Stan, I measured 20 ma DC at each led, then 120 ma DC right after the bridgeand cap, so this makes more sense. I was feeding the breadboard with a Lionel 4150 MPC starter set transformer. When I read ma AC from transformer to bridge, I get 150 to 200 ma AC. This is in the DMM's 220 ma range band. So it may be partly the meter, and it maybe the power dissipated by the bridge (DB107 1A 700V) and eBay Chinese 100uf cap?
Originally Posted by stan2004:
Originally Posted by RRaddict2:
Here is a schematic for a simple circuit that will give you constant lighting. It is similar to the others posted.
I took the liberty of including your attached schematic below. Perhaps you attached the wrong schematic? The following is not a constant lighting circuit; the brightness will vary with track voltage. Also the capacitor voltage (20V) is under-rated for O-gauge command voltage operation. I realize this thread is about low-voltage conventional so maybe your transformers can't reach command voltage levels. But higher voltage capacitors (e.g., 35V, 50V) are about the same price so irrespective of control circuitry used, if modifying an O-gauge AC-powered car for DC LED operation, I'd use a higher voltage capacitor.
Stan
I think that resistor is undersized. With the capacitor at maximum throttle there would be over 24 volts DC into the LED and resistor. You would need at least a 1000 ohm half watt resistor. 1200ohm would be better. If you install a Cl2-n3 chip instead of the resistor you would get CV lighting
Dale H
Note that the CL2 will be running pretty hot dropping 21 volts, the TO-92 package doesn't handle a lot of power well, temperature rise with that drop will be over 50C. I've done this on the bench, it gets REALLY hot.
Originally Posted by CK:
Stan, I measured 20 ma DC at each led, then 120 ma DC right after the bridgeand cap, so this makes more sense. I was feeding the breadboard with a Lionel 4150 MPC starter set transformer. When I read ma AC from transformer to bridge, I get 150 to 200 ma AC. This is in the DMM's 220 ma range band. So it may be partly the meter, and it maybe the power dissipated by the bridge (DB107 1A 700V) and eBay Chinese 100uf cap?
Understood. I believe you're seeing a measurement artifact when using the AC-current function of some DMMs. I'm sure someone else can state it more simply but here's how I see it. If an AC voltage drives just a resistor, the AC current follows the voltage...double the voltage, double the current; if the voltage is a sinewave, so too will be the current. But with that bridge rectifier and capacitor, the cap gets charged up to some DC voltage and current only flow through the bridge when the input voltage exceeds the charged capacitor voltage. This occurs at the peaks of the AC signal so current from the transformer into the bridge is actually zero most of the time. In other words you can have a smooth AC sinewave voltage going into the bridge, but the AC current will be something different. It is this something different current signal that I suggest is causing the apparent measurement error. A discussion of how to accurately measure complex AC waveforms with the high-frequency electronic signals in modern O-gauge circuits can get somewhat nerdy.
Originally Posted by stan2004:
A discussion of how to accurately measure complex AC waveforms with the high-frequency electronic signals in modern O-gauge circuits can get somewhat nerdy.
Probably a lot nerdy! 
This is why I sometimes test with DC for these measurements. I can measure the current accurately using DC.
