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Converting 18VDC to +/- 9VDC

TheZeke

Nov 26, 2012
2
Joined
Nov 26, 2012
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2
Driving stall motors and LEDs with half rectified AC current

Hello,

I'm new to the forum. I've been searching it as well as googling for information but I'm not finding anything conclusive.

I have drawn up the attached schematic in iCircuit (Mac) and according to the scope when I switch one of the SPDT the current through the LEDs spikes to more than I'd like and I think the current will blow the LEDs. The voltage also spikes presumably because of the polarity change on the capacitor releases then normalises again over the discharge period (5 t). I did make a prototype circuit and actually blew LEDs.

I need the capacitor because the half-rectified wave is too dirty and makes the motors hum. In all there will be 10 motors attached to this circuit. I'm told that 500mA is enough for 30 of these motors all stalled.

The motor's that are being driven are actually Tortoise switch machines for a model train. Unfortunately I don't know the exact specifications but I've read that the internal resistance is something like 800 ohms.

Adding the resistors in-line with the capacitors to limit current to the LEDs (and to stop a capacitor loop) drops the voltage on the motors which makes them run too slowly. 9VDC is already slower than I wanted to run them.

ALTERNATIVELY... If I were to take two off the shelf 12VDC walwart type power adapters could I achieve a symetrical +12VDC and -12VDC and a common conductor by connecting one of wires a certain way?

Any advice would be appreciated.

Thanks for reading!
 

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davenn

Moderator
Sep 5, 2009
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Hello,

I'm new to the forum. I've been searching it as well as googling for information but I'm not finding anything conclusive.

.....................................................
ALTERNATIVELY... If I were to take two off the shelf 12VDC walwart type power adapters could I achieve a symetrical +12VDC and -12VDC and a common conductor by connecting one of wires a certain way?

Any advice would be appreciated.

Thanks for reading!

hi there
Welcome to the forums :)

yup thats not a problem :) It may help to use ones that are regulated to 12VDC rather than unregulated ones, so that you do end up with a symetrical + and - supply

connect the negative of one to the positive of the other. that common point will become you 0V rail. the other positive and negative leads become your + and - rails relative to the 0V rail :)

Dave
 

TheZeke

Nov 26, 2012
2
Joined
Nov 26, 2012
Messages
2
Hey Dave,

Thanks for the welcome and thanks for the advice! I hadn't considered regulated vs. unregulated. Still having trouble imagining connecting - of one to the + of the other and it not making sparks! I guess that's why 120V plugs have one prong bigger than the other so this kinda thing matches up.

Thanks again. I'm going to go this route and buy some walwart type power adapters instead of trying to rectify 18VAC into a noisy symmetrical DC current.
 

CocaCola

Apr 7, 2012
3,635
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Apr 7, 2012
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3,635
I guess that's why 120V plugs have one prong bigger than the other so this kinda thing matches up.

That was an early attempt at 'safety' trying to keep polarity consistent across devices (but sadly a lot of houses are wired backwards negating it's purpose), and thus was replaced by the 3rd ground plug...

Still having trouble imagining connecting - of one to the + of the other and it not making sparks!
You have probably done this 1000s of time when putting batteries in devices and never given it a 2nd thought... :p

You might find Googling 'batteries in series vs parallel' a good read... And yes I said batteries only because I know there are more articles about that vs a power adapter, the same rules apply...
 

davenn

Moderator
Sep 5, 2009
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Hey Dave,

Still having trouble imagining connecting - of one to the + of the other and it not making sparks!

it cant make sparks, you havent shorted anything out :)


attachment.php


Dave
 

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