Duty cycle and frequency to drive LED for best energy efficiency  - Page 1
Forum section: electronics / projects

author: Wilson__
author_rank: Frequent Contributor
timestamp: May 02, 2024, 10:36:24 am
content: For battery capacity limited device, what is the 'sweet spot' duty cycle and frequency to drive LED at highest energy efficiency (high brightness using low energy) for human active observer for attention-seeking guide light.Instead of ON all the time, human visible persistence mean one can see the light even if it is a short pulse.My question is how short (duty cycle) the pulse that huamn can 'feel' good brightness compare with 100% on time. Extreme example is xeon flash in film photography to freeze motion. On time is kind of 0.1 to 1ms. Traditional LED bed side clock is 4 digits driving 25% duty cycle.To clarify, this is NOT blinking rate. Say, 3 blinks per second then 3 seconds off.For three blinks per seconds, "167 ms light up and 167 ms no light", repeat 3 times.Here, I am seeking advises for during the 'light up' 167 ms. One way is LED on for 167 ms. Another way is says, "167 ms with 25% duty cycle", "42 ms on, 125 ms off"Many thanks

author: Someone
author_rank: Super Contributor
timestamp: May 02, 2024, 10:53:21 am
content: Depends on the specifics of the LEDs, most have a minima in their efficiency vs current drive.90% the same as the previous post:https://www.eevblog.com/forum/projects/brinking-light-pattern-for-maximum-attention-distance-at-night/

author: Wilson__
author_rank: Frequent Contributor
timestamp: May 02, 2024, 01:23:02 pm
content: Many thanks. Sorry not clear in my question. Edited.

author: Someone
author_rank: Super Contributor
timestamp: May 02, 2024, 11:34:05 pm
content: Quote from: Wilson__ on May 02, 2024, 01:23:02 pmPlease dont aggressively edit a post so that it bares little resemblance to the original. Then the replies make no sense.Otherwise we have to fully quote everything you say:Quote from: Wilson__ on May 02, 2024, 10:36:24 amThere is no magic number, persistence of vision depends on many factors. In general you won't do better than DC operation of LEDs under 100ms.

author: David Hess
author_rank: Super Contributor
timestamp: May 03, 2024, 12:25:05 am
content: The LED has a maximum efficiency at a specific current. Highest efficiency at lower brightness means chopping the signal while operating at this relatively high current.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 03, 2024, 01:32:25 am
content: Quote from: Wilson__ on May 02, 2024, 10:36:24 amIf you want attention seeking, you need to study human eye flicker sensitivity.Peripheral vision is also more flicker sensitive, so you might end up with hundreds of ms of period and tens of ms of ON time for best perception.Set something up that lets you adjust period and on time, in that ballpark and do some field tests.

author: Wilson__
author_rank: Frequent Contributor
timestamp: May 06, 2024, 08:04:03 pm
content: Many thanks for advises from the group that gave a narrow-enough ball park range for doing field test.

author: Tnixsw
author_rank: Contributor
timestamp: May 08, 2024, 01:43:05 am
content: Just some early days info for LED driving.In old HP calculators, circa 1970's, they used LED displays which were driven with relatively high currents for short periods of time and was a preferred method as the LEDs lasted longer than driving with straight DC. Typical ON times were around 40uS and one 8 segment digit would be refreshed in about 312uS. Some of the models had 15 digit displays having 120 LED segments to drive, but there was only ever one LED segment on at any given time. The displays in these devices were quite bright and easily readable with no sign of flicker.For power efficiency, some early models had LEDs that were driven by dumping stored energy directly from a small inductor (around 130uH) and required no series resistance.These systems turned out to be very reliable and many of these are still working perfectly well 50+ years later.The LED data sheets usually have pulse times and safe currents to use. As mentioned, different LEDs behave differently so some experimenting may be required to suit your requirements, but it is easy. If you are using a micro controller to control the LEDs with the higher currents, you may need an external driver.cheersTony