# Why can't we use diodes in place of batteries?

1.why we cant use diodes in place of battery?we are getting 1.4v in a cheaper cost than battery.but why we dont use?

2.if electrons are moving in a single direction it releases many energies but why we are calling it as "current"?

Probably can be put as follows. Opriiginal poster may wish to check this wording:

• Why can diodes not be used instead of batteries as a source of energy. Bothe batteries and diodes have current flow in one direction and diodes have potential voltages of up to 1.4V and are cheaper than batteries - so they seem like better choice.

• Electron flow in a single direction provides a source of energy.
Why do we call this energy flow "current"?

• The original question is needed to understand the questioners mindset. They are confused BUT the question makes less sense when edited that it did originally. – Russell McMahon Mar 5 '12 at 3:40
• Consider the direction of current flow relative to the drop across the device; in a battery, the current is out of the positive terminal, whereas with a diode, the current is into the positive terminal. This tells you that the battery produces power, whereas the diode consumes power. Which holds up to the point where you can somehow cause charge dissociation in the diode's depletion region with an external energy source. With typical diodes, this is nigh on impossible. A solar cell allows it, and indeed, current is out of the positive terminal in that case, but these aren't strictly diodes. – JustJeff Mar 6 '12 at 2:42

As originally asked, the question was:

• 1.why we cant use diodes in place of battery?we are getting 1.4v in a cheaper cost than battery.but why we dont use? 2.if electrons are moving in a single direction it releases many energies but why we are calling it as "current"?

• Diodes are not an energy source.
They modify how energy may flow.
Their function is to allow current or electrons or charge to flow in one direction in a circuit much more readily than in the other direction.
A good analogy is to think of them as a non-return valve of one-way-valve in a water flow system.

• Batteries are energy sources - usually a means of converting chemical energy into electrical energy.
A good analogy is to think of them as a "pump" in a water flow system.
Just as a pump is NOT a pure energy source in its own right, but a way of converting some other form of energy to "flow", so a battery is a converter, turning some other form of energy into electrical energy. usually chenmical energy is the source.

• Electron flow is termed "current flow" because of the traditional and useful analogy with flow of water in a water flow system.

• Many of the elements in a basic electrical system have equivalents in a water flow analogy. The comparisons are NOT EXACT but are useful.

• Voltage: Pressure, water head.

• Current: Water flow

• Resistance: obstruction to flow, pipe resistance,

• Capacitance: inline chamber with a elastic diaphragm across it.

• Inductance: inline chamber with walls made of elastic material.

• Diode: One way valve.

• please edit the question to make the users mindset more clear. This gives a higher quality question and allows higher quality reads for later users. – Kortuk Mar 5 '12 at 4:13

The difference in band energies in semiconductor diodes does indeed provide a small potential difference, but it is matched by Fermi potentials between the contacts (leads) and material and the Coulomb forces in the depletion layer, so it isn't independently useful. Regardless, for a source to provide energy it must easily transmit particles (ie: electrons, phonons, photons, etc.).

• A P-N junction can provide energy - if you spread it out wide and thin so that light can get in there and dissociate charges in the depletion layer.. but that's called a 'solar cell' – JustJeff Mar 5 '12 at 4:56
• @JustJeff oh I didn't see your comment :) – clabacchio Mar 5 '12 at 8:59

Actually diodes, if seen like P-N junctions, "can be" used as electrical sources: photovoltaic cells are nothing more (actually something more ^^) than flattened P-N junctions, where photons composing the light hit electrons in the highly doped N++ region, causing them to become energetic and creating a potential difference which can supply a load.

But they can't replace batteries, maybe work in sinergy :)

As has already been stated plenty well enough, batteries contain stored energy which they can release electrically to a circuit, diodes don't. I think this is the correct answer for the OP.

However, although I don't think this is what the OP was asking about, and it would probably confuse him if he ever came back and read this, diodes can supply power and are sometimes used for this purpose. These special diodes are called solar cells.

