Advice on LED's, resistors and current draw.

Dieseldonkey

Registered
16 Mar 2010
152
0
Near Edinburgh, Scotland
Looking for advice from anyone in the know about using resistors with LED?s

I have a Newqida Harz 2-6-2. I?ve taken out and thrown away the 27MHz R/C kit and replaced it with 2.4Ghz equipment. Power is supplied by a 7.2v battery pack. The loco comes with 3 LED?s at each end that I now need to wire up.
I?ve tested one LED and found that it will light up satisfactorily with two 1.2v AAA batteries in series. So I?m guessing the the LED?s are rated at 2.4 to 3 volts each. I?ve read that you should not wire LED?s direct to a power supply without a resistor as this is required to limit the current. However if I wire three LED?s in series then the voltage drop should be 2.4v across each LED. This would total the 7.2v from the battery pack making me think I don?t require a resistor in the circuit. I believe a filament bulb will only pull the current it requires. Do LED?s work differently and burn out if the current is not regulated? Also if wired in series, if an LED fails, will the circuit be broken and prevent the other two LED?s working, in the same way filament christmas tree lights fail. Would wiring them up in parallel with a higher value resistor be better?
Any knowledge / advice would be gratefully received.
Thanks
 

Glengrant

Registered
24 Oct 2009
11,031
11
NE Scotlamd
Look chaps, here's a guy who wants advice about resistors hahahahahahehehehehehehhohohohohohohohmad of course I'm mad, I was driven mad by resistors. Hope there is a sane person out there who will speak to you, but it's not me:rofl::rofl::rofl::rofl::rofl::rofl::rofl:
ba610308484c4d2e9133ce0954ba4c0f.jpg
 

Neil Robinson

Registered
24 Oct 2009
9,598
558
N W Leicestershire
Country flag
IMHO it is always wise to include a resistor, even if it is quite a low value, with an LED.
There are several helpful sites on the web, I use the one below because I'm familiar with it, others may suit you better.
I recommend you experiment with the site, if you click on a question mark you will see some suggestions to help with choice of value.
http://led.linear1.org/led.wiz
 

Dieseldonkey

Registered
16 Mar 2010
152
0
Near Edinburgh, Scotland
Hi Neil,

Thanks for the link. Very informative website and with the best calculator tool I've found so far. Explains the importance of having a current regulating resistor in the circuit. Looks like a trip to Maplin's to source some 1 ohm items. Still need to find out what happens if an LED fails. ie does it still make a circuit even though it doesn't light up on failure.

Thanks again.
 

Gizzy

A gentleman, a scholar, and a railway modeller....
26 Oct 2009
34,464
1,798
61
Cambridgeshire
www.gscalecentral.net
Country flag
I concur with Neil.

Also Donk, in answer to your question about if one of the LEDs fails when wired in series, then yes, as a blown LED is open circuit like a filament. LEDs though tend to have a far longer life than a filament lamp.

Wiring in parallel would prevent this but you would need to find another suitable resistor as you surmised....
 

Glengrant

Registered
24 Oct 2009
11,031
11
NE Scotlamd
I think if you go to Maplins what you'll get is a bag like the one I show in "2" above, oh it's cheap enough, then you've got to find out which is which, and that's when you start to go mad
 

ntpntpntp

Registered
24 Oct 2009
7,410
271
59
UK
Country
United-Kingdom
Country flag
Multimeter and a resistor colour code chart (plenty to be found online) soon sorts that out! Write the value on the paper strip once you've identified one of each bunch.
 

Glengrant

Registered
24 Oct 2009
11,031
11
NE Scotlamd
ntpntpntp said:
Multimeter and a resistor colour code chart (plenty to be found online) soon sorts that out! Write the value on the paper strip once you've identified one of each bunch.
I know what you are saying Nick and no doubt you have the patience to do just that, but that's where insanity set in with me. Oh, I got 'em to work in the end, but not scientifically. I just wrote wee labels which said, this one OK, this one a bit bright, this one a bit dim, you know, technical langwidge like that
 

TerrySoham

Registered
20 Jan 2010
172
0
If you are trying to put two or more LEDs in series will you please note that the calculation is not as shown in the calculator on the web site that you were directed to. Let me know if you are wiring them in series and I will go into more detail about how to calculate the resistor.
 

whatlep

Registered
24 Oct 2009
15,232
1
Worcestershire
www.facebook.com
Rather than drive yourself mad trying to compute the various values of resistor and then solder them together, why not simply get prewired LEDs from eBay? They come in most relevant colours and voltages (3v, 6v, 12v) and come in 3mm or 5mm diameter sizes. Packets of as few as 5 or as many as 1000 seem to be on general sale. I find 6 volt prewired LEDs work fantastically as a drop in replacement for 5 volt bulbs.

See, for just one of umpteen examples:
http://cgi.ebay.co.uk/5-x-Pre-wired...al_Components_Supplies_ET&hash=item518b40ee76 < Link To http://cgi.ebay.co.uk/5-x...mp;hash=item518b40ee76
 

whatlep

Registered
24 Oct 2009
15,232
1
Worcestershire
www.facebook.com
ROSS said:
Well that will save a lot of problems and queries for people...:party:
I give that 5* Pete.
Cheers!

One more thought. If you are using DCC, most decoders allow you to programme the voltage being supplied to the lighting and function terminals. Typically CV49/50. Adjust the output voltage and you can be sure you are not stressing the LEDs by supplying too much voltage. You could use bare LEDs (no resistor at all) rated for 1.5 volts by adjusting the decoder output(s) appropriately (though I prefer to use 6volt LEDs as you don't have to do any fiddly soldering). Naturally, it's worth checking the actual current "on tap" at the chip with a voltmeter before connecting the LED, though a blown LED will confirm that you've supplied too many volts! :rolleyes:
 

Dieseldonkey

Registered
16 Mar 2010
152
0
Near Edinburgh, Scotland
TerrySoham said:
If you are trying to put two or more LEDs in series will you please note that the calculation is not as shown in the calculator on the web site that you were directed to. Let me know if you are wiring them in series and I will go into more detail about how to calculate the resistor.


Hi, I'll most likley wire them in series
 

Philbahn

Registered
24 Oct 2009
12,687
3
72
Swinton Manchester
Thats the one I use Neil very helpful. Not blown one yet using it

Neil Robinson said:
IMHO it is always wise to include a resistor, even if it is quite a low value, with an LED.
There are several helpful sites on the web, I use the one below because I'm familiar with it, others may suit you better.
I recommend you experiment with the site, if you click on a question mark you will see some suggestions to help with choice of value.
http://led.linear1.org/led.wiz
 

Glengrant

Registered
24 Oct 2009
11,031
11
NE Scotlamd
That's what I have ended updoing buying- prewired ones
 

tramcar trev

all manner of mechanical apparatus...
Ok yes I'm fine with Ohms law ( mind you I don't dream about it) but I'd like to know has anyone ever machined a LED in a lathe? I'd like to make some 5mm bi colour leds about 4mm with a flat end....
Yes..... I know, the chaps inthe white coats are on their way....