Off Topic - Need Basic Electronic Circuit Assistance

ddavelarsen · Feb 19, 2006

This is off topic to knife making but I know that several folks here will be able and possibly willing to lend this old fool a hand.

I'm trying to figure out how to reduce 12v dc to 3v to run LEDs. I have an adjustible voltage regulator but cannot decifer the schematic and formula to determine how to hook it up to do that - can't even tell if it will do that.

Anyone willing to go offline and handhold me through this? If you're local enough I'll buy you lunch, if not I'll send you a box of old issues of Blade.

Thanks for any help!

rashid11 · Feb 19, 2006

You can just use resistors to accomplish that. Typical LED is 5ma of current or thereabouts. To get 5ma out of 12V supply, you'd need
something like 2.4K resistance - from + side of 12V to LED's anode.

You can not hookup LEDs straight to the power rail and ground, even
if you reduce the voltage to 3V. LEDs won't be able to regulate the current
to where they are safe, you'd still need current limiting resistor (.8K in this
case, to get 5ma).

If you still want to get 3VDC, you'd use a three-legged voltage regulator.
12VDC in, 3 VDC out and ground. Depending on exact power reqs, you
might also need to mount it on a heat sink.

To get 3V out of 12V, 9 of them will have to "stay" on the regulator.
With current rating in 1A (that's a whole lotta LEDs, but just for example),
the voltage regulator will need to dissipate 9W . Something like this definitely requires heat sink.

Some models are "fixed" to generate preset voltage over fairly wide
range of input voltages. Others are programmable, requiring extra
components to set the output voltage.

What's the exact model you have ? Digikey most definitely will have
a specs sheet for it .

ddavelarsen · Feb 19, 2006

Thanks, that is helpful. I've been a bit concerned about the current too. The voltage regulator I have is from Radio Shack, http://www.radioshack.com/product/index.jsp?productId=2062601 . I do have a few small heat sinks, I just hope I can work them into the overall design of the armor.

Here's a scan of the back of the packaging. Please let me know what I should be doing with this. The primary LED I'm using is rated at 3.6v, 20 mA, and 1100 mcd, whatever that means...

Thanks again!

Ripper · Feb 19, 2006

Dave,
At 12 volts, if you plug in those numbers for the LED here, it determines you need a .5 watt 470 ohm to properly drive it.The wizard draws a wiring diagram as well.
Regards,
Greg

rashid11 · Feb 19, 2006

Dave,

Will is correct, all you need is a .47K resistor. .5W is a conservative rating,
in reality it will dissipate (9V * .020A) = .18W, less than a 1/4W. Depending on the space you have, 1/4 will be a bit smaller.

mcd stand for "mini candell".

ddavelarsen · Feb 20, 2006

Thanks guys, that does help. Greg, the LED wizard is a godsend.

However. (Surely you knew that was coming)

For power for my experiment I took an old 12v game or radio transformer and hooked the positive lead to the positive side of my typical LED. I clipped a 470 ohm resistor to the negative side of the LED and that to the negative pole of the transformer: nada.

I did test the transformer for power, it has output though I don't know how much. My "testor" is just an old lamp in a screwdrive kind of thing. I tested the LED off a 3v camera battery and it works.

So I'm still screwing something up.

rashid11 · Feb 20, 2006

LED needs DC. A cheap way to get that is to add a diode to your circuit.
The LED won't shine as brightly, since it is not a true DC and LED will be
turned off about 55% of the time. But human eye will avg it out.

Polock · Feb 20, 2006

LEDs need to be connected in the correct polaraty, also if the current draw exceded the LED ratings it can blow so fast you may not have seen it. Most LEDs run around 2.5 volts maximum. If you put 6 in series they will divide the 12 volts equally (2 volts each. I had a 7 LED series array as an extra light on one car's lower console for 8 years with out any resistors and the nominal voltage there was 13.6) and their internal resistance will self regulate the current.

A 1/4 watt resistor at 470 to 560 (470 will be brighter) ohms is recomemded in most cases in the practical realm. A 1/8 (.125) watt is technically correct but may run a little warm. Carbon resistors have a negative temp response, the higher the temp the lower the resitance (ohms), the lower the ohms the more current is drawn, more current equals more heat. If you are running close to the max ratings an 1/8 watt might cause a premature failure.

Hope this helps