Here’s a test probe for adjusting LED lamps with a CAT4101 driver. It uses pogo pins on the end of a stick that connects the LED module to an ampmeter and voltmeter. It also has a LED voltage indicator on board. I haven’t yet found a workable ampmeter indicator circuit, and since regular hardware meters work that end of things so much better anyway, I probably won’t try.

This circuit, importable to Paul Falstad’s Circuit Simulator, will give a green LED when the input voltage is between 0.55 and 0.6 volts:

$ 1 5.0E-6 10.20027730826997 49 5.0 50
r 272 288 272 352 0 220.0
r 272 352 272 416 0 220.0
r 272 416 272 480 0 470.0
r 272 480 272 544 0 1800.0
g 272 544 272 576 0
R 272 96 272 64 0 0 40.0 5.0 0.0 0.0 0.5
w 272 288 336 288 0
w 272 352 352 352 0
w 272 416 336 416 0
w 272 480 336 480 0
174 272 160 272 224 0 2000.0 0.42080000000000006 calibrate
w 272 224 320 224 0
w 288 192 320 192 0
w 320 192 320 224 0
a 400 272 480 272 1 5.0 0.0 1000000.0
a 400 336 480 336 1 5.0 0.0 1000000.0
a 400 400 480 400 1 5.0 0.0 1000000.0
a 400 464 480 464 1 5.0 0.0 1000000.0
w 336 288 400 288 0
w 352 352 400 352 0
w 336 416 400 416 0
w 336 480 400 480 0
w 400 256 368 256 0
w 368 256 368 320 0
w 400 320 368 320 0
w 400 384 368 384 0
w 368 384 368 448 0
w 400 448 368 448 0
w 368 368 144 368 0
172 144 368 144 320 0 6 0.55 1.0 0.0 0.0 0.5 CAT LED Voltage
w 368 320 368 368 0
w 368 368 368 384 0
162 480 464 528 464 1 2.1024259 1.0 0.0 0.0
r 528 464 528 528 0 100.0
g 528 528 528 560 0
162 528 400 576 400 1 2.1024259 1.0 1.0 0.0
162 576 336 624 336 1 2.1024259 0.0 1.0 0.0
162 624 272 672 272 1 2.1024259 1.0 1.0 0.0
r 576 400 576 464 0 100.0
r 624 336 624 400 0 100.0
r 672 272 672 336 0 100.0
g 576 464 576 496 0
g 624 400 624 432 0
g 672 336 672 368 0
w 480 400 528 400 0
w 480 336 576 336 0
w 480 272 624 272 0
r 272 96 272 160 0 19100.0
w 272 224 272 288 0
o 9 32 0 35 0.625 9.765625E-5 0 -1
o 8 32 0 35 1.25 9.765625E-5 0 -1
o 7 32 0 35 1.25 9.765625E-5 0 -1
o 6 32 0 35 1.25 9.765625E-5 0 -1

The circuit will also need op-amps with some serious overvoltage protection, hence the ADA4091’s and the DIAC.

The wand is made to be powered with a 9V battery, the switch at the top enables the voltage indicator circuit, the switch has an LED which glows when there is sufficient power, and the red, yellow, green, red cadence LEDs at the bottom of the wand near the test pins indicate the voltage.  Wires pads on the top are for connnecting the voltmeter and the ampmeter, which do not require the voltage indicator circuit.  The wand can be made without the voltage indicator circuit components, in which case it only functions as a test jig connector.

The switch mounts on the bottom side of the board, and the 9V battery can be taped to the center of the bottom of the board just below the switch.  The end of the wand fits a 3x right-angle through-board machined gold pogo pin array, plus one standard 0.1″ header guide wire at the end to fit the blind hole on the end of the test harness pads on the LED module.

Partlist exported at 11/27/10 9:31 PM

Qty Value Parts
1 0.1uF C1
1 1.80K R2
1 2K R10
2 10uF C2, C3
1 19.1K R5
4 180 R1, R3, R4, R8
1 220 R9
2 220.0 R7, R11
1 470.0 R6
2 ADA4091-2 IC1, IC3
1 AP1117Y50 IC2
1 GRN LED D3
1 LP4 S100
1 ORN LED D1
1 POGO PINS SJ1
1 RED LED D4
1 TMMDB3 DIAC1
1 YLW LED D2

How to calibrate the wand:

  1. The four op-amp test points should measure +0.4, +0.5, +0.55, and +0.6 volts when the power supply is +5.00 volts.
  2. The power supply voltage regulator chip isn’t likely right on +5.00 volts, but it should be within 2% (4.9 – 5.1).
  3. Using the R10 trimmer potentiometer, adjust op-amp 3 voltage to be +0.55 volts.  The calculated value of R10 is 1046 ohms, so it should be right around mid range.
  4. Assert that op-amp 4 voltage measures around +0.6 volts.

Adjustment sequence using the wand:

  1. First, with the lamp disconnected, the power supply is preset to the sum of all diodes’ forward voltage drops plus one volt.
  2. The lamp is energized to full brightness (no PWM dimming), and the wand is held in place manually.
  3. Using the trimmer potentiometer on the lamp module’s CAT4101 RSET pin, the LED amperage should be adjusted to desired current.
  4. The power supply voltage should be adjusted until the wand’s green LED (op-amp 3) just lights.
  5. Subsequent LED modules using the same power supply should have their LED amperage adjusted per an ampmeter, then the amperage fine tuned until the green voltage indicator LED just lights.
  6. Ultimately, the voltage on the LED pin of all CAT4101’s on the same power supply should be identical, between 0.55 and 0.6 volts, and all the module amperages should be within a few milliamps of each other.  If there are wide variations in the forward voltage drops of the LEDs, or the lamp is not up to normal operating temperature, these adjustments may be less precise.  Any and all voltage over 0.6 volts on the LED pin of a CAT4101 driver causes waste heat.

11-MAR-2011

Here’s a quickie movie showing the finished test wand.  I’m still not quite sure if it works.  The op-amps leak a TINY amount of current, enough to light the LEDs when not engaged.  If I hold the end of the wand tightly, my skin resistance is enough to turn off the LEDs, so it sure looks like it works.


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