24 March 2016

Model Railways: Running Flashing LEDs from DCC power

LEDs require a stable DC power supply (Direct current) (apart from a few special LEDs). This means the current only flows in one direction and it’s a very stable supply. Mains power is AC (Alternating Current) and that flows back and forth very rapidly. 

Flashing LEDs have a small inbuilt chip to make them flash and they are very sensitive to instability in current or not enough current so occasionally they cause problems. Typically, they will light up but not flash.
If you find your 12v flashing LED won’t flash I can guarantee it will flash if hooked up to a 9v PP3 battery. This is because battery power is incredibly stable. 

DC output transformers convert AC into DC usually via a full wave bridge rectifier and this is usually sufficiently stable to run flashing LEDs but occasionally you can get problems. This is especially prevalent with DCC power supplies because their output can be less stable and is more akin to AC. LEDs are sensitive to reverse polarity so with DCC is essential to use a bridge rectifier between the power supply and the LEDs to stop any reverse polarity. Bridge rectifiers only allow current to flow in one direction. 

I’ve never used DCC power supplies but a customer found that his Belisha Beacons wouldn’t flash when used with 12v and 14-15v DCC power supplies but when we worked out the problem he provided the following diagram and information for the benefit of others. 

“Most transformers that say they are DC are full wave rectified and produce a saw tooth voltage, which the led does not like. 

The answer is to put a smoothing capacitor across the output, before the led. I found that a 2200uF 35V electrolytic worked perfectly. It smooths out the rough DC to a near perfect straight line and the beacons happily flash away.” 

I hope this is helpful