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Help With Circuit

Hello everyone,

I have designed this circuit, a voltage regulator at 30ma. It uses zener diodes to regulate the voltage and resistors to lower Amperage to 30ma.

Now I know it might sound stupid to lower the voltage this way, but I am new to this and I wanted to start with something like this.

The PIC16F877 receives signals from the buttons. Each time you press one of the buttons the pin that is high changes, activating a diferent transistor and so making 12v flow through a diferent zener.

Before etching this I would like to know what do you think about it, if there is anything wrong if something could be made diferent.

(I have checked the transistor part, but not the seven segment display, since the code isnt finished yet. )

Finally am I in need of a crystal or oscillator here? Because of the seven segments? What would the value be then.



Green box= Zener diode

Orange Box= resistor

Blue box= 7 segment display

Pinkish Box= jumper wire


VoltageSel30ma.jpg269.88 KB

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A schematic shouldn`t need an entire legend to be legible. Surely your drawing program has actual part symbols?

From what I can tell you are simply running current through the zeners to somewhere else. A zener diode doesn`t "output" a regulated voltage like you might think. Zener diodes allow excess voltage to flow through them, essentially removing anything above their rated voltage.

To use them as a regulator make a voltage divider with a current limit resistor on the high side and the zener on the low side (anode towards GND). Apply 12v to the top of the resistor and the middle tap provides your voltage output.

Thanks, I knew I was doing something wrong with the diodes, because I remmember having the voltage lowered with them but now i couldn´t...

So I need to modify the pcb, and by the way, how do I keep the resistor from getting hot? Or am I doing it wrong?

They will get hot no matter what if you are burning power on them. Your only choice is to heat sink them. Burning power is a bad way to regulate though. There are regulator IC's with 3 pins that will provide different outputs and they regulate voltage much more efficiently. The reason you don't want 90 degrees in your PCB is due to back-flow EMF, perpendicular traces will induce magnetic currents (eddy currents I think they are). This will make your lines more noisy and is bad practice. Use 45 degree angles or curves.

I am no PCB expert, not even close actually. I have heard however, that 90 degree corners are a no-no. I think it has something to do with EMF's or noise in general. You can save a lot of space too with the corners chopped off with 45's.