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I'm making a simple 555 timer circuit to drive a flyback transformer. The 555 circuit controls an N-Channel mosfet which is what drives the flyback. I have an old receiver mains transformer that I wish to use to power the entire device. It has multiple isolated secondary taps. One of these taps is 12v and the other is a 50v supply.

I want the 12v supply to drive the 555 circuitry and the 50v supply to drive the flyback. I was going to use two full wave rectifiers, one for each supply. But then I figured without some sort of common the mosfet wouldn't work (no voltage potential right?). So can I simply connect the two taps in parallel or series and then that would work fine? I've read other places that this is acceptable to do but I have to then do half wave rectifiers as a full wave causes issues. Is this true? How would this impact the ripple on my rectified DC supplies?

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With isolated secondary windings, your transformer should easily support making two voltage rails. Assuming that they are simple secondaries as I attempted to draw in the circuit editor, two rectifiers should work as diagrammed. Note that the voltages are just for reference, you won't see that exact voltage in practice.

The two diode bridges are used to create the desired common ground by connecting the negative DC output of the bridges together. This prevents the two secondary windings from being directly connected together. The bypass capacitors shown as C1 and C2 are also connected to this common ground.

schematic

simulate this circuit – Schematic created using CircuitLab

You do not want to put the secondaries in parallel or series. It would be possible to connect the 'bottom' leg of each secondary together and then use a single diode for each supply (as you pointed out), but then you would have significantly higher ripple for the capacitance you are using.

As a side note, since the windings are isolated and only connected through the diode bridges, it is simple to convert to, for example, +50V and -12V by reversing the diode directions D5-D8.

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  • \$\begingroup\$ Thanks for the speedy reply! So two isolated power rails will work fine? The MOSFET will work despite them being isolated? \$\endgroup\$ – Manmax75 Jul 19 '16 at 12:12
  • \$\begingroup\$ No, the circuits are not isolated. Notice how both diode bridges and capacitors use the same ground. Same ground = wire between all of them. \$\endgroup\$ – user2943160 Jul 19 '16 at 13:16
  • \$\begingroup\$ I have added additional text to clarify this in my answer. \$\endgroup\$ – user2943160 Jul 19 '16 at 13:20
  • \$\begingroup\$ Thanks for the clarification! On a final note, when picking capacitor values for C1/C2 what is an acceptable ripple voltage for the 555-IC? \$\endgroup\$ – Manmax75 Jul 20 '16 at 5:02
  • \$\begingroup\$ @Manmax75 sounds like a good topic for a new question. \$\endgroup\$ – user2943160 Jul 20 '16 at 5:30

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