# Amps available - amps draw ?

Discussion in 'Electronic Basics' started by NJM, Feb 25, 2007.

1. ### NJMGuest

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.

2. ### Homer J SimpsonGuest

Voltage is like water pressure.

Current is like water flow.

A resistor is like a tap - the higher the resistance the more the tap is
closed.

So if your supply was 12 volts at 1000 amps that would still be the right
resistor.

3. ### JamieGuest

This has been describe in many ways .. I'll give mine, i'm sure many
will give you their own version. But to put in a simple means for now.

Picture your self wading in water (a river maybe), lets say the
water is moving 5 MPH (Voltage), and you are standing in 1 foot of
water that is moving against you (Current).
Now step in 2 feet of water. The mass that is pushing against you has
now doubled (current), the speed is still the same(speed). Of course if
you repeat this action and have a big hang over like me, when it reaches
the mid section, mass will most like go up 10 times.

The Power could be looked at as the combination of mass (current) and
Speed(Voltage) as Voltage * Current = Wattage.

using the same analogy with pipes. water moving at a set speed in all
different size pipes. the larger volume pipe will have more current of
water in it and will be able to push a heavier weight at the same speed.

Now, picture a very small pipe with water moving at high
pressure/speed(voltage). The impact on a surface could cause damage but
will not be able to push that massive area. if you have ever gotten a
High voltage arc shock of a few thousand volts with no much amps you may
have notice a pin hole or some kind of skin damage.

Now, think of a Resistor as a pipe, the lower R values would be the
same as large pipes that allow less restriction of mass and the higher
values would be like small pipes, restricting mass.

Lets say you have a volt meter(Speed of water), this meter has no
mass for the water to resist against(very small object in water), you
can get the same voltage on both sides of that resistor how ever, in the
real world, the resistor will be connected to an item like the LED's.
These LED's are lets say the blocking mass, much like you standing in
the water. the water will slow down as it finds its way around you.
This slowing down of water (speed)= Voltage will be seen by
your volt meter when testing the speed (voltage) at that point.

Now you can increase this flow by using a smaller resistor how ever,
this is like increasing the volume of water which puts more pressure
against you in the water. this pressure is being converted to energy
that you're expending keeping your self balanced, much like the LED
emitting light.
At some point, there will be too much mass for you to handle and you
will fall in the water much like the LED going to LED haven!

Now lets touch base a little on Speed (Voltage) even though you can
have high voltage with no mass behind it, it's does not mean it's safe.
Take a thin sheet of plastic and put it in the river with slow moving
water but a large area. The plastic sheet will most likely handle that
how ever, placed in fast moving stream of water (Voltage) with no Volume
(current) and it can blast a hole in the sheet. just picture this as
the maximum voltage a component can handle before it breaches or shorts
out from over voltage(Speed)..

Ok, I've gave my explanation. Years ago i used this example teaching
the basics back when they use to have Cub, Boy,Eagle scouts etc... I can
say that many of them ended up in a electrical/electronic couriers.
Hope that gave you some insight!.

"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

4. ### Peter BennettGuest

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
GPS and NMEA info: http://vancouver-webpages.com/peter

5. ### ehsjrGuest

+12---+
|
[50] 1.2V
|
[LED] 3.6v
|
[LED] 3.6v
|
[LED] 3.6v
|
Gnd---+

The 3 LEDs hold the voltage at the bottom of the
50 ohm resistor at 10.8 volts (3.6 + 3.6 + 3.6)
The voltage at the top of the resistor is 12 volts,
so there is a 1.2 volt drop across the resistor.
The resistor limits current through the LEDs by
dropping voltage - that means it affects both
current and voltage.

The current and voltage mathmatical relationship is
shown by the ohms law formula E = I*R. If the voltage
is fixed, then current _must_ increase when resistance
is decreased, and vice versa. Intuitively, think of
running in water (say 2 feet deep) as resistance. It
takes more energy and you go slower than if you run on
dry land with no resistance. If you lower the resistance,
less energy is wasted fighting the water, and you go
faster. In the electrical circuit, the resistance
wastes electrical energy by converting it to heat and
it lowers the voltage proportionately.

Ed