Re: Amps available - amps draw ?

On Sun, 25 Feb 2007 16:08:27 -0600, NJM <guitchess@xxxxxxxxxxx> wrote:


Hello.

I am trying to teach myself the basics, but I am confused about ampage.

For instance, say I have a power source that is 12v at 2 amps. I want to
power a simple LED circuit that I found on the internet that is designed
for a 12v supply but there is no mention of the current. The circuit is a
50 ohm resistor and 3, 3.6v ultra bright white LEDs. The resistor value
was calculated for a 25mA current using ohm's law. What is the resistor
modifying? The current or the voltage? Will this circuit be usable
regardless of the supply ampage? The voltage/current relationship has me
confused.

TIA for any insight you may be able to provide.

The current rating of a power supply is the maximum current that the
supply can safely provide.

The current actually drawn by any load will be determined by the
characteristics of the load (as long as its demand doesn't exceed the
capability of the supply).

I assume that the three LEDs and the resistor are all in series. If
the LEDs truly drop 3.6 volts, there will be 10.8 volts across them
when lit, leaving 1.2 volts to be dropped across the resistor. Ohm's
Law says that you will get 1.2 volts across a 50 ohm resistor when it
is passing 24 mA.

The voltage rating for LEDs is often a "typical" value - actual parts
may vary from that, and the voltage will vary to some extent with the
current. The rated current for a LED is usually the maximum current,
beyond which the life of the part may be shortened. LEDs will work
fine at much lower than rated current, but will be a little dimmer.

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
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.

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