I don't understand this light bulb circuit (car switch illumination) - Page 1
Forum section: electronics / beginners

author: GnatGoSplat
author_rank: Regular Contributor
timestamp: April 27, 2020, 09:45:58 pm
content: I have a 1989 car (Oldsmobile Cutlass Supreme) with a non-illuminated door lock switch and wanting to retrofit an illuminated switch out of an identical 1990 (addition of illumination was a design change). I thought it would just be a simple 12V grain of wheat bulb connected directly to ground on one end and PWM dimmer (0-12Vp-p) output for the interior lighting on the other end, but when I dismantled the switch, I found it's much more complicated than I assumed.I've attached images of the board. The amber LED and 1K SMD resistor can be ignored as it's just a standard LED with series current limiting resistor. It's what's going on with that tiny bulb that I don't understand. I've attached a schematic which I've checked about 6-times and am pretty certain it's accurate.D3 (small diode), reverse-biased, my guess is zener? So probably the bulb isn't a 12V bulb? 150-ohm "151" SMD resistor is in series, I assume for current limiting. I have no idea the purpose of D1, D2, and 50-ohm power resistor (on the backside of the board). Wild guess is it's acting as some kind of current sink?In fact, testing this out, I fried D2 when I applied +12V to input labeled LI (it got very hot and glowed), although D1 survived. The bulb did light up and looked normal and didn't immediately blow.Unfortunately, I have no context as to how this board was originally wired as it was pulled from a switch assembly for where no schematic exists, and I don't have the switch assembly. However, the voltages going into the assembly itself are +12V, GND, and PWM illumination.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 27, 2020, 10:07:13 pm
content: I suspect that D1 and D2 are to absorb negative going spikes on the 12 volt supply during system high current transients (like starter motor or headlights on and off). Use of two in parallel implies that expected surges were too much for one. Availability of board for examination might hint that it had a failure, like maybe a short in one of the diodes giving you the result you see.Can't confirm your schematic from the picture, but I didn't see anything wrong. I would replace the glowing diode, verify that the other one actually is a diode still and try again. Probably worth checking the 50 ohm resistor too as it was probably stressed pretty hard.It is strange they didn't use a 12 volt bulb. If you looked at later model years I bet this is simplified a lot.

author: Renate
author_rank: Super Contributor
timestamp: April 28, 2020, 03:05:25 pm
content: I'm not familiar with courtesy light dimmers.Have you looked at the signal to see if it's raw PWM or real variable voltage?

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 28, 2020, 03:11:14 pm
content: Occasionally, those MELF diodes would have the part number printed on the body. They were derived from the ubiquitous DO-35 package.Can't see anything from above, would the printing be on the board side?

author: GnatGoSplat
author_rank: Regular Contributor
timestamp: April 29, 2020, 03:40:51 pm
content: I've attached a photo of the other side of the board. 50-ohm resistor is actually a 51.I've removed the glowing diode (shorted) and the circuit still works with the only thing getting hot being the 150-ohm resistor, but I guess it's supposed to. Before, it actually worked, but the shorted diode would glow and get very hot. That makes sense D1 and D2 are for absorbing transients, but not sure why that's needed when other bulbs don't have them. To protect the zener? The zener measures at 4.10V, so I'm guessing maybe it's a 5V bulb.The diode is closer to the size of a DO-41. It's approx. 2.5mm diameter. Photo attached showing it next to a standard DO-41. There are no markings on it. Since they are parallel, would it be necessary that both diodes are identical in order to balance the transients between them? If so, I was thinking I could just replace them both with 1N4001. Would that work?I don't have a scope to confirm the courtesy lights are PWM, but the schematic of the light switch (photo attached) indicates it (GRY wire).Another question I have is, if LI is where I should apply +12V to power the bulb, what is LP for?I also wonder why did D2 fry just hooking it up to a 12V PSU? It's possible it was already fried before, but the 51-ohm resistor looks fine and I think it would at least be discolored if the diode was fried before.