Lighting control circuit, this time for sure
February 22, 2009
Every system is a second system, or so it seems.
Update: it really, really helps to use the right chip. After being incredibly careful about every orientation of every weird transistor, I designed the board around a “LM387N”, which is not the chip I intended or the one that I had ordered — LM258 (usually referred to as LM358, but the 258 works in the cold). I was testing the board before putting on the big components, and it was just not working, and in the end, it seemed like the op-amp was on drugs. In the end, I got out a magnifying glass to check the label. DOH!
All is not lost, but the board needs to be rerouted. I’ll sort out the pins and test again.
Up-update: After installing a pin-scrambler, it works roughly as predicted. The most important part of the circuit, the high-end shunt control, delivers 3.85 volts to the base of the shunt transistor at 31.1 volts. This is a perhaps a half-volt earlier than planned, but it’s a half-volt to the safe side. In the high 20s, the shunt is not activated, as planned.
Anomalies:
(1) the LED control sends a little more power to the lights than I had planned. Not lots, and it’s mostly a low-end effect. At 31.1, the control voltage is 1.7, which is exactly on target. At 22.5 it is 2.6 (plan, 3.1) and at 12.9 it is 3.24 (plan, 3.6) and at 9.6 it is 3.4 (plan, 3.8).
(2) the off-level shunt base voltage is 0.6, where the plan was 0.2. This should not be an issue with the particular Darlington shunt transistor; it claims to have a Vbe of 2.8 volts.
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