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Some of the books I have state that power comes from the negative terminal on the power supply.If that is the case, the circuit starts from the negative right?
Say I have a simple circuit with a 6v supply, an 1K resistor and an LED. If I wire the resistor to the positive source, then wire that to the anode of the LED and back to ground, the circuit works fine.
How can this be the case? Shouldn't the resistor be between the cathode and the negative source?
The definition of anode and cathode is different for different devices. The anode of a device is considered to be the terminal that the conventional current flows into and a cathode is where the current flows out from. For a conducting LED the current flows into the longer leg (positive terminal), which is why it is considered the anode, and then flows out of the shorter leg (negative terminal), which is then considered the cathode. For a discharging battery however, the cathode is the positive terminal (where conventional current flows out from) and the anode is the negative terminal (where current flows in).
You could look at it a different way and say that the cathode repels cations (positive charge) which is why conventional current flows away from it, and anodes repel anions (negative charge) thus conventional current flows towards it.
You could do this (resistor only shown for one channel). You could use a single ULN2003A in place of all the transistors and all the resistors except for the 500R, and that is precisely what I would suggest doing.
simulate this circuit – Schematic created using CircuitLab
Best Answer
It is true that, in most conductors, the actual charge carriers are negatively charged electrons, which leave the negative terminal of the power source, pass through the circuit, and return to the positive terminal of the source.
However, early scientists studying electricity didn't know about electrons, so arbitrarily declared that current flowed from the positive terminal of the battery, through the circuit, reeturing to the negative terminal of the battery. Today, almost everyone uses this "conventional" (positive charge) current, and you will avoid confusion by using it also.
Circuits work equally well whether you wish to think of them using conventional (positive) or electron (negative) current.
For your LED and resistor circuit, it doesn't matter which component is connected to the positive terminal of the battery, as Kirchoff's Current Law says that the current is the same at all points in a series circuit. That is, the resistor will limit current through the LED, whether it is placed "before" or "after" the LED, regardless of which way you think the current is flowing.