Re: HELP: Need an Electrical Expert!
- From: "MOSFET" <ntanner@xxxxxxxxxxx>
- Date: Sun, 5 Mar 2006 17:33:55 -0800
Oh, wait. I think I get the diagram now. I hook up all the LED's in
parallel, but then I hook up the resistor IN SERIES with the 5 LED's.
RIGHT?
MOSFET
"Matt Ion" <soundy@xxxxxxxxxxxxxxx> wrote in message
news:cEIOf.107123$sa3.93267@xxxxxxxxxxx
MOSFET wrote:
I need help with some LEDs. I bought this Clarion dash-mount EQ on Ebay
and the blue LED's that illuminate all the dials were dead. I went to
Radio Shack and bought five 12 volt LED's and installed them into the
unit. They are NOT wired to any of the EQ wiring. They have their own
positive and negative leads coming out of the EQ (obviously, this is
something I did, not Clarion). Now this worked GREAT on my 13.5 volt
bench power supply. But in my car hooked up to my ignition voltage, it
SUCKS. It is CONSTANTLY changing brightness with engine voltage going
from nearly off when the car is idling to VERY BRIGHT when the car is at
a high RPM. I KNOW there must be a way to even-out this voltage, so it
maintains a CONSTANT 13 volts (it uses hardly any amperage, of course),
but I don't know how to do it.
Also, should I use the REM wire from the HU for power? I didn't want to
as that wire must already activate the EQ and four amps.
Any help would be GREATLY appreciated.
Alright, there's a lot of speculation and half-assed information floating
around here; let's try to clear a few things up. Note: this is based on
the assumption that these are just straight-up LEDs with no built-in
resistors or other circuitry (what is the RS part number?)
First, a voltage regulator is overkill. The problem you're having with
fluctuating brightness is because wiring them in series is dropping the
voltage across individual LEDs to below each one's forward voltage.
Any diode will not begin conducting current until the voltage reaches a
certain level: for germanium diodes, it's typically about .7V; for silicon
diodes it's 1.4V; and for LEDs it's usually around 2-3V. Once you reach
that voltage level, it will begin conducting the full voltage through with
next to no resistance to the current. That means if you just hook up an
LED across your car battery, it will appear as pretty much a dead short,
and will glow very very brightly... for a few milliseconds.
Now what you did in your orignal setup was to split the car's voltage
evenly across the five LEDs - if each one drops exacly 3V, then to run
them all, you need a solid 15V; any less, and the voltage per diode drops.
As it drops below the avalanche point, they'll stop conducting; if ONE
stops conducting, they ALL shut down.
So what you want to do is wire the LEDs in parallel, so each one gets the
full available voltage. Again, this will normally allow all available
current to flow through each one until they melt down, so you need to
limit the current with a resistor (or resistors). Most LEDs require
around 12-20mA, so using Ohm's Law, we can calculate, assuming an average
14V and a median 16mA (check the package specs):
14 volts / 0.016 amps = 875 ohms.
So an 875-ohm resistor in series with EACH LED will give you a good
mid-level brightness; now the closest standard resisitance to that is 820
ohms, which will be a little brigher, but not noticeably so; assume a 10%
tolerance and it's well within range. Figure 14V*0.016A=.224W, so a
quarter-watt unit will be fine (1/4W, 10% resistors are about the most
commonly available, so it makes for easy shopping).
You could simplify things a bit by using one resistor to feed all five in
parallel. Since this would require 5*16mA, or 80mA, you need
14V/0.08A=175 ohms total (or just do 875/5). A 150 or 180 ohm resistor
will do, but keep in mind, the power draw will increase accordingly, to
about 1.25W, so you'll want a resistor that will handle at least 1W.
So your wiring would look like this:
820ohm LED
+----/\/\/\/--|>|----+
+----/\/\/\/--|>|----+
+12V--+----/\/\/\/--|>|----+--ground
+----/\/\/\/--|>|----+
+----/\/\/\/--|>|----+
Or
+--|>|----+
150ohm +--|>|----+
+12V---/\/\/\/--+--|>|----+--ground
+--|>|----+
+--|>|----+
(Use a fixed-width font or the diagrams will look nasty)
Brightness may vary a little with voltage variations, but shouldn't be any
more than the dash lights normally vary. If it's too much, you can go to
the voltage regulator (use the same setup, but put a 12V regulator at the
input of the whole thing, so you don't have to muck around with changing
all the resistor values), but I don't think that will be necessary.
Actually, I'd be very surprised if all five original LEDs in the EQ were
actually fried; I'd expect it's more liklely just a broken connection
feeding them, or at worst, whatever was feeding them (probably a resistor,
maybe a resistor/transistor setup for a constant-current supply) was
toasted. I'd give the EQ a good going-over for a broken circuit trace or
loose wire before going to all the effort of rigging up a completely
separate display.
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