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Discussion Starter · #1 ·
Greetings to all,
I am trying to understand why the LEDs I replaced the incandescent bulbs in my Caravan's HVAC Control center with suddenly aren't illuminating. Rather than dink around, hook it up, see if they work, I'd greatly appreciate it if someone out there could tell me which of the pins needs to have power applied to it and which of the pins needs to be grounded in order for me to test the control panel lighting at my work bench.

Many Thanks!
 

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When you say suddenly aren't illuminating, I assume they had previously been illuminating?

Did you replace the bulbs directly with LEDs by themselves, or LEDs with an appropriate current limiting resistor?
 

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1997 Plymouth Grand Voyager Rallye
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I'd check the LEDs first.

Pin 14 on C1 connector with manual temperature control

60764




Pin 3 on C2 with automatic temperature control
60765





(ATC)= Automatic Temperature Control
(MTC)= Manual Temperature Control

60766

Source: 2005 RG/RS-SM; Chrysler RG & RS Platform Service Manual; Property of DaimlerChrysler Corporation
 

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Discussion Starter · #4 ·
When you say suddenly aren't illuminating, I assume they had previously been illuminating?

Did you replace the bulbs directly with LEDs by themselves, or LEDs with an appropriate current limiting resistor?
Hi ScuzziOne,
That is correct. When I first installed the LEDs in place of the (burnt out) incandescent bulbs in the (manual) HVAC control panel, much to my shock and amazement, all 4 of them worked! To be honest, I cannot recall when I first noticed that they were no longer lighting, but it was 2 or 3 months after the switch over at least.
Your question about an appropriate current limiting resistor is interesting to me, because I was unaware that this was required. Would you please expand upon this including such details as what size (ohms and watts) resistors need to be installed and precisely how I should install them? The LEDs I purchased fit directly (though imperfectly-a bit too small) in the openings in the PCB where the incandescent bulbs had been. In fact, if you know of a specific LED replacement that is straight-on 'plug and play', I would be ever so grateful if you would share that information with me as well. I recall looking high and low across the net before I found the replacements I have presently.
 

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Discussion Starter · #5 ·
I'd check the LEDs first.

Pin 14 on C1 connector with manual temperature control
Hi SpecialEdy,

Thanks for your reply and most especially for the PinOut diagrams you included. Forgive me for removing them from my reply, but it seemed superfluous to keep posting them over and over, ya know?

Just to make sure I understand you correctly, I will connect the 12 VDC positive lead from my bench top power supply to Pin 14 on the C1 connector and the negative lead to Pin 1 (Ground) of the C1 connector, correct?
 

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I also thank you Edy for that valuable info! Bookmarked this thread, because I have at least 3 of the automatic controllers that have burned-out bulbs in them. Hope to one day fix them all, and then sell off the extra.
 
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Hi ScuzziOne,
That is correct. When I first installed the LEDs in place of the (burnt out) incandescent bulbs in the (manual) HVAC control panel, much to my shock and amazement, all 4 of them worked! To be honest, I cannot recall when I first noticed that they were no longer lighting, but it was 2 or 3 months after the switch over at least.
Your question about an appropriate current limiting resistor is interesting to me, because I was unaware that this was required. Would you please expand upon this including such details as what size (ohms and watts) resistors need to be installed and precisely how I should install them? The LEDs I purchased fit directly (though imperfectly-a bit too small) in the openings in the PCB where the incandescent bulbs had been. In fact, if you know of a specific LED replacement that is straight-on 'plug and play', I would be ever so grateful if you would share that information with me as well. I recall looking high and low across the net before I found the replacements I have presently.
Incandescent bulbs and simple LEDs are not directly interchangeable due to the way they behave when voltage is applied to them. Incandescent bulbs offer their own inherent resistance to limit the flow of current and don't care which way the bulb is installed; however, LEDs will only flow current in one direction (once their forward voltage has been reached... and as long as their maximum reverse voltage is not exceeded!) with little (ideally no) resistance to current flow for voltages beyond their forward voltage, unless a series resistor is added to limit current appropriately. The size of the series resistor to be used depend on the LED you've selected, the source voltage, and how bright you want it to illuminate. Typical LEDs may operate anywhere from ~3mA to ~30mA, but the specific datasheet for your LED should be consulted to determine an acceptable current level to achieve the brightness you want without exceeding the maximum current level the LED can withstand.

