Roger Halstead wrote:

On Sat, 20 Mar 2004 00:25:32 GMT, Scott Gettings
wrote:

Sorry this was not so "obvious".

Scroll about 1/4 way down the page and click on the "more" beside the picture
of the nav/strobe light demonstration.

Just went through the information on the page and it looks good as
well as interesting. However I'd like to offer a warning.

The math and resistor rating concern me a tad. I may have misread, or
mis-calculated, but...

It talks about using two LEDs and dropping 7 volts at 0.35 A or ( 350
ma) and using a half watt, 20 ohm resistor.

It mentions the resistor getting hot. It should, they are over rating
the resistor by over a factor of 4 and that is with no safety factor.
It's not uncommon for a resistor to get hot, but they should not get
so hot you have to be concerned about them except in the case of wire
wound power resistors which may actually run quite hot.

R does = E/I and in this case 7/0.35 = 20 ohms but it needs to be
carried one step farther and figure the power as well.

Two ways, the first is simply the voltage times the current. You are
dropping 7 volts at 0.35 A = 2.45 watts.
Another way is P = I^2*R, or current squared multiplied by the
resistance. or (0.35 * 0.35) = 0.1225 * 20 = 2.45 watts.

Normally, we double the rating for safety so you need a 5 Watt
resistor, not a half watt.

You are dissipating 2.45 watts in a half watt resistor.
You could get away with that if the lights were pulsed, but in this
case they are on all the time. That much heat will cause the
resistor value to change and is a definite fire hazard.

Normally the resistor gets too hot and breaks, but I have see the
things start glowing. When they do that the resistance can become
quite low, rather than high and you run the risk of shorting the LEDs
which would increase the current even more.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com


Sound like good advice. My calculations were based on all the sources I've
found, but the above makes sense with its safety margins.

A 1/2 watt resistor doesn't get scalding hot, but hot enough. Using a
larger-wattage resistor (such as going to 5-watt) is easy, safer and certainly
cheap!

Scott