Electronic Circuits and Projects Forum

you forgot... (9V - (0.7V drop))/680 ohms = Current .... is what the current would be.
That is important for LED's that have a high Vdrop such as supper brights (which can be as high as 3.7V) Thus for a supper bright it would be (9V-3.6V)/x ohm's.. If you just use the 9V/680, your LED would be ON but dim and you wouldn't realize the full brightness.

Originally Posted by unix60959

In this video I show you how to calculate the R value needed for your LED circuit ...

No you did not show how to calculate the resistor value, you showed how to calculate the current when the resistor value is known.
Not the same thing at all.

While doing the calculation you forgot to take into account the forward volt drop of the LED.

The forward voltage drop (Vf) of a basic red LED is not 0.7 volts, it is nearer to 2 volts.
I just did a quick test using some LEDs I have to hand and they showed a Vf of 1.9 to 1.95 volts.

As edman pointed out, not all LEDs are the same, the Vf varies with the colour of the LED, some can be as 3 or more volts which makes big difference to the required resistor value.

I realise that this video clip was made for beginners, but your description of the purpose of the resistor was poor, almost to the point of being wrong.

Sorry, but your video just does not come up to scratch.

JimB

Sorry, but your video just does not come up to scratch.

Sorry Jim but I disagree..... At least he has tried... Its quite clear in the video that it's a bog standard LED... People have to start somewhere..

unix60959... Don't worry about the negative comments.. The video is a good start... You can always edit it to take these comments onboard.

No disrespect intended to both of you.. You are both highly qualified, but constructive comments go a lot further.

but since most LED's are .7V forward bias

Incorrect.

A silicon diode has a Vf of about 0.7v, but Vf of an LED is significantly higher, depending on the colour and the doping used in the semiconductor used to give that colour.

JimB

Unix,

Hang out a little, snatch some new material and remove and replace Video 1 with Video 2. While Video 1 does have inaccuracies the new and improved (I love that term) Video 2 will be accurate and on target. Live and learn as the saying goes.

Ron

As the main critic of the video, I am in complete agreement with Ron:

...snatch some new material and remove and replace Video 1 with Video 2.
...Video 2 will be accurate and on target.

JimB

http://www.oksolar.com/led/led_color_chart.htm

Might help new users approximate the VF of the diode, keep in mind all the listed voltages are at 20ma current, which is nominal for typical LED's, higher power LED's can handle more current but generally need to be heatsinked to maintain the same voltage drop, heating (including ambient temperatures higher than the data sheet) can drastically increase the diode's voltage drop often catastrophically, if not with a significantly shorter life.

Your specific tutorial approximated pretty good for a low power RED diode, but if you made the same assumptions on something like a bright white LED, you'd lose 50% or so of hte brightness because it wasn't being fed the appropriate current. The devil is in the details.

I'm definitly with Stratus, the production was decent, but you really have to work out a script, the number of throat clearings and umms makes for BAD video. If you want good quality tutorials you either have to offload the video/audio production on to someone else or learn it yourself.

If being a teacher were simple the whole world would be smart because the most intelligent could instruct those with public speaking abilities as to how to inform the masses. Such is not the case.