mg: I have a 5VDC transformer with 30 amp rating. It also has a 12VDC output with 11 amp rating. I plan on using this PS for turnout motors, lighting etc. My MRC Dual 1400 has AC low voltage if I HAVE to have AC for some device. Where I need help is in getting the 5VDC down to something under 3VDC for a ScaleShop motor. It is rated at 1.5amp. With the goal of 2.5VDC, my resistor calculator came up with 1.667 ohm/3.75 watt. The nearest I could come to that at the LES was two .86ohm/10watt. I soldered them in series, and confirmed with my multimeter that there is 1.7 ohms of resistance in the resistor combo. Here's the puzzlement. When I put the resistor combo in an unloaded circuit attached to the 5VDC output, I get an output reading of 5VDC! I reversed my test leads and got -5VDC. Thinking that it might be a peculiarity of the PS (from a PC), I took a toy train PS that was laying around that puts out 17VDC. When I put the resistor combo in series with it, it still read 17VDC. I know that the meter is not 1.5 amp, so I won't see the 2.5VDC. But shouldn't I see some decrease? Or, are meters built to avoid affecting the circuit? In any event, I know the math is right - but I still don't want to chance frying the motor? Any suggestions on how I concoct something to test the output voltage of this configuration?
When you are using a meter to measure voltage, it measures potential and doesn't draw any current so you won't see a voltage drop. Put some load on it like a cheapie spare motor or appropriate light bulb then measure across the leads. You should see a drop. Your math is correct by the way.
First, as MK pointed out, the resistors will have no voltage drop accross them unless the circuit is complete. Second, have you established that the motors will even run on 2.5V? There web site says they are rated for between 3 and 12 Volts. 2.5 may not even operate them. Why is it that you want to drop the voltage so low?
The spec sheet that comes with the kit says 1.5 to 3VDC. The owner, Bruce suggested lower voltage for slower movement (more prototypical). Since my first post I followed MK's suggestion about testing with something else. I have now operated the motor and am probably going to add to my resistance to slow it down even more.
