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Author Topic: LED Problem  (Read 388 times)

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Offline BlythPower

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LED Problem
« on: August 26, 2017, 09:35:07 pm »
Hi,

I suspect the answer to my problem may simply be that I'm trying to do something that shouldn't be done but here goes...

I'm trying to knock up a small mimic diagram to show the state of the points. Rather than going for a simple on/off of a green LED, I thought it would be good to use bi-colour LEDs to give a nice positive red light on the track that's not accessible in addition to the more usual green for the one that's set. I'm using a latching relay which is set by the switch that fires the point motor. I had the thought that the outputs could power the red of one LED and the green of the other and when switched would power the green of one and the red of the other. Hopefully the crude diagram should give an idea of what I've done.

It sort of works but the problem I have is that that whichever red light is on glows very brightly whilst the green is barely on at all. Is there something I can do to correct this or I just not be doing it at all..?

Thanks for any help you can give!



Offline ntpntpntp

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Re: LED Problem
« Reply #1 on: August 26, 2017, 09:47:14 pm »
You're using a common resistor with both LEDs in parallel. Probably the red and green LEDs have different forward voltages and hence one diode is fully on and conducting before the other reaches its forward voltage.

This link kind of explains it (I think):
https://electronics.stackexchange.com/questions/22291/why-exactly-cant-a-single-resistor-be-used-for-many-parallel-leds
« Last Edit: August 26, 2017, 09:50:31 pm by ntpntpntp »
Nick.   2016 celebrating the 20th anniversary of "Königshafen" exhibition layout!

Offline BlythPower

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Re: LED Problem
« Reply #2 on: August 26, 2017, 09:51:57 pm »
Aah..! Thanks! I did wonder if putting s resistor and the red leg might be something I'd need to do and it looks like that's the case. Time for some experimentation...

 :thankyousign:

Offline ntpntpntp

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Re: LED Problem
« Reply #3 on: August 26, 2017, 09:54:32 pm »
I think try a red and a green LED in series instead of parallel, if you can figure out how to do that with the 3 wire bi-colour LEDs? That might sort it out, if it's possible. You may need a slightly lower value resistor because the total forward voltage will be higher, but try it with the one you have first.

Otherwise I guess it's a case of a separate resistor for each red and green LED?
« Last Edit: August 26, 2017, 10:08:49 pm by ntpntpntp »
Nick.   2016 celebrating the 20th anniversary of "Königshafen" exhibition layout!

Offline NeilWhite

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Re: LED Problem
« Reply #4 on: August 27, 2017, 10:02:24 am »
I think the safest way is to have separate resistors for the red and green parts of the LED. LED testers (e.g. https://www.google.com.au/search?q=led+tester&rlz=1C1CHJX_enAU748AU748&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwjc_JPzhffVAhVHKpQKHSn-CiAQsAQIQg&biw=1366&bih=638)

can be helpful, but possibly a better way is to use a resistance wheel (such as https://www.jaycar.com.au/resistance-wheel/p/RR0700) to work out what resistance value gives you the look you want for one LED and then do the same for the other colour LED. Once you'be done this for one pair you can use the same values for all of the others.

Neil

Offline Bealman

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Re: LED Problem
« Reply #5 on: August 27, 2017, 10:12:04 am »
That's not a bad idea... select a resistor by trial and error with that useful little bit of kit.

I have both (resistance wheel and led tester from same firm), and they are some of the best bits of gear I've got.
« Last Edit: August 27, 2017, 10:14:55 am by Bealman »
Vision over visibility. Bono, U2.

Offline Quinn

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Re: LED Problem
« Reply #6 on: August 27, 2017, 10:53:00 am »
Using a lot of leds on dioramas I had to learn how to calculate the resistance for a required current. It may need adjustment to get the same perceived brightness so it's useful to have a few values to hand. I bought a range from ebay sellers.

A good idea to have separate resistances, the red side would need more resistance than the green side. The red forward voltage I take as 2 volts, the green as 3.5 volts.

