I have a transformer with four taps and I use two relays to switch between them:


simulate this circuit – Schematic created using CircuitLab

When both relays are energized, they consume ~60 mA which really is a waste. I want to switch between taps of transformer with something that draws less current.

I was thinking to use four logic-level mosfets but mosfets have an internal diode which will do unwanted half bridge rectification...

What options do I have other than relays and mosfet to switch between taps of transformer?


12 V Relays are on the secondary side of transformer, relays and controller (MCU) are powered from another winding of same transformer which is independent from this four tap winding.

  • \$\begingroup\$ have you looked at SSRs (solid state relays?) \$\endgroup\$ Mar 14 '20 at 12:19
  • \$\begingroup\$ @BrianDrummond they use triacs, is it suitable? \$\endgroup\$ Mar 14 '20 at 12:22
  • 1
    \$\begingroup\$ SSRs will probably require 10 to 15 mA on the LED side so I don't think you'll save much. Which side of the transformer are the relays on? That makes a big difference to their power consumption. Add the coil specification into your question - not the comments. Also, indicate what your planned controller will be powered from. The DC output or something else? \$\endgroup\$
    – Transistor
    Mar 14 '20 at 12:28
  • \$\begingroup\$ @Transistor Added the extra info. \$\endgroup\$ Mar 14 '20 at 12:35

You may use magnetic latch relays which require only a short pulse to set or reset.

Here's a schematic, using a diode matrix, 3 push button switches and 3 magnetic latch relays.

enter image description here

Freewheeling diodes for the relay coils and LED indicators are not shown.

'NO' contacts of the relays K1, K2 & K3 are to be used to select the taps.

How it works:

  • The 'S' (set) coil of a relay switches the coil one direction and it remains there by magnetic action.
  • The 'R' (reset) coil of a relay switches the coil in the opposite direction and it remains there by magnetic action.
  • Pressing S1 momentarily 'sets' K1 and 'resets' K2 and K3.
  • Pressing S2 momentarily 'sets' K2 and 'resets' K1 and K3.
  • Pressing S3 momentarily 'sets' K3 and 'resets' K1 and K2.
  • The diodes prevent backfeed from one switch into the circuit of either of the others.
  • \$\begingroup\$ I can't use diodes, they'll do half wave rectification. let alone voltage drop and heat... \$\endgroup\$ Mar 14 '20 at 12:42
  • 1
    \$\begingroup\$ Those diodes have nothing to do with the rectification. They are pure there to control the latching relays. \$\endgroup\$
    – Oldfart
    Mar 14 '20 at 12:45
  • \$\begingroup\$ Only the normally open contacts of the 3 relays would be used to select the taps, as you have already done using 2 relays and their change-over contacts. The diodes are used only in the relay control circuit. So there's no question of half-wave rectification, voltage drop and heat. \$\endgroup\$
    – vu2nan
    Mar 14 '20 at 12:46
  • \$\begingroup\$ Current drawn, only with the push button kept pressed, would be less than 200 mA. \$\endgroup\$
    – vu2nan
    Mar 14 '20 at 13:02
  • 1
    \$\begingroup\$ @ElectronSurf: See the update. \$\endgroup\$
    – Transistor
    Mar 14 '20 at 15:53

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