Re: Coach Lighting
Vollrath, Don <dvollrath@...>
John, what I don’t know is what is inside the LED light strips you have in your hand. I’m guessing that there are 4 or 5 LEDs wired in series with a current limiting resistor (or maybe an active current regulator IC, NOT the same as a voltage regulator) with the intention of the strip being powered from 12-14Vdc (think auto-car battery). There probably is a (+) or (-) mark where you are to attach power wires. Yes, each LED is indeed a diode supposedly to conduct current on only one direction. However as others have pointed out the rated reverse breakdown voltage of each LED is limited… and particularly low for blue and white ones. If/When voltage is applied in the reverse direction, like directly from DCC on the rails, which is actually alternating in polarity, some current may flow backwards through the parts. It may appear to work OK for a long time or they may fail catastrophically without warning. One way to prevent that is to put some sort of rectifier in the circuit so as to protect the LED cells against reverse voltage and current flow. There may already be a diode or other protective device inside the lighting strip.
Either way… To add a flicker-free power carry-over mechanism, you have only three choices: 1) purchase some commercial product and pay their price or 2) utilize independent battery power or 3) become a D-I-Y expert enough to figure out what is inside the LED strip and come up with an external carry-over mechanism using readily available low cost components. The thought of using a 5.5V supercap is a good one, but you are not going to get more than 2 LEDs in series to light up [If you try this don’t forget the series resistor to limit current.] You will need to trace out the circuitry inside the lighting strip to be able to cut PCB traces to connect directly to 1, 2 or possibly 3 LED cells and wire them up to external circuits.
Each external circuit should start with a 50V rated full wave rectifier with the AC inputs attached to power pick-ups from the truck(s). the DC side of the rectifier needs to power the charger for the supercap or rechargeable battery. It must be current limited and have over-voltage protection for the supercap or battery (whatever that maximum working voltage may be) to protect from catastrophic failure of the storage cell. Yes a good method is to place a zener diode across the battery or supercap. From the storage cell, supercap or battery, there needs to be a current limiting resistor in series with the 1, 2 or possibly 3 LEDs to light and control current. Adjust the resistor to determine lighting intensity. LED current is likely to be 8-20 milliamps. For more LEDs in the same coach, connect up multiple LED and resistor circuits as just described in parallel to be powered from the energy storage cell. Sounds simple enough, but without knowing anything else about what components you are likely to use and their maximum voltage rating it is difficult to show actual wiring or resistor values.
Contact me directly by email if you wish
From: WiringForDCC@... [mailto:WiringForDCC@...] On Behalf Of John Cahill
Sent: Thursday, January 30, 2014 11:57 AM
Subject: Re: [WiringForDCC] Coach Lighting
Don, I may have mis-described initially. I was trying NO rectification and feeding direct track DCC voltage into the LED strip. You have explained the need for a 50V diode in line with a suitable rated resistor to that circuit to protect the LEDs. I will use this going forward. Don't really want to put a big cap in coaches (weight, size, cost factors}. However, I recall a drawing of a lighting circuit I found on line a few years ago which showed a zener across the rectifier outputs and a small supercap in parallel for flicker free lighting. Does this make sense? Voltage was around 5.5V out of zener, if I remember correctly, but my knowledge of electronics is rather basic, I'm afraid.
On Thu, Jan 30, 2014 at 7:52 AM, John Cahill <johncahill25@...> wrote:
Thanks Everyone for your helpful suggestions. I'm learning slowly! Will now add 1N4001's + resistor in circuit.
Can anyone add thoughts on using super capacitors in the circuit for flicker free operation instead of a battery?