I just acquired a portable RRampMeter to test voltage along the tracks throughout my layout. I am testing for voltage drops under load by connecting an 1156 automotive bulb to the RRampMeter.
The layout is divided into two booster districts and further divided into seven power districts, three of which are controlled by PSX units and four of which are controlled by PSX-AR units.
All seven power districts perform as expected. However, one power district controlled by a PSX unit shorts when I test voltage under load. But, if I test voltage without load, I get normal voltage readings, no short.
I do not understand what is happening here. Any ideas or thoughts as to why this one power district shorts under load when measuring voltage with the RRampMeter under load? I can run locos in this power district with no problems.
Going to have to check the settings on that PSX. It must be at a lower setting than the others. A loco or two will draw less than an 1156 bulb, so thweey will run fine, but the 2.1 amps or so of an 1156 bulb may trip if the breaker is set to something less than that. The 2.54 amp option could be marginal.
ahh, I hadn’t thought about that. Thanks Joe and Randy. I will check the PSX settings tomorrow. The power district in question is the Engine Servicing Facility, and it is packed with locos although none are idling but power is present.
I checked the PSX unit in question. The trip current setting is set at 3.81 amps, just like all of the other PSX and PSX-AR units.
I was able to watch the PSX that controls the engine servicing facility while I tested that power district under load. The PSX shorted out, while the booster showed no adverse effect. So, it appears that the PSX recognized the short in timely fashion before the booster could short.
I then did the quarter test at various points inside the engine servicing facility. It shorted that power district at each and every point tested, momentarily shutting down the PSX, while the booster remained unaffected by the short.
So, here is what I am thinking. The effect of the load from the 1156 bulb, combined with the number of locos sitting in the engine servicing facility must exceed 3.81 amps. Wouldn’t this make sense if the load produced by the 1156 bulb is 2.1 amps plus the 1.65 amps reading without load on the in-line RRampMeter?
are you suggesting that the combined load of 3.75 (2.1 + 1.65) shouldn’t trip the breaker since it is less that the 3.81A value stated in the manual by 0.06A?
do you believe the circuit breaker and RRampMeter are that accurate?
the lamp may momentarily draw more that 2.1A as the filaments heat up
Nope, Not suggesting that. I am in agreement with you that the likely current draw is exceeding the 3.81 amp setting on the PSX, thus tripping the breaker.
Got to thinking about this a bit more, so I just went down to the layout and powered it up. Pulled a couple of locos off the tracks in the engine servicing facility and applied load with that 1156 bulb. The power district shorted out. So, I kept on removing more locos until I reached 11 locos off the tracks and the PSX no longer shorted whenI applied the 1156 bulb load.
I took one of those locos and placed it on the mainline which increased the current draw by 0.05 amps. Using that measurement, when I pulled 11 locos off the tracks in the engine servicing facility, I reduced current draw by 0.55 amps. That was enough to prevent the controlling PSX from shorting.
So, to recap, the engine servicing facility was measuring 1.65 amps without the 1156 bulb load. When I removed 11 locos, the amps dropped from 1.65 to 1.10. Add 2.10 amps for the 1156 bulb load, and I get 3.20 amps in the engine servicing facility - - less than the 3.81 trip current setting on the PSX.
If all of this is accurate, and if my reasoning is correct, I am a little surprised that I needed to remove as many as 11 locos to end the shorting.
The only one that MIGHT is an old 8060, it’s one of those old classics with the buttons ont he side to select the ranges. It’s TrueRMS but up to 100KHz. The specs show the accuracy dropping way off as you appreach the limit though. I need to clean mine up again, the LCD started acting up again so I can’t really read it - it worked for a while after I cleaned it up last time.
A cheap meter with absolutely NO RMS compensation will give the best results for DCC square wave signals. Or an oscilloscope.
If a meter says RMS - those are generally only calibrated to work for sine wave, 60Hz or maybe a little extra. True RMS, they can handle sine, square, and triangle waves, but for the most part are still limited in frequency. If limited to a few hundred Hz at most, to test DCC it might as well not even have the True RMS feature, it’s going to be way off at the far higher DCC frequency.
As far as accuracy at low current levels - that’s another one of those, does it really matter that much kind of things. It’s not like we’re designing the loco ourselves (and the decoder) so that it draws the absolute lowest current. Designers of some things are concerned with that - sleep mode on some microcontrollers can be in the nanoamp range, and that requires specializ
then it would have to know how much current it draws and subtract that from the measured value. a fluke, for example, is powered from batteries.
The RRampMeter may be able to measure voltage +/- 2%, but as I said, if the loco draws 40 ma, the meter draws 10 ma, and it reports 50 ma, then the error is +25%. of course, if the loco draws 200 ma, the error is only 5%.
Every one of my engine terminal tracks have kill switches, especially when there are close to 30 engines down there; some with sound. The engine terminal itself has it’s own 5 AMP booster, plus circuit breakers. During an op session I have certain tracks powered that have engines to be used during the session. Otherwise, power is off the other tracks.
Highly recommend adding them. Worth the investment…
