DCC power surge to bulbs??

[JRP]

I have an Athearn GP35 with a Soundtrax DSD-AT100LC decoder. I decided to use 12 volt mimi bulbs (as LED's are not recommended with this decoder) for the front and rear lights and connected them with 820 ohm 1/4 watt resistors with each bulb as instructed and they were working fine. But I noticed right away that if I pushed the "light" on button after first starting up power, the lights would come on very bright until I quickly turned the button off, then waited a few seconds and turned on the lights again (rear or front did not matter). Then the lights appeared as they should in their "normal" brightness or until I pushed the dimmer. This "extreme" brightness happened each time I first started up the power. The other day one of the front bulbs burned out. They haven't been used that long (maybe 3 hours max). What would cause the bulbs to come on overly bright in the first seconds of power on? PS. I'm using a Digitrax Zephyr system. Merry Christmas everyone! Thanks JRP

[Hamltnblue]

It sounds like a decoder setting. Here's a link to the manual

I see something in CV51 and 52 that say FX hyperlight. Maybe it's that or something in the other cv's that will fix it.

http://www.soundtraxx.com/manuals/lc_techref.pdf

[Hamltnblue]

After doing a quick scan of the manual I see the decoder has a dimable light feature.  I'm sure others will chime in but to me it looks like you might need a higher value resistor.  When in full brightness the voltage to the bulb might be a little too high.

[Hamltnblue]

After searching some more here's someone using LED's with that decoder

The value of resistor they note doesn't match the picture though.  The picture shows a 750 ohm which is close to yours.

http://www.trainbuddy.com/Reference/Decoding%20Engines/Sound/Kato%20SD90.htm

Also here's a soundtrax application guide which notes using 1k resistors which once again is close to your value.. Their example is hooking up to an LED

http://www.soundtraxx.com/documents/appnotes/katoac4400.pdf

[rrinker]

820 ohm seems a bit high to use with 12v bulbs. If anything, the light should be dim. 1/4 watt is also probably too small, perhaps it overheated and failed, although typically a resistor would fail open, not in a short-like condition.

 Not sure on this one, unless maybe one of the connections was touching the chassis or a pickup wire somewhere.

                                    --Randy

 

[JRP]

We'll Everyone, Hold the phone....I goofed...sorry...wrong information. I am actually using 1.5 volt mini bulbs (not 12 volt) and a 560 ohm 1/4 watt resistor as recommended by Soundtrax. However, I was only installing one resistor for both bulbs. After reading the Soundtrax instructions again carefully, they advise to use 2 resistors, in this case one for EACH bulb in series. I got that corrected, and the bulbs work again, but both bulbs still light up too bright when the power is first turned on. So, I will take a look at the CV's you mentioned and see if I need to tweak them. Otherwise I just don't know why the bulbs get so bright in the beginning, even using the recommended resistors. JRP

[CSX Robert]

Are you sure you're not seeing the normal reaction of an incandescent bulb? When power is first applied to one, it will come on brighter than normal and then dim. It is caused by the bulb's filament having a very low resistance before power is applied, then once power is applied and the filament heats up, it's resistance will increase, causing the bulb to draw less current and dim shortly after having power applied.

[rrinker]

 You need to know the current rating for the bulbs to calculate the correct resistor. All 1.5 volt bulbs are not created equal. 15ma and 30ma are common ratings. For 15ma 1.5 volt bulbs, assuming 12 volts at the function output, a 680 ohm resistor would be closer, if over 12 volts, 820 ohms would be the choice.

                                 --Randy