United Pacific LED taillight problem

[Leddie]

Jon from Wexford, Pennsylvania visited my LED site and asked this
question which I am unable to answer. Could someone help? Thanks guys
-- Leddie host of fun-led-light.com

"Purchased LED taillight units for '31 Model A ford (12volt). I bought
three units so I could fabricate a matching 3rd brake light for the
rear window. The lens/light units are made with upper and lower
sections which are wired together to work as one. The turn signals
are not part of the system. After installing them I found that the
park lights work the way they should and the brake lights also. What I
noticed was that with all switches off, there are three LEDs in the
lower section of all three lights which glow dimly....The same three
on each of the lights. All the other LEDs are off. How or why would
these be lit with the switches controling them open? The United
Pacific Company could offer no answers. Is this anything you've seen
before? Thank you for any information you might provide."

 

[Leddie]

Just a guess: First of all, does this vehicle feed +12V battery to the +
side of the brake lights at all times and ground the brake - to
illuminate the lights?

If so, I'd look for a sneak circuit that allows a small current to flow
from the brake light - side to ground. Something like a car alarm would
do it.

I'm guessing that the 3 LEDs are used as crude voltage references in a
regulator circuit that drives the remaining LEDs. The forward voltage
across an LED is roughly constant for a wide current range. The LEDs in
each light are broken up into 3 groups with one regulator (and LED) for
each. The sneak circuit current is sufficient to cause these three to
illuminate dimly, but not enough to turn on the others controlled by the
regulator circuits.

Thanks Paul, much appreciated.
I asked Jon to reply so we know if this is what he experiences.
Am featuring your response on FLL also -- Leddie
http://www.fun-led-light.com/tail-light-led.html#United_Pacific_LED_taillight_problem

 

[Leddie]

Just a guess: First of all, does this vehicle feed +12V battery to the +
side of the brake lights at all times and ground the brake - to
illuminate the lights?

If so, I'd look for a sneak circuit that allows a small current to flow
from the brake light - side to ground. Something like a car alarm would
do it.

I'm guessing that the 3 LEDs are used as crude voltage references in a
regulator circuit that drives the remaining LEDs. The forward voltage
across an LED is roughly constant for a wide current range. The LEDs in
each light are broken up into 3 groups with one regulator (and LED) for
each. The sneak circuit current is sufficient to cause these three to
illuminate dimly, but not enough to turn on the others controlled by the
regulator circuits.

Thanks Paul, much appreciated.
Posted your reply for Jon on FLL also, see how this may help . --
Leddie
http://www.fun-led-light.com/tail-light-led.html#United_Pacific_LED_taillight_problem

 

[Leddie]

Just a guess: First of all, does this vehicle feed +12V battery to the +
side of the brake lights at all times and ground the brake - to
illuminate the lights?

If so, I'd look for a sneak circuit that allows a small current to flow
from the brake light - side to ground. Something like a car alarm would
do it.

I'm guessing that the 3 LEDs are used as crude voltage references in a
regulator circuit that drives the remaining LEDs. The forward voltage
across an LED is roughly constant for a wide current range. The LEDs in
each light are broken up into 3 groups with one regulator (and LED) for
each. The sneak circuit current is sufficient to cause these three to
illuminate dimly, but not enough to turn on the others controlled by the
regulator circuits.

Thanks Paul, much appreciated.
Posted your reply for Jon on FLL also, see how this may help . --
Leddie
http://www.fun-led-light.com/tail-light-led.html#United_Pacific_LED_taillight_problem

 

[Leddie]

Just a guess: First of all, does this vehicle feed +12V battery to the +
side of the brake lights at all times and ground the brake - to
illuminate the lights?

If so, I'd look for a sneak circuit that allows a small current to flow
from the brake light - side to ground. Something like a car alarm would
do it.

I'm guessing that the 3 LEDs are used as crude voltage references in a
regulator circuit that drives the remaining LEDs. The forward voltage
across an LED is roughly constant for a wide current range. The LEDs in
each light are broken up into 3 groups with one regulator (and LED) for
each. The sneak circuit current is sufficient to cause these three to
illuminate dimly, but not enough to turn on the others controlled by the
regulator circuits.

Jon wrote back saying "Paul is exactly right. The circuit is designed
just that way."
Thanks Paul. He wrote a bit more about what he unearthed and I feature
it at the thread
http://www.fun-led-light.com/tail-light-led.html#United_Pacific_LED_taillight_problem
Thanks again -- Leddie