dropping resistors

[Anonymous]

Running some Lionel accessories on 16V AC
Need to reduce voltage to control accessory using resistors etc.
Any Suggestions
Thanks!

[lionelsoni]

You can do it with a resistor if the load current doesn't vary while the accessory is on.

The simplest way is to put an AC voltmeter on the accessory and experiment with different resistor values, starting with large values, until you get the voltage you want. To calculate the resistor's power rating in watts, figure the difference between the voltage that you want and 16 volts. Square that value and divide by the resistance. It's a good idea to use a resistor with at least twice that rating, so that it doesn't get so hot. However, when you are experimenting, its no big deal to use a rating somewhat smaller than you calculated as long as you don't leave it on any longer than it takes to read the voltmeter.

On the other hand, you may not have on hand or want to buy the collection of resistors that the experimental approach requires. In that case, get a 1-ohm 10-watt resistor from Radio Shack (2 for $1.37) and use it with your adjustable transformer output, the one that you would use to run a train. Adjust the transformer so that you have the voltage you want on the accessory. Then measure the voltage across the resistor, which, for that resistance, is equal to the current in amperes that your accessory draws. Now go back to the voltage difference between 16 and what you want on the accessory and divide that by the current that you just measured. This is the resistance that you will need. Multiply that same voltage difference by the measured current. This is the power rating in watts that you will need. Double it to be safe.

If you are very lucky, these will be values that you can find in the store. If not, you will have to make up a combination of resistors to do the job. When you wire resistors in series (in a chain), the resulting resistance is the sum of the individual resistances and, if the resistors are all the same, the power rating is the sum of the individual power ratings. So, for example, if you need 4 ohms at 2 watts, you could connect 4 1-ohm 1/2-watt resistors in series, or, better, 4 1-watt resistors for a 4-watt rating.

If you connect identical resistors in parallel, the resistance is divided by the number of resistors, but the power ratings still add up. So 2 10-ohm 10-watt resistors in parallel have a resistance of 5 ohms and a power rating of 20 watts.

[Anonymous]

Thanks, BOB ill give it a try

[wrmcclellan]

subway1,

Power resistors are a good way to drop some voltage to accessories.

Be careful where you place the resistors. Depending upon the current being drawn, they can get quite hot, enough to deform or melt plastic and singe wood.

Make sure the final mounting configuration places the resistor away from wood and plastic and that you have good airflow all around the resistor to disapate the heat. If the resistor overheats in a confined space, it may burn up.

Regards,
Roy

[daan]

Best is to mount resistors one a plate of aluminium. I have a huge amount of them on a aluminium cooling plate in my powersource and the can stand even shortages that way without overheating.