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:
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
|4||180||R1, R3, R4, R8|
How to calibrate the wand:
- 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.
- The power supply voltage regulator chip isn’t likely right on +5.00 volts, but it should be within 2% (4.9 – 5.1).
- 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.
- Assert that op-amp 4 voltage measures around +0.6 volts.
Adjustment sequence using the wand:
- First, with the lamp disconnected, the power supply is preset to the sum of all diodes’ forward voltage drops plus one volt.
- The lamp is energized to full brightness (no PWM dimming), and the wand is held in place manually.
- Using the trimmer potentiometer on the lamp module’s CAT4101 RSET pin, the LED amperage should be adjusted to desired current.
- The power supply voltage should be adjusted until the wand’s green LED (op-amp 3) just lights.
- 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.
- 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.
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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