Further question on Using LED’s with DCC

[CDRC]

Hi,

I have recently added 3 additional Peco electrofrog turnouts to my DCC layout. Each controlled by a DCC Concepts IP Digital motor powered from an separate accessory bus and using 2 push buttons on each point as a control as per DCC Concepts guidance here-

https://www.dccconcepts.com/manual/cobalt-ip-digital-controlled-by-push-button-switches/

Happy with the result I ballasted the additional track and turnouts, again testing each with a run with a loco. I then decided as per my larger mimic board to add LED indicator lights to the buttons. As per DCC Concepts guidance here-

https://www.dccconcepts.com/themencode-pdf-viewer-sc/?tnc_pvfw=ZmlsZT1odHRwczovL3d3dy5kY2Njb25jZXB0cy5jb20vd3AtY29udGVudC91cGxvYWRzLzIwMTYvMDIvTEVEcy1hbmQtaG93LXRvLXVzZS10aGVtLnBkZiZzZXR0aW5ncz0xMTExMTExMTExMTExMTExMSZsYW5nPWVuLVVT#page=&zoom=auto&pagemode= 

Each of the turnouts has 2 push buttons and 2 green LED's indicating open track (green) 2 of the turnouts (19a & 19b) are stand alone in operation and are not connected in anyway other than sharing the same accessory bus. The other turnout (10) connects to a small goods yard.
Following the guide I ran a small jury rigged setup to ensure my wiring was sound. I connected to the frog polarity for 10 and happily the rig worked the LED's switched as the turnouts switched. I wired up the LED's on the others 2 as per the rig and included on all an additional higher rated resistor on the correct side to reduce the brightness. (The LED's came prewired with a small resistor so the larger is on the same side)  The LED's for turnout 19a and 10 both work perfectly The LED's for 19b not so. On turning on my Elite 1 LED comes on but when I operate the turnout both stay on! I have tested the LED's using a 9v battery and both work. I decided to reverse the wiring link to the bus, the LED's glowed but didn't switch. I reversed and changed the diode no luck. I tested for continuity from the frog connection at 19b all the way back to the LED and there is no break in the wire so power was travelling from the turnout. I took the existing LED's out of the set up and used 2 others, no change. The power to each on the LED's pairs is via the accessory bus to a small distribution board. Somewhere along the way I have wired these LED's incorrectly but I cannot figure out how or where. This was yesterday. I decided to sleep on it and tackle it fresh this morning. I repeated my steps of yesterday but without success. Suggestion, advice, ideas, large scotch would really help.

thanks in anticipation  

Charles

[2e0dtoeric]

You need to download a circuit diagram of YOUR set-up, so we can possibly see what you have done.

It's usually something silly that you have missed.

-

(Similar to writing a long speil about something, and no matter how many times you re-read it, as soon as you have 'posted' it, an error glares at you, because you check-read what you thought you put, not what you really put!)

[Chrissaf]

What Eric says. Plus your second link opens a page documenting 7 pages of wiring options. We would be just guessing which option you are referring to, but I assume it is the option on Page 7 as that appears to be the only DCC one documented.

[CDRC]

Thanks for your observations. I'm not very good at circuit diagrams so I have attached 4 photos of the actual setup.

1 shows the LED's with the turnout thrown in one direction 

2 shows the LED's with it thrown in the other direction 

3 shows the LED with the diode reversed and the turnout thrown

4 shows the LED thrown in the other direction - please note the difference in the brightness

/media/tinymce_upload/b3d5ff3ce6c799c8053efb28690372c2.png

/media/tinymce_upload/679f5d13f61965866f49255ba2976401.png

/media/tinymce_upload/6bcb433333f124272288d0490a32e670.png;/media/tinymce_upload/835a6551ff922dd9b4af91e58694f60a.png

[Chrissaf]

Sorry for the delay. I needed to undertake a full technical evaluation of your photos, this took some time.

.

NOTE:

First of all a question. Did you purchase the LEDs pre-wired or did you connect the red and black wires yourself. My assessment and evaluation of your issue is that one of the green LEDs has been wired in reverse - see first set of schematics below. Such that the red wire on the LED is actually the negative side of the LED and not the positive side.

.

The second conclusion I have come to and can prove with my second set of schematics below, is that there is an error on the DCC Concepts diagram on Page 7 of the instructions. DCC Concepts have drawn the diode reversed. They show the triangle facing upwards from the frog, whereas the circuit will only work if the triangle faces downwards towards the frog (as indicated on my schematics below).

.

