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To determine the input voltage needed to power one or more series-connected LED modules using a BuckPuck or PowerPuck driver, use the following formula:
Vin = Vt + (Vf x LEDn)
Where:
- Vin = Minimum required input voltage to the driver
- Vt = Driver Threshold Voltage (2.5V for BuckPuck, 2.0V for PowerPuck)
- Vf = Maximum Forward Voltage rating of the LEDs
- LEDn = Number of LEDs in the series string
Important Considerations
Always use the maximum Vf rating from the LED datasheet, not the typical value. Forward Voltage varies from LED to LED within a production Bin, and designing to the maximum ensures the driver maintains regulation across the full range of parts you might receive in any given shipment.
The formula gives the minimum required input voltage. Any voltage from this minimum up to the driver’s maximum input voltage rating will work, and a margin of one to two volts above the calculated minimum is good practice to account for voltage sag in the power supply, wiring, or input filtering.
If Vin drops below the calculated minimum, the driver loses regulation. The LEDs will continue to operate, but at reduced current and noticeably lower light output, and the driver may run warmer than usual.
BuckPuck Driver Example
To power 6 series-connected LXML-PWC2 LEDs with a BuckPuck driver:
- Vt = 2.5V (DC driver threshold)
- Vf = 3.25V (max rating for LXML-PWC2)
- LEDn = 6
Vin = 2.5 + (3.25 x 6) = 22V
A 22 VDC source is the minimum. A 24 VDC supply is a common practical choice for this configuration, since it provides a small margin above the minimum while staying well within the BuckPuck’s input voltage range.
