Voltage drop due to signals/turnout motors?

[AndrueC]

Hi,

 

I added four signal LEDs and three turnout motors to my layout this weekend. They are working but I've noticed a reduction in the speed of my locos. After investigating I've found out that the LEDs and motors are apparently causing an ~5v drop. I'm measuring the voltage using a multimeter in AC mode which I know won't give an accurate absolute reading but relative measurements should be fine I'd have thought.

 

• LEDs/motors disconnected: ~22v
• LEDs/motors connected: ~17v

 

Is this to be expected? I'm using an NCE Powercab and according to it the LEDs/motors are drawing about .3 amps. With one loco and the LEDs/motors powered the Powercab is reporting just under half an amp. Without the LEDs/motors it's reporting less than .2 amps.

 

I'm hoping to add another four LEDs and three turnout motors so I'm a bit concerned about this. I was also under the impression that the Powercab could handle up to 2A so I'm puzzled why a .4A draw should bother it.

Edited October 24, 2021 by AndrueC

[WIMorrison]

Sounds to me that your wiring may be too light on either the bus or your connection to the bus, possibly both.

Edited October 24, 2021 by WIMorrison

[AndrueC]

The accessories are on their own bus, or at least comprise their own bus. The controllers and mimic are chained together and they are fed from a terminal strip which also branches off to the track bus. I can't use thicker wire because of the terminals on the Powercab PSU and the controllers/mimic.

[WIMorrison]

You also need to check all your connections - you have a resistance somewhere to show such a voltage drop when current is drawn.

[KingEdwardII]

3 hours ago, AndrueC said:

I'm measuring the voltage

Measuring the voltage where, exactly?

 

For a 0.3 amp current to cause a large voltage drop of this size implies a significant resistance somewhere in the circuit. You should try to find out where this is.

 

I suggest that you check the voltage in multiple places like:

 

o The point where the wires leave the controller

o The track 

o The terminals on the point motor(s)

 

I assume that the 0.3a current is not entirely down to the LEDs so is it correct that your point motors have a current draw when not operating?

 

Yours, Mike.

[AndrueC]

2 hours ago, KingEdwardII said:

Measuring the voltage where, exactly?

 

For a 0.3 amp current to cause a large voltage drop of this size implies a significant resistance somewhere in the circuit. You should try to find out where this is.

 

I suggest that you check the voltage in multiple places like:

 

I assume that the 0.3a current is not entirely down to the LEDs so is it correct that your point motors have a current draw when not operating?

The point motors shouldn't pull any current when stationary as far as I know. I'll check for issues with the connectors although it's all either push fit (for the motors/LEDs) or screw terminal for the power. I suppose I can also try disconnecting each item in the chain to try and narrow it down. The wire connecting each controller and the mimic is 22 gauge and I don't think I can use anything thicker. It's also probably less than a metre long in total.

[AndrueC]

Sorry, I forgot to say that I'm measuring the voltage from track to track on the layout itself.

[ISW]

6 hours ago, AndrueC said:

The accessories are on their own bus, or at least comprise their own bus. The controllers and mimic are chained together and they are fed from a terminal strip which also branches off to the track bus. I can't use thicker wire because of the terminals on the Powercab PSU and the controllers/mimic.

Why not use a separate power supply for the Accessories bus? I'm using an old XBox 360 power supply that gives 12v at up to 14amps. They can be picked up quite cheaply (mine cost me £1 at a car boot). If size is an issue, small 12v / 2 amp 'laptop' power supplies are also available cheaply (I use one for LED lighting strips).

 

Ian

[RobinofLoxley]

5 hours ago, AndrueC said:

The point motors shouldn't pull any current when stationary as far as I know. I'll check for issues with the connectors although it's all either push fit (for the motors/LEDs) or screw terminal for the power. I suppose I can also try disconnecting each item in the chain to try and narrow it down. The wire connecting each controller and the mimic is 22 gauge and I don't think I can use anything thicker. It's also probably less than a metre long in total.

I am less than 100% sure but my understanding is that stall point motors do pull a bit of residual current. So its not an issues when you have only a few but as the number increases, obviously it gets more significant. From the sound of how you have cabled, putting them on an auxiliary supply wont be difficult

[AndrueC]

3 minutes ago, RobinofLoxley said:

I am less than 100% sure but my understanding is that stall point motors do pull a bit of residual current. So its not an issues when you have only a few but as the number increases, obviously it gets more significant. From the sound of how you have cabled, putting them on an auxiliary supply wont be difficult

Yeah I don't mind doing that but out of an abundance of caution I'm just wondering if this 'sounds right'. I know my PowerCab is a starter system but it seems a bit drastic that an extra .3A draw results in a 20% reduction in track voltage. One my locos - a class 56 - which never was particularly fast crawls pitifully with the accessories connected. The other locos also become fairly pedestrian which is how I first noticed the issue. The PSU claims 1.8A output but my layout currently seems to suggest that pulling .5A renders the track voltage almost unusably slow.

