Board DC supply using BRUSHES (like slot cars)

[emontllo]

Hello!

I am designing a circuit that is powered from a low voltage DC power supply but will have brushes instead of cables. The board moves along an electrified rail. I have already tested and used brushes with the required voltage and current ratings.

However I still have some doubts on the electronics. Could you please comment or correct me?

1) A capacitor in parallel to the supply input will be added for voltage stability. A huge capacitor however may cause large current peaks after transient disconnections, therefore I will try to minimize the capacitance.

2) Do you think some kind of current limiting element would be useful in series with the brushes to minimize spark current?

3) Do yo think it is recommended to have some sort of input filter (such as on board ferrite beads)

4) Could adding a diode in series with the input be useful (apart of providing reverse polarity protection)?

5) The board must meet some EMI standards, any additional consideration? Do the brushes cause radiated or conducted EMI?


Thank you!

 

[chuckey]

So you are picking up the supply for the electronics from the rails. If you use a 1NF cap brush to chassis and brush to brush, then a 1000MF cap with current limiting resistor in series across the brushes, taking you electronic supply from across the cap, would seem a good start. You do not say what the supply voltage is, what the electronics supply minimum is or how much current is drawn or even if you are powering a motor from within the carriage. You do not state how you are going to get sufficient brush pressure.
Frank

 

[emontllo]

Thank you Frank,

Nominal voltage is 48 V and current 3 A. I will try starting with the proposed approach but with using smaller bulk capacitor. Regarding brush pressure, the brushes have springs to maintain pressure on the rails.

 

[godfreyl]

I'd expect EMI problems from sparking. From my memory of slotcars, the metal rails on the track and the brushes on the car somehow always managed to acquire a layer of tarnish that prevented a good connection and caused sparking. The brushes didn't last too long either.

Part of the problem may have been too low a pressure pushing the brushes down, but the type of brush may have been a factor too. For example, carbon brushes (as used in some motors) may be better than copper braid (as used in slot cars).