The item below is one of two components of an old (c 1950?) electric chisel. (The other part is a reciprocating solenoid.) The name tag on this part says "Electric Converter", with a 6 amp rating. Inside the case are 10 thin disks, separated by plastic-looking spacers.

The output socket has 4 pins: frame ground; 'neutral' (connects to one AC input line); and two which (unloaded) each measure 120 Vac relative to the 'neutral'. The latter two do not show continuity to any other pins, or to each other. No DC, at least on my DVM.

I'm wondering if this is a dual half-wave selenium rectifier (Wikipedia). However, if the 10 disks are the "plates" described there, the 20v reverse voltage per plate rating seems marginal.


  • Any idea what it is?
  • If it is a rectifier, it's broken, yes? (AC on the output)
  • If it is a selenium rectifier, is the 20v per plate Vrev not a problem?
  • If it is selenium, would that material's properties (e.g., current-limiting (per wikipedia)) be advantageous in this application (large switching inductive load), compared to a diode-based rectifier with a similar power rating?

EDIT: The output cord from this device feeds the solenoid section of this tool. The nameplate on that says 120 V, 60 cy [hz]. So, not a rectifier.

Size: 6" long, 4" diameter

case - 6x4"

Disks & spacers: disks & spacers

  • \$\begingroup\$ There was a time when copper oxide rectifiers were also in use and they were superficially similar and also lossy with large heat dissipation plates. - en.wikipedia.org/wiki/Metal_rectifier It may be possible to replace it with a compact silicon version if the rest of the circuitry can handle the improved characteristics. \$\endgroup\$
    – KalleMP
    Oct 25, 2018 at 7:08

3 Answers 3


Any idea what it is?

Your guess of a selenium rectifier seems likely. Based on your analysis, it may have failed. This is not surprising; selenium rectifiers were rather fragile and prone to failure already, and using one in a high-vibration environment (like an electric chisel!) would have placed it under a lot of stress.

This device probably used a selenium rectifier because silicon rectifiers did not exist at the time it was constructed, or were not large enough to use in this application. If a similar device were to be built today, it would use a silicon diode -- there is no advantage to the use of a selenium rectifier.

Selenium is rather toxic. If you have touched the disks inside this device, wash your hands thoroughly. Do not place this device in the trash; please bring it to a local hazardous waste center for disposal.

  • \$\begingroup\$ Failed selenium rectifiers have a characteristic rotten egg smell, caused, I believe by (rather toxic) hydrogen selenide gas. \$\endgroup\$ Oct 31, 2018 at 11:43
  • \$\begingroup\$ @SpehroPefhany How long would that smell persist, though? Selenides stink to high heaven, but if it's been on the shelf for a couple of decades it might have dissipated. \$\endgroup\$
    – user39382
    Oct 31, 2018 at 16:32
  • \$\begingroup\$ Sure, I think it would only last days or weeks. But he's apparently powering it so it might make a new smell. \$\endgroup\$ Oct 31, 2018 at 16:58

A selenium surge suppressor has construction and appearance similar to a selenium rectifier. It is possible that the pictured component is a surge suppressor. If you could trace the wiring and draw a schematic diagram, you could probably determine which it is.

  • \$\begingroup\$ Per new info (edit, above), it seems not to be a rectifier. Wiring: one AC input line is wired straight through to the output plug. ('neutral', so to speak.) The other goes to the center of the disk stack. \$\endgroup\$
    – George
    Oct 25, 2018 at 14:13
  • \$\begingroup\$ ... Also, the outside ends of the disk stack are wired to the hot contacts on the output plug. So, it looks like 1/2 the stack (5 disks) is in series with each output feed path. \$\endgroup\$
    – George
    Oct 25, 2018 at 18:22

It is a rectifier, and appears to work. The 120 V line input connects to the center of the disk stack, and the outputs are connected to its two ends. When loaded (100 W bulbs), one output measures 37 Vdc (relative to AC neutral); the other measures -37 Vdc.

I don't know for certain that it's selenium, but I think that's the most likely, given its design and vintage.


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