[AndrueC]

The controllers and mimic don't have separate signal and power inputs so how do I wire up a separate PSU for them?

[Dungrange]

2 hours ago, AndrueC said:

The controllers and mimic don't have separate signal and power inputs so how do I wire up a separate PSU for them?

 

I think the separate PSU would only be to supply power to the point motors.  They would still pick up the signal from the DCC accessory bus.

 

My understanding is that Voltage drop (V) = Current (I) x Resistance (R).  That means that R = V / I and if you are noticing a 5 volt drop with a current of 0.3 Amps, then you have a resistance in your circuit of 16.7 Ohms.

 

I would say that 22 AWG wire is a bit on the light side (my own layout will use 20 AWG for droppers and 13 AWG for the bus wire) but 22 AWG wire should have a resistance of about 55 Ohms per kilometre, so your resistance is what I would expect over 300 m of 22 AWG wire.  I doubt you have that much wiring, so I think the issue must be a poor connection somewhere - a badly soldered joint or a screw connection, where contact with the wire is poor (eg only a limited number of strands are making contact with the screw connection).

 

I think the approach that I would take would be to measure the voltage at various locations starting with the output from the booster (controller) - I'm assuming this will be about 22 volts.  Then work away until you find a high resistance joint by identifying where the voltage first drops to circa 17 volts.  If you have a multi-meter that measures resistance, then you could step round the wiring to try and identify a double digit resistance between track and bus, if the problem lies with one of your droppers.

[AndrueC]

9 hours ago, Dungrange said:

I think the approach that I would take would be to measure the voltage at various locations starting with the output from the booster (controller) - I'm assuming this will be about 22 volts.  Then work away until you find a high resistance joint by identifying where the voltage first drops to circa 17 volts.  If you have a multi-meter that measures resistance, then you could step round the wiring to try and identify a double digit resistance between track and bus, if the problem lies with one of your droppers.

I have had to fix up a few fishplates over the summer but I had assumed that if the voltage drop goes away (or at least trains return to what seems normal speed) when I disconnect the acc. bus it was safe to lay to the blame there. Could it instead be the case that the acc. bus draw is just the final straw?

Edited October 25, 2021 by AndrueC

[WIMorrison]

It is just that it becomes visually apparent when you include the accessory bus as the speeds are dropping to a noticeable level.

 

the fact that you have had to replace ‘fishplates’ means that you have a track issue also.

 

all in all, seems like rewiring to a better standard is being called for.

[ISW]

12 hours ago, AndrueC said:

I know my PowerCab is a starter system but it seems a bit drastic that an extra .3A draw results in a 20% reduction in track voltage.

I have the same PowerCab 'starter system' and can easily run 4 trains, and that's with some long gradients of ~1in45. According to the PowerCab display I've been up to ~1.0 to 1.5amps continuously with that many trains running. If I try to run another train, there is still no significant impact (ie: no volt drop).

Maybe your PowerCab power supply is faulty and cannot provide the requisite amps?

Ian

[RobinofLoxley]

34 minutes ago, AndrueC said:

I have had to fix up a few fishplates over the summer but I had assumed that if the voltage drop goes away (or at least trains return to what seems normal speed) when I disconnect the acc. bus it was safe to lay to the blame there. Could it instead be the case that the acc. bus draw is just the final straw?

Is your track supplied with droppers throughout or do you have some/many/all connections via fishplates? Apart from contact issues at the rail joins, replacement fishplates arent as good quality as the original fishplates supplied with setrack items, and corrode quite rapidly in some cases. We dont know but it could be that there has always been some resistance in the payout somewhere but it has only become important since the extra items were installed

[AndrueC]

33 minutes ago, RobinofLoxley said:

Is your track supplied with droppers throughout or do you have some/many/all connections via fishplates? Apart from contact issues at the rail joins, replacement fishplates arent as good quality as the original fishplates supplied with setrack items, and corrode quite rapidly in some cases. We dont know but it could be that there has always been some resistance in the payout somewhere but it has only become important since the extra items were installed

I soldered my own droppers on but initially only did 'a few here and there'. Over the summer I've soldered three or four 'failed' joints and added two more pairs of droppers. I'm probably averaging one pair of droppers every couple of metres but I'm using settrack and none of my curve sections initially had droppers.

