Simple Circuit Design Question

Thread Starter

sonnyangell

Joined Mar 1, 2017
31
Hello!

I have a simple experiment fun project. Modifying spy glasses to have LEDs and Lasers for a small amateur film.
The glasses come equipped already with 2 BLUE LEDs with a 6v (4 AAA battery) powersource.

I would like to have (1) Blue LED and (1) Violet Blue Laser on each side (2 each)

Blue LED - Voltage 3.4v // Current 30mA
LEDs Here

Violet Blue Laser - Voltage 3v - 5v // Current <55mhA
Laser Here

I created this experiment already and tested it, worked fine, now ONE of the blue LEDs seemed to have burned out.

I am guessing there is too much current going thru and slowly kills the LEDs and I would be smart to put a small 100 ohm resistor at the powersource?

or would I have to place a resistor on each LED?

Please explain what the problem may be
 

Audioguru again

Joined Oct 21, 2019
1,914
The cheap Chinese LEDs are No-Name-Brand so they do not last long.
One spot on the sales sheet says "20mA maximum".
Without a current-limiting resistor in series with each LED the current can be 50 times too high. If the LED is 3.2V with a 6V battery then if you use 100 ohms the current will be (6V - 3.2V)/100 ohms= 28mA which is higher than the maximum allowed current.

The laser is extremely dangerous (mar and char surfaces like eyes and skin). Don't use it.
 

Thread Starter

sonnyangell

Joined Mar 1, 2017
31
The cheap Chinese LEDs are No-Name-Brand so they do not last long.
One spot on the sales sheet says "20mA maximum".
Without a current-limiting resistor in series with each LED the current can be 50 times too high. If the LED is 3.2V with a 6V battery then if you use 100 ohms the current will be (6V - 3.2V)/100 ohms= 28mA which is higher than the maximum allowed current.

The laser is extremely dangerous (mar and char surfaces like eyes and skin). Don't use it.

I figured the LEDs were garbage. SO I should get better quality LEDs and use a 100OHM resistor on EACH LED?

I wont use the laser.
 

djsfantasi

Joined Apr 11, 2010
6,811
I figured the LEDs were garbage. SO I should get better quality LEDs and use a 100OHM resistor on EACH LED?

I wont use the laser.
The choice of a 100 ohm resistor is not an arbitrary one. The resistance depends on the actual LED used. I’ll explain in a moment.

But first, in a simple wiring diagram with each LED in parallel with the others, each LED needs its own resistor. LEDs are current controlled; not voltage controlled. Due to slight manufacturing variations, with only one resistor, one LED could get too much current and burn out. Then the next weakest LED will get too much current and burn out. And so on down the line.

So, you need to use one resistor, of the proper value, for each LED. Here is how to calculate its value.

You need to know a few values. Vs is the supply voltage. Vf is the “forward voltage” of the LED. And Iled is the current required by the LED.

A note about the required current for the LED. On the LED datasheet, the current listed is the maximum current allowed, before the LED burns up. Typically, 50%-70% of this value is sufficient to illuminate the LED.

In this example, I’m going to assume the supply voltage (Vs) is 6V, the forward voltage (Vf) is 2V and the desired LED current is 10mA (50% of 20mA).

The equation you use is this. It is based on Ohm’s Law.

R = ( Vs - Vf ) / Iled (in Amps)​

Plugging in the values for our example, we get:

R = ( 6V - 2V ) / 0.01 A​

or R = 4 / 0.01 = 400Ω
 

Thread Starter

sonnyangell

Joined Mar 1, 2017
31
The choice of a 100 ohm resistor is not an arbitrary one. The resistance depends on the actual LED used. I’ll explain in a moment.

But first, in a simple wiring diagram with each LED in parallel with the others, each LED needs its own resistor. LEDs are current controlled; not voltage controlled. Due to slight manufacturing variations, with only one resistor, one LED could get too much current and burn out. Then the next weakest LED will get too much current and burn out. And so on down the line.

So, you need to use one resistor, of the proper value, for each LED. Here is how to calculate its value.

You need to know a few values. Vs is the supply voltage. Vf is the “forward voltage” of the LED. And Iled is the current required by the LED.

A note about the required current for the LED. On the LED datasheet, the current listed is the maximum current allowed, before the LED burns up. Typically, 50%-70% of this value is sufficient to illuminate the LED.

In this example, I’m going to assume the supply voltage (Vs) is 6V, the forward voltage (Vf) is 2V and the desired LED current is 10mA (50% of 20mA).

The equation you use is this. It is based on Ohm’s Law.

R = ( Vs - Vf ) / Iled (in Amps)​

Plugging in the values for our example, we get:

R = ( 6V - 2V ) / 0.01 A​

or R = 4 / 0.01 = 400Ω

Wow! Thank you so much for explaining this! I am still connecting the dots as I go with my beginner experiments. Makes perfect sense now!

My next question would have been do I add a resistor to the power source or each LED, and you've answered it quite eloquently, Thank you!!
 

Thread Starter

sonnyangell

Joined Mar 1, 2017
31
Hello!

I have a simple experiment fun project. Modifying spy glasses to have LEDs and Lasers for a small amateur film.
The glasses come equipped already with 2 BLUE LEDs with a 6v (4 AAA battery) powersource.

I would like to have (1) Blue LED and (1) Violet Blue Laser on each side (2 each)

Blue LED - Voltage 3.4v // Current 30mA
LEDs Here

Violet Blue Laser - Voltage 3v - 5v // Current <55mhA
Laser Here

I created this experiment already and tested it, worked fine, now ONE of the blue LEDs seemed to have burned out.

I am guessing there is too much current going thru and slowly kills the LEDs and I would be smart to put a small 100 ohm resistor at the powersource?

or would I have to place a resistor on each LED?

Please explain what the problem may be

May I ask for a link to some GOOD QUALITY LEDs?
 

SamR

Joined Mar 19, 2019
2,586
Well, I was going to give you a very good link to LEDs but it has disappeared into the cobwebs of the net. Look at the Wiki LED page. The 20ma is considered max and with the superbright LEDs today it doesn't have to be that bright. So think of it as the "no higher limit" rather than the target. It also depends on ambient light. Bright noonday sun vs. moonless dark night.
 
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