Re: HELP: Need an Electrical Expert!

Yes, well said. You guys are soooooo smart. I LOVE RAC!!!!

MOSFET



"Austin Becker" <beckerhead@xxxxxxxxx> wrote in message
news:U4JOf.596553$084.292747@xxxxxxxxxxxx
Well said!

--
- AUSTIN BECKER
"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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