 

Given that the PowerCab reports less than .3A with the acc. bus disconnected when running one loco and with six others idle (no lighting other than what the locos come with) it doesn't sound to me like the track is a problem. From what I've read that's about right for N scale locos. Nonethless I'm reviewing the track in the light of this.

 

I've already decided that my next layout (in the design phase, lol) will have droppers on every single piece of track.

Edited October 25, 2021 by AndrueC

[AndrueC]

1 hour ago, ISW said:

I have the same PowerCab 'starter system' and can easily run 4 trains, and that's with some long gradients of ~1in45. According to the PowerCab display I've been up to ~1.0 to 1.5amps continuously with that many trains running. If I try to run another train, there is still no significant impact (ie: no volt drop).

Maybe your PowerCab power supply is faulty and cannot provide the requisite amps?

Ian

That's what I'm beginning to suspect. I've measured the DC output and it's 13.79v so bang on but of course putting it under load is another matter. Whether the draw from the acc. bus or track is excessive is one thing but it doesn't seem right that a device that claims to be able to provide 2A struggles when asked for .5A.

[ISW]

1 minute ago, AndrueC said:

but it doesn't seem right that a device that claims to be able to provide 2A struggles when asked for .5A.

Have you tried to operate a few heavily loaded trains at the same time, to put some 'load' on the PowerCab and then watched the amps in the display? (you need to use the setup to change the display from 'time' to 'amps'). This could be very informative.

Ian

[AndrueC]

4 minutes ago, ISW said:

Have you tried to operate a few heavily loaded trains at the same time, to put some 'load' on the PowerCab and then watched the amps in the display? (you need to use the setup to change the display from 'time' to 'amps'). This could be very informative.

Ian

I leave the PowerCab set to show amps and I have two separate loops so can operate two at a time easily and from what I remember the current got closer to .3A but the trains didn't seem to run any slower. For reference all my trains are a bit less than a metre long and I have one approximately 2% incline that one of the trains would have to negotiate.

Edited October 25, 2021 by AndrueC

[AndrueC]

.3A for the acc. bus doesn't sound too bad to me either. Five decoders (four for the turnout controllers, one for the mimic), numerous LEDs (four signals + LEDs on the PCBs) and whatever current the turnout motors use when idle.

[Harlequin]

Track joints shouldn't "fail" at the rate you seem to be experiencing and I remember that you've reported various other "summer/winter" inconsistencies in running your layout in other threads.

 

My guess is that there's nothing wrong with the PowerCab but there is something fundamental about your layout, which you need to fix to make everything work reliably.

 

[Robert Stokes]

In your first post you mention voltages of 22V and 17V at the track. This implies an input voltage of about 24V DC assuming that these readings are correct. Is this the case? If so then it is much higher than the NCE recommended input maximum of 15V DC.

 

I use an input voltage of 15V DC and get 13,8V AC (not regular AC of course) at the track.

[Brian]

Coming to the "Party" a bit late.. Can I ask ...

Are you measuring the rail to rail voltage with the meter set to its AC voltage range?  DC is not correct for measuring DCC rail voltages.

Assuming AC is used be aware that most 'Domestic' style multimeter do not display an accurate reading. But what is read should be constant all around the layout. Any major loss of voltage indicates a issue that needs resolving ASAP

 

Is every section of track fitted with two dropper wires that connect to a larger wire size DCC Bus pair?  Or are you relying on the metal rail joiners (Fishplates) to pass data and power rail to rail?  If the latter, I would recommend you consider soldering droppers to as many rails as possible and just allow the rail joiners to align the track together.

 

Edit to correct typo.

Edited October 25, 2021 by Brian

[AndrueC]

Yes I'm measuring using VAC on my multimeter. I'm aware that the absolute values will therefore be inaccurate but I'd have thought they are at least correctly proportional to each other. Thus if the MM reports a drop from 22 to 17 I can fairly safely say that the track voltage has dropped by about 20%. My PSU is providing 13.8v and apparently the PowerCab 'costs' .5v so it seems to me that we can say that my track is going from 13.3v to 10.6v.

 

I don't think the track has serious problems. As I mentioned previously running two metre-long N-scale trains has never pushed the PowerCab above .3A which seems reasonable. Trains seem to run at a constant speed on all parts of the track and I only clean the track if I've been doing scenery work in an area.

 

Oh and although I use settrack curves and turnouts the straights are streamline. There is no change in loco speed while they are traversing a straight that has its own droppers which I assume rules out dodgy joints..or does it?

Edited October 25, 2021 by AndrueC