A semiconductor diode does produce a small potential inside. This is about 700 mV for silicon diodes. This potential ballances the diffusion of positive charges from the P region into the N region, and negative charges from the N region into the P region. Diffusion pushes the charges accross the junction, which then builds up a potential, which opposes the diffusion of more charges. 700 mV is about the potential for P and N silicon where the diffusion pressure is offset by the voltage and equillibrium is reached.

There is a finite amount of energy bound up in this migration of charges, but it is not a power source. However, when a photon hits within this region where the potential gradient exists, then a charge is sortof released, which is then propelled by the potential field to flow to one side. If photons hit the junction continuously, then these charges flow continously, which in the aggregate is a current. In this way a diode can be used as a power source, although it doesn't contain stored energy like a battery but rather converts light power into electrical power.

Again, I don't think this is what the question was about, but I wanted to point out that diodes as power sources isn't as silly as it first sounds, although most likely not for the reasons the OP was thinking.

Unless I'm missing something here, this question is nonsensical.

A battery is an energy source. A diode is not.

The voltage rating of a diode does not mean a diode is a voltage source.

• it sounds like you have found the users fault in understanding, but saying it makes no sense in 2 sentences could be attempted by further explanation, however I think this question is bordering on too basic. – Kortuk Mar 5 '12 at 4:13

As a couple of others have already pointed out, the reason is that diodes (except for some special cases where there's an external source of energy from light) don't work as a power source in a circuit, whereas batteries do.

I'll expand on how you can see that is true.

First, you've probably already seen the power formula:

P = IV

Power is the product of current and voltage. But both P, I, and V are all signed quantities. The meaning of positive or negative P is based on a common convention: When P is positive it means the circuit element is absorbing power, using it to do something useful (like emit light if it's an LED or turn a motor) or just turning it into heat (as in a resistor). When P is negative it means the circuit element is delivering power to the rest of the circuit.

For this convention to work, we have to be careful about how we define the direction of I and V:

If we define the positive terminal arbitrarily, and say that the voltage is positive when the positive terminal is at a higher potential, then current is positive when it goes in to the the positive terminal.

To the point, the power flow is into the device when current flow is into the terminal with higher potential.

If we look at a diode in comparison to a battery cell we see that the relative directions of current and voltage are opposite:

In fact, in a circuit theory context, we would say that the current through the battery is negative. In day-to-day use we rarely make this distinction, because everyone knows which way the battery current is meant to flow, although there are certain manufacturers (NXP?) who carefully observe the circuit theory sign convention in their datasheets.

To question 2: The energy of a single electron is too tiny to be practical as a unit of measurement. So instead we look at the sum of those energies over a unit of time and call that current. What we call 1 Ampere (current) is equivalent to the energy of roughly 6.24x10^18 electrons (1 Coulomb) per second (see this Wikipedia article).

In a nutshell, we do it to make the units easier to grasp and keep the numbers in a more practical range.

Diodes do in fact generate a small electric potential when they are not attached to a power supply. It is caused by heat or vibration mostly, or induction from near by radiation and electromagnetic fields. This DARK CURRENT is too small to be of practical use in batteries, but large enough to cause problems in video circuits unless compensation is made in the design. There have been discussions about how to accumulate enough dark current in diodes to be useful. For a static source of one watt, it requires about 700 stacks of diodes with 500 diodes in each stack, or about \$35,000 worth of components. That is the answer to your original question. It's just too expensive to do except as a demonstration of principles. The cost goes down when an alternating source is constructed with strong induction field effects built up from LC resonators, but it is still too costly to use in every day life. Patents have been issued on how to build diode stacks in groups such that one diode failure does not disable the entire assembly. So far the inventions have not been commercially successful. Maybe some benefit will be found in future technologies to lower the cost. Then supposedly the resulting power supply will get cold as it sends out electric power. This is the third law of thermodynamics related to non random systems.