can I drive a 3W led for 40ms without current limiting resistor

Thread Starter

South

Joined Apr 7, 2017
18
I am building an electronic metronome for my mum. She has stipulated that she wants a short beep every clock pulse and a light flashing every 3 or 4 clock pulses. so I have built one for her. Unfortunately the array of four superbright white 5mm leds I am currently using is not bright enough! I have decided to employ the services of a logic level, n-channel, power mosfet to drive it. I am running the metronome from 3 AA batteries. What I am wondering is will i need a current limiting resistor if the on time is less than 40ms at a mark:space ratio of 100:1? or will the internal resistance of the power pack limit the current sufficiently?
 

bertus

Joined Apr 5, 2008
20,881
Hello,

Have a look at the datasheet of the led for the maximum allowed current.
Always keep the current below the maximum allowed.
You can use a resistor to limit the current.
Also use a logic gate mosfet on the 3 X AA batteries.
Regular mosfets may not switch on at 3 X AA.

Bertus
 

ronsimpson

Joined Oct 7, 2019
1,017
It is not good design practice. But you will probably get away with it. The current will be unpredictable, depends on why makes the batteries and how new they are, room temperature.
 

BobTPH

Joined Jun 5, 2013
2,587
The datasheet should have a spec for pulsed operation at higher current.

Also, have you tried a longer pulse? The eye is an integrating device, and a very short pulse may look less bright at the same luminous intensity intensity.

Usually, metronomes have an accented beat to mark the measures audibly, which corresponds more closely to actual music.

Bob
 

Thread Starter

South

Joined Apr 7, 2017
18
Thanks for your help guys I will take it all under consideration.

and thanks for replying so quickly.

South
 

Thread Starter

South

Joined Apr 7, 2017
18
umm yeah I had planned on that but abandoned it when mum reinforced that she wanted the lighted accent instead of the audible accent. I think i will play it safe and somehow add an audible accent as well!
 

Hugh Riddle

Joined Jun 12, 2020
71
I am building an electronic metronome for my mum. She has stipulated that she wants a short beep every clock pulse and a light flashing every 3 or 4 clock pulses. so I have built one for her. Unfortunately the array of four superbright white 5mm leds I am currently using is not bright enough! I have decided to employ the services of a logic level, n-channel, power mosfet to drive it. I am running the metronome from 3 AA batteries. What I am wondering is will i need a current limiting resistor if the on time is less than 40ms at a mark:space ratio of 100:1? or will the internal resistance of the power pack limit the current sufficiently?
A (logic-level e.g AO3400) MOSFET, 1 low-power NPN transistor and 2 resistors can drive the LEDs with constant-current. Used that for a 120mA flashing 40 (parrallel) LED placard using 3 AA batteries - stabilises brightness and really saves on batteries. Maybe solder MOSFET to thick copper wire or 2 sq cm of copper-clad board to avoid overheating.
 
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