In-car heater control panel, LEDs not dimming as they should

[Leuven]

I have this issue that's been bugging me for a while.

First of all, the LEDs behind the heater display are quite a bit brighter to begin with (see radio panel vs heater panel). They're sitting at 1.9V at maximum output and drop to 1.7V when the dash dimmer switch is on - hardly any dimming at all.

For comparison, the display LEDs on the radio are at 1.4V dropping to 1.33V with the dimmer on full. I know 0.7V doesn't seem much of a drop but that's how it measured on the voltmeter and the dimming is quite radical.

Strange thing is all other LED on the heater panel will dim appropriately and in sync with everything else in the dash. What could have gone wrong on this board that only affected that cluster of 8 LEDs behind the display?

[T4P]

1.9V .... that's running it a little over the mark
1.4V to 1.33V makes a big difference actually, even if PWM or not

[Leuven]

1.9V .... that's running it a little over the mark
1.4V to 1.33V makes a big difference actually, even if PWM or not

Good to have that confirmed. Whatever that blown component is, it has a massive failure rate. If I go on the car forums and complain, about half of the replies will reassure me it's normal for the heater display not to dim. Their panel is busted too 

[Jimmy]

You could just find out how much current the board is drawing and then put an appropriate pot in series with supply to adjust the brightness of the led's or just install correct resistor.

[Leuven]

Made a little progress, the 7 pin component is a Siemens power switch (BTS 621 L1).

Pin 1 is definitely feeding all the LEDs, I've severed it and cut the power to all 8 of them.

The voltages are as follows:
Pin2 ground/Pin7 output - 5.93V dimmer off
Pin2 ground/Pin7 output - 5.94V dimmer actuated

Pin2 ground/Pin6 input - minus 4.6V dimmer off (polarity reversed)
Pin2 ground/Pin6 input - minus 4.6V dimmer actuated (polarity reversed)

Do these figures look right?

[amyk]

The voltage is slightly higher with the dimmer actuated because there's less voltage drop from the supply feeding this and the other LEDs due to decreased current.

Trace where the input is coming from.

Posting some high-resolution perpendicular pictures of the board would also be helpful.

[Leuven]

Sorry, I read the power switch pins the other way around. I was measuring garbage 

I've also managed to follow most of the dimming circuit, unfortunately with a gap in the middle. Anyway, here are high rez picts:

http://www.hotpen.co.uk/others/panelfront.jpg
http://www.hotpen.co.uk/others/panelreverse.jpg

This is the trace with the voltages (dimmed-non dimmed).
1) 0.8V - 12V … direct entry from the dash switch
2) 1.8V - 12V … after SJ35
3) From here it goes to various clusters of 2 resistors and then feeding the other non-display LEDs. Voltage after the 2 resistors is 0.7V - 3.8V

gap

5) 5V - 5V … after pin 71 on the Motorola MC912D
6) 4.7V - 4.7V … after 4991 resistor (goes to pin 3 power switch input)
7) 12V - 12V … pin 1 power switch output

Edit: this seems to be more complicated than I thought, there is a light sensor in the dash which acts like a switch - in broad daylight it deactivates the dimmer. Maybe that's the problem right there, that panel is stuck on 'daylight' regardless of what signal is receiving from the dash sensor. There's no wire with a voltage difference corresponding to this ambient change in the plugs so the signal might be transmitted through the k-bus wire (networking).

Also, the microprocessor feeding the switch doesn't look like it has a dimming function, I think it's just turning the fan on and off along with the LEDs by sending voltage to the switch.. When the fan is off, the display goes off too and that is controlled by the microprocessor.

[max010273]

Hello Leuven,
did you find the defective component after all? It was BTS621L1 or something else?