Gas station without pumps

2022 October 27

LED board schematic

Filed under: Uncategorized — gasstationwithoutpumps @ 12:40
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I did a series of posts about the LED boards that I designed for lighting and strobes (starting with Summer project in 2014), but I just realized in going over the posts that I never posted the schematic nor explained how the board worked. I had to dig out the Eagle files on my old laptop to find the schematic, then redo it in Scheme-It to make it look reasonable.

Schematic for the LED board. This design is version 8, and is the one I actually had made.

The resistor R1 is a current-sensing resistor, measuring the current through the nFET Q2 and the LED. When the current is high enough, the voltage on the base of Q1 becomes large enough to turn on the NPN transistor Q1, lowering the gate voltage on Q2 and starting to turn the nFET off.  When the current is low, Q1 is off and the resistor R2 pulls the gate voltage for Q2 up, turning on the nFET more strongly. The Schottky diode D1 is just there to protect the LED from large reverse voltages if the board is hooked up backwards.

Power is dissipated in 4 places: the LED itself, Q2, R1, and D1. Based on the measurements in LED board I-vs-V curve,  the current is limited to about 118mA, so the voltage on the base is about 0.555 V when Q1 starts to turn on and R1 dissipates about 65mW. There is some current going through R2 and Q1 that doesn’t go through the LED (probably about 1–2mA), depending on the voltage applied to the whole board, but it is only about 1% of the total current, so I’ll ignore it—we may have another 30mW dissipated in R2.

I probably should measure the voltages across R1, on the base of Q1, on the gate of Q2, on the drain of Q2,  and across D1 to get detailed information about where the power is really being dissipated.  I think that the voltage across the LED should be about 6V when the board is fully on, and the voltage drop across the Schottky diode should be small (maybe 0.1V for 12mW).  For board voltages before the current limitation cuts in, almost all the power goes to the LED.  At higher voltages, the extra voltage drop and power dissipation is all in Q2, with around 700mW dissipated in Q2 when the board voltage is 6V higher than where the current limitation starts and 700mW delivered to the LED. That’s about as high as I’d be willing to go for continuous lighting, even with a heat-sink on the board.

I should be able to make the measurements fairly easily with the Analog Discovery 2, and I might do so later this week. One thing I’m curious about is whether the drop in current with higher board temperature is due to changes in the characteristics of the NPN transistor or the nFET.  The nFET is dissipating most of the power, so its junction temperature is probably changing much more, though the LED and the nFET together warm up the whole board, so the NPN transistor is getting warm also.

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