Power for camera from track

[paulherman]

I want to supply power to my miniature car-mounted camera from the track, instead of a battery. I have the following circuit in mind:

The four diodes form a full-wave rectifier to make sure I have the right voltage polarity at the camera regardless of which way the train is on the track. The 8.2V Zener diode (Z1) acts as a voltage regulator. And the resistor limits the current through the Zener diode.

The capacitor acts as a storage device to provide temporary power to the camera when going over switch frogs or dirty spots in the track.

I haven't calculated any of the component values yet, but that shouldn't be hard, depending on the current draw of the camera.

My concern (and question for electronics gurus out there) is whether the capacitor will act as a filter and remove the DCC signal superimposed on the DC track voltage? Or will the full-wave rectifier isolate the capacitor's filter action from the track?

 

[capt_turk]

The capacitor will smooth out the variations(ripple) from the output of the rectifier. I would suggest that you ad a small choke to the output. It will clean up the voltage even more. You can make your own small choke by wrapping as many turns of small wire around a toroid core as you can, or just get a pc board choke at Radio Shack. One winding of any small transformer can also be used.

 

[Fred_M]

Why not use a 7808 voltage regulator? The hard part is to get track power to the circuit reliably, as in pickup shoes. This has been tried before and is problematic at best. Most have ended up just purchasing a 9 volt rechargable at the end. That seems to be the cheapest route. I will also remind you that the track voltage will need be around 10 volts to overcome the voltage drop of the full wave. So, the camera will only work at speed. so why not use a full wave bridge and a limit resistor and a rechargable battery? The rechargable will act as a cap and allow the camera to work at low speeds and charge at higher speeds, and will also act as a regulator. Fred

 

[paulherman]

I may end up using a rechargable battery if this circuit does not work out. But I think I can get it to work. Since I am using DCC, there is a constant voltage to the track. It is actually a square wave 28V peak-to-peak signal, but once rectified, should be close to 14V DC. So I have plenty of voltage to work with either to charge the capacitor or a battery. Since there is voltage anytime the power is on (regardless of whether or not the train is moving), there is really no need for a battery. I just need something to power the camera during momentary power dropouts (from dirty track, etc.)

If I get it working, I'll post the final schematic. My calculations so far indicate that the capacitor will have to be a very large value (22,000uF or larger) to power the camera for a half-second loss of power, which should be ample time (If the loco loses power for longer than that it is probably dead in the water anyway).