I have a reflective vest with flashing LEDs. The lights can be set to always on, flashing 5 times/second, or flashing 2 times/second. Assume the time on and off are equal for the flashing modes, so the light is on for exactly half the time no matter how fast it flashes. Which mode is going to use the most battery?
More generally, how much extra power (if any) is needed to cycle an LED light on and off? Is it more than the savings of the light being on only half the time?
I’ll include to note that the electronics associated with LED’s are intended to have them constantly flashing at a specified rate that may vary from application to application. This saves power and probably the lifetime of the LED so its not ‘constantly’ on. Persistence of vision makes them appear to be constantly on.
With a constant duty cycle (in this case 50%), lower frequency tends to equal lower overall power usage, all else being equal. This is because the brief act of switching uses power, especially for MOSFETs. In other words, during the few microseconds or milliseconds when going from on to off, or off to on, power is dissipated in the circuit because one or more capacitors needs to be charged or discharged; power is dissipated in the conductors going to the capacitors, and in the ESR for each capacitor. Having said that, switching on and off will almost always use less energy vs. constantly on, unless the switching frequency is very high.
Agree. But now I’m curious about something. If, for example, you set it for two flashes each second, are the LEDs 100% on during the time intervals that they’re “on”? Or are they still flashing, but at a high enough frequency so that can’t be perceived by the human eye & brain?
Does it do this using capacitors? Is it the same mechanism as longer-period flashing sequences?
I suppose it depends on what level of brightness you set in the firmware: full brightness means a 100% duty cycle when it is on. A 50% cycle will not appear as bright.
There is probably at least one capacitor somewhere in the circuit… the “mechanism” ISTM is switching the current on and off. Some kind of transistor could do it.
There is some small amount of power dissipated during the transition either way between off and on. The time is more typically in the nanosecond to possibly a few microseconds. Any capacitance involved is just stray characteristics of the components involved or part of a clock circuit for the electronics.
It’s reasonably easy to determine whether a bright-ish light is steady or rapidly flashing (as is common with LEDs): With the light visible in otherwise moderately dark conditions, simply sweep your view horizontally past the light and note whether you see a steady streak or a bunch of dots.
I engineer LED systems in my day job. Pulse Width Modulation is the most common way of controlling the flash and dimming of the LEDs, such as in household dimmable bulbs. Changing the frequency can also be done, such as in your example, or my dog’s nightime blinky collar which can blink 1x/sec or 5x/sec. There is a very small impedence loss from changing the square wave between on and off, but it is nearly always done within a chip package at a micro/nano level on a DC current where energy loss is too insignificant to notice, assuming the sum total on vs total off state of the LED remains the same.
Not necessarily a straight answer as it is totally dependent on how the person set up the circuit design of the LED doodad. but in your blinky vest case or my dog collar, it should be no difference to your battery.
The classic fundamental solid-state oscillator timing circuit is a resistor-capacitor astable multivibrator.
I imagine there are timing and clocking circuits that don’t use capacitors, but other than electromechanical implementations like a relay buzzer I’ve never seen one.
Thank you all for the answers.
A question. A billion years ago when I was a kid playing with basic circuits, to get a really bright LED the trick was to use short pulses of high current. Much higher than the constant current the LED would survive. The LED was happy and the visual brightness was many times higher.
So, do modern LED systems operate like this, or has the underlying physics of LEDs changed enough that this isn’t needed?