate the desired load. Maybe start with a maximum load (so the current drawn would be 450 mA if the supply provided 7.5V DC. Then take a look at your output and see what else you might need to do.

 

[rude man]

Your diagram unfortunately shows the xfmr secondary being short-circuited! Lotsa smoke, maybe. Not good smoke like pipe tobacco, either.

As gneill says, put diode in each secondary leg (not the center-tap). Cathodes connected together, anodes to each secondary high (pointing to the right). Then, the 100 ohm CT resistor is too big - at max output current = 7.5V/120 = 62.5 mA, the CT voltage is already at - 6.25V so each secondary high winding can't go above -6.25 + 115/(2*5)V. Moreover, these voltages are squirrely - combination of sine and chopped-off full-wave rectified sine.

I would ground the CT and move R to between the diode cathodes and your zener/resistive load. To avoid overheating the zener, 7.5*(11.5-7.5)/R = 1W. This limits the zener to 1W peaks in the no-load condition.

Unfortunately, your output voltage will not be constant; the zener will not always be turned on. To achieve that goal, you need a capacitor shunting your load. I would guess RC = 8ms with worst-case R = 120 so C = 68uF. This would be a polarized cap so make sure you get the + end on the output & the - end to ground, not vice-versa.