Monday, July 28, 2014

Automotive LED Indicator Lamp Analysis

I've been helping put together a small solar set-up for my father, and he wanted a light connected to the regulator to give a bit of light at night outside.  I thought the best thing for the job would be a white indicator LED that's used on a car or truck, that way it's already waterproof.  So I ordered a cheap one off ebay and it was fine.  It did the job perfectly and gave off plenty of light, but I was curious as to how it worked.  The specs claim it operates from 12 or 24 volts and I wanted to know how it went about doing this.

LED Light
12/24 Volt 2 LED White Indicator Light
The module contains 2 white LEDs, and from previous experience I know that white LEDs operate somewhere around 3 volts and if they are in series it means you have to supply about 6 volts.  When the module is powered from 12 or 24 volts there are two ways to go about supplying the power, the first is some sort of switch mode power supply, and the second is basic linear regulation.  Due to how cheap the lamp was, it was most likely going to be linear regulation

LED Light
4 terminal LEDs under the diffuser
Even though the PCB was encapsulated to protect the circuit from water, it was done with a clear resin, so although we can't probe the board directly it can at least be mapped out.  At first glance it was obvious that it was going to use linear regulation.  There's only a handful of components on the board, a 39 ohm resistor, capacitor, diode, and T0-92 package that was only partly encapsulated in the resin which isn't ideal, but I think it's for a reason that I'll come to later.  There aren't any components you'd expect to see in a switched power supply.

Encapsulated PCB
 The schematic was easy enough to draw after visual inspection of the board.

Circuit Schematic
Reverse Engineered Schematic
With the circuit in a more familiar topology it reminded me of an LM317 current limiter circuit, with the LEDs as the load.  As I couldn't read the part number on the T0-92 package I'm going to assume it's an LM317 or an equivalent with similar properties, take a few measurements and see if they match my theory.  Luckily the 39 Ohm designator of the resistor is still visible as this allows the current limit to be calculated.  The diagram below shows a reference of 1.2 volts, but according to the LM317 data sheet it can range from 1.2 to 1.3 volts. Setting R1 to 39 gives a current range of 30.8 to 33.3 mA.  The current flowing into the adjustment pin is in the order of 0.1 mA, so it can be ignored in this analysis.

Circuit Schematic
LM317 Precision Current Limiter Circuit
To test if the circuit used this configuration, I measured the current at several operating points.  It became obvious that the current is limited to around 33 mA starting at about 10.25 Volts.  This agrees with what was calculated earlier assuming that the circuit uses a LM317 or something similar.  At this operating point the LM317 has just started to regulate the current so the voltage across it will be close to its drop-out voltage of 1.7 Volts.

Vin = V(diode) + 2*V(LED) + I*R + V(LM317)
10.25 = 0.7 + 2*V(LED) + 0.033*39 + 1.7
V(LED) = 3.3 Volts

An LED forward voltage of 3.3 Volts agrees with my experience with white LEDs.  As the supply voltage changes in the regulated region, the current remains constant, as does the voltage across the diode, LEDs, and resistor.  This voltage is equal to:

V(diode) + 2*V(LED) + I*R  = 0.7 +2*3.3 + 0.033*39 = 8.6 Volts

Therefore the voltage across the LM317 is the supply voltage less 8.6 Volts.  This means that the waste power dissipated in the regulator as heat in Watts is equal to:

(V(supply) - 8.6) * 0.033

Current Consumption of an LED Automotive Indicator Light
Now that's all well and good, but it's a little wasteful.  If the supply voltage is 12 volts it's not too bad, the power dissipation is equal to (12-8.6)*0.033 = 112 mW, but if the supply voltage is 24 volts the power dissipation is equal to (24-8.6)*0.033 = 508 mW.  That's getting a little high, particularly when you consider the thermal constraints on the design.

The thermal resistance, junction to ambient of a T0-92 LM317 is 160 degrees Celsius per Watt.  At 24 Volts the device dissipates 508 mW of heat, this means the junction is 0.508 * 160 = 81 degrees Celsius above ambient temperature.  In Australia a device like this might be operating on a hot day mounted to a material that is hotter than the air temperature.   This means that ambient temperature might be around 50 degrees Celsius, and the junction would be at 131 degrees Celsius, which is above the junction operating temperature of some versions of the LM317.  It's borderline, but when you then go ahead and encapsulate everything in resin this could make things worse depending upon the thermal conductivity of the potting compound.  I think that's why they left half of the LM317 package protruding from the resin.

It's fine for the 12 Volt supply I'm using, but I wouldn't expect it to have a long life at 24 Volts.  It would have been nice to see something more efficient used, but for a cheap low power device you can't expect much.


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  2. In the current limiting circuit, what happens when the wiper for R1 is all the way to the right? The R1 resistance is now close to zero. By the formula I_limit = 1.2/R1, aren't we going to draw the maximum amount of current? (a bad thing)

    1. Spot on. The LM317 is rated for 1.5 Amps output. If R1 drops below 0.83 Ohms, the output current will rise above this limit and the device could be damaged by overheating. Odds are the thermal protection will cut in and it'll shut down. You still don't want that happening though.

      If using a variable resistor in a circuit like this you can limit the current by adding a fixed resistor in series before it to limit the output current.

      At the other end of the range the LM317 needs to output approximately 10mA at all times or it won't be able to maintain regulation. The means that R1 shouldn't be above 125 Ohms either. This can be ensured by putting a fixed resistor in parallel with variable resistor.

      It's kind of academic though. A variable resistor that can handle that amount of current and has a resistance that low is going to be quite expensive.

      I hope this helps.


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