works of Carl Pisaturo
Electrical Notes: High Current LED Pulsing
A system which uses LED pulsing, the Transmutascope, is block diagrammed at left.
In order to get the very bright flashes needed, 8 LEDs are pulsed at high currents: 3 amps per LED. The pulses are brief, and only on only a small percentage of time, therefore installing a power supply capable of supplying the full 24A peak load would be wasteful. Instead, a large power buffering capacitor is used to supply this peak load. The 1 ohm power resistor (left of cap) prevents the power supply from "dropping out" during the capacitor's recharge. It also effectively isolates the 2 circuit boards from major powerline noise caused by the LED pulsing.
The 1 watt white LEDs each have a 1 ohm resistor (not strictly necessary) to even out load sharing.
The key element of the system is the LED Pulse Controller circuit. It handles the important task of making sure the LEDs don't stay on too long. Keep in mind that unlike a typical LED circuit, there is virtually no current limiting resistance here. If the LEDs were commanded to stay on a bit too long, they would all be destroyed. The LEDs are low-side-switched on for about 800uS each time a trigger occurs. The trigger is provided by an optical slot sensor.
In order to get the very bright flashes needed, 8 LEDs are pulsed at high currents: 3 amps per LED. The pulses are brief, and only on only a small percentage of time, therefore installing a power supply capable of supplying the full 24A peak load would be wasteful. Instead, a large power buffering capacitor is used to supply this peak load. The 1 ohm power resistor (left of cap) prevents the power supply from "dropping out" during the capacitor's recharge. It also effectively isolates the 2 circuit boards from major powerline noise caused by the LED pulsing.
The 1 watt white LEDs each have a 1 ohm resistor (not strictly necessary) to even out load sharing.
The key element of the system is the LED Pulse Controller circuit. It handles the important task of making sure the LEDs don't stay on too long. Keep in mind that unlike a typical LED circuit, there is virtually no current limiting resistance here. If the LEDs were commanded to stay on a bit too long, they would all be destroyed. The LEDs are low-side-switched on for about 800uS each time a trigger occurs. The trigger is provided by an optical slot sensor.
LEDs can withstand far greater currents than the "absolute maximum" specified by the manufacturer - provided they are on BRIEFLY and for a small percentage of the time.
For example, 3A pulses 1ms in duration at 20 Hz (2% duty) can be withstood.
To achieve these high currents, little or no resistance is put in series with the LED and a 12VDC supply is used. This puts a burden of responsibility on the control system: if the LED is commanded "on" too long and/or too frequently, it will be killed in the blink of an eye.
For example, 3A pulses 1ms in duration at 20 Hz (2% duty) can be withstood.
To achieve these high currents, little or no resistance is put in series with the LED and a 12VDC supply is used. This puts a burden of responsibility on the control system: if the LED is commanded "on" too long and/or too frequently, it will be killed in the blink of an eye.
