Getting 12 volt AC from my MRC Tech4 that puts out 16 volts AC......??

[fwright]

3rd rail

Hi folks, I have wired up my lighting on the layout with all 12 volt fixtures and as I have added more and more streetlights, building lights, etc, the old "trainset" powerpack that I was using for this lighting source will no longer handle the load. I have a MRC tech4 350 pack that has more than enough "balls" to handle the load, but it is pushing 16 volts at the ACC terminals. I suppose I need to install some resistors or something, but I know almost nothing about that kind of thing. What's the best way to get that down to 12 volts AC?  The "straight DC' terminals are at close to 20 volts, so I don't want to use them. Any advice would be greatly appreciated.

Thanks,   Todd

 

The voltage will drop a little under load, but it won't solve your problems.

On an upscale MRC power pack, there may be a switch to reduce the maximum DC voltage.  The switch positions are generally labeled "G" and "HO or N", or something similar.  The G setting provides the extra DC voltage used by large scale trains (and MTH locos run on DC   ).  The other setting should limit maximum DC voltage to around 12.  But that still doesn't solve your problems.

The problem is not your bulb fixtures, but your rated bulb voltage.  Chances are that 12V rated bulbs will be too bright and too short-lived when run at 12 volts for you to be happy.  Bulbs like to run at 10-25% under their rated voltage, and most bulbs are too bright for our purposes at rated voltage.  There are bulbs of various voltages available in most standard socket/base styles.

If you are going to use the 16V AC terminals of your power pack, you want at least 18 volt bulbs.  Or put two 12 volt bulbs in series, so that each bulb gets 8 volts.  As suggested, you can run off the variable DC terminals of your power supply and adjust the voltage to your liking.  This is probably the simplest and best alternative - you can use whatever bulb rating suits you and then set the voltage for the desired effect.  Bulbs don't care (for our purposes) whether they get AC or DC.

The diode solution is the other recommended way to get a voltage drop from your supply.  Resistors waste power as heat (a bulb is a very hot running resistor), and the voltage drop depends on the current going through the resistor.  Diodes have a pretty constant voltage drop within their current rating - about 0.65 volts/diode.

hope this helps

Fred W

[Vail and Southwestern RR]

dsabourne

Todd:

 Voltage (V) = Current (I) x Resistance (R).

You need to confirm the current supplied by the powerpack (check spec book) and then determine the resistance (R) required by the above formula.  Not sure of the next step though ... I'm civil engineer not an electrical engineer ... LOL.

 

Not quite so simple, since the current is going to depend on the load, not on the supply (unless it's maxed out). I lean towards using diodes to drop the voltage. I'd be tempted to use sets of four parallel to each other in both directions, to drop the voltage about six Volts. If that dims the lights too much, you could try with three. The other possibility would be to use the DC output and wire the bulbs in sets of two in series, with the disadvantage of having any single failure taking out two lights.

[dsabourne]

Todd:

 Voltage (V) = Current (I) x Resistance (R).

You need to confirm the current supplied by the powerpack (check spec book) and then determine the resistance (R) required by the above formula.  Not sure of the next step though ... I'm civil engineer not an electrical engineer ... LOL.

 

[3rd rail]

Hi folks, I have wired up my lighting on the layout with all 12 volt fixtures and as I have added more and more streetlights, building lights, etc, the old "trainset" powerpack that I was using for this lighting source will no longer handle the load. I have a MRC tech4 350 pack that has more than enough "balls" to handle the load, but it is pushing 16 volts at the ACC terminals. I suppose I need to install some resistors or something, but I know almost nothing about that kind of thing. What's the best way to get that down to 12 volts AC?  The "straight DC' terminals are at close to 20 volts, so I don't want to use them. Any advice would be greatly appreciated.

Thanks,   Todd