|Topic Review (Newest First)|
|05-27-2020 11:55 AM|
Originally Posted by jimbocker View Post
|05-27-2020 10:58 AM|
That's a cool wizard. Looks like you're on track!
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|05-27-2020 09:46 AM|
so I found a good online calculator to help me nail down the resistor sizes I should be using.
My blue LEDs are 3.2V 20mA
if I use 12V as my source voltage (with 2 blue LEDs in series), the calculator suggests a 330Ω resistor.
if I increase to 14V for the source... this is the output:
based on my testing.... I estimate that a 390Ω resistor will run the LEDs around 90-95% full brightness.. (3600-3800mcd). This should ensure a long life out of them as well.
|05-26-2020 03:23 PM|
Originally Posted by John Strenk View Post
(that's actually my normal field of expertise... IT Consultant for a large computer company. )
|05-26-2020 08:36 AM|
I doubt if the forward current at 14 volts would push you over the edge on the Maximum current allowed with a 300Ω resistor.
Usually the typical current listed is far below maximum current. It's not like 20mA will be bright and 22mA will leave a smoking hole in the dash.
(Cool you found the "Ω" symbol" )
|05-24-2020 10:44 AM|
The LEDs should have a maximum forward current rating. If you're lucky, the datasheet for the LED will also have curves that represent forward voltage vs forward current. That can be used to find the voltage on the LED at a specific current. In the absence of those curves, just assume the specified forward voltage.
So, for a 14V supply, the voltage across the resistors will be 14-V(LEDs). Then choose the resistance such that I(LED) <= I(LEDmax). That, with some additional margin, will make sure the LED is not damaged by the alternator output. Since you're connecting it to a dimmer, the brightness shouldn't be an issue for you.
I haven't read this whole thread exhaustively, so if I misunderstood the problem, let me know. I'm an electrical engineer by trade, so I'm happy to help out on this kind of stuff.
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|05-24-2020 09:46 AM|
or do I just need to reverse the math to calculate 8V on a 300Ω resistor into current. (.026A)... and make sure that I'm still within a tolerance range for the LEDs?
|05-24-2020 09:29 AM|
yes... 3 volts across each LED.
Not sure if I understand the duty cycle question. I think you are asking the "on" vs. "off" time on the dimmer's square wave output? If so, it's variable. It has a click dial (off click, potentiometer dial adjusts to 100% on at full turn).
The approach I've been using is to design the circuit (resistors inline with LEDs) without the dimmer in the mix. Then, since the dimmer simply outputs a modulated (on-off) square waveform of the same voltage that is input to it, the result should work to effectively dim the LEDs. In my tests, it seems to be working fine... but my biggest concern is that all of my testing is using a 12V power supply. Unfortunately, I don't have any way of upping the voltage output to 14V so I can simulate the vehicle voltage when the alternator is spinning. My concern is that voltage drop across the resistor.
The math I'm using is R=E/I
Let's assume simplified numbers for the sake of comparison.
LED = 3V / .02A
If I'm running two LEDs in series, that's 6V takedown across the pair. that leaves 6V for the resistor to own.
6V/.02A = 300Ω
If I do the same math using a 14V circuit however:
3+3V = 6V LED Voltage takedown leaving 8V for the resistor:
8V/.02A = 400Ω
running the 300Ω resistor in that circuit leaves 2 extra volts that I need to figure out where they go. This is the essence of my concern.. I think the only way I'll be able to figure this out is to plug (my test circuit) into the cigarette lighter on a vehicle with the engine running and see what happens. That or I do my testing with the forward voltage across the LEDs running low as a safety margin, expecting they'll go up (get brighter) once powered by the alternator.
|05-23-2020 07:46 PM|
Is that 3 volts across each?
Do you know the duty cycle on the PWM? Some DMM can measure it.
That will also determine the resistor size,
Say duty cycle is 50% you can safely cut the resistance you calculated by ohm's law by 1/2, if it's 25% then you can cut it by 1/4
|05-22-2020 04:20 PM|
so here's my current illumination design. Can you take a stab at what I should expect?
|05-19-2020 08:29 PM|
Lets skip the forward voltage drop, PWM control and all that stuff for the moment.
Look at the specs on the LED.
Say one in particular has a typical Forward Current of 10 mA or .010 Amps and Maximum Forward Current at 50 mA.
You can find this info on a spec sheet. Digikey has all the info on each LED they sell as do other places.
To figure out what size resistor for 12 volt you would use R=E/I or 12/.010 or around 1200 Ohms to limit your current.
So now the voltage increases by 2 volts, what is the current now? I=E/R or 14/1200 = or 0.012 Amps. Just an increase of 0.002 amps or 2 mA. You are still well below the 50 mA limit.
What is the maximum Voltage well E=I*R so 0.050*1200= 60 Volts maximum.
And now we went completely through Ohm law. E=IR, I=E/R and R=E/I
It get's more interesting using PWM control, then the specs change. Typical forward current can reach .050mA if Duty cycle is 10% at 1kHz
Oh, What watt size resistor? well Watts=I^2R so .010^2*1200 = 0.001*1200 = 0.12 watts. So you can go easily with a 1/4 watt resistor.
Oh, use 1 resistor per LED.
|05-19-2020 05:35 PM|
|WadeHiersSr||hey... another thought that occurred to me....for anyone that wired their own LEDS using raw bulbs and resistors.... did you have any issues with the change from 12V battery voltage to 14V when the alternator is up to speed? did you account for the voltage uplift in your resistor selection? I've been performing my testing by using a 12V power supply... but I can't help but wonder if moving to alternator power at 14V will blow out half of my LEDS... I guess I need to take my test circuits and plug them into my Wrangler (actual working vehicle).|
|05-19-2020 07:42 AM|
Originally Posted by uw91 View Post
|05-19-2020 04:06 AM|
With the OEM ones you could just remove the mask. And put in a new one.
You would not have to sand any black off.
You could print the new label design on a laser printer using the clear plastic sheets they use to use for overhead projectors.
|05-18-2020 11:56 PM|
incidentally.... I wonder if anyone has tried to make custom labels for these indicator lamps???
I'm building a (6) switch panel to mount to the bottom of the dash for auxiliary lighting... and to carry the styling queues from the original dash design, I'm using push/pull toggle switches and plan to take a set of these lamps and modify the label to match my switch requirements... (fog, winch, rear, Fr Flood etc).. I'm thinking my best bet is to use the labels from the new set, sand off the black and use vinyl stickers as a mask to repaint them.... anyone have any other ideas on how I could replicate the style of these indicator lamps easily?
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