Using LEDs as DCC power indicator

[kenkal]

I'm currently wiring the control panel for my turntable. 

I have 16 storage tracks around the turntable.  Each track has it's power on or off via a SPST toggle switch.  I plan on using LEDs with the appropriate resistor to indicate when the tracks are powered. When the switch is thrown to "on", the left rail power will be connected to it's matching left rail track and would also go through the LED and it's own resistor to the right rail. The track's right rail is always connected to right rail power. Anyone see a flaw in this? 

I thought maybe I should put one diode between the 20 LEDs tie point and the right rail connection as I wondered about the effect of the constant rise and fall of the DCC voltage on the LED. If that is a concern, any problem with using just one diode?  Or should each LED have it's own?  Since the total current draw (16 x 10ma) will likely be less than 200ma even if, while unlikely, all tracks are on, then a 1N914 (200ma, 75PRV) should be adequate.

Thoughts?  Thanks.  Ken

[Vail and Southwestern RR]

I'd start by saying that it seems you are going to a lot of trouble and expense to do something that isn't necessary (to me), but given that you want to do it...

Yes, you want a diode to protect the LEDs against reverse voltsge.  DCC track voltage is beyond the reverse voltage rating of most LEDs, and eventually, you would probably have failures, eventually.  I don't see any problem with a single diode in that role, though obviously a failure there brings down the whole network.

[larak]

Ken,

It sounds fine in theory. Being a bit conservative I would use a 1 amp diode though. They are cheap and I like to rate solid state devices at 200%. You are cutting it close with an expected 160mA on a 200mA device. You might want to expand some day too.

Also remember to design your resistors for the PEAK voltage of the DCC waveform or current draw may be more than you expect. Some meters measure "average" some "true RMS" and some peak to peak. Not an issue if you overrate the passive components.

Karl 

 

[kenkal]

Yeah, a lot of work and perhaps not really necessary, but I enjoy it.  I figured since I have to wire each track anyway, (most of my locos are sound powered and I don't like them all on at the same time because of the power consumption and noise) the 15 cents per switch for the LED and resistor was not a lot. And I get a quick visual status of each track.

Since you mentioned it, I think I'll go with a 1N4001 1A diode for a little over protection since I'd hate to have to mostly re-wire the LEDs should a lesser rated diode fail.

Thanks for your input, Jeff.

Ken

[kenkal]

Hi, Karl,

Good point Karl.  As I mentioned in my reply to Jeff above, I decided to use a 1A.  I did allow for conservative current draws by using a 1500 ohm resistor with each LED.  With about a 14.2 track voltage, that's about a 9-10ma draw.

Thanks for your thoughts.  Ken

[stokesda]

There's an article in the current issue of Model Railroader (the May issue?) that explains how to use the DCC bus wires as a power source for LED's to light structures and passenger cars. I suppose if you changed the white LED to a different color, you could use the same setup as a DCC power indicator on your control panel.

[cacole]

I took your idea one step further and used some 1.5 Amp 400 PIV miniature bridge rectifiers puchased in bulk from All Electronics, Catalog Number PWB-11, and a 1K Ohm resistor.  Using a bridge insures that the LED always gets voltage of the correct polarity, and the 1K resistor insures a very long life for the LEDs.