N Gauge Track Voltage

[Ch4lkst3r]

Hopefully this is a quick question.  I am building my layout and track is almost all laid and tested.  Control is via an NCE Powerpro/JMRI.  Testing the track voltage with my basic multimeter shows it as 14.6 volts.  Should I open up the Powerpro and twiddle this track voltage down a bit?

[jpendle]

No, the DCC chip in the loco will bring it down to 12v or less to drive the motor in a loco.

One of the benefits of DCC is that it runs at a higher voltage than DC, lights are brighter, and they stay on all the time. Unless of course you choose to turn them off.

 

Regards,

 

John P

[terrysoham]

I agree with John.

You should also be aware that ordinary multimeters are very unlikely to give you an accurate reading when measuring dcc voltage. For example, when I read the voltage (always use the AC scale) on my N scale layout, Neely, from my MERG command station using a cheapo multimeter, I get about 9 to 10 volts. However, on checking (very carefully, I should add) the DC voltage across the light tags on the decoder, I see about 14 volts which is about 1 volt less than the input voltage.

So the multimeter readings should only be used on dcc to check that you are seeing the same voltage at all positions for diagnostic purposes.

Regards

[Butler Henderson]

One way of taking an accurate voltage reading is off the accessory outputs (lights etc) of a decoder (although simpler  to do with one that has to be hard wired rather than plugged in)  and essential if you have lights in a loco but have removed any of the existing circuity as new resistors will probably be needed.

[Phil S]

Separate the speed of locos and track voltage, from the effects of track voltage on other devices...

 

A 'lower' dcc track voltage means slightly higher currents for the same speed of travel (same power requirement), and the effect of voltage drops in the track and wiring will be 'greater' because it is operating closer to the startup/dropout voltage of the decoders. This is more likely to be experienced when multiple locos are running, lighting is present in coaching stock, and sound is in use in locos.  I am assuming that power for solenoid point motors is not taken directly from the dcc supply, but either externally or via capacitor charging charge/discharge modules, or lower-current-demanding and end-off switching point motors.

 

A higher track voltage reduces the current-per loco to run at the same speed - because it is your choice of speed step/Vmax in each decoder which determines how fast the loco runs.   Whilst possibly reducing the 'average' current, the STALL current would increase due to higher voltage.

 

If, for example, the basic Hornby decoder is installed, which cannot change CV5 (Vmax), then there is possibly a good case for dropping the track voltage to limit train speed for 'children' to operate who like to turn the knob to full speed.  [With our LGB shunting puzzle, which is operated by visitors, we use  16V dcc track voltage (standard Roco output with an 18Vdc power supply) but have the decoders programmed down to 1/4 -1/2 the 'max' possible speed].  [LGB decoders use an internal scaling of1024 steps for speed changes- so speed steps are still invisible]

 

N gauge users have narrower track cross sections than 00/H0 users, and LGB G-Scale users have a larger cross section than Peco G-Scale track: LGB track is also brass.

This means that voltage drop in N gauge is more significant than 00 or G - although longer runs in adverse conditions may be more likely in G outside in all weathers.

Therefore N gauge dcc modellers are more likely to feel the need to add droppers 'to every rail section' than 00 or G scale users, to minimise that voltage drop.

 

NOTE that the adequacy of the wiring should always be tested to ensure both the maximum current is available with minimal voltage drop (for lots of locos with sound and lighting),

AND then that the SHORT CRCUIT PROTECTION of either the Central Controller/Booster output, or suib-district's OWN current limiting protector (such as a PSX) is operative - by using the 'Coin Test' at all parts of a layout.  WIRING REQUIREMENTS therefore  INCREASE with the INCREASE IN OUTPUT CURRENT of the Central Station/Booster:

(A 1Amp DCC controller is probably okay with 'existing analogue wiring',  - but check the short-circuit protection! - whilst a 3A, 5A or 10A Central Controller will be installed to provide for more simultaneous running trans, or larger gauges - and the wiring (lack of voltage drop_ needs to bebetter to match - BUT THEN the SHORT CIRCUIT PROTECTION again needs to be considered - as the short circuit currents will be maybe 6A, 10, or 20A .... meaning an increasingly low resistance wiring loop is required!!!!

