Author Topic: Resistor and capacitor paralleled across LEDs in car instrument cluster  (Read 358 times)

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Offline jono80

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I’ve been looking at car instrument clusters (this one’s out of a 2010 Toyota Hilux).  The circuitry to drive some indicator LEDs (eg. for low oil pressure, check engine, brake etc.) is a common 12v positive supply with the ECU (or sensor) providing the path to ground via a current limiting resistor (on another part of the PCB, not shown in the pic).  There is a capacitor and resistor in parallel with some of the LEDs and I’m curious as to what this would be for?  For some LEDs there is no capacitor or resistor, for others there are both or only the capacitor or resistor.

If I was guessing it might be ESD protection?  Or to prevent capacitive coupling causing the LED to light from neighbouring traces (unlikely at this voltage?)

Anyone know or care to hazard a guess?  It might be related to how each LED is driven (eg. by ECU or by a simple switch like oil pressure)…

 

Offline sigma_xi

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Could be a snubber to protect the LED from the large transients that can occur in car supply rails (e.g. load dump). The parallel resistor discharges the capacitor in case there is no path to ground. Although usually TVS diodes are used to protect circuits from these transients.
« Last Edit: June 23, 2021, 02:43:30 pm by sigma_xi »
 
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Offline Benta

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It doesn't even have to be a load dump (which is worst case).
A car is an incredibly "dirty" environment when it comes to spikes and other noise.
If the indicators are incandescent bulbs it's no problem. But with LEDs, flashing or ghosting from noise is an issue.
The wire harness from ECU to dashboard can easily be a meter or longer, plenty for picking up noise.

 
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Offline Roverfan

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My guess is its purely for aesthetics, softer user experience. The capacitors turn off leds slower. This is expected in the modern world, as a led going off instanly is too much of a shock.On some circuits it would be expected that the light turns off instantly, others would need to be soft close.
 
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Offline bigk1

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My guess is its purely for aesthetics, softer user experience. The capacitors turn off leds slower. This is expected in the modern world, as a led going off instanly is too much of a shock.On some circuits it would be expected that the light turns off instantly, others would need to be soft close.

This ^ If you notice, the capacitor / resistor are for the LED's with user functions (cruise, 4wd etc). The one's without are for errors (diesel low, engine oil etc). User functions are a normal notification, but you want that "shock" to draw attention to problems.

Cars are dirty environments but most things now communicate over CAN bus with the power filtered at every major electronic device such as instrument clusters, HVAC ECU etc.
 
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Offline krzysssztof

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Resistor might be there to bleed residual current (potentially induced by other circuits in the proximity?).
Even microamps can cause LEDs glowing what would be irritating, especially at night.
 
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