What voltage should I expect when decoder set to LED lighting

[Mark R.]

The "LED Mode" setting on the decoder has nothing to do with the actual output of the function terminals in terms of voltage / current. It is a setting within the decoder to alter the output for different lighting effects.

Bulbs and LEDs re-act very differently. When using a bulb, the brightness ramps up to full on and dims out to off. An LED is instant on and instant off. To replicate different effects (strobe, mars, etc.) the LED mode allows the output to the LED to re-act more like a filament bulb.

Mark. 

[richg1998]

My decoders all measured about 12 to maybe 12.5 volts DC. Used 1k resistors.

I do not have any that had LED terminals.

The decoder instructions were very clear.

Rich

[rrinker]

 If open circuit (no LED connected) and you are using a decent meter with a high input impedence like 10 MOhm or more, and the decoder uses a current source instead of simply including resistors, your voltage reading may be way off. The high impedence of the meter means it puts practically no load on the circuit unde test (usualyl a GOOD thing) but that means a constant current source set for LEDs will eb nowhere near its limit, so the voltage will go as high as it can - I wouldn;t be surprised to see a 12V or more reading. That doesn;t mean you can;t conenct an LED directly. The voltage to an LED doesn;t really matter, it just needs to be more than the junction drop, which for a white LED is about 3.5-3.7V. LEDs ARE sensitive to current, too much and the LED blows up.

 Some meters, like one of the ones I have, have a "Low-Z" volts mode. This is a low impedence input which actually DOES put a noticeable load on the circuit under test - something liek this might give an accurate volt reading froma  constant current source.

 Basically - don't worry about it. If the decoder specifications and instructions say you can hook up an LED directly withotu resistors - you can hook up an LED without resistors. If it says you cna do both, pay careful attention to the wiring diagram. Usually you connect to one terminal for the current limited output and a different location for the full power version of the output. Don't mix them up, or you will either have LEDs that pop like flashbulbs because they were hooke to the fiull current outputs, or you wil have incandescent bulbs that won't light because they are connected to the limited output.

                                  --Randy

 

[gmpullman]

Don't assume that because a decoder has an "LED" mode that the output of the function is the correct voltage.

 

In many cases (you really have to dig into the decoder's specific specification) the LED "effect" merely ramps up and ramps down the "on-off" function so that the LED mimics the look more like an incandescent lamp.

 

If there are on-board pads for LED's or traces to jumper, then you might be OK. TCS has a motherboard with solder pads for LEDs.

https://www.tcsdcc.com/Customer_Content/Literature/Motherboards/AS-MB2.pdf

 

For instance, from the ESU Loksound select manual:

LED mode: The LokSound Select’s outputs are pre-set to be used with incandescent bulbs. However, because of their differences in brightness characteristics, some lighting effects will not look prototypical if you use LEDs. If you wire LEDs on the output, you should set the LED compensation mode bit in order to get satisfying characteristics.

It would be helpful to know exactly which decoder you have.

Regards, Ed

[passenger1955]

Great. Thanks.

[tstage]

Generally, 3mm LEDs are rated at 3V but some are 12V.  So, whatever the LED is rated at, the voltage (plus resistor) should be less than that rating.  And, the lower the voltage, the better for longevity of the bulb.

Tom

[passenger1955]

What voltage should I expect to see on a voltmeter between the 2 leads when the decoder set to LED lighting mode?