Trailer Light Indicators

[smittypaddler]

After reading the original post again, I realized his LED
light had a built-in resistor, and further reading based on Google
searches made me realize this is so because the resistance of an LED
is negligible, typically 22 ohms for a red LED and a turn-on voltage
of 1.8v in a circuit involving resistances in the thousands, so the built-in
resistor is needed to keep from burning out the LED. So I've changed my circuit diagram to include it:



I had a lot of fun and spent more time than I probably should have
this morning, trying to figure out how big the resistor in parallel
needs to be. I'm hoping one of you electronics gurus can tell me
if I've figured it right. Given -

Vt = Voltage drop across the trailer light.
Vp = Voltage drop across the parallel circuit for the LED indicator light.
Vs = Voltage of supply = 12

Vs = Vt + Vp = 12

Let At = 3 be the amperes needed to light the trailer light properly,
with the voltage drop (Vt) no more that 2 volts. So the resistance
(Rt) of the terminal light is

Rt = Vt / At = 2 / 3 = .667 ohms

This means we must design the parallel circuit containing the LED to
pass 3 amperes as well.

Rr = Resistance of the resistor in parallel with the LED.
Rl = Resistance of the resistor in series with the LED = .0025.


The resistance of the parallel circuit (Rp) is given by

Rp = (Rl * Rr) / (Rl + Rr)

The maximum amperage (Al) of the LED light with resistor Rl in series,
according to the RadioShack tag is .0025 amps with a voltage drop
voltage (Vl) of 3, so the resistance of Rl must be no smaller than

Rl = (Vs - (Vt + Vl)) / Il = (12 - (2 + 3)) / .0025 = 2800 ohms

If the amperage necessary for the trailer light At is 3, and the
voltage drop Vt is 2 volts, Al is .0025 amps, Rl is 2800 ohms, then
the resistor in parallel must be capable of passing

Ar = At - Al = 3 - .0025 = 2.9975 amps

The resistance (Rr) of the resistor in parallel with the LED must
be no greater than Rl, or too much current will be forced through
the LED. The voltage drop (Vr) across the resistor in parallel
must be at least

Vr = (Vs - (Vt + Vl)) = (12 - (2 + 3)) = 7

This means the resistor in parallel must be no greater than 2800 ohms
and rated for 3 amps (rounded) and at least 7 * 3 = 21 watts, assuming
all my guesses are close.

[journey on]

smittypaddler,

the trailer under Journey On has 12 Vdc lights, so I assume that's what you're working with. I certainly don't know what the voltage is for the LEDs you're showing (and I admit I might have missed that.) but I had assumed that you would get 12 Vdc LEDs there.

Putting 2.3K ohms in series would prevent the taillights from showing. Lets see, the current at 12 V through 2.3 K ohms is 5 mA which seems somewhat low.

Boris

[damason]

The way I hooked up the Trailer Light Indicators is parallel with the trailer lights. The connection is at the front of the trailer to keep the connections out of the water. My trailer is a Float On by Pioneer and has LED trailer lights. When I start out I do a walk around and check that all LED lights are working and again recheck when I stop for a break. The Trailer Light Indicators just reaffirm while I am driving that the lights are still working.

Being hooked up parallel if one of the LED trailer lights quit working while driving the Trailer Light Indicators would still show that everything is OK. However while driving I feel that there is a good chance if the Trailer Light Indicators are working the LED trailer lights are also working.

[smittypaddler]

Dave,

If your LED indicator is indeed in parallel with the trailer light, it merely tells you there's current to the light, but it won't tell you if the trailer light itself is burnt out. Again, I'm not an electrician, so hopefully an expert will chime in.