For example, if you had a 12V power source (Vs), an LED with forward voltage (Vf) of 1.4V, and you wanted to flow a current (I) of 10mA across that diode in an ideal case then the series resistance equation for the needed resistor would be:

R = (Vs - Vf) / I

Plugging in our example numbers:

R = (12 - 1.4) / 0.01 = 1060

So, you would ideally need a 1,060 Ohm series resistor for this example. Necessary power rating would depend on current flow, voltage across the resistor, and a derating factor (Fd) for environmental operating temperature. Keeping it simple with a 50% derating of the resistor, the equation for power rating would be:

Pr = (I * (Vs - Vf)) / Fd

And plugging in our example numbers:

Pr = (0.01 * (12 - 1.4)) / 0.5 = 0.212 Watts

So, for this example, I would recommend a 1/4 Watt resistor to be conservative. Again, your numbers will vary based on the specifics for your LED and the margin/derating you select for your application.

Lacking a current limiting resistor, either added or built-in, the LED will typically over-current/over-heat and "burn-up." And, if not the LED itself that burns up, then some other more vulnerable trace, fuse, wire, or so forth may be what burns up.

All that said, you can buy LEDs that already have a resistor built-in for an intended working voltage and environment such that you simply have to pick the right one...

Before diving into the mess above, let's figure out what LED you purchased and installed. Can you provide the manufacturer and part # for the LEDs you installed?

Pretty sure this A/C Control bulb replacement with LEDs has been beaten to death elsewhere on this forum, but I can't seem to find the thread at the moment... anybody??

Hmmm.... here's one of the old threads:

 

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Incandescent bulbs and simple LEDs are not directly interchangeable due to the way they behave when voltage is applied to them. Incandescent bulbs offer their own inherent resistance to limit the flow of current and don't care which way the bulb is installed; however, LEDs will only flow current in one direction (once their forward voltage has been reached... and as long as their maximum reverse voltage is not exceeded!) with little (ideally no) resistance to current flow for voltages beyond their forward voltage, unless a series resistor is added to limit current appropriately. The size of the series resistor to be used depend on the LED you've selected, the source voltage, and how bright you want it to illuminate. Typical LEDs may operate anywhere from ~3mA to ~30mA, but the specific datasheet for your LED should be consulted to determine an acceptable current level to achieve the brightness you want without exceeding the maximum current level the LED can withstand.

For example, if you had a 12V power source (Vs), an LED with forward voltage (Vf) of 1.4V, and you wanted to flow a current (I) of 10mA across that diode in an ideal case then the series resistance equation for the needed resistor would be:

R = (Vs - Vf) / I

Plugging in our example numbers:

R = (12 - 1.4) / 0.01 = 1060

So, you would ideally need a 1,060 Ohm series resistor for this example. Necessary power rating would depend on current flow, voltage across the resistor, and a derating factor (Fd) for environmental operating temperature. Keeping it simple with a 50% derating of the resistor, the equation for power rating would be:

Pr = (I * (Vs - Vf)) / Fd

And plugging in our example numbers:

Pr = (0.01 * (12 - 1.4)) / 0.5 = 0.212 Watts

So, for this example, I would recommend a 1/4 Watt resistor to be conservative. Again, your numbers will vary based on the specifics for your LED and the margin/derating you select for your application.

Lacking a current limiting resistor, either added or built-in, the LED will typically over-current/over-heat and "burn-up."

All that said, you can buy LEDs that already have a resistor built-in for an intended working voltage and environment such that you simply have to pick the right one...

Before diving into the mess above, let's figure out what LED you purchased and installed. Can you provide the manufacturer and part # for the LEDs you installed?

