Is it necessary to have an automobile type lightbulb accross the track feed to get an accurate amperage reading for locomotives? Most of my engines read between .09 and .10 amps but the amperage still reads .06-.07 without engines on the track, due to the presence of three Frog-Juicers for my turnouts. It seems "the DCC Guy" mentioned this at one time.
John in South Carolina
I had a reading of .05 on the track with many engines on staging tracks, no sound. I didn't think this was any type of issue since the locos weren't moving and that reading was in multiple areas. I do have some auto reversers, but they shouldn't affect that rating. I could be wrong...
Neal
No, because then you would need to know the amperage of the bulb to subtract from the reading.
There seems to be this idea, mostly left over from the 40's and 50's, and the big open frame motors used in Bowser locos, and the original black motors used in older Blue Box locos (before the gold motor) that HO locos draw closer to an amp of power each. Unless you're running vintage equipment witht he original motors, this couldn;t be further fromt he truth. Especially if using some of the modern and better quality locos. The motor MIGHT go over 1/2 amp if stalled, but normally working, with a train it can actually pull, without spinning its wheels, the numbers you are getting are completely reasonable.
Where you need the light bulb is to check voltage drop - if you just go to the most distant part of the layout and put the Rrampmeter on the tracks and check the voltage, it will probably be exactly the same as is you connected it directly to the booster output (unless your wiring is REALLY bad). Voltmeters by design put as little a load on the circuit being tested specifically to avoid changing the reading. If you want to see how good your wiring really is - adding the load of a light bulb will then show how much the voltage drops under that amount of load at that point on the layout.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
If you want to check your loco's do them one by one on a separate test track, not where you have your juicers. They draw only 17 MA per unit so they will add up.
Thanks for the helpful info. All my locos have good quality can motors so it seems my readings are good after all. One loco had a consistently high reading and is now on the way to Bowser for evaluation and repair. It was not the Loksound 5 decoder nor the motor or drive train so I suspect something in the circuit board. It would just stop occasionally and then the current draw would rise from .25 (higher than any of my other engines) to .45 or higher.
Amperage is a very different quality from voltage, especially at the low levels being discussed here.
Amperage is the current being drawn -- and it is proportional to load and to voltage, it is not 'determined' on its own unless you are using something comparatively exotic like a constant-current power supply.
The light bulb has a value in WATTS (at the voltage rating it was designed for) and consequently serves as a load that produces a known current at a given test voltage. A power resistor could be used, but would cost more and wouldn't double as a handy optical current indicator. You wouldn't use it at all when testing amp draw for locomotives.
Voltage can be tricky -- it can show high or normal when even a small actual load or current draw makes it sag dramatically. So sometimes you want to measure voltage when a substantial 'enough' current is going across the circuit, so you load it with something of known wattage, and then add back to the measurement the amount of 'voltage drop' occurring across the load.