Why are my LEDs burning out?

[marconi]

For Brouhaha: The LEDs have arrived. I have just connected 4 LEDs with a 500 Ohm resistor ( made from two 1kOhm 1/4 W resistors in parallel) across a 24V dc regulated power supply to do a 24 hour soak test. Current 20 to 30mA. Turned on at 1115am.

 

[DPG]

For Brouhaha: The LEDs have arrived. I have just connected 4 LEDs with a 500 Ohm resistor ( made from two 1kOhm 1/4 W resistors in parallel) across a 24V dc regulated power supply to do a 24 hour soak test. Current 20 to 30mA. Turned on at 1115am.

I would put a fiver on that being absolutely fine.

(And I'm tight!)

 

[ColinACJL]

I would put a fiver on that being absolutely fine.

(And I'm tight!)

A typical forward voltage for a red LED is 2 volts, so for 4 LEDs would be about 8 volts. So that would be say 16 volt across the 500 ohms of the ballast resistor, that means 32 mA forward current.

 

[marconi]

For Brouhaha:


TruOhm CR-50 560R 0.5W Carbon Film Resistor - Pack of 100 - https://www.rapidonline.com/truohm-cr-50-560r-0-5w-carbon-film-resistor-pack-of-100-62-0550

The forward voltage drop Vf I measured for each white LED were: 2.85V. 2.99V, 2.89V and 2.87V (or about 3V). These were measured after 22 hours being on. See attachment.

At link are the details of the required series voltage dropper resistor R which one calculates as follows:

PSU voltage - (number of series LEDs x Vf) =

24- (4 x 3) = 12V

R = 12/Led current

R = 12/0.02 = 600 Ohms.

Nearest value is 560 Ohms.

Power dissipation is (24 - 12) squared/560 = 144/560 = 0.26W Next higher power rating is 0.5W.

LED current is 12/560 = 0.021mA

You could use a much cheaper single 560 Ohms 0.5 Watt resistor instead of the Dynaohm to run 4 white LEDs using a 24V power supply. Other colour LEDs have different nominal forward voltages Vf.

See:
How to calculate the series resistor for an LED – Stompville - https://stompville.co.uk/?p=37


I will still do the test on the dynaohm + LED circuit when it arrives from the USA.

 

[DPG]

For Brouhaha:


TruOhm CR-50 560R 0.5W Carbon Film Resistor - Pack of 100 - https://www.rapidonline.com/truohm-cr-50-560r-0-5w-carbon-film-resistor-pack-of-100-62-0550

The forward voltage drop Vf I measured for each white LED were: 2.85V. 2.99V, 2.89V and 2.87V (or about 3V). These were measured after 22 hours being on. See attachment.

At link are the details of the required series voltage dropper resistor R which one calculates as follows:

PSU voltage - (number of series LEDs x Vf) =

24- (4 x 3) = 12V

R = 12/Led current

R = 12/0.02 = 600 Ohms.

Nearest value is 560 Ohms.

Power dissipation is (24 - 12) squared/560 = 144/560 = 0.26W Next higher power rating is 0.5W.

LED current is 12/560 = 0.021mA

You could use a much cheaper single 560 Ohms 0.5 Watt resistor instead of the Dynaohm to run 4 white LEDs using a 24V power supply. Other colour LEDs have different nominal forward voltages Vf.

See:
How to calculate the series resistor for an LED – Stompville - https://stompville.co.uk/?p=37


I will still do the test on the dynaohm + LED circuit when it arrives from the USA.

The traditional methods are often the best.

 

[marconi]

The dynaohms I ordered from digikey Minnesota USA on Monday arrived at lunch today Friday. Great postal service!

Now in circuit instead of the conventional resistor I used before. See attachment for dynaohm voltage drop and led current. I will leave it on for 24 hours and then do the tests I mentioned a few posts ago.

 

[marconi]

For Brouhaha: After 24 hours all 4 LEDs lit and current steady at 23mA using a dynaohm.

I then varied the applied voltage from the psu and filmed the two meters measuring current flowing and voltage drop across the dynaohm. Current remains nice and constant at 23mA until dynaohm’s minimum voltage drop criteria is not met. See attachment.

I cannot find anything wrong with using a series circuit of 4 Leds in series with a 20mA dynaohm fed from a 24 V dc power supply.

I hope this gives you confidence in your project‘s lighting design.

(You could wire more LEDs in series and then in series with one dynaohm provided that you make sure 24V - (number of LEDs x Vforward) is a voltage greater than the minimum dynaohm voltage drop of 2.6V - assuming a 24V psu. Off the top of my head I would only design a circuit which ensured the dynaohm voltage drop is no less than say 6V to take into account variations in individual LED forward voltages and PSU output voltage. Always check first though before you assemble in your project. A 6V dynaohm volt drop would drive 6 LEDs each dropping 3V fed by a 24V psu. If the same LEDs each dropped 3.5V, then 6 would in total drop 21V leaving 3 volts across the dynaohm - greater than 2.6V but not by much margin. You get the idea. Probably best to stick at 4 or 5 LEDs in series maximum to ensure at least 6V across dynaohm if your LEDs all drop typical 3.5V.)