In my observation car I have a bridge rectifier before the LED marker lights and drumhead--which work fine. The problem is a flashing LED I want to mount in the roof--It flashes just fine with a 2.3k resistor when tested with a 9 volt battery, but won't run off track power, even thru the bridge rectifier, whether DC powerpack or DCC. What do I need to do to get it to work off DC track power, at least (so I can run it at the DC club I belong to.) I would have thought that the bridge rectifier would produce pure DC like the battery, but I guess not. Thanks.
Tom Cockle
McKinleyville CA
Hi Tom,
What you get from a bridge rectifier is full wave rectified DC. This has either sine wave or a pulsed form depending on the type of DC controller you are using. This pulsing output may be upsetting the built-in pulsing circuit for the LED. I suggest you add a small electrolytic capacitor on the output of the bridge rectifier then add the resistor and flashing LED to it. Most flashing LEDS need only 5Volts so a better way is to use a 5 Volt regulator as well, otherwise you may burn out the LED. Of course the 5 volt LED won't flash unless the track voltage averages more than 5 volts.
Alan Jones in Sunny Queensland (Oz)
Most flashing LEDs require from 3 to 5 Volts input.
The bridge rectifier is reducing the input voltage by 1.2 volts.
On a DC layout, track voltage varies according to train speed, so your LED will require additional components to receive a steady voltage, or you will have to use a battery to power it.
It's imposslbe to power an LED directly from DC track voltage without some type of filtering to insure a constant voltage is applied, and it will not illuminate until the track voltage reaches a certain threshold.
Sort of a combination of both answers, is what your problem is. A bridge rectifier won;t produce battery-smooth DC, you need filter capacitors along with it. What using a bridge rectifier does is amke sure the LED polarity is correct regardles of the direction you are moving. The other problem is voltage - to get 9V DC into your circuit, that's 3/4 or more throttle - so your train will be moving at a pretty good clip before the LED lights up and flashes. To get it to work at all speeds, you'll have to make it battery powered. A reed switch and a little finagling can make an on/off switch that you wouldn;t have to touch the loco, just wave a magnet at the right spot. You can also use a rechargeable battery and build a charging circuit, so when it runs at high enough speed it will charge the battery up.
Reminds me somewhat of an issue I had at the club I used to belong to. I forgot who made it, but there was a flashing ditch light circuit for DC operation, that took advantage of the fact that the DC on a typical power pack is NOT smooth filtered DC and used those pulses to get it to light up at low speeds. I helped a fellow member install this in his loco, and we went to test it. We had 2 mainline power supplies, one the big MRC with walakaround, and the other a Starr-Tec Hogger. One one main, the ditch lights worked great. On the other - not until the loco was going relatively fast. Neither unit was smooth DC, they both had pulse power, but the way the did it was difference, and only one was compatible with the design of the circuit.
--Randy
Modeling the Reading Railroad in the 1950's
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