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Connecting LEDs: in series or in parallel?
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Offline Brakepad  
#1 Posted : 21 April 2009 21:05:47(UTC)
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Brakepad

France, Metropolitan   
Joined: 25/08/2008(UTC)
Posts: 633
Location: Montlouis sur Loire, France
Hi all,

I've just converted to digital a 3021 and will replace the original bulbs by LEDs.

Will carry out a test with 3mm yellow LEDs. I wonder what is the best way to connect the leds: in series (resistor-led-led) or in parallel (each led with its resistor).

For connecting them in series, had thought about using a 800 ohm resistor, while the value for parallel connection should be 900 ohm.

Is this OK?
check out http://maerklin-back-on-track.blogspot.com if you like to see how old Märklin locos are brought back into life! (in spanish by the moment)
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Offline dntower85  
#2 Posted : 21 April 2009 21:29:55(UTC)
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dntower85

United States   
Joined: 08/01/2006(UTC)
Posts: 2,218
Location: Shady Shores, TX - USA
marklin has there surface mount boards in series 3 yellow/white led's circuit board should have about a 1k ohm resistor for the red lights and a 780ohm for the 3 front lights.

see if this link helps
https://www.marklin-user...98&SearchTerms=37090
DT
Now powered by ECoS II unit#2, RocRail
era - some time in the future when the space time continuum is disrupted and ICE 3 Trains run on the same rails as the Adler and BR18's.
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Offline DaleSchultz  
#3 Posted : 21 April 2009 22:12:34(UTC)
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DaleSchultz

United States   
Joined: 10/02/2006(UTC)
Posts: 3,997
resistor - led - led will consume less current, but the voltage will need to be sufficient to drive both - about 6V or more for two white LEDs. You will have plenty of voltage so I would go serial.

for calculating the resistor we need to know the forward voltage of the LEDs and the supply voltage and how much current you want to run them at. I suggest 16mA or less.

I have a table at http://layout.mixmox.com/1/Resistors for 12V DC supply and white LEDs that use 3V each.
Dale
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Offline intruder  
#4 Posted : 21 April 2009 23:14:22(UTC)
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intruder

Norway   
Joined: 16/08/2006(UTC)
Posts: 5,382
Location: Akershus, Norway
I normally use a somewhat bigger resistor than actually required, as it doesn't have a great influence one the brigthness, and reduces the current draw.

For normal LEDs (2 Volt) I use a 1,2 KOhm resistor. That should give 600 ohm if two LEDs are connected in series, 400 ohm with three LEDs and 300 ohm with four.

For the bright white LEDs in my camera car I used 1000 ohm per LED, LEDs connected in parallell. Two LEDs in series gives 500 ohm, three 333 ohm, four 250 etc.

The only disadvantage I can see with LEDs connected in series is that if one LED is broken (very rare) all the LEDs go dark. Trouble shooting is required to find the defect one.

When LEDs are connected in series, make sure that all the LEDs are identical, otherwise their brighness may be unequal.

Edit: added LEDs to avoid misunderstanding.
Best regards Svein, Norway
grumpy old sod
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Offline perz  
#5 Posted : 22 April 2009 00:08:22(UTC)
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perz

Sweden   
Joined: 12/01/2002(UTC)
Posts: 2,578
Location: Sweden
Quote:
[size=1" face="Verdana" id="quote]quote:Originally posted by intruder
<br />
For normal LEDs (2 Volt) I use a 1,2 KOhm resistor. That should give 600 ohm if two are connected in series, 400 ohm with three and 300 ohm with four.



If you mean 600 Ohm for two LEDs etc. it is not correct.

But maybe you mean that you can connect two 600 Ohm resistors in series to get 1200 Ohm ?
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Offline intruder  
#6 Posted : 22 April 2009 01:32:40(UTC)
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intruder

Norway   
Joined: 16/08/2006(UTC)
Posts: 5,382
Location: Akershus, Norway
Sorry, Per.

I did not explain myself good enough.
You are correct. I meant one 600 Ohm resistor if two LEDs are connected in series and so on.
Best regards Svein, Norway
grumpy old sod
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Offline perz  
#7 Posted : 22 April 2009 02:05:48(UTC)
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perz

Sweden   
Joined: 12/01/2002(UTC)
Posts: 2,578
Location: Sweden
Quote:
[size=1" face="Verdana" id="quote]quote:Originally posted by intruder
<br />Sorry, Per.

I did not explain myself good enough.
You are correct. I meant one 600 Ohm resistor if two LEDs are connected in series and so on.


Your clarification avoids misunderstanding.

But the calculations you propose are incorrect.
For one normal LED you can use a 1.2 KOhm resistor, if it is used in a digital lok where the total voltage is 20 V.

2 V is over the LED and 18 V over the resistor. This gives you 18/1.2 mA = 15 mA of current, which is reasonable.

If you have two LEDs in series, you get 2 x 2 V = 4 V over the LEDs and 20 V - 4 V = 16 V over the resistor. To get the same current and thus the same brightness you should then have 1.2 x 16/18 kOhm = 1.067 kOhm.

With three LEDs you get 2 x 3 V = 6 V over the LEDs and 20 V - 6 V = 14 V over the resistor. The resistor should then be 1.2 x 14/18 kOhm = 0.933 kOhm

And so on.

Note: 2 V over the LED is only an example. It depends on what type of LED it is.
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