LED Failure? Partly solved but not quite!

[austinbob]

Most of the ebay sold leds sold with pre soldered resistors are covered in opaque heatshrink so you have to rely on the seller getting the correct value of resistor.

If it is the block signalling indicator then the wiring diagram suggests using only one resistor for a pair of leds, and suggests the cathodes of each led are connected together then to a 1K resistor. The other side of the 1K resistor then goes to the supply common (or negative).

http://www.blocksignalling.co.uk/index.php/points-position-indicator-ppi1

However the leds with pre soldered resistors can still be used. Just wire the anode (+ lead) to the ouptut from the board and all the cathodes (- lead) to the common terminal.

Steve

Steve - I think we established in a previous thread that Peter has th PPI 4 which, from memory, has a different arrangement.

[Steve.T]

Oh, okay, sorry.

Then ignore my last post then.

Would I be right in then thinking the PPI 4 does not require any series resistor.

Steve

[Steve.T]

Peter,

Hope you get there in the end.
If you want to try some regular green and red leds then pm me your address and I can post a few for you to try, then if that works you are confident to order the rest.
If you need them with wires soldering onto them let me know.


Steve

[petercharlesfagg]

Friends,

Thank you for all your replies, as usual, it appears that I have omitted tons of information!

(1) I am NOT fitting the PPI's, ONLY the LED's into the top panel of my console.  The PPI's will not be installed until ALL the connections to switches, LED's etc are up and running following the PPI manufacturer instructions.  (Block Signalling PPI4 DC)

(2) The PPI manufacturer only recommended the use of a 12 volt DC supply instead of my own 16 volt AC supply.

(3) I have to assume from the advertisers information that the resistors are suitable for use with 12 volts DC. (I purchased the LED's from several suppliers on Ebay so perhaps I have done a doodoo in my own nest, as usual I was trying to spend the least amount of money?)

(4) Question:  If I test the LED's at 9 volts DC and then raise the voltage to 12 in situ, am I not likely to have the same result but with even more hassle involved in removing them again?

(5) According to an email from Steve Heaton @ Block Signalling, having resistors already fitted to the LED's will make no difference at all!

I apologise and hope that this clarifies SOME of the unmentioned problems.

Peter.

[Malc]

Peter, just to confirm. You are not connecting the PPI yet. You are connecting the short leg (cathode or side with the flat) together on one set of LEDs. This common connection will go to C1-. You do the same with the other set of LEDs and this will eventually go to C2-.  If you connect your 12v-  to the common wires and the touch the other legs of the LEDs to the 12v+ in turn, they should light.

[Bealman]

[smg id=22373 type=preview align=center width=400]
You need one of these, Peter! One of the best bits of test gear I ever bought. It's especially useful if you have a bunch of clear leds and want to find out what colour they light up before you hard wire them into a circuit.

[petercharlesfagg]

Peter, just to confirm. You are not connecting the PPI yet. You are connecting the short leg (cathode or side with the flat) together on one set of LEDs. This common connection will go to C1-. You do the same with the other set of LEDs and this will eventually go to C2-.  If you connect your 12v-  to the common wires and the touch the other legs of the LEDs to the 12v+ in turn, they should light.

Malcolm, thank you.

There is no short leg and no long leg (Pre-wired LED's) just a Red wire (With resistor) and a Black wire, if I offer one of the wires from the feed to the wrong wire the LED does not light but the other way round it does!  My total knowledge!

I have been doing some digging on the 'net before our crappy laptop seized solid!

240 volt AC transformers that merely plug into the mains and give out 12 volts DC are, as I understand it, uncontrolled (Meaning that the output varies???), if that is the case then the one I have bought is likely to give a surge, Yes/No??, which could be blowing the LED's?  (The transformer in question cost me £4.99 on Ebay, Made in China)

Should I therefore bite yet another bullet and consider a more expensive transformer from one of the model railway suppliers?

Warmest regards, Peter.

If your answer is Yes, which one should I be considering?

[Malc]

Hi Peter, you mean unregulated rather than uncontrolled. I use something like this.
https://www.amazon.co.uk/dp/B00ENZUQO8/ref=cm_sw_r_awd_UB6LvbES0T9BP

I don't know how much current your PPI will draw, but it shouldn't be much.