Because one of the LEDs is reversed, the schematics below show the two green LEDs facing the same direction. Whereas the two LEDs should be facing each other 'cathode to cathode' as shown on the DCC Concepts Page 7 drawing. The LED on the right of the LED pair is facing the wrong way.

.

/media/tinymce_upload/bd124c412761da74a04b462b026bda32.jpg

.

How to read the schematic above:

.

The power for the LEDs is DCC Bi-Polar track voltage which means that the voltages on the 'DCC Leg A' and 'DCC Leg B' will be alternating at a nominal 7,000 times a second. Thus the 'what is happening' assessment has to take into account both of these opposite DCC voltage states. Thus Fig B represents the opposite rail polarity to Fig A and Fig D represents the opposite rail polarity to Fig C. Because the polarities will be alternating at a nominal 7,000 times a second. The human eye cannot see the flashing and perceives the LED as always being on. Red and Blue on my schematics represent Positive & Negative.

.

OK, that's the background .... now to the explanation.

.

PHOTO 1) - Fig A

In this schematic. Positive is on DCC Leg A and negative on DCC Leg B. The LED on the right is in essence 'short circuited' by the diode which is 'forward biased'. Being 'forward biased' a current can flow through the diode via the LED on the left as indicated by the red arrows. The left LED lights, but the right LED is extinguished.

.

PHOTO 1) - Fig B

In this schematic. The DCC polarities are reversed. Both the LEDs AND the diode are now 'reversed biased'. No current can flow in any path and both LEDs remain extinguished.

.

The result is that the left LED is being pulsed 7,000 times a second and appears to the human eye to be permanently on.

.

PHOTO 2) - Fig C

In this schematic. The DCC Leg A is positive. This 'reverse bias' the diode so that it cannot pass any current. The two LEDs however are both in 'forward bias' by the DCC polarity. So they both light up.

.

PHOTO 2) - Fig D

In this schematic. The DCC Leg A is now negative. But although the diode could potentially be in 'forward bias' the two LEDs are both in 'reverse bias' so neither of the two LEDs will light up.

.

The result is that both LEDs are being pulsed 7,000 times a second and appears to the human eye to be permanently on.

.

Conclusion & Suggested Fix.

To correct this issue I would expect that reversing the red and black wires to the LED on Photo 1 that IS NOT lit will make the circuit work. Note the direction of the diode on Photo 1) ... make sure that you have the diode as shown on the photo.

.

---------------------------------------------------------------------------------------------------------------------------

.

Just for additional info. The next set of schematics show the two LEDs the correct way round (cathode to cathode) and demonstrates how the circuit is supposed to work. I won't provide a detailed text description, the schematics should be self explanatory if the previous descriptions on the first set of schematics were understood.

.

/media/tinymce_upload/d2959a5aff75cf000c22e14f56012f20.jpg

.

Please DO NOT use the 'Blue Button' to reply. If you do, I will remove it after it has been approved by Admin. Scroll down to the 'Reply' text box.

 

[CDRC]

Hi Chrissaf,Thanks for what is a comprehensive response. I think I have followed what you are saying. It took a number of readings but I got there in the end. In answer to your opening question, I did purchase the leds ready wired.

When I removed the paired LEDs on the problem point and repeated the test with 2 different pairs (and diode) the problem was the same. I then used the original (problem) pairs of LEDs on one of the points that was working correctly it worked fine. I then rewired 4 separate LEDs with resistors and tested those and the problem was still there so my conclusion was at that stage there was a problem with the switch motor.

In a previous post concerning the wiring of 2 aspect light signals to DCC Concepts IP Digital point motors a wiring schematic was suggested either by yourself or Flashbang as to how to go about using the connectors 7,8 and 9 on the switch motor to operate the signal lights. Using this method, the leds (the original problem led pairs from the original wiring setup) work perfectly and so I have used this method and have now completed the additional lighting on the mimic board.

Thank you so much for going to such lengths to explain what the problem was. I think at this stage I'm going to leave the 'Problem point' wired the way it is as I don't wish to tempt fate by taking it down. 

Thanks again

Charles

[Chrissaf]

Changing the contacts on the iP Digital from the frog to the metallic ones was going to be my next suggestion.

.

Is the problem point motor by chance using a separate accessory BUS from the main track connection for the DCC power input. If it is, then using the frog switch contacts is not advised. I'll only explain the 'reason why' if you say it is a separate accessory BUS to save my wasted effort if that is a 'red herring'.

.

In the context of this reply ... I am defining a separate Accessory BUS as being one that is fed via a different DCC power supply. Such as a 'Booster' module for example or Controller B in RailMaster.