VU Meter LED backlight voltage help...

[3rutu5]

I will probably post some pictures, and some of the details, but since it's pretty non-standard and nobody will ever duplicate it, I don't think I'll go too crazy with it.
And I'm pretty sure you'll win this race.

TomZ

Haha don't let that stand in your way, I doubt anyone wants to recreate any of my random creations

[tomzarbo]

Oh very cool, looks like you and I are in a race to build a ghetto blaster by the end of the year

Are you going to post a build log for this?

I will probably post some pictures, and some of the details, but since it's pretty non-standard and nobody will ever duplicate it, I don't think I'll go too crazy with it.
And I'm pretty sure you'll win this race.

TomZ

[3rutu5]

Oh very cool, looks like you and I are in a race to build a ghetto blaster by the end of the year

Are you going to post a build log for this?

[tomzarbo]

So I received some various values of 1 watt resistors from PE and it looks like a 470 + a 100 ohm resistor in series cuts the light to a decent level, and still stays cool to the touch. I think I'm good there. Now I just have to figure out the rest of the doo-dads and get them to talk to each other, I'm having more noise issues, I may just end up using the amp built into the car radio. More to come...

TomZ

[DrewsBrews]

Power supplies fail eventually and they don't tend to go quietly. An outboard brick might make a problem easier to catch before the flames come.

[AEIOU]

A long time ago, I installed a blinking red LED in a car (12 VDC). Anyway, I connected the LED to a wire wound rheostat of a few thousand Ohms, I dialed in the amount of resistance until the LED brightness was where I liked it. I then measured the adjusted resistance and used a fixed resistor of that same value. Easy-peasy.

[tomzarbo]

Thank you guys...

I'm going to revisit this this weekend if I can find some time, crazy busy lately.

I'm looking forward to getting this part of the build working, I think the rest will be fairly straightforward.
Just need to decide too weather to have a built-in power supply, or do the power brick thing and make life easier. I'm leaning towards easier, but I do have a few decent inboard PS units that would work well. It just makes me a little nervous to have 120V floating around inside something I'VE built. : )

TomZ

[randyohoh]

Here's a picture of the xl spreadsheet that I had previously mentioned. I AM NOT an electrical engineer, but as a mechanical worked with indicator LEDs for several years, had an EE help me.

[neildavis]

Whoops...I didn't see your voltage measurement of 2V, which is a typical value of a yellow LED at 20ma.

At 9.5V, there will be about 7.5V across the resistor, which means you have 7.5/570, or 13ma flowing through the LED. You could probably drop down to a 390-ohm resistor if you want the display brighter (that would result in 20ma through the LED), or use a larger value than 570-ohm if you want it dimmer. With 13ma flowing through those 2 resistors in series, the 470-ohm resistor will dissipate (.013^2)*470 watts, which is .08W. So, the 1/4 resistor size is fine. A larger resistor (1 watt) will get just as hot since it will dissipate the same power, but it might feel cooler due to the larger surface area.

[tomzarbo]

Thanks guys for the good information.

Neil, I'll read this again when I get home. A 1/2 hour lunch break isn't enough for this to sink in.

I have a decent assortment of 1 watt resistors coming from PE -- they handily come in packs of 10 -- between that and the ones I already have, I should be able to get this working soon.

I'm really looking forward to getting the size and space requirements for the electronics part of this figured out. Then I can finally start to build it out.

Thanks again for the guidance.

TomZ

[neildavis]

You need to focus on the current through the LED, not the voltage. Look up "diode equation" on Wikipedia and you will see that the voltage across a diode, including an LED, is a function of the current. The curves are different for different types (and colors) of diodes. You need to keep the current within the rating of the LED, which is usually on the order of 10 to 40ma. At these currents, the voltage drop for a red LED is typically around 1.8V, and white LED's are 3-3.5V.

Calculating the required resistor is easy. Suppose you want to operate the LED at 20ma and your source voltage is 9.5V. At this current, the voltage across a white LED will be about 3V, so the voltage across the resistor will be 9.5V - 3V, or 6.5V. To get 20ma, just use Ohm's Law, where V=RI, or R = 6.3/.02, or R = 315 ohms. A 330ohm resistor will work fine. Since there is about 20ma going through the resistor and the power dissipation is I^2R, the resistor should be at least 0.132W. So, you should use at least a 1/4W resistor.

You can adjust the brightness by using more or less current. But don't worry too much about the voltage--it will vary with the current but usually you can assume the voltage to be constant in the useful operating region. Note that if you can find 5V somewhere, the power dissipated by the resistor will drop quite a bit. For a 5V supply and 20ma through the diode, you would need a 100-ohm resistor, and it would only dissipate .04W.

[unclejunebug]

You may also consider using a voltage regulator instead as their outputs are often adjustable. You may even find one with a fixed voltage output value that matches what you need. If there are any additional resistors/caps needed they would be clearly documented in the data sheet.

[AEIOU]

It's times like this that an adjustable DC power supply comes in handy.

[tomzarbo]

White, Blue, & 525 nM "Traffic" green LEDs normally require 3 volts to get them to the rated light output. Red, orange, yellow, and the yellow-green "Grellow" 565 nM LEDs normally require 2 volts to get them to the rated light output. Running the LEDs hotter will increase the light output and decrease the life, and that can be dramatic or even instantly if substantially overdriven. Most T1 (3mm), T1 3/4 (5mm), and small (0603, 0805, & 1206) surface mount (smt) or (smd) LEDS run at 20mA. If LEDS are wired in series, their voltage ratings must be added, and in parallel, the currents added. I have an old version xl spreadsheet to calculate resistors which I was not allowed to attach.

Thank you randy, that sheds some light on my situation. I may be able to guesstimate a better solution with your help. Thanks!

TomZ

[Steve Lee]

Sounds like fun - Good Luck!!