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Your cars will run okay on 15 volts. You may even enjoy the inline motored cars a lot more at that voltage because of the track size. TJets do tend to favor slightly higher voltages but will run okay. As long as everyone running on your track is running at the same voltage it's still a fair race.

As far as current is concerned, (in laymans terms) the cars determine how much current they want to run properly. If you give them a power supply with an "unlimited" amout of current they will only draw as much as they need. This would pretty much describe the case when using car/marine batteries which can supply hundreds of amps. In the case of a DC power supply, the cars may want more than the power supply can deliver. They are out of luck.

But there's also a big "twist" in cases where the unregulated power supply can't deliver what the cars demand. In most cases when the car demands more current from the power supply the only way the power supply can deliver it is to drop the output voltage. The degree to which this happens has everything to do with the "VA" rating of the power supply. Low end unregulated power supplies like wall warts have low VA ratings. The Tomy 22V supply has a VA rating of 7 VA. This means that the product (multiplication) of the output voltage and output current can never exceed 7. This means that at 22 V the power supply can deliver about one-third of an amp. When the current demand increases the output voltage will sag.

I know 7 VA sounds pretty low (and it is), but keep in mind that a typical stock motored slot car will demand less than one amp when running around the track at a fixed speed. The peak current demand occurs only when starting the motor from a dead stop. Once the motor is running the current demand drops off substantially. The real issue with the wall warts like the Tomy is when you power both lanes with the same unit the total demand on the unit exceeds what it can deliver. With two cars running at the same neither one is getting all the current it wants. When one car falls off the track the other car suddenly gets more of its current demand satisfied, which usually leads to a nice little wall shot for the car that's still on.

Regulated power supplies are able to maintain the output current (and voltage) at a steady level, so as long as the unit can meet the total demand at all times you will not have the surge problem. A 9 amp uint should be more than adequate for running 4 lanes with most any stock or somewhat hotter setup. If there is any deficit it would only occur very briefly when all 4 cars are starting from a dead stop at the same time. I doubt you would even notice.
 
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