dual power supply for large countdown timer?

[koko]

Hi, I'm doing a project at the mo- setting a large digital countdown timer into a mobile unit. The timer was sold with a 220v dc power supply plug with output of 12v 1.5A the unit draws less than 5w. All works as it shoud when plugged into mains- I picked up a suitable lead acid 12v battery and power input and it powers up and works no prob via battery power. - What I am trying to do now is figure out a system to have a simple switch to allow the user to alternate between dc and ac power supply- there is only one power socket in the timer and when project is finished this wont be acccesable!
Any help or thoughts would be great, cheers!!!

 

[dorke]

The simplest SPDT switch to select the source of the 12VDC to the timer.

 

[koko]

Thanks dorke! That's exactly what I'm after! Id imagine this could also be rigged with some kind of '3 position switch' so the operator could easily from - 'battery' to 'off' to 'mains' (left, centre, right respectively) ?

 

[dorke]

Yes, absolutely.

You can also add a protective fuse ,
and a series schottky diode to protect against reverse polarity,
both on the "timer -leg".

 

[AnalogKid]

That is called a center-off toggle or rocker switch, sometimes noted as ON-OFF-ON on a configuration chart.

Something to consider is adding a 12 V relay, powered by the output of the AC supply. This way, the system changes over automatically. Wnen AC power is present, the system uses it even if DC power also is available. This does not replace the on/off switch, just the select function.

ak

 

[koko]

Thanks a million ak and dorke. I'm nearly finished out the project now and it seems to be working as it should. I got a 'center-off rotary switch' and hooked it up to the battery and mains today. After i left the studio i had a niggling thought that both 'on' positions on the switch are powering timer from the battery. I'll check this in the morning by disconnecting battery and see if mains position works on its own. Wil put up some pics of the piece if that's a done thing on this forum.