# Linear Power Supply Design Tap Switch

Computer science & math major here.

I'm designing a linear power supply to introduce myself to electronics. To keep things cool and cheap, I am trying to limit the amount of power dissipated in the LDOs when delivering high currents (1 amp) at low voltage.

My plan is to use a relay (or maybe a pair of traics) to switch which tap of my center tapped transformer feeds my rectifier and filtering caps to reduce the voltage drop across my LDOs.

Is this a totally misguided idea and need I be concerned about transients caused by the change in current of the secondary coil of my transformer when I switch from the center tap (12 VAC) to the outer tap (24 VAC)?

I've read some articles and done some modeling of the behavior of a transformer feeding into a full-wave bridge rectifier and then into filtering cap as a pre regulator for my LDO (image not mine).

Because, as I understand it, kickback transients are proportional to the current flowing across the inductor; I've started to design an "interrupter" using an opto (to detect 0 points of the AC wave) and a couple of logic gates to force the transition to happen when the capacitors are supplying current. Is this enough precaution to protect my puny circuit from high voltage transformer transients?

Wolfram alpha output of "interrupter" logic also for those interested. Where U is high when user needs to use the outer tap, O is the opto (high when AC is near 0v), and C is high when the outer tap is in use.

[edit 1] changed a word.

• I have some bench power supplies that do add/remove transformer windings, however not in such big steps. Commented May 24, 2017 at 20:51
• @PlasmaHH - how do they change taps under load? Commented May 24, 2017 at 20:53
• I have never looked, but likely in a make-before-break fashion Commented May 24, 2017 at 21:03
• @PlasmaHH What's the difference between changing taps under load and switching the full load on or off? Those are both large current changes. Commented May 24, 2017 at 21:47
• @PaulUszak: but not so large impedance changes. When you switch the load off, you have a huge voltage spike due to EMF because the current can go nowhere. If you switch in/out some winding you still have a good path for the current to go. This is btw. also done at power distribution substations when the voltage changes outside the limits they switch in/out some windings. Google image for "tap changer" Commented May 24, 2017 at 21:52