Help me build a power supply for this application

[dog80]

I have a device (remote controlled relay) that operates on 12VDC and consumes less than 70mA. At the place I am going to install it, all I have available is 13.5V AC @ 50Hz.

My first though was to build a typical linear power supply with a full wave rectifier, a smoothing cap and a LM317 regulator. But just the bridge rectifier alone will drop the voltage to 12V, then there's no way to use a voltage regulator, even a low dropout one.

Can I get away powering the device with just the bridge rectifier and a big enough smoothing cap?

[AndrejaKo]

It seems to me that you forgot to take into account the effects of the smoothing capacitor or alternatively you have very low current output from the power supply.

Basically, the peak voltage of that 13.5 power supply is going to be 13.5*sqrt(2), which gives us around 19 volts. Two diodes give us around 1.5 V, so the peak is 17.5 V. This gives us plenty of room for a 12 V regulator, unless you run the supply at its full output current.  So what you can try out it to make a rectifier, place a big enough smoothing capacitor after it, make a dummy load from a 150 ohm resistor (make sure it's 1 W or higher!) and see the voltage you have on it once it's connected. If there's room for a regulator drop-out voltage, then use a regulator too.

[dog80]

Forgot about the sqrt2 multiplication. 

Then there's abolutely no problem with the power suply, I can include the LM317 allright

[SeanB]

If it is a remote control relay most likely it already has a bridge rectifier and such, as most I meet are rated for operation from 12-24VDC or AC, and work well from 8V to 20VDC.