Offline Malc

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Re: LED Problem
« Reply #7 on: August 27, 2017, 03:19:07 pm »
LED brightness depends on the current. I generally use 10mA as the current and adjust the resistor to suit. If I am using a 12v PSU, 1k resistor works well. Different LED types draw differing amounts of current. Super bright ones being the worst. As previous posters have said, one resister per LED is best, then you can adjust for minimum smoke.
I'm not sure if life is passing me by, or trying to run me over.

Offline BlythPower

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Re: LED Problem
« Reply #8 on: August 27, 2017, 03:32:17 pm »
Thanks everyone! I'm going to get a resistor assortment off good old eBay and play around with a test harness until the desired results come up!  :thumbsup:

Offline Quinn

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Re: LED Problem
« Reply #9 on: August 28, 2017, 06:55:13 pm »
I made up a range of 680, 820, 1k, 2.2k, 3.3k, 4.7k and 10k. I work from 6 volts. #

Sometimes there's a call for a very dim led. I'm going to experiment with preset resistors "in series" with fixed ones (so the led doesn't get the full-on supply by accident).

Offline ntpntpntp

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Re: LED Problem
« Reply #10 on: August 28, 2017, 07:25:26 pm »
For my recent lighting project I used the tiny SMD LEDs which don't require as much current as the more usual 3mm and 5mm packages. So for a given output connection I drive

4x 0603 LEDs in series from 12V via a 4K6 resistor = 0.7mA
(these are vehicle head/rear lights so don't want them too bright)

1x 0402 LED from 5V via a 1K resistor = 1.4mA
(these are flashing beacons so need to be brighter to be effective)

So definitely experiment with a selection of resistance as suggested in the earlier posts. Always start with the highest value.

Also, remember there are various online LED resistance calculators available. These have been posted before in other threads.  Here's the one I use:
http://led.linear1.org/1led.wiz

I use this chart to find typical forward voltage for different colours of LED
http://www.oksolar.com/led/led_color_chart.htm

I agree with @Malc that 10mA is usually a good starting point for the target current for the 3mm and 5mm LEDs.
« Last Edit: August 28, 2017, 09:01:32 pm by ntpntpntp »
Nick.   2016 celebrating the 20th anniversary of "Königshafen" exhibition layout!

Offline bgennette

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Re: LED Problem
« Reply #11 on: August 29, 2017, 08:22:10 am »
There are LEDs and there are LEDs.

Best choice for this use is a duo-LED which is 2 closely matched LEDs in a single dome package that has just 2 legs.

When positive voltage is applied to the longer leg (with negative to the shorter) the primary LED will light.  When the voltage is reversed the secondary LED comes on.  Red/Green duo LEDs have 2 volt primary Red and 2.2 volt secondary Green.  When the voltage is swapped faster than human persistence of vision (about 50 times a second) the LED appears to be Yellow.

They are available in 3 and 5mm sizes and require just one resistor to limit current to 10-15mA.  eg for 6V the resistor is (6V - 2V) / 0.012A = 333, use either 330 or 270R.

You already know how to wire frogs for reversing polarity, just apply the technique TWICE to completely swap polarity to a duo-LED.  Or use a cheap microcontroller to do it in fancy ways.

bye.

Offline BlythPower

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Re: LED Problem
« Reply #12 on: August 29, 2017, 08:24:24 am »
Thanks everyone! I have a resistor assortment on order and a test harness rigged up. I'm off on holiday shortly so will have the experiments to look forward to on my return. I'll post the results in case they're of use to anyone else.  :thankyousign:

Offline BlythPower

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Re: LED Problem
« Reply #13 on: October 09, 2017, 10:51:48 am »
Well, the answer to life, the universe and everything may be 42, but after a bit of trial and error a 47 ohm resistor has balanced the red and green out nicely. Thanks to everyone who's offered advice!

:thankyousign:

 

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