climate control bulb blown

[cmm303]

I have followed the fact sheet on Climate Control Illumination and found one of the bulbs is dead - open circuit. The fact sheet indicates that Mazda replacements are expensive and poses the possibility of using an LED. I bought a packet of LEDs at Maplin yesterday ( when buying a resistor to put on my temperature gauge).

Now I am a bit stuck. It seems I need to incorporate a resistor, get the polarity right and that's before I work out how to connect it to the circuit board. I wondered about cannibalising the bulb holder of the dead bulb. I welcome any advice, perhaps somebody has also explored the options?

[cmm303]

Found that 12v LED can be bought in all sizes. So really now need to work out which is the positive contact on the circuit board. Can't see any clues.

[Darkstar]

Nornally the longer tail of the led is positive,
Cant remember but think it will only light when wire correctly + to positive supply

[Simon Jones]

A multimeter will allow you to check the polarity on the circuit board. Generally, the negative side of the circuitry on the PCB will have larger tracks.

[cmm303]

Nornally the longer tail of the led is positive,

agreed

Cant remember but think it will only light when wire correctly + to positive supply

Mystery is the polarity of the contacts on the PCB and worried that if I get it wrong the LED fries itself. I'll ask tinterweb.**

** tinterweb says connecting an LED the wrong way to 12v will probably break it. With a voltage <5v, it is likely to survive but just doesn't light up.

[Simon Jones]

... if I get it wrong the LED fries itself.

Depends, some LEDS can withstand that sort of reverse voltage, others won't. Check the tech spec for the one you have found, or buy a spare one or just get multimeter for about a fiver which will allow you to test loads of other things in future.

[cmm303]

Depends, some LEDS can withstand that sort of reverse voltage, others won't. Check the tech spec for the one you have found, or buy a spare one or just get multimeter for about a fiver which will allow you to test loads of other things in future.

on order from maplin. Spec seems typical with max reverse = 5v, could not find anything higher. Have multimeter but was not overly happy about powering the unit up whilst in bits then prodding around with probes. Say it ... I'm a big wooz! In the absence of the answer on a plate, I plan to cannibalise the dead bulb holder, solder a couple of fly leads which I can use to measure voltage with unit semi reassembled and live. Then spot the holder and PCB, mount the LED. Simples. It avoids soldering directly to the PCB track, which feels like a good thing. Depending how easy it turns out, i might spot positive on the other two bulb tracks and whack LEDs in all of them.

[cmm303]

Gave it a go. Went ok, but LEDs too dim, so need brighter ones. I used part no. CJ69A from Maplin:
CJ69A 3mm 12V Green SB LED
Light output min.@ 12V: 8mcd
Light output typ.@ 12V: 20mcd

These 3mm LEDs fitted easily into the old bulb holder


wrap the leads, much like the original bulb leads and though a bit tight the holder can be pressed and screwed into place on the circuit board, making sure LED positives line up with the PCB positive track.



My concern about using a larger LED is that its leads will be too chunky to wrap round the holder and screw onto the PCB. Obviously thinner leads could be soldered on. Any advise on a brighter LED is welcome.

[cmm303]

Sorted. Had to buy LEDs and resistors in bulk but only cost a fiver so a cheap fix and have loads of spares if anyone interested. I have wired 3 LED in series with a single 220 ohm resistor and I can for the first time see the controls in the dark. LED spec is:
Forward voltage = 3.2 - 3.7 v
Forward current = 20mA typical (30mA Max)
Luminance = 10000 mcd
3mm T1 clear package

Before wiring them into the climate control, I rigged up a test on the vehicle. With the engine running, the voltage read 14.4v (can this be right, seems high?), the current was 17-19mA which is good. So I am happy that I am not going to overheat the resistor or fry the LEDs. Glad I erred on the high side with the resistor.

[g8dhe]

The voltage is correct! The nominal 12 volt supply is just that nominal! Actual voltages can range from 10.8v-14.4v steady state, with spikes at points within the wiring loom to several 10's volts for millisecond periods.

[cmm303]

Cool. Thanks