Does current matter more than resistance when building circuits?

Question

I am trying to understand how to build simple circuits as well as work with Arduino. My interpretation is that voltage doesn't matter as much as current that passes through a given electronic component. In other words, whether I have a 100 volt battery or 1 volt battery, if I have enough resistors, my LED won't blow up. Is that correct? Do we really just care about the current that will pass through a component?

I know if I put 8V into the 5V pin on my Arduino board, it will break some components. Is that because the resistance on the board is quite low, meaning the current passing through a component will be high, therefore destroying that component?

Answer 1

What's more important is somewhat of a subjective topic. The concept of duality is one way to frame it: every electrical thing has a dual. One of the more obvious examples: Capacitors are the dual of inductors. Everything that's true about inductors (ideal ones, at least) is also true of capacitors, if you exchange voltage and current, series and parallel.

There are also many electrical machines that have duals. For example, the common loudspeaker is driven by magnetic attraction or repulsion between an electromagnet (voice coil) and a permanent magnet. These are low impedance speakers (usually \$4\Omega\$ or \$8\Omega\$, meaning that an amplifier designed to drive them will be designed to output a large current over a small voltage. But, there are also electrostatic speakers which are high-impedance devices (\$>10M\Omega\$, easily), driven with small currents at high voltages. Rather than being a mostly inductive load, they are a mostly capacitive load.

The world is full of these duals. So as far as theory goes, mostly voltage and current can be exchanged and you end up with a different circuit or machine that accomplishes the same thing.

However, we live in a biased world. Voltage sources are more common than current sources. When we represent physical quantities electrically (like sound pressure) we tend to analyse them as voltages, not currents. When we think of a mechanical actuators, we think of magnetic solenoids before we think of electrostatic ones. I'm not entirely sure why this is true, but it is. Maybe it has to do with the practicalities of constructing things with the materials we know about. I've actually thought about framing it as a question on this site, but I couldn't think of a way to do it that wasn't too subjective.

Here's the lesson to take away: because voltage sources are so common, it's common to only need to consider the current, because the voltage has already been decided for you. If you have say, an Arduino that runs from a 5V supply, then you don't think about the voltage. The voltage is 5V, and there's nothing you can do about that, if you want to use that Arduino. All you can change in your design is how much current you require that 5V supply to provide.

However, there is no theoretical difference in importance between current and voltage. They are two sides of the electrical coin, equally important. And, in many cases, you can trade one for another. Consider both equally in your thinking.

Answer 2

It's not quite as easy as "current matters, voltage doesn't". Depending on the parts involved, there are different failure modes:

One of the most common problems is failure due to overheating, which means that components simply melt or burn (interesting videos of burning resistors can be found on youtube). Overheating is caused by excessive power being dissipated. In the case of an ohmic resistor (like a standard resistor, or to first approximation a small lamp), power is \$P = I^2*R\$ or equivalently \$U^2/R\$, so you can look at it both from the current and the voltage side.

As a simple example, consider a resistive device that can dissipate 1W of power and has 100 Ohms resistance. That means you can have at most 10V across it. You could run this device from a 100V voltage source, but you'd need a 900 Ohms resistor in series, which would have 90V across it and therefore would dissipate 9W of power. Not only does this drop the efficiency of your circuit to 10%, it may also require a fairly good heatsink solution. If you run it from a 1V power source, it won't overheat, but you will only have a power output of 10mW - that may not be enough for the intended purposes. So even for a really simple device, you can't arbitrarily pick the voltage of your power supply.

Then there is damage caused by electric breakdown of insulators. This is due to strong electric fields. Usually, we think of air/plastic/epoxy as insulators, but they can only withstand a certain electric field. For example, the breakdown voltage for air is about 3000V/mm. This (plus other electrochemical effects) cause capacitors to die when subjected to excessive voltage.

Semiconductors like diodes and transistors are more complicated and there are more effects to consider. For example, most LEDs will not allow any current to flow at 1V. You need a minimum voltage for them to conduct (and emit light) at all.

Answer 3

Talking about "resistance" in relation to semiconductor devices like LEDs and microprocessors (on the arduino) will lead you astray, as they don't behave like resistors. Instead, these types of device have a voltage threshold and behave differently above and below the threshold.

For red LEDs, this is about 2V. So if, for example, you connect an LED to a variable bench power supply and wind it up gradually you might observe (values very approximate):

• 1.5V no light, near zero current
• 1.7V dim light, few ma flow
• 1.9V normal brightness, current flow near datasheet value
• 2.1V bright, starting to get a bit warm
• 2.3V very bright, colour shifts, LED destroyed after short time

Adding a resistor gives you a system with two components that react differently to voltage and current. The resistor has strictly linear behaviour - current and voltage exactly proportional to one another - and so the system stabilises with a particular current through the LED that's sensible for it.

Something similar applies to the base-emitter voltage of bipolar transistors, which is generally about 0.7V across a wide range of currents.

The thing that kills microprocessors when subjected to overvoltage is a similar nonlinearity: the current that flows with increased voltage will be much greater at some internal spot, which will then burn out.

(This effect may be exploited selectively within a device: some devices have programmable fuses in them which may be deliberately blown to activate security features or set the serial number.)

Semiconductor devices can actually be built to handle very high voltages (power MOSFETS, IGBTs) but this makes them large and expensive.