Re: Light on turntable arch


Vollrath, Don <dvollrath@...>
 

Rich,

 

Not having reverse polarity protection to an LED powered directly from the DCC supply can cause a shortened lifetime of the LED. Yes, the LED seems to light correctly with current flowing in one direction, as when the DCC voltage reverses the LED does not light up but a human does not notice the blinking. HOWEVER most LEDs will suffer reverse voltage breakdown and conduct current in the reverse direction. Rated reverse voltage varies with LED color and manufacturing batch… usually only 5-7V. [look for the PIV spec on the data sheet.] So even with the resistor still in series to limit current… the forward plus reverse power dissipation creating internal heat in the LED is far greater than intended when operating in this mode. An extra hot semiconductor junction (the tiny part that gives off the light) shortens the lifetime. White or blue LEDs will be affected the most. Putting a diode in series or anti-parallel with the LED will work to protect the LED.

 

DonV

 

From: WiringForDCC@... [mailto:WiringForDCC@...]
Sent: Sunday, June 26, 2016 11:03 AM
To: WiringForDCC@...
Subject: Re: [WiringForDCC] Light on turntable arch

 



A 1k, 1/4 watt resistor works. My NCE Power cab panel has a common 3mm red led with resistor across the DCC output.

 

Rich

On Sunday, June 26, 2016, 10:20 AM, riogrande491 riogrande491@... [WiringForDCC] <WiringForDCC@...> wrote:



David -

 

Making it work is pretty simple. The DCC track signal actually has both positive and negative polarities, so the LED can light up full-circle, except for the dead-bands when the polarity on the bridge reverses.

 

However, there is a very important detail to which we must first attend. LEDs don’t like being connected backwards to a voltage greater than about 5 volts, and a typical DCC installation runs about 15 volts. If we don’t do something, about half the time the LED will experience a 15 volt reverse voltage. Soon they can grow dim or entirely fail. Others might be happy for the life of the layout. So if we don’t want risk tearing into the turntable to replace the LED, we need a way to prevent the reverse polarity from causing a future problem.

 

For the sake of discussion, let us assume that you will use a 1K ohm a/k/a 1000 ohm resistor in series with the LED. The particular resistor value is relatively unimportant. Let us also assume that the resistor is located underneath the turntable deck. Connect a small diode, typically a 1N914 or 1N4148 (a small diode with a glass body and two leads) across the two wires running up to the LED. Polarity is important! If it is wrong, the LED won’t light, and it is time to swap the diode end-for-end. If the LED lights up, and connections are secure, life is good!

 

The usual polarity of a 3mm LED can be found by the length of the leads. The longer one is usually positive or +. On a diode, one end has a band. That is the cathode or - connection. Just solder the cathode or - of the diode to the wire running to the longer lead of the LED, or +. Then it should outlast both of us.

 

All the best!

----------------------------------------------

Bob

 

 




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