# Basic question

Discussion in 'Electronic Basics' started by saketram, Oct 16, 2006.

1. ### saketramGuest

A simple question, just to know about theory.

Suppose I have 12v DC @ 1A, 1.5 LED.
How much power resistor do I need to use to glow the Diode?

Should I use 12 ohms as,
R = V/I = 12/1 = 12 ohms

or should I use 10.5 ohms as,
R = V/I = (12 - 1.5)/1 = 10.5 ohms
(as 1.5 LED will consume 1.5v).

Thank you.

Saketram

2. ### Guest

diodes aren't ohmic, so can't use v=ir rule (ohms rule).

you need to add a few resistors because the diode can only handle 1.5
amps. Also don't overload the current otherwise the LED will blow. this
is a pretty basic problem - lots of people have solved it - look online

3. ### redbellyGuest

Questions for you:

Is the "1.5" for the LED refering to it's voltage or current? If
current, is that in A or mA or other?

You give the current spec for the power supply, but we need the current
spec for the LED.

Mark

4. ### Costas VlachosGuest

This has been asked and answered at least 1 million times. From what you're
saying it seems that you have a 12V power supply rated at up to 1A, and you
want to use this to power an LED with a forward voltage of 1.5V. Standard
LEDs need about 20mA, so your resistor would be

R = (12V - 1.5V) / 0.02A = 525 Ohms.

That's because the resistor needs to subtract 10.5V from the 12V supply to
leave the remaining 1.5V for the LED. All this must be calculated at the
LED's operating current (20mA), not the supply's maximum current capability
(1A). To calculate the power at the resistor, just use any of the following
equations:

P = V * I = 10.5 * 0.02 = 0.21 Watts
P = V * V / R = 10.5 * 10.5 / 525 = 0.21 Watts
P = I * I * R = 0.02 * 0.02 * 525 = 0.21 Watts

A standard 560 Ohms 1/4 W resistor would be fine. Now, if your LED actually
needs 1A to work (unlikely), then your second calculation is correct and the
power consumed by the resistor would be P = 10.5 Watts.

--
Regards,
Costas
_________________________________________________
Costas Vlachos Email:

5. ### feeboGuest

first thing, you 12V at 1 Amp power supply - this current is what it
*can* supply. I don't know where this notion that you can "push"
current through something comes from.

Your PSU will give 12v (forget the current for now) an LED will "drop"
around 2 volts and a common red LED will need between 0.015 and 0.02
Amps to get it to light nicely. I put the current like that so you can
relate how tiny it is in relation to the 1 amp maximum of your PSU.

so 12 - 2 =10v to drop across the resistor. As you are after 20mA (or
so), the maths is simply 10/0.02 which gives 500(ohms) Generally
resistors all come in set sizes, either E12 or E24 - google this. What
this means is that you can't generally go and "buy" a 500 ohm resistor
so you have to choose a value near to it. If you drop the resistor
value, more currnt will flow into your LED and this might shorten it's
life so a lot better to limit the current still further. I would go
for a 560 ohm resistor - this is a standard value and will be about
\$0.02 each.

You might like to work out what the current will be using a 560 Ohm
resistor (instead of 500) and if you are sure your LED will drop 1.5
volts instead of the more usual 2, you could work out the exact value
required for your 20mA and then choose a better one from the standard
ranges

6. ### ehsjrGuest

Use the formula:
R = (Vsupply-Vf)/I
where Vf is the forward voltage rating of the LED,
Vsupply is the supply voltage, and I is the current
you want the LED to draw.

Note that the current - I - should be less than the
maximum current rating for the LED. The spec sheet for
the LED will give you Vf and maximum I .

Ed

7. ### JamieGuest

you need to know the I (current) of the LED ?
lets assume for now, it's 20ma at 1.5 volt
across the diode.

R = (12-1.5)/I = (12-1.5)/ 0.020 = 525 ohms

like i said, you need to know the (I) of the LED..
its also best to operate the LED at a little less than
spec. so a 680 Ohm would do just fine, which is a more
common value.
that's the closes i can come up with for you.

9. ### saketramGuest

Thank you all very much.

I like to study in this way the Electronics as learning by creativity.
I just want to become proficient in electronics in designing any
application on my own.

So I request you people give me some links where I can learn by this
way.

Once again I should thank you.

10. ### Guest

You need a power supply of some type and a solderless breadboard and a
multimeter. Then I would find a cheap book on basic electronics. It
doesn't matter how old it is.

Then, I would do the calculations on the basic circuits in the book and
prototype then on your breadboard and do some measurements to insure
they agree with the book.