Infuriating LEDs

[Lawrence]

Back in the darkness of the years passed, I probably covered this on a course somewhere, but I'm damned if I can work this out now.

I have a piece of stripboard (vero board), dc power supply connected to it to provide 0-12v dc supply busses, from a H&M controller.

In the station canopy I have 2 sections of the flexible LED strip (like this http://tinyurl.com/p8zldbc) split, but wired together with some very fine armature wire, which work fine on their own.

I decided to put a single LED (http://tinyurl.com/p5dachq)  in the little shop along with a 470 ohm resistor, also wired to the supply bus mentioned above, now when power is applied the second section of flexi does not illuminate but the first and shop LEDs do.

If I disconnect the shop led it all works again 

To my mind, both circuits are in isolation merely drawing power from the same bus, I fail to see how there is insufficient current being produced by the controller to fire all of these.  I am sure I have missed something obvious here, but don't see it at the moment

 

[E6系]

Hello Mr Lawrence,

 

Please supply wiring diagram.  Please rename LEDs to LED1 LED2 LED3 etc. as I cannot follow your first post.

[cteno4]

lawrence,

 

from your description its sounding like you dont have enough amperage going to the second strip thru the armature wire when the single led is also drawing power. have you tried jumping between the two strips with a heavier gauge wire? is the controller putting out 12v at a good amperage? is it putting out 12v? have you tried any other 12v source just to be sure? Solid connections all the way down the line?

 

agreed a simple diagram would help to make sure its what we think you are doing.

 

jeff

[Lawrence]

 

The lower LED with 470 ohm resistor is the green one in the shop, the wire used to link the 2 strips is armature wire out of an old N Bachmann motor

I think it may have something to do with the series/parallel wiring of the strips

[cteno4]

Lawrence,

 

ahh now i see. how the strips should be wired is both the +/- wires to the end of one strip and then two jumper pieces of the armature wire between the two strips. if you do this i think you will be fine. right now the strips are set up to draw 12v each and putting them in serial drives this down. not sure why one lights and not the other like that, but the strips are not wired up the way they want to be. bet if you fix that it will work. if you have a big spool of leds whack off two more strips and wire it up temporarily and bet it will work fine.

 

the strips are built in units of three leds in series with a single resistor. then there is a pass thru of both poles of the power (a power bus if you will) as well to pass full power down to the next module in the strip.

 

cheers,

 

jeff

[E6系]

Hello Mr Lawrence,

 

From your wiring diagram it appears that you have 2 x LED strip in series.  The series is then wired in parallel with a single LED, which you call a "Shop LED".

 

Without knowing the load drawn by the strips it is impossible to calculate a solution for you.

 

Mr Jeff may be correct.  The problem may be that there isn't enough power.

 

However, I more suspect it is simply a matter of an unbalanced circuit.  Put simply, if the total resistance on the Shop LED circuit is less than the total resistance of the strip LED circuit then most of the current will flow through the Shop LED circuit. 

 

To keep it simple, try replacing the 470 ohm resistor with 820 ohm or 1.1k ohm.  My hunch is that will solve your problem.

[Lawrence]

Fixed  :D

Really annoyed with myself actually I clearly got rather lax on the dynamic checks because all the static checks were good, anyway here's what happened

Removed the station canopy and laid it on the bench upside down, at this point, everything was still connected as my original diagram, applied power and it was as described.  I did voltage checks and as I put the probe on the +ve o/p of the first strip, the second strip lit up, I removed the probe and after a second or two they went out again, I pushed down on the tracks of the first strip and strip 2 lit up again.  I de-soldered the wires from the first strip o/p cleaned it all up and re-soldered them.  It would appear there was intermittent contact between the point where the wire was soldered on and the track, when I put the probe on to con check it it made good contact and all looked ok but once the probe was off contact was broken.

I knew the circuit theory was ok and there was enough power available, just annoyed that I was not thorough enough with my checks, thanks for your input folks.

[E6系]

Good job Mr Lawrence,

 

Have you done the math to check that the circuit is balanced?

[kvp]

 

Have you done the math to check that the circuit is balanced?

You don't have to balance it if the current source has enough power. Each led gets the set amount of current based on the source voltage and the current limiting resistor's value. In this case enough is more than the circuit needs based on the sum consumption of the parts.

 

ps: The circuit diagram above is not really accurate, each section actually connnects to the bottom rail though the leds and their resistor and the top and bottom rails are just passed through so each section is parallely connected to the two rails. This means the number of total sections only modify the current consumption.