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Discussion Starter #1
I've read a couple of threads on ways to power these built up and lighted buildings, but they all seem to mean running power busses (120VAC, 24VDC, and 4.5VDC) under the layout. I definitely don't want to run household power, and the other two mean a lot of wires. After reading some of the threads on here and other resources, I have an alternate idea I'd like to bounce off of the forum.

Since I have track voltage (18VAC) available from a powerhouse so I'm assuming a clean sine wave, I thought using a full wave bridge and a 3300 microfarad capacitor would give me 25.5VDC. I can run this through a buck DC to DC converter and get the required 4.5V or 24V at the point where I need it.

This seems like a simple solution, so I'm wondering if anyone has tried it or if there might be a problem I'm overlooking?
 

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My Menards buildings all run on 4.5V DC.
 

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That's a good plan, you can pick up reasonably priced DC-DC converter modules that will handle several amps, usually for a dollar or two.
+1.

You need a DC supply with a higher voltage than the target DC voltage (losses in the DC-DC step-down converter/regulator). For 24 V DC, the rectified 25.2 V may work well, may not. Might be load dependent.
If you have one old laptop "brick" circa 18 V is a great switching DC supply for accessories. They're robust providing 3.5+ amps and highly stable up to their max load. You can buy DC-DC step-up converters for the same for a buck or two as the DC-DC down converters. They'll supply 24 V DC off the lower brick voltage while the DC-DC step-down converters supply lower voltages for other accessories.

Using a AC house current to low voltage DC switching regulator will provide a highly stable DC supply for the accessory DC-DC converters regardless of load. If you're an electronic hobbyist and have the bridge/capacitor, go for it. If not, look to eBay for ~$10 24 V DC LED strip brick power supply. Won't cost much more that the electronic parts after shipping. And you'll get the 24 V DC by default only needing the step-down converters.
 

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I supplied track power (around 18 VAC) to this $2 Evans rectifier and hooked up three raw 3V LEDs in parallel to light a station. Shelly at Evan said the rectifier will power up to 4 LEDs brightly, but add any more and the lights are less bright. I don’t know how well it would work to power a pre-lighted building, but it worked out great for the station.

Rectifier for LEDs

544157
 

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I supplied track power (around 18 VAC) to this $2 Evans rectifier and hooked up three raw 3V LEDs in parallel to light a station. Shelly at Evan said the rectifier will power up to 4 LEDs brightly, but add any more and the lights are less bright. I don’t know how well it would work to power a pre-lighted building, but it worked out great for the station.

Rectifier for LEDs
The reason the $2 Evans is only good for a few LEDs is becuase it is not a voltage regulator. It's just a diode, a cap and a resistor. The resistor is sized to power 3 to 4 LEDs. If you add more LEDs the voltage drop across the resistor increases (due to the extra milliamp load) thus the LEDs get less voltage. The DC-DC converters that GRJ mentioned work differently. They hold a constant voltage up to their max load of several amps. You can power 100+ LEDs off a single DC-DC converter.
 

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+1.

You need a DC supply with a higher voltage than the target DC voltage (losses in the DC-DC step-down converter/regulator).
In this case, he's trying to power Menard's buildings at 4.5VDC, so I don't think the rectified 18V is too low a voltage. ;)
 

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You didn't read the whole original post. He wants to run an 18V filtered DC bus around and tap off it with DC-DC converters to generate the proper voltages for each application, obviously 4.5 VDC for the Menard's buildings. I know sometimes we appear stupid, but not that stupid! ;)
 

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Discussion Starter #11
I have completed my prototype with success. The bridge + capacitor combination gives me 24VDC from the 18VAC track power for the Woodland Scenic's Just Plug lighting, and the buck power converter steps that down to the 4.5VDC. I hooked it up the Menards Vetter Sash and the Woodland Scenic Tavern, and both worked just fine. Now I can tap off of track power where ever I need DC without running long DC busses under the layout.

You are correct, Mike: the 18VAC gives me 25.4VDC in theory, and the two diodes conducting in the bridge give me a 1.4VDC drop, so I'm right at 24VDC.
 

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Discussion Starter #12
I thought I'd share a picture of the power supply as I've implemented it. I currently have 3 of these on the layout - one at each of the three towns modeled. This photo shows the power output (4.5V and 24V) as well as the power for 4 uncoupling tracks. The only thing not shown is that I've put a diode in series with some of the Menards buildings to drop the voltage from 4.5V to 3.8V to slightly dim the LEDs on those buildings.
545224
 
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