led resistor

frank

frank

Club Member
i need some sparky help please :confused:.
i,m trying to knock up some bright front sidelight leds, using the parts from some of those £1.99 for 3 cheapo lights.
ive wired them in series to make it 12v, but i think that because they were in parallel before, the resistor is the wrong value (it,s 10 ohms) and after about 1 hour, one of the leds has burnt out.
what value resistor do i need guys ?
http://www.micra.org.uk/attachment.php?attachmentid=11110&stc=1&d=1227569784
 

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You need to measure the current. At a guess I reckon you need about a 33ohm resistor. if they are too dim try a 22.
 
ollie240585 said:
...the voltage going through them originally?
Click to expand...

"voltage (electrical-pressure) applied 'accross' them"

'current (movement of charge) flows 'through''

__________________

its tricky without the data sheet for 'those' LED's. The difficulty comes in 2-fold with diodes

(the datasheet normally lists: what is the forward voltage drop range for the non destructive current range)

those diodes look like 5mm packages...and for a 5mm package continuous non destructive currents rarely exceed 20 to 25 milliamps. Less constent (depending on the particular diode) the forward voltage drop can range from around 1.2 v to as high as 4 v in some cases...so for 3 in a row that could be 3.6v through 12v dropped accross the diodes...(unbeleivable I know!!!)..

so If your supply is only 4.5 volts...and the LEDS are in any way bright then the total voltage drop for all 3 of 'these' LEDS in series must be less than 4.5 volts....(less than 1.5 volts each)..but now we're at the bottom end

( what a clart on !)

any how...more to the point < I must stop drinking so much>...you may have only 4.5 volts on the bench; in which case I think only "tens of ohms" may be too many to be bright enough(depends on 'those' diodes...BUT on the car you will have between 12.5 and 14 volts (ish)...so to limit the current on the car (to stop one of them popping) I would aim for around 500 ohms (ish)....but EXACTLY as Ed said:..you may have to reduce the resistance a bit if they are too-dim <like me> (since we have no idea of what the forward drop for 'these' LEDs are)...and they may be 2.4v diodes (377 ohms - ish required on the car) or even 4v diodes (100 ohms - ish on the car)

fizz pop: im going to sleep now
 
assuming there 25mA led,s running off a 14 v supply you need a 14/0.025 ohm resistor = 560 ohm.

above using ohms law, V = I x R
 
i think the superbright white ones are max 5v and max 100mw, so ill try a 30 ohm resistor and leave them on for a day to test them eh.
my £2 something 21 led 4.5v spotlights, wired in series only survived a week (should have wired a 3rd one in *doh*, 7v each was a bit ott, but you could drive down an unlit road with them tho)
 
To work out the correct resistor value you need to know how much voltage will be across the resistor, i.e the voltage across each diode. Your calculation gives 25mA with no diodes, so the current with a diode in the circuit will be less. Very safe resistor value, but not optimal if you want them bright.
 
ok then if you want a bright resistance and not a 'safe' one then using the attached datasheet for bog standard white led's >>>click<<<

30mA max current 3.6v typical drop

14 - ( 3.6 x 3 ) = 3.2volts

i dont like using max values so ill use 28mA

3.2v / 28mA = 114.28 ohms.

closest prefered 120ohm but will be un-noticably dimmer.

so using 120ohm.

3.2v / 120ohm = 26.66mA

so use 120ohm for the optimum safe brightness.

the reason they burned out cos you were using 10ohms

3.2v / 10ohm = 320ma

10 times more than the absolute max, im suprised they lasted an hour tbh
 
yeah thats right, you need to find out / measure the volatge across the diodes frank. Then you can work out a value as above.
 
Frank just do what I said, I know I'm right ;) 5v per led is way too high. Most are around 3.4v max and red are usually lower at about 2.5, give or take.

Ed
 
ed

i will ;), i,m going to get some 30 ohm ones from maplin tomorrow, i had to get some more lights from the pound shop (these other ones have got a slightly blue tint :doh: ) and these have got a 5 ohm resistor (albeit wired in parallel)
 
no offence to ed or anyone but puttin a 33 ohm resistor there will give you 115mA through the led's, where the absloute maximum is around 30. i wouldnt do it that way, but thats just me id sooner keep my led's running at there optimum than be replacing them every few hours.


superls

whilst at maplin also get some 120 ohm, and some more lights from the pound shop for when they burn out at 30 ohm
 
TBH this whole exercise is pointless without knowing the specification of the LEDs. Without that none of us will be right, and even then you should physically measure the current draw to be absolutely sure.
 
completely 100% agree with you there ed. which if im honest i do every time with my decade resistance box, adjust it till the current reads wot i want it to.
 

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Frank measure total voltage across the leds and the resistor, then the voltage across just the resistor. Post up the difference and with the aid of the original circuit, we can give a resistor value to match the original circuit. 100 ohms does sound about right though.
 
just measure the voltage across 1 led that should be enough, is that off of a car battery or on the car running?

cos if its only on a car battery then the 30ohm circuit is running at 73mA, only twich the max so will last a while but the real test is while the car is running (normal situation) there it will be running at 153mA alot more than the max and will not last long.
 
superls without knowing the voltage across the leds, those figures are a complete (all be it educated) guess. Anyways, if you are happy with 100ohms Frank, glad we could help lol. Otherwise, need the voltage value.
 
superls said:
just measure the voltage across 1 led that should be enough, is that off of a car battery or on the car running?

cos if its only on a car battery then the 30ohm circuit is running at 73mA, only twich the max so will last a while but the real test is while the car is running (normal situation) there it will be running at 153mA alot more than the max and will not last long.
Click to expand...

