Building a Power Supply by Tim Surtell

Excellent article, but as I'm new to trying to electrocute myself... and electronics....

I need a power supply with one 5v DC out and one 18v AC out for a model railway project (controlling LED display and powering a capacitor discharge unit). Would I be correct in assuming I can take an 230-18v AC transformer and use it as the 18v AC power, then take a spur from the 18v AC output to the 5v DC circuit as shown in the article?

If so how do I know what the transformer rating has to be? For example 25 LEDs in parallel (20mA 2.5v) and the CDU which I have no idea of the current it requires but it will use the 18v AC.

Regards

Newbie Angie

Posted by Angie Mindyerown :) (emptyone@blueyonder.co.uk) on 20/07/2006, at 14:42 GMT

Building a Power Supply.

You are correct on most issues, but I would connect a 9-12v pre regulator in front of the 5v regulator to minimise the 5v regulators dissipation due to the input/output voltage that would otherwise be present the if the 5v regulator was connected straight onto the 18v supply. The rectifier diodes and the smoothing capacitor will not need to be fitted on the 5v regulators board if the 9-12 regulator is fitted first onto the 18v supply ie 9-12v followed by the 5v regulator.

Posted by Robert Neill (varient412@yahoo) on 12/02/2010, at 15:41 GMT

Transformer rating- Building a PSU.

I would suggest a transformer rating of 40-50/80VA for your psu.

Posted by Anonymous (varient412@yahoo.co.uk) on 12/02/2010, at 15:56 GMT