# Completing a circuit, how is the necessary voltage determined?

When you have some device, like a TV, why is there a necessary Voltage-or current for that matter. Whenever I see circuits, it always says to find the current from the voltage and resistance given, so presumably with any battery there should be a current. Why do some things require bigger current and bigger voltages? And how would one calculate the voltage required? Furthermore, even with some deemed insufficient Voltage, there would always be a current, so I don't know why a device wouldn't work.

The reasons vary greatly, but I think a good way to summarize them is this: a device does more than simply passively draw current. Often times the actuators and transformers in a device rely on electrostatic phenomena, so in order for it to accomplish a task, a varying range of voltages is required for various devices.

For example, the CRT of a television: the deflection plates use an electric field to deflect the electron beam to a given pixel:

The voltage applied to the vertical and horizontal plates determines the angle of deflection--the greater the voltage, the greater the deflection. So a larger screen would require a higher voltage in order for all the pixels to be reached.

This is just one example, but you get the gist: in most electronic devices the actuators are driven by voltage. Even when they're driven by current, such as with speakers, the signal is usually transmitted throughout the circuit as a voltage.

Another thing to consider is the behavior of transistors. Transistors, which drives all the switches, amplifiers, active filters, and logical elements of a circuit, have a threshold voltage. This is usually fairly small, around 0.7 volts or lower, but if a transistor does not receive at least this voltage on either of its channel terminals, the transistor will essentially be in an "off" state, functionally indistinguishable from a transistor with no voltage applied at all.

• In the case of speakers, what do you mean by "the signal is usually transmitted throughout the circuit a voltage?"
– user25228
Jun 15, 2013 at 5:50
• @Tony I'm not sure how else I could put it. Signals are usually voltages throughout a circuit, and if the actuator is controlled by a current, the voltage is converted to a current at the final stage before the actuator. Jun 15, 2013 at 5:58

Mostly you need a certain amount of current to make it physically work. For example, if you need an electric motor to have a certain torque, you have to make a magnetic field of a certain strength, which requires a specific amount of electric current.

For logic gates on computer chips, a 1 is created by charging a capacitor up to a certain voltage. The more current you have, the faster that charging happens, and the faster your computer computes.

A light bulb needs a certain amount of current in order to create a certain amount of light.

All these things have an intrinsic resistance. That means if you want to induce a certain minimum current, you require a certain minimum voltage, by Ohm's law.

In practice, designing circuits is often done kind of in reverse. A lot of household devices require 120 or 240 volts, because that happens to be the amount that comes out of the sockets in your wall. Portable electronics get designed for the voltages of batteries that happen to be readily available at the store. But they often will convert internally to a voltage that better matches their actual needs.

I read your question as something like this: "why does a circuit that is designed to operate on 12VDC need 12VDC to operate?"

Circuits are designed to work with a particular power supply voltage. For example, older logic ICs were designed to work with 5V power supplies. Later, more efficient ICs were designed to work with 3.3V power supplies.

Older vacuum tube circuits sometimes required power supply of hundreds of volts because, well, that's how much plate voltage is required to operate a vacuum tube.

Any reasonable answer to the question of why these devices require these power supply voltages will assume a certain level of knowledge of circuit theory and device theory.

The fact is, active devices aren't resistors and, in general, are quite non-linear. There may be essentially zero current for any voltage up to some operating threshold. If you exceed the maximum voltage, there may be lots of current and probably smoke too.

Why do some things require bigger current and bigger voltages?

While not the most comprehensive answer, a reasonable start to one is to consider that, depending on the purpose, electrical power is converted to some other type. For example, an amplifier and loudspeaker convert electrical power into acoustic power

Since the product of voltage and current is power, the more power that is converted, the larger the product of voltage and current required of the power supply.

In many cases, the electrical power is converted to heat due to losses. For example, digital circuits that switch at very high frequencies convert considerable electrical power to heat (think of the large CPU coolers required for high end gaming computers).

A Class A audio power amplifier may convert hundreds of watts of electrical power into heat just while it is idling, i.e., not producing any electrical output to the speaker.

And so on...

Tony,

Concerning the part of your question about voltage being lower than specified. In many cases an electronic circuit will work with lower than specified voltage. As an example: a transistor radio running on a 9 volt battery. You'll notice that as the battery drains, it will continue to operate, drawing less current, and will function to an extent with a pretty loww battery voltage. At some point there is just not enough voltage to turn on the transistors.

My advice: Take a basic electronics course it will be fun.

Dave