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Paul M

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Due to a slight mishap, I've managed to blow 3 LEDs on one of my coaches. There are fitted in parallel and I don't really want to do much rewiring.
I have 3 LEDs from a kit which is saying they need a continuous current of between 8 & 12mA. The battery voltage is 12V looking at my chart I would say that the forward voltage is about 2.1 and I've calculated that would need something around a 165ohm resistor. But I have absolutely no confidence that I've done it correctly. Can anyone help?
Thanks in advance. Paul
 

at lower voltages (like 12v) it is important to use a calculation that also specifies the forward voltage of the LED itself.

white leds are usually about 3.5 volts forward voltage drop .... this is a significant part of the 12v source.

calculating for 30 ma, you need about 283 ohms... (3 in parallel 10 ma each)

if you put a forward voltage drop of 2.1 (wrong, good for red led), then 330 ohms would be calculated.

You have an error in your calculation somewhere...

(if you wanted to be really precise, you could measure your forward voltage drop)
 
The problem with LEDs in parallel, is they can fight each other for current..
Better if you can wire them in series, or fit a resistor per LED.
I can help you with the calculations for either of these, depends how you can rewire them?

<edit>
I see Greg is already on the case..

PhilP
 
Thanks chaps, I had a horrible feeling it would be wrong! My LEDs are orange so according to the calculator chart they should have a Vf or 1.9-2.2, but I'll splash out and get some new ones so I'll know for sure the correct value. In the meantime I'll have a play and see what happens. If you see any smoke over the North Herts area, you know what's happened :rofl:
 
Paul M said:
Thanks chaps, I had a horrible feeling it would be wrong! My LEDs are orange so according to the calculator chart they should have a Vf or 1.9-2.2, but I'll splash out and get some new ones so I'll know for sure the correct value. In the meantime I'll have a play and see what happens. If you see any smoke over the North Herts area, you know what's happened :rofl:
Click to expand...
I'll keep looking SW of here then....
 
Ahh, sorry for assuming they were white... so, hook one up with a higher resistor.... measure the voltage across the LED with a voltmeter... now use that value in your calculation...

it's actually pretty simple:

V=IR Ohms law...

V(led) + V (resistor) = 12

say the led forward voltage is 2 volts..

then V(resistor) = 10, and with V=IR you have 10 volts = current times resistance

Say your current desired is 30 milliamps (3 in parallel)

so 10 = 0.03 times the resistance or

R = V / current or R = 10 / 0.03 or 333 ohms ....

the key is to subtract out the forward voltage of the LED.... since an LED is a diode, the forward voltage is pretty constant irrespective of the current.

So this is why you can measure the LED forward voltage under a lower (and safer current) situation, thus not risking damaging it.

Greg
 
Paul M said:
Thanks chaps, I had a horrible feeling it would be wrong! My LEDs are orange so according to the calculator chart they should have a Vf or 1.9-2.2, but I'll splash out and get some new ones so I'll know for sure the correct value. In the meantime I'll have a play and see what happens. If you see any smoke over the North Herts area, you know what's happened :rofl:
Click to expand...
Fosworks does supply LED’s ready fitted with resistances for up to 24v. Just scroll down to lighting.
fosworks.co.uk

Fosworks

Garden Railway Radio Control and Sound Radio DCC for Gauge 0 to Gauge 3 Analogue DC and live steam systems too
fosworks.co.uk fosworks.co.uk
 
Greg Elmassian said:
You forgot to subtract the forward voltage, or you used a different forward voltage...

So, how about posting your calculations, we will all learn something.

Greg
Click to expand...
I will try to, but I've never "got" maths. If there was such a thing as dyslexia for maths, I probably have a touch of it.
 
It works, but it rounded to the nearest value instead of giving the exact answer... also you need to put 30 ma in for the led, since it has no option for parallel

The important thing to remember is that you need a calculator that has an "input" for the forward voltage drop of the LED.... otherwise the calculation will be wrong.

There's a lot of wrong stuff out there...

Greg
 
I could not get the parallel option to work, let me try again... ahh, it does work, thanks... you have to kick the number of leds, and sometimes it appears to take some extra clicks...

so, I had to reduce the supply voltage down below 12 volts to force it to use a parallel arrangement, select 12 volts (as per this thread) and it only wants to give serial...

Not explained, and since this site is for novices, I give it a thumbs down there are better ones that tell you more and give you control over series / parallel.

(right back at you, you do not like to be disagreed with)

Greg
 
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