Powering a PICAXE system from a model railway DCC supply

Evening all,

I'm wondering if it's possible to power a picaxe from a model railway style DCC supply?
If you're unfamiliar, a DCC supply is a variable square wave DC supply, rapidly switching between - 15-17V to + 15-17V (dependant on controller).

If it were a simple DC supply, I obviously wouldn't hesitate in simply adding a voltage regulator and smoothing transistors to the circuitry, but it being a square wave, would this work...? Unfortunately, when it comes to power supply circuitry I'm almost completely stumped :)

Cheers in advance :)
 

techElder

Well-known member
rapidly switching between - 15-17V to + 15-17V
Sounds like a viable AC signal there if you can get it capacitor-coupled to a bridge rectifier and filter. Don't know about the power available from the DCC system, but it could work. You might just duplicate some of the circuitry that the locomotives use to tap this power.

Also, if it could work, then someone else has already done it. You might try looking.
 
I've had a quick look, but there's nothing specific to the PICAXE system that I can find. Also, the circuitry used by the locos is on a separate chip, a decoder, and I'm not sure if it even properly regulates to DC - motor control using this system is PWM, and for LED outputs a simple diode would work....
I suppose my main question is, how smooth does the supply need to be to work for PICAXE - otherwise a simple diode coupled with a medium sized capacitor would surely do?
 

westaust55

Moderator
In terms of power requirements a PICAXE based circuit no no different to any other electronic circuit with an Microcontroller and logic ICs.

You need:
1 a bridge rectifier using high speed diodes (UF4001)
2. an electrolytic capacitor (say 100 uF)
3. a 3.3 or 5 V voltage regulator and
4. A tantalum capacitor (maybe 10 uF)

The reason for the high speed (ultra fast) diodes is that typical/conventional diodes do not switch off fast enough. With the 5 to 8 kHz frequency of the DCC power the conventional diodes will keep conducting longer and may get warm.
That said, some folks do use conventional 1N4000 diodes and appear to have no problems.

Might I ask why you wish to power from the DCC bus?
If only to source power then keep in mind if there are many such "accessory" devices you might be consuming a reasonable percentage of the available DCC output capacity. Consideration to a separate 12 Vdc or 9 Vdc plug pack / wall wart might be worthwhile.

If trying to detect the signal superimposed upon the DCC bus then I suggest you search the forum and read up on my DCC accessory project.
 
Yeah... Unfortunately it ain't that simple. I'm already using "land-based" chips for signal control etc. So powering them isn't an issue.
To make it interesting, these chips are going to be inside trains :) The DCC decoders I'm using can be sneakily programmed to output a logic (no current) pulse of a set length when a function button is pressed - allowing me to have as many function outputs as I can reasonably attach to a PICAXE :) instead of being limited to 4-6 outputs, I can attach umpteen more, and just need to program the decoder appropriately!

Thanks for the supply system though - I'll get to work designing and testing, it will be interesting to see how this turns out :)
 
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