What resistor do I need then

[amateur_24]

Forgive me for my rusty electronics. The led's I've brought have a voltage drop of 2.5 and takes 25ma of current. Doesn't this mean if I supply it with 4.98V it will blow? I used a led resistor calculator to work out what resistor I needed. My supply voltage is 5.05V (according to my multimeter). Because the avr chips can only sink 20ma of current , I rated the led at 20 instead of 25. It told me that I needed 150ohm but the voltage has barely changed and the current is the same.

I'm only using one led. why hasn't this worked out?

 

[ericgibbs]

Forgive me for my rusty electronics. The led's I've brought have a voltage drop of 2.5 and takes 25ma of current. Doesn't this mean if I supply it with 4.98V it will blow? I used a led resistor calculator to work out what resistor I needed. My supply voltage is 5.05V (according to my multimeter). Because the avr chips can only sink 20ma of current , I rated the led at 20 instead of 25. It told me that I needed 150ohm but the voltage has barely changed and the current is the same.

I'm only using one led. why hasn't this worked out?

hi,
Subtract the Vled from the Vsupply, eg: 5V - 2.5V = 2.5V,,, so for a LED current of 20mA, you require a resistor of 2.5V/0.02 = 125R.

For a genareal purpose LED 20mA is a little high, I would use a current of 15mA, so thats 2.5v/0.015 =167R, nearest preferred value of 180R.

 

[efron]

Hi,

Also the type of LED has to be considered and even its emission colour. For standard LEDs (general purpose LED), you have 5 and 3mm round dimmensions.

As a general rule, 5mm LEDs can be supplied with 20mA forward current, while 3mm usually uses 10mA.

Under these conditions, the voltage drop between the LED terminals will remain +/- constant. 2.5V is a typical value, but once again, the type and colour of the LED will drive this value.

Light Emitting Diodes (LEDs)

 

[amateur_24]

Ok I tried the value you gave me but my leds are still getting too much juice since it's getting very very hot when I plugged it up. Hot enough to burn my fingers.
that's exactly wht happened when I used 150ohms. I ended up frying two leds.

Am I connecting the resistor wrongly, it's connect directly to the positive (5v) of the usb connector then the other end to the longer leg of the led. The shorter leg to the negative. I measured the voltage between resistor and led and it's 3.5 ! The spec says 2.5V max if the current is 20ma

Also the 5-2.5 = 2.5 so is that the voltage the resistor is meant to be resisting hence why you minus 5 -2.5? I never really understood ohms law at school

 

[audioguru]

What is getting very hot? The LED or the 150 ohm resistor? If they each have 2.5V across them then they each dissipate only (2.5V squared/150 ohms= 0.042W which is almost no heat.
What has a voltage of 3.5V? The LED (a blue or white one) or the resistor (when the LED is a 1.55V red one)?

Ohm's Law says that the current in a resistor and anything in series with the resistor (the LED) is the voltage across the resistor divided by the value (number of ohms) of the resistor. 2.5V/150 ohms= 16.7mA.

 

[amateur_24]

Correct me if am wrong,
ohms law says V= current*resistence. Say my circuit has a resistor and a single red led. My supply voltage is 5v at 0.029 milliamps. Therefore 0.029*180ohms resistor gives me 5.22V. The led spec said 2.5 forward voltage. i.e. 2.5v to switch it on right? so therefore thats too much voltage right?

 

[Reloadron]

Correct me if am wrong,
ohms law says V= current*resistence. Say my circuit has a resistor and a single red led. My supply voltage is 5v at 0.029 milliamps. Therefore 0.029*180ohms resistor gives me 5.22V. The led spec said 2.5 forward voltage. i.e. 2.5v to switch it on right? so therefore thats too much voltage right?

Based on the nformation provided and Eric's suggestion things should look like the attached. With a 5 volt supply and a forward voltage on the LED of 2.5 volts for 15 mA of current through the LED you would place 167 Ohms in series with the LED. However a standard resistor of 180 Ohms was suggested. That should further limit the current. You have the LED polarity correct? If the LED data was correct the LED should not get hot. How are you deriving the supply current @ 29 mA?