I use several like this for lights, arc welders etc

[petercharlesfagg]

Hi Peter, you mean unregulated rather than uncontrolled. I use something like this.
https://www.amazon.co.uk/dp/B00ENZUQO8/ref=cm_sw_r_awd_UB6LvbES0T9BP

I don't know how much current your PPI will draw, but it shouldn't be much.

I use several like this for lights, arc welders etc

Thank you I will keep that one in mind!

Would this one be of any use?

http://www.ebay.co.uk/itm/290883688146?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Peter.

[Malc]

It might be a little on the low side for current. 1amp should be OK though. It does need wiring up to the mains. It is really meant for those under cupboard LED lights and would be wired into the ring main.

[Webbo]

Peter

It seems that your LEDs are blowing because you are trying to pass too much current through them. This is pretty much the consensus of this thread. LEDs have a maximum current rating - small LEDs will have a smaller current rating than larger ones because they have finer wires and are less able to dissipate excessive heat. We need to know what the maximum current rating is in order to calculate the size of resistor required. The voltage drop across an LED is colour dependent and might range from 2 V (red) up to more than 3 V (blue). For LEDs run off a 12V supply, let's say 3 V is dropped across the LED itself leaving 9 V to be dropped across the resistor. If the LED is rated as having a maximum current of 10 mA say, then the resistor required to limit the current to 10 mA (= 0.01A) is = 9 / 0.01 = 900 ohms. If the LEDs being used have a maximum current of somewhat less than 10 mA, then a 1000 ohm resistor will not be enough to limit the current and prevent the LED from blowing up.

My electronics catalogue lists 3 mm LEDs mostly with a maximum current of 20 mA or more. Going through the resistance calculations  for these suggests that a 500 ohm resistor would be plenty large enough to limit the current for these if a 12 V supply is used to run them. Peter, can you tell us what the maximum current rating is on your LEDs, please.

Webbo

[austinbob]

It seems that your LEDs are blowing because you are trying to pass too much current through them. T

I don't think that is the problem Webbo. The PPI4 instructions show the LED's connected with no resistor added. So the current must be limited on the PPI.
http://www.blocksignalling.co.uk/index.php/points-position-indicator-ppi4-dc
If the LED's were running at too high a current they would be extremely bright until they blow. I'm sure Peter would have noticed that.
The most likely problem, in my opinion, is that the LED's have been connected the wrong way round. There seems to have been some confusion in previous posts about which lead the resistor is connected in - I'm not sure if that has been resolved.
I still think the best solution, if Peter thinks he is capable of it, is to use plain LED's as described in the PPI4 instructions.
If this is practical for Peter then he could just wire up one set of LED's and see if they work reliably. If not then we have to think again.
Regards

[Webbo]

Yes, I understand that there is a current limiting resistor in the PPI4 of 1000 ohms. I suppose that what I'm suggesting is that Peter may have bought LEDs with maximum allowable currents that are less than 10 mA in which case 1000 ohms would not be enough on a 12 V supply. Seems unlikely for 3 mm LEDs but possible. And I agree that he would see the LEDs burn brightly for a short period before giving up the ghost if excess current were the problem (sort of like a supernova).

Webbo

[petercharlesfagg]

Hes not using the ppi at the moment accordinng to his replies so my original (unanswered post) still thinks the resistors are the issue.

Paul, the resistors are an unknown since I have chucked the blown ones!

I have tried removing the sheathing or whatever it is on one of the others and only managed to get a bit off before slicing my finger!

I am thinking of cutting my losses and purchasing a whole set of new from one supplier, preferably in the UK as I have hinted in another thread. 

http://www.ngaugeforum.co.uk/SMFN/index.php?topic=28560.msg314919#msg314919

I would appreciate your thoughts on those LED's before I go any further with my originals?

Warmest regards, Peter.

[austinbob]

Hes not using the ppi at the moment accordinng to his replies so my original (unanswered post) still thinks the resistors are the issue.

Ok - you're right about not using the PPI - missed that (too much information!!)
What I don't understand is that assuming Peter used the same test voltage before and after he mounted the LED's, why would they be fine to start with and then fail after mounting them? Unless he accidentally got the power supply reversed?
Running out of ideas here.
Regards