Set a bench supply to 350 mA, switch it on into an LED string, and the meter settles at 350 mA. What the meter does not show is the first few milliseconds, when the current can run several times higher before the current limit takes control.
Yellow: output voltage. Blue: current through the LED string. Red dashes: the current you set. Illustrative model, not a measurement of a UDP5000.
Why the spike happens
With constant-voltage priority, the supply’s voltage loop is in charge at switch-on. It drives the output toward the voltage setting as fast as it can. An LED string draws almost nothing until the voltage reaches its forward voltage, then the current climbs steeply, because the string’s resistance above that point is small. By the time the current loop sees too much current and takes over, the output is already well above the voltage the string needs at 350 mA.
The excess lasts a few milliseconds. LEDs and laser diodes carry a peak-current rating, and a spike like this can go past it on every power-up. Setting the voltage only slightly above the string helps, but it leaves no margin as the string warms and its forward voltage drops.
Two ways to remove it
CC priority. Here the current loop is in charge from the start. Voltage rises only as far as it takes to reach the current setting, so the current comes up to 350 mA and stays there. On the UDP5000 this is IHS (high-speed) or ISR (with a set slope).
A slower voltage slope. In CV slope priority (VSR) the voltage ramps at the rate you set. The current still crosses the limit, but slowly enough that the current loop catches it with a small overshoot.
Both are front-panel settings on the UDP5000, and both are available over SCPI with the :OUTPut:MODE command.
Pick a model for the current, not only the power
LED strings draw little current. The data sheet gives current ripple in CC mode, and it is lowest on the high-voltage, low-current models: 6 mA rms on the UDP5800-2 and 12 mA rms on the UDP5250-6, against 80 mA rms on the UDP5040-40. For a 350 mA string, choose the model whose voltage covers the string and whose current is closest to what you need.
Try this
- With CV priority, 8 LEDs and 350 mA, the current peaks well above the red line before it settles.
- Press CC priority. The current rises to the setting and stops there.
- Press CV, slope and drag the slope down. The spike shrinks as the ramp gets slower.
The traces come from a simple model: white LEDs at 3.0 V each with 0.5 Ω of dynamic resistance, a voltage loop that settles in about a millisecond, and a current loop that reacts about a millisecond after the current crosses its limit. Real numbers depend on the LEDs, the wiring and the supply. To see your own start-up, put a current probe on the LED lead and trigger a scope on the output switching on.
CC priority and slope control on all 20 models.
See the UDP5000 seriesRelated: Constant voltage vs. constant current. UNI-T. Design. Debug. Deploy.™
