Cheap skate block detector questiongreenspun.com : LUSENET : Wiring for DCC : One Thread
I want to try out some of Allen's cheap skate block detectors to indicate occupancy in a hidden staging yard. I built a test circuit using 1N914 diodes that I had lying around. I realize these are probably inadequate even for N-scale locos like I have, but this was just for a test. The detector works as Allen says except for a problem with the locos speed in the detection block. The loco creeps with my Digitrax throttle full on. It has no power to even pull cars. I realize there is going to be a voltage drop with this circuit but this seems extreme. I am also curious what voltage LED I should use. Right now I am using one rated at 12 volts in my test circuit. I really would like to make these things work. I don't need anything more complex or costly at this time. Anyone know what I am doing wrong? Thanks, Michael
-- Michael Tondee (W4HIJ@msn.com), August 12, 2001
1N914 diodes definitely will not work. These are signal diodes NOT designed to carry much current. I'm surprised they haven't burned out. (I once had too much current flowing through one. It briefly glowed orange before burning out. It was fun to watch! :) ) Eventhough these diodes should destroy themselves in this application in a matter of seconds, I doubt this is your problem.
When you say you have a LED rated for 12V, I am wondering if you are using one of those special LED that already has a current limiting resistor built in. This would make sense as to why your cheap skate detector isn't working.
Note! Normal LED's are not rated to work at 12V! There are several specs to watch for, though for the cheapy detector, neither matter much. Just for the record, there is the forward voltage drop rating and the reverse voltage rating. It's the forward voltage drop that you have to take into consideration when selecting an LED for any application. LED's are typically about 2.6V; but they can be a little higher or lower. The reverse voltage rating is how much it can withstand when reverse biased. Normal diodes are designed for this. LED's typically have a low reverse voltage tolerance, but survive in DCC applications better than I'd expect. In any event, if you get one good for 30V, you'll never have to worry.
Briefly try your Digitrax booster set to HO. This circuit was designed on a Digitrax system set to HO for HO. You may have to leave out a pair of diodes or two for N. I don't have N so I can't try it.
My first advice is to make sure you are using an ordinary LED and NOT one with a resistor built in.
-- Allan Gartner (email@example.com), August 12, 2001.
Thanks for the pquick response Allen. My immediate suspicion was the 1N914 diodes. I knew they were a bit small but they were in my ham radio junk box. I had already tried taking the LED out of the circuit and it didn't seem to help. I m going to try some stouter diodes and a different LED. One follow up question if I may.... you mention possibly removing diodes from the circuit. My operaating scheme calls for double heading engines. In theory two N scale locos should not draw much more than one H.O. scale loco but I think I may get in trouble removing diodes. What do you think?
-- Michael Tondee (W4HIJ@msn.com), August 13, 2001.
The reason I SUGGEST that you might remove diodes isn't because of the current draw. It's because you have the booster on a lower voltage setting - since you are running N.
Try putting your booster on HO while your double headed locos are in a block with the cheapskate detectors. This will raise your track voltage enough that with your doubleheaded N locos - will give you a situation like HO. Then if things still run bad, something else is wrong.
-- Allan Gartner (firstname.lastname@example.org), August 13, 2001.
-- javed abbas (email@example.com), May 12, 2003.
-- qaisarabbas (firstname.lastname@example.org), September 03, 2003.