 

HOWEVER - the effect on UNREGULATED COACH LIGHTING is probably the most obvious and possibly damaging effect of inappropriately high track voltages- particularly for older 'analogue era' stock with bulb lighting.  Full voltage was rarely reached- only for short periods at maximum speed.

Back in analogue days,  '12V' bulbs were used on 'notional 12{-16V}' track (not regulated - simple transformers,  Triang controllers quoted 14V or 16 as their maximum, but I have an H&M 12Vdc output which is 20V with a light load.

 

NOW any bulbs offered for replacements are usually 16v, because when used on dcc; they may be on all the time, and at the full track voltage !!

PRIOR to modern regulations, which require energy-efficient SMPS power supplies (with their regulated outputs), a Roco transformer, off-load (ie no trains running) could produce a maximum permissable dcc track voltage (22Vdcc) !!!  This was visiible if UNREGULATED bulb-lit coaches were placed on the track... they would get brighter when trains stopped.

The rooves of the coaches would also get noticeably warm !!      Bachmann Voyager Head/Tail ligths, if not modified for DCC melt the surrounding plastic ! (in my experience)

 

Many LGB coaches are lit - and make use of 5V regualtors - so that constant lighting is achieved from the earliest application of power.  Designed-for DCC/dc/ac LED lijghting boards usually onvlude regulayion of the curent - so no problem

 

If using '12Vdc' LED lighting strips, then remember the non-linear nature of LEDs - when the applied voltage rises to 14V, the current into the strips (20mA per 50mm at 12.00Vdc) has doubled- andso has the heat output !    When I use them on 16Vdcc track, I add either half-wave rectification (a single diode 1N4148) and extra resistance to get the desired lighting level, or full wave and the strips in series, halving the applied voltage - (24Vdc strips are available atidentical prices)

 

So, in summary; the track voltage in dcc is not a primary concern for locomotives (speed / motor requirements) but IS of relevence in other matters.

 

Without an oscilloscope, Rrampmeter,or some specialist digital meters such as certain Fluke Models, the 'dcc track voltage'is often MISREAD (high OR low) by cheaper meters which are calibrated for 50-60Hz ac sinusoidal waveform. A 'cheap meter' CAN be used to read the DCC voltage, with a small correction, by using a bridge rectifier, the dc range,  and adding-back-in the voltage drop of the bridge rectifier

[Butler Henderson]

The Roco transformer not only put out 22v but can, at switch off, put a far higher voltage spike across the output. Discovered the hard way when a couple of accessory models failed; thankfully they were under warranty and have been replaced. Ditched the Roco transpormer for a laptop psu.

[Nigelcliffe]

Hopefully this is a quick question.  I am building my layout and track is almost all laid and tested.  Control is via an NCE Powerpro/JMRI.  Testing the track voltage with my basic multimeter shows it as 14.6 volts.  Should I open up the Powerpro and twiddle this track voltage down a bit?

Your PowerCab will be fine as it comes from the box.   Others have pointed out the difficulties in measuring track voltage with cheap multi-meters - it gives you a number, but its probably not accurate except to compare with another situation of being "higher" or "lower".

 

There can be issues with high track voltages from some manufacturer's systems - the older transformer power supplies which Roco used, and the Bachmann EZ system are two such.  But that's not an issue here.   The long list of issues which Phil raises are, in my view, not that important.

 

If you find an N loco runs too quickly, then you can tame it with CV changes. If the loco is lacking low speed control, then some decoder makers are markedly better at controlling small motors than other makers.

 

 

- Nigel