What would be the circuit equivalent be of a uniselector, aka rotary relay, aka stepping relay?

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Uniselectors were commonly used for phone switch boards (and thus why we used to hear individual pulses). The uniselector:

  • has high-voltage galvanic isolation between inputs (easily hundreds if not thousands of volts)
  • carries bi-directional, high current at low resistance
  • incrementally steps between inputs, guaranteeing input pairs stay exclusive
  • has a floating ground (it really has no ground at all)

Potential equivalents

  • An array of bistable relays, controlled by a decade counter and an overlap protection circuit, is the closest I can come up with. However, this feels very unsatisfactory as its needlessly complex in part count.

  • Transistors and FETs do not provide isolation, so are not a replacement.

  • Analog switches are not an equivalent as they have relatively high internal impedance (>1ohm), and thus cannot carry much more than signal current.

  • \$\begingroup\$ An analogue cross-point switch might be the term you are looking for. \$\endgroup\$
    – Andy aka
    Apr 2 at 15:13
  • \$\begingroup\$ The cross-point switches I saw are all variants of analog switches, which suffer from (relatively) high internal resistance. Do you know of one which could carry more than a few hundred mA? \$\endgroup\$ Apr 2 at 15:16
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    \$\begingroup\$ Telephony audio routing (whatever the method) requires no more than 5 mA peak for the audio signals. Your question made the comparison with the old Strowger relays and, subsequently, cross point switches put Strowger relays on the scrap-heap for those applications. So, before you go any further, add the full technical details of your requirements to your question. \$\endgroup\$
    – Andy aka
    Apr 2 at 15:22
  • \$\begingroup\$ I used the Strowger relays as the easiest form to find clear pictures. However, those are just a single example of uniselectors. In general, they are capable of carrying as much current as standard COTS mechanical relays, which can go into the tens, or even hundreds, of amps. i.ebayimg.com/images/g/6lkAAOSw8gVX67k4/s-l1600.jpg is an example of one used in MiG-29 aircraft. \$\endgroup\$ Apr 2 at 15:33
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    \$\begingroup\$ If you want a replacement, you must provide specifications of what you want to stop people guessing. Add that to your question else the answer becomes, replace X with another version of X made by somebody else (and note that shopping questions are off-topic). \$\endgroup\$
    – Andy aka
    Apr 2 at 15:38

Opto-MOS type photo-voltaic opto-isolators (some are rated for 2 A), driven by a string of CD4017-type Johnson counters.

  • \$\begingroup\$ Thanks for the idea. Wouldn't the Opto-MOS internals allow current to flow backwards even when the isolator is de-energized? \$\endgroup\$ Apr 2 at 15:43
  • \$\begingroup\$ No.. there are AC rates ones which have back-to-back Mosfets. \$\endgroup\$
    – tobalt
    Apr 2 at 16:54
  • \$\begingroup\$ Thanks, looking around I found the TLP241B, mouser.com/datasheet/2/408/…. Mouser places it in the opto-isolator category, but digikey lists it under SSR. Good to know! \$\endgroup\$ Apr 2 at 17:52

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