Question pls: How does a buck driver like Convoy 5A reduce brightness? By decreasing voltage?

cannga · 2025-09-22T17:41:57.820Z

Sorry for this basic question, not modding anything, just curious in the vein of “how things work.”

I recently bought Convoy T8 with Buck 5A driver. No PWM noted. How does this driver reduce brightness for lower levels? Does it lower voltage along the Vf vs currrent line as seen in @koef3 's SFT-25R review?

Or does it keep a fixed voltage, say 3.5 v for a current of 5A per Koef’s chart, then reduces the amount of current available for lower brightness? If V=IR and V and R are fixed, could current be set to “amount available”? Hope I’m making sense at all, no electronics training whatsoever and last physics course many years ago.

From Koef’s review here:

INeedMoreLumens · 2025-09-22T18:11:02.024Z

cannga:

Does it lower voltage

Short answer = Yes

Qubeet · 2025-09-22T18:12:35.796Z

In closed circuit voltage and current are related. You can’t change one without changing the other.

MCU can drive the input of buck converter: increase it or decrease. Then the MCU can measure the output current. If it’s lower than the programmed level the MCU increases the input and vice versa. It happens step by step dozens of thousand times per second and soon the current comes close to the programmed level.

Quadrupel · 2025-09-23T05:36:16.981Z

This is an important driver test, but rarely found. Usually cheap drivers have lame firmware with programmed too early stepdowns. Low Vf leds can keep power at low voltage but driver usually programmed for High Vf leds … Thin wires, spring , PCB tracing, high resistance mosfets does not help too. At worst case you can get same performance as good linear driver have or even worse.

cannga · 2025-09-23T17:38:54.681Z

Qubeet:

MCU can drive the input of buck converter: increase it or decrease. Then the MCU can measure the output current. If it’s lower than the programmed level the MCU increases the input and vice versa. It happens step by step dozens of thousand times per second and soon the current comes close to the programmed level.

Thanks everyone for the answers. Do I understand it correctly that in fact in all 3 common types of flahslight driver, buck, boost, and CC, it is the current that is programmed/set by the designer, and an MCU adjusts voltage to maintain this current?

LED drivers are basically “constant current”, not “constant voltage” driver? TIA.

Quadrupel · 2025-09-23T17:48:00.844Z

cannga:

LED drivers are basically “constant current”, not “constant voltage” driver?

Usually, yes.