Forgive me, as I am far from an electrical engineer, but when rectifiers were explained to me I found that a rectifier allows current only in one direction, which allows it to filter out negative voltage of an AC power source and only output the positive, but where does this negative voltage go? Is it just lost in the conversion process? If so is there a circuit that can "recycle" this "lost" negative voltage?

  • 5
    \$\begingroup\$ Yes. useful search terms: "full wave rectifier", "bridge rectifier". \$\endgroup\$ – user_1818839 Apr 22 '20 at 21:05
  • 1
    \$\begingroup\$ You are most likely talking about a single diode (rectifier) half-wave rectifier. The negative voltage or current doesn't "go" anywhere, it simply doesn't flow or pass, it is not allowed to pass by the rectifier, so i's not like it's wasted and needs to be "recycled". It's just that it hasn't been put to use and the circuit after rectifier only gets to use the AC supply current half of the time or during one (positive) half of the AC cycle. You could turn the rectifier around and use only the negative cycle, but I don't want to confuse you any more. \$\endgroup\$ – Edin Fifić Apr 22 '20 at 21:21
  • \$\begingroup\$ Your title contradicts your inner question. \$\endgroup\$ – Andy aka Apr 22 '20 at 22:35

The simple case you describe is a half-wave rectifier circuit, configured for a positive DC voltage output. In the most basic sense, the rectifier diode acts as a switch, either allowing electrons to flow through it (on) or not (off). During the negative half-cycles of the AC input, there is no electron flow. It is the same as you flipping a switch on and off 60 times each second, perfectly timed for the positive half cycles.

Current (electron motion) happens only when caused by external conditions. A battery, or a wall outlet, sits there with a potential difference (voltage) between its terminals. If there is no connection between the terminals (an electric load like a light, motor or whatever), no electrons flow; there is no current.

Pedantic note: "current flow" is not correct; current is the motion (flow) of electrons. Blah blah x 10^18 electrons per second moving past a point is called 1 ampere.

  • \$\begingroup\$ Exactly what I hoped for and needed, eloquent in its simplicity. Thank you! \$\endgroup\$ – August H Apr 23 '20 at 0:11
  • \$\begingroup\$ I'm torn between editting your post directly, and commenting in the hope that you'll modify it. I'm giving you the benefit of the doubt. Bringing in electron flow to noobs is one of the largest conceptual problem we face as engineers trying to undo old wrong habits. To be pedantic, conventional current is charge flow, and in metal wires, charge is carried by electrons. We try to emphasise the use of current, because it's the simplest 'lie to children' that works well, it's more correct than you think. 'Electrons' do not work well, even it they look like they do, from your perspective. \$\endgroup\$ – Neil_UK Apr 23 '20 at 8:42
  • \$\begingroup\$ Within the rectifier, charge is carried by electrons and holes, so going to electrons too early is counterproductive. Holes are fully paid-up members of the quantum mechanical particle club, just as real as an electron, they have momentum, energy, mean free paths etc. I think it would complicate an answer about rectifiers to start on the minutiae of the charge carriers. That's why we say charge flow for noobs, and use quantum mechanics when they're ready. I know one model for holes is a 'missing electron', it's a poor model, as poor as Drude \$\endgroup\$ – Neil_UK Apr 23 '20 at 9:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.