Zener diode + H-Bridge + Full wave rectifier = ...voltage switcher?

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.

H-Bridge Controlled Zener Regulator

How?#

I’ve omitted the flyback diodes that usually run parallel to the transistors Q1-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 D1.

For high output - i.e. Vin - both Q2 and Q3 must be saturated, ensuring that the zener diode is forward biased. Whilst for regulated output - i.e Vz - both Q1 and 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.

Usage Notes#

  1. The value of the current limiting resistor (R1) protecting the zener needs to be calculated with the output load in mind. (electronics.stackexchange.com )
  2. A fuse should always be utilised on Vin due to the potential for a short-circuit to occur if the incorrect transistor pairs are saturated.
  3. 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.

Why?#

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:

A more sensible solution

  1. 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. 🤷 ↩︎

Disclaimer: There's a lot of poor - bordering on dangerous - information online with regards to electronics. From infuriating schematics where the maths doesn't add up, to dangerous interfaces that risk component damage or even hazards like shocks and fires. And I'd hate to add to this mess.

That's why this disclaimer is on all my posts tagged "electronics"; although there's really little in the way of "original" or "novel" ideas, I can be more than a little absent-minded! Please email me if you have any concerns with the technicalities of what I've written, and I'll ensure it's updated in a timely manner.