Basic Ohm's Law Question

I’m trying to use a 12 volt battery to power two USB ports to charge phones. My question is about how to get 5V, I know resistors in series can get a “voltage drop” of 5V (see attached image for circuit). But does this mean if I put a USB connection where the resistor is I am getting the needed 5V? Or am I completely misunderstanding the law here? Sorry for the simple question I’ve looked everywhere for an answer with no luck!

Ohms law is not applicable for power sources - the voltage developed varies as the current used varies.
You need an active regulator. A switching regulator makes the most efficient use of the available power source, with 85-90-95% of the incoming power converted to outgoing power. Go to and look at the available regulators.
A linear regulator, like the 7805, converts 12V to 5V by dissipating the 7V as heat. The more current used, the more heat dissipated.
Switching regulators chop up the DC into AC, and basically PWM the output and filter it to create a DC output at a lower level. As the output current demand goes up the PWM width gets wider, up to the current limit of the design, when it is 100% on.

Here is an example of one I use.

Ohms law is not applicable for power sources - the voltage developed varies as the current used varies.

Just to clarify for you - Ohm's Law is a law of nature and it's always true. But a voltage divider (2 resistors) is not practical in this application. (And, it's just a crappy engineering design. :wink: )

The circuit you're powering is in parallel with one of the series resistors. That changes the equations/calculations. Worse, that load is rarely constant so the voltage can change which makes it unreliable as a power source.

And, you'll usually end-up wasting more power in your voltage divider than your actual circuit is using. That means you need a bigger power supply, or your batteries don't last as long, you end-up generating extra heat, etc., while not working all that well...

Voltage dividers are used all the time when you need a "voltage reference" and very-little current is required. In that case, it can be a GREAT engineering design!

Ohm's law is not a law of nature at all. It certainly is not always true. If it were than we wouldn't
have diodes transistors or any active circuit components.

In fact Ohm's law is a (usually very accurate) approximation to the behaviour of charge carriers in
uniform conductors, ionic liquids and uniformly doped semiconductors. It doesn't apply at all to insulators,
gases, vacuum, non-uniformly doped semiconductors, superconductors, electrochemical cells, plasmas,
quantum-tunneling materials or living cells...

Please, give Mr. Ohm a break His law is very useful for a multitude of applications and for the most part cover almost everything here on the forum. Calculating internal resistance and current flow in heating semiconductors that may or may not be properly heatsinked is not one of the applications for ohm's law , and all the other cases you cited would never come to mind in the same sentence with Ohm's Law. Personally , I'm a big fan because it covers just about everything I work with, both on and off work.

Everything but diodes, LEDs, transitors, MOSFETs, microcontrollers, sensors, transducers...

It applies to resistors and conductors, basically, that's it. [And at fixed temperature too,
come to that, tungsten filaments don't obey it]

I don't disagree but almost every case you cited involves some use of obm's law for support components. Also, everything you need to know for fhe devices you cited is on the datasheets.

I am reminded of the xkcd cartoon: xkcd: Duty Calls

I am reminded of the xkcd cartoon: xkcd: Duty Calls

:slight_smile: :slight_smile: :slight_smile:


:smiley: :smiley: :smiley:
Yeah, that about sums it up, huh ?