Explain Current and Voltage to me

[bobsmith1492]

You're driving an LED? That changes everything. You must not run an LED directly from a wall wart. You need a resistor to set the current through the LED; otherwise you will either not turn on the LED or burn it out.

LEDs run at a particular voltage, say 1.5V for your LED. You can't just apply 1.5V, though it might work, but you won't know how much current it will take and the brightness will be wildly variable from LED to LED because of differences between LEDs and differences in the output voltage from the supply. Chances are you might still burn out the LED.

What you want to do is the following:

- Get a 3V or 3.3V wall wart
- Decide how much current you want to supply the LED - say 100mA for yours
- Know the LED voltage (say 1.5V)
- Calculate the resistor needed: (Vsupply-Vled)/(Current) where current is in amps
So, for this example, with a 3.3V wall wart, to power the LED, you want (3.3-1.5)/(0.1) = 18 ohms.

Check the LED voltage vs current graph below. See how steep the curve is (on the right of the graph)? If the voltage is a little too low, no current will flow and the LED won't turn on. If the voltage is a little too high, too much current will flow and the LED will burn out. So you need a resistor to control the current (or a constant current driver which is more complicated).

http://upload.wikimedia.org/wikipedia/commons/a/a5/Diode-IV-Curve.svg

 

[DrPizza]

Just out of curoisity - do you know what resistance a typical human body is? I thought it was around 10K or so...

I think that number goes way down for AC because the skin becomes less resistive, so at 12VDC, it's even less of a concern.

Huge range that can be googled. But a lot of factors matter - how dry the skin is, how far apart the contact points are, etc. Once through the outer skin, resistivity drops off significantly.

 

[William Gaatjes]

Just out of curoisity - do you know what resistance a typical human body is? I thought it was around 10K or so...

I think that number goes way down for AC because the skin becomes less resistive, so at 12VDC, it's even less of a concern.

The skin resistance between 50k and a few Mega Ohms.

It depends on the conductivity of your skin. And the type of skin.
Sweat glands produce salt water that is great in conductivity. It depends of the time of day. If you are tired or not. If you are having a fewer(sweating more). The temperature of your environment. The humidity of your environment.

For some reason my skin resistance is around 90 k (my hands only )most of the day but it rises up quickly when i am tired at the end of the day.
But the weird thing is do not have sweaty or oily hands when comparing with other people. Anyway, if you put a large voltage across your skin, you will quickly reach a breakdown voltage and your skin becomes a good conductor.
However, the cellular fluid in your cells and surrounding your cells is a excellent conductor. The current will primarily flow here until it reaches your nerve paths. These conduct even better but still have resistance. The end result is , you will have a lot of damage.

Why 120V 60Hz or 230V 50Hz kills you is not primarily the voltage but because your heart tries to follow the 60Hz/50Hz. That means it tries to pump 60, 50 times per second. Afcourse it can not handle it and will seize to function.

 

[William Gaatjes]

I don't really like shorting wall warts. I have destroyed a couple doing just that. My main motive is that I have an LED flashlight I wanted to upgrade to a better LED and I just wanted to learn a bit more on how the driver is providing the power to it and whether or not the new LED or driver will be safe after the upgrade.

Usually when you destroy a cheap 50Hz(60Hz) transformer adapter, you simple burn an internal fuse inside the transformer itself(primairy high voltage winding). The better ones have a thermal polyfuse or multifuse that restores itself. The cheap ones just break down and do not function again. Another way is to simple embed a normal current fuse inside the transformer. This way you do not need a fuse on the pcb. Making it cheaper. If you unwind such a fused transformer you should be able to see the fuse embedded.

If you want to make a perfect led light. Go see linear technology or maxim or catalyst semiconductor (now aquired by onsemi).
They have great chips for optimal driving a led. It extends the life of your led(s) and minimizes power consumption increasing efficiency...

