I am a bit of a sophomore in my knowledge of electronics. It’s part of the hobby that I’m finding I really enjoy, but I’m certainly in the learning stage.
I am installing about 20 Tortoises. Was planning to use an old wall-box type power supply marked as 12V DC. I have an old Radio Shack multimeter which appears to read about 19V on that supply. I thought I’d better try something smaller, so found a 9V DC wall-box. It reads 15V on the meter. What am I doing wrong? (I don’t have an instruction sheet for the meter – that’s probably the problem).
To get an accurate reading of the power supply output, you need to measure it under load, as in attached to at least one of the motors you’re going to use it to drive. Most modern multimeters add very little load to the circuit, a desirible trait when measuring voltages in most solid state circuits.
Another thing to consider is the Amps output of your power supply. Each Tortoise should have a current rating in amps or milliamps (1/1000 of an amp). Add the rating for each Tortoise you’ll have in the circuit, you have to be sure you won’t exceed the current output of the power supply
I had the same problem. I use the two power supply version with spdt switchs and find 12v power supplies work just fine. I do not remember what they finally read on the meter. I did try a bunch before I settled on 12v. Part of my motivation was that they were $1.00 at a surplus store and I could experment.
A typical cheapy wall wart is unregulated, so the voltage will swing rather wildly from no load to maximum load. Even Tortoises won’t put that much of a load on the power supply - they draw about 15ma each - takes 100 of them to equal 1.5 amps. However, for the use in powering Tortoises, a 9V unregulated wall wart is fine, if if you have enough to load it down to actually putting out 9V, the Tortoise will move just fine.
A more expensive regulated power supply will maintain the stated voltage across the load range. But this is unecessary for Tortoises, they aren’t that picky. However, for powering structure lighting, it’s a good idea. Say you have a supply driving a bunch of lights, under load it puts out 12 volts, and you used 14v bulbs. Sounds good so far, right? Running the bulbs at less than rated voltage will greatly extend the life. However, if this is not a regulated supply, and you disconnect some lights, or some do burn out - the load is reduced, and maybe the output of this unregulated supply shoot up to 16 volts. Ooops.
Instead of risking voltage fluctuations from an unregulated wall-wart transformer, I always use a computer power supply for my layout lights and Tortoise switch machines. If you look around, you can usually find a new computer power supply for little more than the cost of a wall-wart transformer, and the computer supply will be much more reliable.