# Trying to figure out resistance for hardwired toy

Discussion in 'The Projects Forum' started by oldpaddy, Nov 2, 2012.

1. ### oldpaddy Thread Starter New Member

Nov 2, 2012
1
0
Hi all, great site!

I took two years of electronics back in college (98-99), but I can't remember a damn thing! Oh well, hopefully you guys can refresh my memory a little.

I've been setting up a O gauge train table and hard wiring everything. Luckily I've been able to backwards engineer a lot of the switches and wiring. One thing I can't remember is how to figure out what resister I need to hard wire a toy skating rink to the table.
I've got a 18vac power supply, the toy is 5vac and 1.2a
What kind of resister do I need? What's the formula?

Thanks

2. ### hexreader Active Member

Apr 16, 2011
250
82
My maths and logic skills are rubbish, but I'll have a go anyway....

Ohms law would suggest R=V/I 13V/1.2A = 10.83 Ohms.

W=VI 13*1.2 = 15.2W

So to prevent resistor running at full power, and overheating, maybe buy an 11 Ohm 30Watt resistor.

But don't!..... This would be crazy, and is likely to cause damage to your toy if the current should ever go below 1.2A (and I bet it will).

Just buy an AC power supply that supplies AC 5V at 1.2 Amps or more.

Or maybe the toy would run from DC? but hard to guess without knowing everything there is to know about the toy. Converting AC18V to DC 5V is much easier if the toy will accept it. Just needs a rectifier, a beefy 3-pin regulator chip and a few capacitors as recommended in the datasheet for the regulator, along with a good size, well cooled heat-sink. - but none of this is any good if the toy will only work with AC.

Buying the right power supply for the job is boring, but is the simplest and safest way to go, and saves overloading the existing 18V supply.

Last edited: Nov 2, 2012
3. ### crutschow Expert

Mar 14, 2008
13,494
3,373
Your are going to waste a lot of power dropping 18VAC to 5VAC at 1.2A. It would require a 18-5/1.2 = 10.8Ω resistor dissipating (18-5) * 1.2 =15.6W of power so you would need at least a 20W resistor. Can the 18VAC supply provide the required 1.2A?

I recommend just buying a 5VAC, 1.5A transformer. It will likely cost less than the resistor and save a lot of power and heat.

Edit: hexreader beat me to the punch by 3 minutes.

Last edited: Nov 2, 2012