Here are some type numbers to google datasheets:

CAT4201 350mA led driver (SMPS switching driver)

LT3474/LT3474-1 1A led driver(SMPS switching driver)

cheap and only a few components needed.

The CAT4201 is great for starters.
But you must be able to solder SMD (it's a small 5 pin ic) or know someone who can do it for you ...

http://www.onsemi.com/pub_link/Collateral/CAT4201-D.PDF

Important :

The thing is, you must drive the led with a constant current. Leds like that. These ic's make sure the led always receives a maximum current that you set. And that maximum current depends on what the led can have as maximum current. This makes sure the leds keep working. If you want less light, just decrease the current.

Intermezzo :
There is Ohms law. U = I*R .

If R is constant and u is constant, then I will be constant.
But the R of a led is not a constant. The more voltage, the lower it gets, consuming even more current. And the hotter it gets, the lower the leds resistance. Consuming more current.
This will continue untill the led burns up with a flash. That is why a led is a current driven device.
You must power it from a current source and not a voltage source. A walmart adapter is a voltage source. When you put a led in series with resistor that's what you do, is you chance a voltage source into a current source. A current source is nothing more then a voltage source with a very high series resistance(read output resistance). That's why you always need a resistor for a led with a voltage source.

Ideal :
A voltage source provides a constant voltage no matter what the current draw of the load is.
Here, the voltage stays constant.
A current source provides a constant current no matter what the resistance of the load is.
Here the current stays constant.

In theory both can go from 0 to infinity. In reality both have a minimum and a maximum.

End intermezzo :


The leds do not mind if you provide less current. The leds do mind if you feed it more current then it can handle. Leds have this way to keep consuming more current when you feed it more then it can handle.

 

[watdahel]

Interesting.

 

[Modelworks]

There are some really crappy power supplies out there. Some are just a transformer, two diodes and a capacitor in a case, no fuse, no protection of any kind except that the winding will fail and maybe not smoke in doing so.

I saw one yesterday that I couldn't believe. It was a transformer design, with one diode, a capacitor about half the voltage it should be and they had tied the neutral from the AC to the ground output of the supply. No fuse or anything for safety. A fire waiting to happen.

 

[OUCaptain]

Think of water. Voltage is the pressure of the water in the pipes. Current is the diameter of the pipe. The pressure does the work but only as much as the supply will allow.

 

[bobsmith1492]

Think of water. Voltage is the pressure of the water in the pipes. Current is the diameter of the pipe. The pressure does the work but only as much as the supply will allow.

More like the current is the rate of water flow, while the diameter of the pipe is the resistance of the load in the circuit.

More pressure means more water flow; a more open pipe also means more water flow but only as much as the source can supply.

 

[Born2bwire]

More like the current is the rate of water flow, while the diameter of the pipe is the resistance of the load in the circuit.

More pressure means more water flow; a more open pipe also means more water flow but only as much as the source can supply.

What? No!

You see, it's a lot like a sewer. What you get out of it depends on what you put in it. But like any good vintage car, you have to perform maintenance. You have to wax and buff the hood, clean the spark plugs, but in the end, always realize that eventually she won't pass the emissions test and then you have to put the old girl down.

And that's pretty much what a woman is like.

 

[William Gaatjes]

What? No!

You see, it's a lot like a sewer. What you get out of it depends on what you put in it. But like any good vintage car, you have to perform maintenance. You have to wax and buff the hood, clean the spark plugs, but in the end, always realize that eventually she won't pass the emissions test and then you have to put the old girl down.

And that's pretty much what a woman is like.

I do not want to seem nosy...

Are you having a lover's quarrel ?

 

[bobsmith1492]

What? No!

You see, it's a lot like a sewer. What you get out of it depends on what you put in it. But like any good vintage car, you have to perform maintenance. You have to wax and buff the hood, clean the spark plugs, but in the end, always realize that eventually she won't pass the emissions test and then you have to put the old girl down.

And that's pretty much what a woman is like.

Well then.