Pretty sure this A/C Control bulb replacement with LEDs has been beaten to death elsewhere on this forum, but I can't seem to find the thread at the moment... anybody??

Hmmm.... here's one of the old threads:

Another thing, LEDs have a very narrow window of voltage to operate in, since current goes up exponentially with voltage.
Thus, you cannot typically control brightness with voltage, you have to limit current or pulse width modulate(think flipping the light switch on and off hundreds of times a second) to dim an LED. The panel lamps are probably dimmed by voltage, your LEDs would either be on or off, and not scale linearly in brightness with the incandescent light bulbs.
 
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Another thing, LEDs have a very narrow window of voltage to operate in, since current goes up exponentially with voltage.
Thus, you cannot typically control brightness with voltage, you have to limit current or pulse width modulate(think flipping the light switch on and off hundreds of times a second) to dim an LED. The panel lamps are probably dimmed by voltage, your LEDs would either be on or off, and not scale linearly in brightness with the incandescent light bulbs.
Agreed. The key is to control current, which is why you need the series resistor to limit current through the LED to something below the maximum rating for the LED but high enough to get the light output you want.

With a series resistor you can achieve some amount of brightness control by varying the voltage across the complete series circuit of resistor and diode; however, as Special Edy pointed out: it won't be linear and it will absolutely cut-off as applied voltage goes below Vf... likely being too dim to be useful well before going below Vf. In this case, you would select your resistor for desired/maximum brightness/current at the maximum possible voltage the circuit might see so that brightness would fall off from there as voltage is reduced.

For better dimming control, you need more complicated LED current control circuitry, likely incorporating switching voltage regulation (buck/boost) and pulse width modulation (PWM) that Special Edy mentioned. If you're lucky, you may find an existing drop-in replacement LED bulb assembly that will do what you want. One source for such a drop-in might be:

 

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Discussion Starter · #10 ·
Before diving into the mess above, let's figure out what LED you purchased and installed. Can you provide the manufacturer and part # for the LEDs you installed?

Hello again ScuzziOne,

Thank you for the refresher course in basic electronics. I mean that sincerely, since it's been more than a couple of decades since I studied electronics theory as a part of my degree program in college!

Apologies for redacting your post for my reply, but again why say the same thing over and over?

I purchased the LED replacements from a merchant on Amazon, named partsam. The original order I placed with them was for the wrong sized replacement LED altogether. I emailed them about this issue and provided them with pictures of the PCB, the original bulbs, and they were able to recommend the ones I have installed now. Here is the information available from Amazon regarding these LEDs:

Partsam T3 Neo Wedge LED Light Bulbs A/C Climate Heater Control Lamps Gauge Cluster Instrument Dashboard Bulbs.

Cross Reference / Compatible socket bulb number: T3, 39397-SA5-003, 35852-SEP-A02, 35853-SDA-A01

Type: Neo Wedge Bulbs, 3014 SMD

Base Size: T3

Color: White, 5500~6000K

Voltage & Wattage: 12-14V DC

Current:0.01mA

Beam Pattern: 180

Lumen: 17lumens, approx 34CD

That is the extent of the technical data available to me. I hope it is sufficient.
 

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Hmmm... does look like what you purchased should have incorporated some form of current control suitable to your application. Before even applying power to the board(s), I would recommend taking a resistance reading across at least one of the LED bulb assemblies to check for any possible short that might have developed and then go from there. This assumes they are all in parallel of course, such that testing across any one would test across all. If this is not the case, then test as appropriate. Probably worth taking a resistance measurement from the power input pin to its' return as well to make sure you are not trying to apply power into a shorted piece of equipment.

I did see some comments regarding concerns for the amount of heat generated by these LEDs, so perhaps that might be a concern as a possible cause of pre-mature failure if they were getting too hot inside the control assembly?

Would be interesting to know what the absolute maximum voltage is that these are designed to operate at. Presumably, there should be enough tolerance to handle whatever a car's regulator might supply... which typically is below 15V.

I have seen some battery chargers go over 15V during charging though...

Looks like you are definitely in for some investigative work!

Please do let us know what you figure out!!
 
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