Ron

 

[amateur_24]

The supply is a usb port. I used my multimeter to test for voltage and current like in the diagram except no components attached. I'm detecting 4.9 v across led somehow.
I can smell burning and it's like a bright orange colour.

 

[Reloadron]

The supply is a usb port. I used my multimeter to test for voltage and current like in the diagram except no components attached. I'm detecting 4.9 v across led somehow.
I can smell burning and it's like a bright orange colour.

OK, something is very, very wrong here. Before I forget, never measure current placing a currenmt meter across a source. You are effectively shorting out the source when you do that. Generally I never even suggest to people to use their USB ports as a power source other than a possible charger device made for a USB port.

Next, if the data for the LED was correct and the voltage was 5 volts, things should have worked just fine as in the circuit I posted. Since we only have two external parts (LED & Resistor) I can only assume the LED has bum data. Something sure is not right!

This link is to a USB pinout, you are sure you have the correct pins though if you get 5 volts I assume you have the correct pins. You don't want to kill a USB port which can happen.

Checjk your USB port voltage per the pinout in the link and make sure there is only 5 volts on that port.

Ron

 

[JimB]

I feel I must jump in here and comment.
Some of the comments may seem harsh, but to me it is the only way forward.

The supply is a usb port.

OK, but you seem to have very little idea of what you are doing.
Is it sensible to use a USB output to provide power for a learning experiment.
Better to use a $5 battery that a $500 computer when you have no idea what you are doing.

I used my multimeter to test for voltage and current like in the diagram except no components attached.

So if there were no components attached, it was not like the diagram! Was it?

Did you connect your meter, set to measure current, across the USB power pins?
I sincerely hope not.

I'm detecting 4.9 v across led somehow.

Presumably you are measuring it with the meter.
Is the LED and the resistor wired in series to the USB power, or are you just connecting the LED without the resistor?

I can smell burning and it's like a bright orange colour.

What is burning, the USB port, the internals of the computer?
What is like a bright orange colour, the LED, the computer, the USB port?

You really must give more precise descriptions if you want some correctly focussed help, we can't see what is in front of you.

JimB

 

[audioguru]

Your schematic is correct with the resistor and LED in series.
But your problem will be caused if they are in parallel.

 

[audioguru]

I think the face of our OP is a bright orange colour because his circuit does not have the resistor and LED in series.

 

[Reloadron]

I think audioguru is into something with the possibility that the LED is in Parallel and not series with the current limiting resistor. That would explain a lot.

Ron

 

[Dr_Doggy]

i always thought led's were like transistors 0.7V, When i'm doing something like this i generally just use a 1K pot....start high go low.

 

[audioguru]

At 20mA, a 1N914 or 1N4148 silicon diode has a forward voltage drop of 0.77V.
An IR LED is about 1.3V.
A red LED is about 1.8V.
A yellow or old green LED is about 2.2V.
A new bright green, blue or white LED is about 3.5V.

LEDs are easily burned out if the current is too high so you must calculate the value of the current-limiting resistor so that the current is not more than the maximum allowed current as shown on the datasheet for the LED. The supply voltage, the forward voltage of the LED and the resistor value determine the current.

 

[amateur_24]

Thanks alot for your help guys. Especially eric you're the man oviously!! I do apologise for being a pain in the butt. See my soldering skills aren't that great because I don't have much experience with it hence I used a breadboard. I have never used one at school. My teacher wouldn't even let us near the soldering iron lol. She did the soldering for us lol.

My board has 5 sets 5x2 blocks at the top. one row marked positive and the other negative. I didn't know they were all connected to each other. I thought they were all separate. I connected my resistor from one block to the next lol!! That's where the 4.5 volts was coming from!! I only found out when I tried stacking the resistor and led upwards (as if they're stand on the boardboard).

So in my components grave yard I have two red leds and one white one.