# LED question

Discussion in 'Electronic Basics' started by Andrew Howard, Mar 3, 2004.

1. ### Andrew HowardGuest

You have to be careful, because a car battery is not normally 12V but a bit
over, like 13-14V (I think). Maybe a regulating circuit could be used?
It should.
I don't think so, because the wire isn't carrying much current through it.

2. ### Ian StirlingGuest

Nope.
To be extra cautious, you might add a reverse biased silicon diode across
the LED, to protect it from any nasty spikes.

3. ### electrickedGuest

So the car battery produces 12 volts. Let's say I want to light up a single
LED with the car battery. Is this possible?

V=12V
I=20mA
R=?

V=IR
R=V/I=12/0.020=600ohms

So if I hook up a circuit in parallel with the rest of the circuitry that
goes off the battery, and I place a resistor of 600OHMs and a single LED,
will it light up? Does the size of the wire I use matter?

TIA!

--Viktor

4. ### Costas VlachosGuest

You need to consider the voltage that's going to appear across the resistor.
This is given by the battery (12V) minus the LED forward voltage (about 2V).
So we have:

R = (12-2)/0.02 = 500 Ohms.

Also, as already said, car batteries are closer to 14 volts than 12, so your
original result of 600 Ohms turns out to be correct (accidentally).

cheers,
Costas

5. ### Peter BennettGuest

Yes, it will light. Since the wire will only be carrying 20 mA, you
could use quite small wire - I'd suggest #22 or 24 for ease of
handling, although they are much larger than really required.

The 20 mA current you suggest for the LED is probably its maximum
recommended current - I usually run LEDs around 8 - 10 mA, and find
them bright enough.

As others have mentioned, a "12 volt" car battery is really somewhat
higher - up to about 14.4 volts when being charged.

6. ### John LarkinGuest

Close enough.
No, the current is very low here.

Should work fine; try it.

John

7. ### John LarkinGuest

There's no reason to regulate the voltage for a simple resistor+LED.

It's not. You are adding the 600 ohm resistor, so you only use about
20 ma.

John

8. ### electrickedGuest

So if I'm regulating the voltage at 12V then it will work so long as the
voltage is constant. Got ya.

Another question now. From my theory understanding, if I connect my LED
circuit in parallel with the other circuits hooked to the battery, wouldn't
that make a lot of current flow in my circuit, since the resistance of my
circuit would be much smaller than in the rest of the circuit hooked in
parallel, then wouldn't all the current choose to go through my little
circuit? This doesn't make sense but that's what the theory says or maybe
i'm not understanding it right. Someone care to explain how all this works
and fits?

Here's a diagram of what I'm refering to:

-------------|---------------
| bunch | |
| of car | |
| circuits | |
| in - battery |
| parallel | |
| with the | |
| battery | |
-----------------------------

Since resistance in MY LOAD is less than resistance in rest of circuitry
wouldn't that cause more current to flow through my circuit? What am
misunderstanding here?

--Viktor

9. ### Peter BennettGuest

In a parallel circuit, the current in each branch is determined only
by the resistance in that branch.

If you connect your LED and 600 ohm resistor across the battery, it
will draw 20 mA (or so), regardless of what other loads may be
connected to the battery (except when you hit the starter - the
starter will draw so much current that the battery voltage will drop,
and the current in all other loads will therefore drop until you stop
cranking.)