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Hi All,

I am fitting a decoder in an HO loco and in the instruction for doing it mention is made of half wave or full wave lighting, can someone define the difference between them for me please.

Many thanks.
 

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Presumably in the context of connecting half wave lighting between the function wire (white, yellow etc) and the red or black, and full wave between the function wire and the blue.

The voltage between the two rails on a DCC system is always the same size* but oscillates about 8000 times per second between positive and negative. Full wave rectification of the DCC signal provides the function voltage 100% of the time when the function is activated, so you will have a steady voltage a bit less than the nominal track voltage. Half wave rectification provides the same voltage but only when the rails are the correct polarity, so you will only get the voltage for half the time.

For bulbs half wave rectification is the same as supplying half** the voltage. However for LEDs you probably still need the same value of resistor in the circuit as the LED may not like having double its rated voltage even for 1/8000 of a second. With the same resistor the LED will only give out half as much light.

*unless you adjust it on your command station or use something called ABC which needn't concern us here.
**this will change a bit if you use loco zero, again you probably don't need to worry about it!
 

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Discussion Starter · #3 ·
Thanks Edwin becomes clearer now
 

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QUOTE (Edwin @ 29 Oct 2008, 18:00) <{POST_SNAPBACK}>Presumably in the context of connecting half wave lighting between the function wire (white, yellow etc) and the red or black, and full wave between the function wire and the blue.

The voltage between the two rails on a DCC system is always the same size* but oscillates about 8000 times per second between positive and negative. Full wave rectification of the DCC signal provides the function voltage 100% of the time when the function is activated, so you will have a steady voltage a bit less than the nominal track voltage. Half wave rectification provides the same voltage but only when the rails are the correct polarity, so you will only get the voltage for half the time.

For bulbs half wave rectification is the same as supplying half** the voltage. However for LEDs you probably still need the same value of resistor in the circuit as the LED may not like having double its rated voltage even for 1/8000 of a second. With the same resistor the LED will only give out half as much light.

The same is true for LEDs. Half wave will effectively be half the voltage (the same voltage for half the time, averaged out). Reducing the resistor value will result in increased current through not voltage across the LED. LEDs can be abused quite a bit and will easily handle increrased current when pulsed this way, especially if the existing resistor was such that the current on full wave was only 5mA or so. You may not need to go down to half the resistor value, anyway as the human eye will be folled by the higher brightness pulses. This is the theory behind red LED bicycle lights (I'm not sure about white LED front lights). They are pulsed at quite high current to get a very bright output. The persistence of vision in the human eye gives the effect of a constant high brightness, rather than averaging it out between on and off periods. This results in longer battery life

Andrew
 

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QUOTE (SPROGman @ 30 Oct 2008, 22:41) <{POST_SNAPBACK}>The same is true for LEDs. Half wave will effectively be half the voltage (the same voltage for half the time, averaged out). Reducing the resistor value will result in increased current through not voltage across the LED. LEDs can be abused quite a bit and will easily handle increrased current when pulsed this way, especially if the existing resistor was such that the current on full wave was only 5mA or so. You may not need to go down to half the resistor value, anyway as the human eye will be folled by the higher brightness pulses. This is the theory behind red LED bicycle lights (I'm not sure about white LED front lights). They are pulsed at quite high current to get a very bright output. The persistence of vision in the human eye gives the effect of a constant high brightness, rather than averaging it out between on and off periods. This results in longer battery life

Andrew

**I doubt that you'll notice any reduction in LED output at all at standard track voltage.

When we install white LEDs for example, we go from a "safe standard" of 1k for a full brightness diesel to a high of around 80k ohms for a steam loco bufferbeam lamp with the same LED.... so this sort of change that maintains the voltage anyway will do B-all in pragmatic terms...

Richard
 
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