Time for another half-baked circuit idea - thoroughly untried, untested, and unbuilt as per usual! Drawing from the the classic ‘H Bridge’, and utilising a full-wave rectifier to maintain polarity, this little schematic demonstrates a rather convulated way of switching between an input voltage and one regulated by a zener diode.
There’s really no reason why it would ever be a good idea (see “Why” below), alas it’s worth a share as a fleeting thought and something to tinker with1.
I’ve omitted the flyback diodes that usually run parallel to the transistors
Q4 in a traditional H-Bridge - a decision that may not be wise if you were to power an inductive load (i.e. a motor) from the output. Instead, there’s a full wave rectifier at the center of the bridge - providing outputs with consistent polarities - clamped to either side of Zener
For high output - i.e.
in - both
Q3 must be saturated, ensuring that the zener diode is forward biased. Whilst for regulated output - i.e
z - both
Q4 need to be in saturation. The above schematic also demonstrates one method (sans resistors on the transistor bases) to control these PNP/NPN pairs via a single control line.
- The value of the current limiting resistor (
R1) protecting the zener needs to be calculated with the output load in mind. (electronics.stackexchange.com )
- A fuse should always be utilised on
indue to the potential for a short-circuit to occur if the incorrect transistor pairs are saturated.
- The capacitor (
C1) across the output lines is there resist ripples in the output voltage, this may be omitted if switching latency is a concern. Higher capacitance will reduce voltage ripples, but will also introduce higher latency.
Honestly, there’s no real reason to do this… that I can think of anyway. A far more elegant - and cheaper - solution is to use a simple SPDT analog switch - switching between two different power lines. For example:
I find tinkering with existing/known circuits to be quite good as a learning exercise, and although the H-Bridge is usually used for DC motor control, and full-wave rectification is typically used for AC-DC rectification, in this particular case I have no real use-case for a DC motor, and have no desire to work with AC… so YOLO. 🤷 ↩︎