3.19v across one led, using 6x2v traction batt,s, and yes thanks for all your help lads (Y)
 
hey frank i am watching a film / bit drunk, this calculation looks right at first glance though. Kinda narrated a bit.


4.5 - 3.19 = 1.31v

v = ir
i = v/r
i = 1.31/10 = 0.131A

current per led = 0.131/3 = 0.043A

In series:

12 - (3.19 * 3) = 2.43v

i = 0.043
v = 2.43

r = v/i

r = 2.43/0.043

r = 57 ohms


(ps try measuring 3 and divide by 3 for assurance). 57ohms will/should give you quite near to exactly the same led brightness as they came standard at 4.5v @ 12v
 
frank said:
i need some sparky help please :confused:.
i,m trying to knock up some bright front sidelight leds, using the parts from some of those £1.99 for 3 cheapo lights.
ive wired them in series to make it 12v, but i think that because they were in parallel before, the resistor is the wrong value (it,s 10 ohms) and after about 1 hour, one of the leds has burnt out.
what value resistor do i need guys ?
http://www.micra.org.uk/attachment.php?attachmentid=11110&stc=1&d=1227569784
Click to expand...

How many LED's are you connecting in series Frank? You need to limit the current through the string
 
thats alright but the led's in the lamp origionally would have been in paralell, therefore the 43mA you say is shared by 3. giving 14mA

so if he did want them as bright as they were in the light at 4.5 volts he needs a

14 - (3.19 x 3) = 4.43

i=0.013
v=4.43
r = 4.43 / 0.013
r=340 ohm, far too high

but if he wants them to run at there optimum i.e. 30mA

r = 4.43 / 0.03
r = 147ohm

the 120 ohm i recommended earlier in the thread should be giving a led current of

i = 4.43 / 120
i = 36.9mA
or 0.0369A,

a little over the max but it will be fine.

please measure the current if you get chance, it would be interesting to know.

scott
 
superls, you got it wrong

current per led = 0.131/3 = 0.043A
Click to expand...

it is 0.131A shared between all of them. As you can see i divided by 3 to get 0.043.

Also, I did write quite clearly the resistor value is for 12v, not 14. I narrated the calculation so that frank can easily plug other numbers in if he wants.
 
sorry i apologise i miss read it, your calcs are sound, i just find it hard to believe 3 aaa batteries will sustain 131mA for any time at all.

frank did the 10ohm come in the lights when you bought them? and how were they wired?
 
superls, that was my first thought also. But that is what the numbers say. Could do with a measure of all 3 leds, and the total voltage as I wrote originally. Would make for a better approximation.
 
it would yes, the only thing i can think of is that the 10 ohm resistor wasnt in the lamps and frank has introduced it, but as i read it , thats wot came with it.
but as you rightly say the numbers are sound and in theory there taking 130mA from aaa bateries
 
superls said:
sorry i apologise i miss read it, your calcs are sound, i just find it hard to believe 3 aaa batteries will sustain 131mA for any time at all.

frank did the 10ohm come in the lights when you bought them? and how were they wired?
Click to expand...

superls
yeh, 10ohm (checked with meter and codes) connected to the 3 parallel wired led,s
edit, i,ll take some more readings
 
it seems quite clear from the pictures that 30ohms is too little and 100ohms is too much. So 50-60 as calculated fits nicely no?
 
to run at what they were 43mA then as you have calculated 50-60ohm, yes.

but imo thats a little high to be running them, i have worked with thousands of led's at the place where i used to work and in tones of different applications and they never got hot as frank says they did at 30ohm. but i agree they dont look bright enough at 120ohm. but i think that the calculated 43mA is too high, but that depends on the specified value in the datasheet, which we dont have.

i mean ive got in my hand now as i type some 3 watt led's which run at 750mA and they will take your skin off with the heat but there supposed to do that and are attached to a heatsink.

frank if youve got one left measure the current draw on a standard lamp with standard batteries, also i could get you white led's cheaper than the 3 for a pound ones your currently using.
 
in which case, the calculation becomes:

4.5 - 3.19 = 1.31v

v = ir
i = v/r
i = 1.31/10 = 0.131A

current per led = 0.131/3 = 0.043A

In series:

12.2 - (3.19 * 3) = 2.63v

i = 0.043
v = 2.63

r = v/i

r = 2.63/0.043

r = 61 ohms
 
yer they did, i just feel that were missing something cos theres no way you could draw 130mA off aaa batteries for anything longer than a few minutes.
 
thats showing 2 or 3 ma id say 10a was bust also? did you change the probe over to the 10a hole?

found this on google
click

try it it backs up the 43mA we deduced but really blows both our series calculations out of the water.
30ma.jpg

43ma.jpg
 
cos the battery should charge at about 14 v.

frank, mend the ma on you multimeter, buy a fer values of resistor around 50 - 60 ohm right to 120, (only a few pence each) measure the current while changing the resistor till it reads about 40ma, then test it if they burn out then higher the value if not then put a lower one in and trial it.

the above is easy for me is i have this little invaluable device called a decade resistance box where i can change the resistance in 1 ohm steps right to 1 megohm, i measure the resistance while i change the value in prefered stages until the curent reads right.

anyways time for bed.

bye and happy experimenting
 

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