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post #91 of 95 Old 05-22-2020, 04:20 PM
WadeHiersSr
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Hey John...

so here's my current illumination design. Can you take a stab at what I should expect?
  • I don't know the max amperage of the LED's I'm using but the forward bias is 20mA
  • I started at determining my resistor size using Ohm's law (the math way)... but then ultimately adjusted it to 390Ω by measuring the voltage drop over each LED. the math suggested I needed something like 300Ω but when I measured the voltage drop it came in around 2.9V. By dropping down to 250Ω I was able to get my voltage drop to around 3.05V.
  • I tried shunting the negative output of the PWM dimmer circuit to ground and then grounding the negative end of the diodes->resistor but the dimmer would not work in that manner so I am running the circuit in a complete loop back to the dimmer.
  • The dimmer works on any voltage level from 1.5V to 16V and outputs the same voltage as what comes in. It can also handle up to 2amps so I should be able to reach the total of 12 parallel copies of this circuit that I'll need without any trouble.
  • I'm thinking that since I'm still using current limiting resistors in this circuit, I should still be seeing current values similar to circuits without the PWM...correct?


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post #92 of 95 Old Yesterday, 07:46 PM
John Strenk
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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


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post #93 of 95 Old Today, 09:29 AM
WadeHiersSr
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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).
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

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.
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post #94 of 95 Old Today, 09:46 AM
WadeHiersSr
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Quote:
Originally Posted by WadeHiersSr View Post

running the 300Ω resistor in that circuit leaves 2 extra volts that I need to figure out where they go. .

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?
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post #95 of 95 Old Today, 10:44 AM
jimbocker
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Originally Posted by WadeHiersSr View Post
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?
This is what I would do.

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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