Strange Switch Wiring?

Hello I have a pre made board which has a switch wired in a fashion I can’t work out.

The switch as far as I can tell is SPST however it has 3 terminals. 2 of these terminals when tested with a multimeter are connected when the switch is on and disconnected when the switch is off. The third terminal however seems to be never connected to either of the other two terminals regardless of the switch position.

Oddly the board has been wired so that the Live wire is connected to both the board and the mystery 3rd terminal. the Neutral wire goes via the two “working” terminals on the switch.

I have attached a sketch of the wiring as well as a picture of the switch.

Can anyone explain what this third terminal might be doing or why is is wired in an apparently pointless mannor?

Thanks in advance

Hi, that wiring would be right for a switch that is illuminated, that is, has a lamp to indicate when the switch is on. So the switch turns the board ON, and the other wire provides the current return for the inbuilt light. So on your diagram you could draw a lamp symbol between the centre and left terminals. However I would be worried about the switch, only rated at 12Vdc and only has a 12V lamp. So If you do not measure any resistance where the lamp should be then its blown, also 250Vac should not be used on this switch. What does the board do, or supposed to do?

Tom...... :)

Hi Tom this explains everything! Except the voltage part. When I moved my multimeter to a higher resistance I got a reading on that pin, I was expecting very low resistance so it was off the scale. I did not realise this is how the lamps work.

As far as the voltage goes I will need to look into this further. The board is a power supply board which then feeds into a PWM motor controller.

That said the switch definately seems to be before all of the electronics and seems to be on the mains side of the PSU circuitry.

See attached diagram.
Note that the position of the terminals with respect to each other may be different but the concept is the same.


You Guys are great!

Hi, does that switch really switch 250Vac? If so then that switch is not suitable. If you purchased it like that I'd be complaining to the seller for using a switch that is not rated for what its used for.

Tom... :)

If that switch is for mains voltage the lamp is likely a neon in which case you will not be able to read its resistance. The strike voltage for those is around 60 V. If it is low voltage its possibly a led which is polarity sensitive and will require about 2 v to get areading. Multimeters do not generally give out enough volts for this. Some however have a diode test facility which might.

The switch in the pictures is rated DC 12 V 10 A, lamp DC 12 V. That's why others have warned about this in case those are the actually used switches and they are in the mains circuit.

Thanks for the info guys, thought the switch is definitely switching 240vac. Perhaps I have miss identified it.

I did have a diode test and managed to verify that this third. pin was for the bulb