# Carnival Project

#### whequeen

Joined May 23, 2013
3
This time I may have bitten off more than I can chew. I was "volunteered" to do an N scale (railroad model) working carnival.

I will be running 3 to 4 12VAC motors and roughly 50 - 100 3VDC LEDs. My plan is to use a wall wart to come into the base and power the motors. I would like to do a branch circuit (using the 12VAC input) which steps down the input to 3VDC to power the LEDs. What to use for this step down?

Or would I better off bringing the house current into the base and then using 2 separate transformers/adaptors to accomplish this? Seems awfully cumbersome.

I can solder and I have used resistors which I researched and decided that was what was needed for another project. I understand basic electric and have wired large dollhouses. As far as you guys are concerned, however, I am a babe in the woods.

After Ive (you) have solved my input problems, Ill probably be back to configure the LED runs.

Thank you so much.

Whequeen

#### Bernard

Joined Aug 7, 2008
5,783
I would use two separate wall warts, 12 V AC & ? V DC. figure out the LED arrangement, series, parallel best. If you can find it a 24 V DC would be nice, or up to 48 V. determine colors & current. Example: One of series string on 24V X 80% [ 20% for ballast resistor ] = 19.2 V / 3V = 6.4 LEDs or 6 LEDs per string. 100 LEDs / 6 = 16.6 strings, add one or drop one. 16 strings @ 20 mA = 320 mA needed + a little reserve, 50 1/2 A @ 24 V.

#### JohnInTX

Joined Jun 26, 2012
4,384
First, get a 12VAC wall-wart to drive the motors. Add an amp or so (12-20VA in AC wall-wart speak) to drive the LEDs converting the AC to DC after the motors to drive the LEDs.

LEDs are DC as you know. You can drive them in 2 ways:
1) use a full-wave rectifier bridge to convert the 12VAC to DC. or
2) Split the LEDs into 2 legs so that approx. 1/2 of them are on each leg.

1) is easier, I think.. so you'll have an unfiltered DC source of about 14-16V (depending on how much the motors pull the unregulated transformer down). How you wire from there is up to you.

One way is to wire LEDs in series until their forward voltage gets close to your applied voltage then use a resistor to make up the difference. For example, if you measure your (effective) DC at 15V and each LED is 1.5Vf, try 8 in series. 1.5Vf * 8 is 12V. You have 3V left. To drive the LEDs at 10ma your resistor is 3V/10ma = 300ohms. The power dissipated by the resistor is I*I*R or .01*.01*300 = .03watts so a 1/4W resistor will be fine.

These calculations are pretty coarse but hopefully you get the idea.

For 2) You can also series up LEDs and run them in inverse parallel to use both halves of the AC cycle but you would still need a diode like a 1N4001 in series with each to protect the LED strings from reverse voltages. I like 1).

Use any bridge with a 50V 2-3Amp rating and you are good to go.

Sounds like fun. Post pix when you are done.

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#### JohnInTX

Joined Jun 26, 2012
4,384
You can get them at Mouser as well.

EDIT: Actually, for a few pennies more you can get one of these or similar. Easier to mount and connect with higher ratings.

Have fun.

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