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### Author Topic: Forward Voltage Drop?  (Read 3933 times)

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#### SeagullOne

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##### Forward Voltage Drop?
« on: October 05, 2009, 02:43:47 PM »
I've been reading the Led Tutorial carefully, but I want to make sure I'm clear on something:

Is the forward voltage the same as the forward voltage "drop?"

Let's say I have a few blue leds and I'm powering them with 5V. These leds take a typical load of 20ma of current at a forward voltage rating of 3.6 V, 4.0 V max.

So if I do the math with what my understanding is of forward voltage drop--or at least what I think I understand--I get this.

(5V - 3.6V)/ 0.02A = 13 ohms

Only 13 ohms?

What exactly is the difference between forward voltage and forward voltage "drop?" If there is a difference...
I've been trying to understand it, but I don't want to blow my leds.

I'm using a Ledwiz board powered by a USB cord.
I think the chauffeur did it.

.......

He did.

#### ArcMan

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##### Re: Forward Voltage Drop?
« Reply #1 on: October 05, 2009, 02:55:42 PM »
Your equation is correct, but your math is faulty.
The correct answer is 70 ohms.
Forward voltage drop is the correct term for anything that consumes power.  Forward voltage is a more correct term for something that produces power, like a battery.

#### SeagullOne

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##### Re: Forward Voltage Drop?
« Reply #2 on: October 05, 2009, 07:02:29 PM »
I see, that makes sense about the forward voltage drop.

I must've gotten careless with my math, maybe hit the wrong button on my desktop calculator or something.

I've visited the store where I'm buying my leds, but they don't seem to show any graphs or charts when I look up the specs. Is there mathematical way to find the forward voltage drop given these specs?:

Blue 45° 5mm LED:  Wavelength: 470nm
Luminous Intensity: 800mcd typ. @ 20mA
Max Forward Current: 30mA
Pulse Current: 100mA for <= 10ms, duty <= 1/10
Forward Voltage: 3.6V typ. 4.0V max @ 20mA
Max Reverse Voltage: 5V
Power Dissipation: 120mW
Operating Temp: -30 to +85 C
Soldering Temp: 265 C for 10 secs
Max Reverse Current: 50uA @ 5V

It's a blue led.
I'm also buying a few that are green.
I think the chauffeur did it.

.......

He did.

#### SmAsH

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##### Re: Forward Voltage Drop?
« Reply #3 on: October 06, 2009, 03:47:48 AM »
Forward Voltage: 3.6V typ. 4.0V max @ 20mA
Thats your voltage drop...
Howdy

#### Soeren

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##### Re: Forward Voltage Drop?
« Reply #4 on: October 06, 2009, 04:46:53 PM »
Hi,

You can allways measure the voltage drop.
The easy way is to use a resistor of 470 Ohm in series with the LED and drive the combo with 12V.
Measure over the pins of the LED.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

#### SeagullOne

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##### Re: Forward Voltage Drop?
« Reply #5 on: October 07, 2009, 01:52:18 AM »
Forward Voltage: 3.6V typ. 4.0V max @ 20mA
Thats your voltage drop...

Okay, so let me make sure I understand this correctly. The supply voltage will be 5V and the led will consume 3.6 to 4V at 20ma before it blows. So I make sure to get a resistor that can handle about 70 ohms +- 50% to keep the led from blowing.

Is that correct?
I think the chauffeur did it.

.......

He did.

#### Soeren

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##### Re: Forward Voltage Drop?
« Reply #6 on: October 07, 2009, 02:05:07 PM »
Okay, so let me make sure I understand this correctly. The supply voltage will be 5V and the led will consume 3.6 to 4V at 20ma before it blows. So I make sure to get a resistor that can handle about 70 ohms +- 50% to keep the led from blowing.

Is that correct?
No.
The LED will be eg. 3.6V, not "3.6 to 4V". For any given LED, the tolerance is very small.

A resistor cannot "handle" resistance, it IS resistance and you most certainly shouldn't have a tolerance of +/-50% (i.e. from 35 Ohm to 105 Ohm which will give a current of from 13.3mA to 40mA).

For a LED with a drop of 3.6V, use 68 Ohms.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

#### SeagullOne

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##### Re: Forward Voltage Drop?
« Reply #7 on: October 07, 2009, 06:26:01 PM »
Oh...Wow, I got pwned...

I studied electronics dedicatedly but I'll have to remember that was a few years back and I've been focusing on mechanics and programming. So my electronics is a little fuzzy lately. I'll have to review before I blow up the house and I'm locked away in the asylum for being a crazed robotics enthusiast......again.

Anyway, thanks for the tips everybody. I think its all sinking in again.
I think the chauffeur did it.

.......

He did.

#### SmAsH

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