Question about contolling polarity

Dan, you said earlier that the sound was powered by a 12 volt battery. Now you are saying it is powered by the motor leads.
It cannot be both, so which is it please?
If there are two wires connecting the battery to the sound and two different wires connecting to the motor leads you will not need the diode.
However I agree with PhilP that it is more likely there is no connection to the motor leads and the speed increase in the sound is controlled by a knob on the sound system.
 
Dan, if you just connect the board directly to the battery it will always have the same polarity irrespective of the direction of the current flowing to the motor.

Obviously you will have a On/Off switch. I don't know enough about electronics but I would say that if you used a double pole switch taking a positive (just linking across to both connections) - one for the motor and one for the board - you could shut down both the loco and sound on one switch but having two seperate feeds.

I don't know if your loco is R/C or manual or if the former would have any effect.

And if you can't run the wiring in any other way then the bridge rectifier should do what you require.

I have a MRails chuff board fed by a PP3 and that is linked to the motor wires (track fed) because it works on the feed/speed of the motor otherwise if I had a baseboard one I would do as above.
 
OK, I see that I must be leaving some information out. The battery is in a trailing car fitted with G Scale Graphics R/C components. Almost all of my locos are fed power in this manner. So the same power that feeds the motor, feeds the sound board. I have the volume and speed knobs set at a level that suits me. I don't touch them unless I feel I want to make a change. And yes, the volume and speed sounds remain constant, even though the speed of the loco changes. As mentioned, I have a diode wired into the leads that branch off to the sound board. Good for forward, but no sound in reverse. This is where I Believe Tony's suggestion of a Bridge Rectifier comes into play.
 
Yes - bridge rectifier required then, Dan. Power from the motor feed goes into the AC (~) inputs of the rectifier, DC (+) (-) output goes to the sound board.
 
Dan,
Perhaps I can try another tack.
What powers the sound when the motor is stationary?
How many leads are on the sound board? Other than the speaker?
Any chance you can take a pic of the wiring instructions that were supplied and post it here?
 
Tony Walsham said:
Dan,
Perhaps I can try another tack.
What powers the sound when the motor is stationary?
How many leads are on the sound board? Other than the speaker?
Any chance you can take a pic of the wiring instructions that were supplied and post it here?
Click to expand...

The power for the sound is the same power for the motor. When the power is cut to the motor, it is also cut to the sound. As I mentioned earlier, two wires come from the battery car into the rear of the locomotive. They split into four wires. Two + and two -. One set goes to the motor, the other set goes to the sound board. The only other wires in this picture go from the sound board to the speaker. The link I posted to Mrrails website shows the clearest picture you are going to see. There are no wiring instructions supplied with the unit as it comes prewired to a battery box.
 
ntpntpntp said:
Yes - bridge rectifier required then, Dan. Power from the motor feed goes into the AC (~) inputs of the rectifier, DC (+) (-) output goes to the sound board.
Click to expand...

Thanks Nick. I see that a few people have the same idea as you, and I am going to give it a whirl.
 
Well, I tried to help but it seems I cannot. Sorry.

I have never known of an after market sound system that did not have at least a rudimentary wiring diagram included.
 
Well Tony, your efforts did not go un-noticed.
 
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