Dash LED Protection

After 3 sets of LED dash light completely died, I did some head scratching as to cause.

SMD T10 WedgesEach T10 wedge bulb consists of an LED wired in series with a small, current-limiting resistor. Initially, I thought vibration was the culprit, but the resistor-LED connection was fine in all cases.

Next was the possibility of voltage spikes.

When the starter disconnects, a voltage transient is generated ('back EMF') and injected into the electrical system. If this is positive, a bit of extra current would flow through the LEDs in the forward direction and shouldn't do very much harm (i.e. low voltage x low current).

If the transient is negative (i.e. driving the nominal +12V bus negative with respect to ground) and large, it might exceed the reverse breakdown voltage of the LEDs, causing high voltage x low current = higher power and blowing the little suckers out.

To check out this theory, I bought a 4th set of dash LEDs and dug a 1N4004 diode out of the electronic junk box. The idea is to wire the diode across the dash light bus so that normally it's reverse biased and does not conduct, but if/when there is a negative pulse, it conducts and maintains the voltage below dash-LED-blowing levels. I scraped the insulation off one blue/red LED wire and soldered the cathode (the end with the white line) to it, and connected the anode under the yellow/black screw terminal on the back of the meter module.

Protection Diode SchematicBest I can say for all this is the LEDs are cheap on eBay and work well before they blow out. Results to follow.

Oct. 2014 Results

Well, 2 of these lights have become intermittent despite the little diode. I think the low-power dash lights are not up to the vibration level of motorcycling. The higher power LEDs in the tail light and turn signals seem fine. Going back to incandescent for the dash lights.

Trying Again - End November 2015.

Reasoning that dash LED failures may be caused by vibration cracking the connections, I decided to try my hand at circuit potting. The idea is to make the circuit more mechanically rigid, so connections don't flex and break.

Bought some more T10 4 SMD LEDs (see above), about $1.50 for 10 on eBay.

Mixed up some JBWeld on the bottom outside of a glass jar, and heated it up in the microwave to thin it. Using a toothpick, I drizzled the epoxy into the center and corner spaces of the mounting plate holding the little SMD LEDs.

Installed and tested four of these in the cold garage. So now the packaging looks a little ugly, but the light is not impacted, and I'm hoping they last more than the typical 'work for one season' of the past.