DCC Voltages for onboard camera

[Lannoc]

Hi,

I've recently added an onboard camera to a train but it is slightly clunky. I've made a hole in the rear, so that a charging USB plug can be inserted. It was only when I was finished that I thought that perhaps I could be powering the camera (and the battery) from the rails.

Does anyone know what the voltage is on the tracks and how I would step this down to be th 5v DC required by the camera?

Thanks.

[Chrissaf]

The DCC track voltage is a 'bi-polar' alternating square wave at a nominal frequency of 7,000 cycles per second, with a variable 'mark / space' ratio. The 'peak to peak' voltage is typically 28 volts.

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If you pass the DCC track voltage through a 'bridge rectifier' you get a pseudo DC voltage of about 13 volts give or take a bit, because the bridge rectifier losses 1.2 volts across it.

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To get 5 volts DC from the rectified DCC voltage you would then need a voltage regulator chip circuit.

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In this example circuit, the regulator chip has to lose about 8 volts to get down to 5 volts (8 + 5 = 13). Therefore before saying whether or not the example circuit below is totally suitable one would need to know how much current the camera is going to draw. Because that will dictate the amount of power & heat that the regulator has to dissipate when dropping the excess 8 volt voltage.

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The LM317L chip in this example circuit is rated at a maximum of 100mA. 100mA at 8 volts means that the regulator has to handle 800mW of power.

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The two capacitors provide regulator stabilisation. Resistors R1 & R2 create a potential divider circuit that defines the output voltage.

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However, this is only half the story. The circuit above does not include any battery charging protection circuitry. Thus it would be possible for the battery in the camera to over-charge if left connected to power for too long.

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Personally, I wouldn't bother trying to use track power and just accept that the camera will need occasional charging off the track.

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TIP: As this is your very first post, just be aware that the 'Blue Button with the White Arrow' is not a 'Reply to this post' button. If you want to reply to any of the posts, scroll down and write your reply in the reply text box at the bottom of the page and click the Green 'Reply' button.

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Particularly as my reply includes an image. If you use the blue button, any reply you write, may be held back for image approval. Even though it is already a previously published image.

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See also – further TIPs on how to get the best user experience from this forum.

https://www.hornby.com/uk-en/forum/tips-on-using-the-forum/

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[Lannoc]

Thanks Chris! That's a lot of information to give before saying, "don't bother"! ...and I think you are right.

 

Connal

[Chrissaf]

Informed decisions are the best ones to make. Particularly the 'informed decision conclusions' you make yourself.

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If I had just said don't bother then you would not really know the reason why. Understanding the 'why' is just as important as understanding the 'how'.

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PS - The circuit schematic came from my personal model railway electronic circuit library, so it wasn't an onerous time consuming reply to create.