What Are AC and DC Power Systems?

By: Jeremy Neisser

The term AC power system is due to the fact that it uses alternating current. The term DC power system is because direct current flows in a single direction only and there is no alternation of polarity.

• Alternating Current - AC power

• Direct Current - DC power

What does AC Stand For?

AC stands for alternating current which is the type of power we get from electricity mains. AC voltage changes its polarity many times in one second.  

For example, if you have 220VAC then it means that there are two points on your circuit with different voltages and because those points are not connected to each other directly (lamp), it can glow by having positive (+) and negative (-) charged particles rushing through it at high speed so they can neutralize each other because they are like opposite charges.  

If you connect a capacitor between those two points then electrical charge won't be able to pass through them since there's no path for current to flow through. It will store energy whereas if we had a wire connecting those two points, electrons would flow freely.  

What are AC and DC Power Systems?

What is an AC power supply?

AC stands for Alternating Current. An AC power supply is a device that provides alternating current to equipment in an electronic circuit.

The electrical current in the wires feeding electricity to your home are the same as those feeding electricity into any other appliance so we can use them as well to power our basic circuits but there's one difference- they don't provide DC power, they provide AC power.

That means that voltage polarity varies and goes from positive to negative many times per second and not just going from positive (+) to negative (-).  

This way AC power works just like the load connected to a capacitor but with no resistance (wire) .  

What is an Electronic Circuit?

The ac electronics circuit consists of capacitors and resistors. They have jacks at each end that connect to a power supply. It also has lamps, motors and other things connected to it. The diagram below shows how they are connected together:

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Why do we need capacitors in electronics circuits?

Here's why- We have charged particles running through our wires which means electricity is flowing.  When there are positive (+) charged particles, negative (-) will come after them and so on which is good because if one wasn't there then electrical energy would stop transferring from the source.  

Now if we have a wire that has electricity flowing through it and there's a diode connected to it then those positive charged particles won't be able to pass through the diode. But they will also not go away, so they'll stay around building up as much negative (-) charge (opposite of positive charges) on one end of our wire while the other end has no charge at all.  

That means that energy is stuck in our wires because there are no negative charged particles coming after the electrons. That's bad because electrons can't move freely like this which means power cannot flow from one place to another but gets stuck instead, not being transferred anymore.  This is what we call an electric current.

It's not really stuff that flows through wires like water but just the fact that electric charges are moving from one place to another.  

To prevent this, we need a capacitor that can hold positive charged particles inside while negative ones pass by it without being able to get in. We also need resistors since electrical energy has different voltages since we have different kinds of power sources such as batteries and AC wall sockets.

You probably know why- if you use a battery to light up a lamp then it will remain dim or not lit at all because the voltage is too low for lamps which usually require more power.  Same with motors where the current flow would stop before reaching its maximum speed because the voltage was limited.  

What does DC stand for?

DC power stands for direct current which is current that flows only in one direction from a positive terminal to a negative terminal and does not change polarity during its entire lifetime (s) : it's just always positive or negative.  

It can be said that AC voltage is like having an obstacle course for electrons in your circuit whereas DC power is when the path of electrons is clear, although all it takes to do that in AC case if you connect a capacitor between those two points so a charge will build upon the plates of capacitor before discharging thus preventing current from flowing directly into the load as they are blocked by stored charges within caps.

All this means that in order for a power source to give the energy to load connected across terminals it has to charge first them up, so for example, if you have a battery with 12V then you need to connect it across another source that generates the same voltage or higher such as AC mains supply in order for current from the battery to flow through wires and lamps.

Otherwise, they would be charged only by batteries, not the mains power supply and your load would glow very dimly if at all (assuming there is enough resistance to prevent most power from flowing back into the battery).

If we had identical loads connected across two different voltages then the highest voltage will always prevail which means that more amp will be drawn from it than the lower one regardless of their state or how many are connected.  

All this doesn't mean nothing happens in DC power circuits since when a conductive wire is connected to two different points through switches there's still something happening but the difference is that it can only change direction.

For example in a circuit with a battery and lamp, we need to flip the switch on the battery terminal to turn the lamp on. The problem is that when you have loads directly connected across a power source (ie not via other components such as resistors or capacitors) then they share the same voltage which causes current sharing among them resulting higher amount of energy being taken from low(er) resistance load than intended since all voltages are the same across those loads.

The importance of circuits with resistors in them AC circuits work significantly different when using resistors within their design because this time voltage across certain parts will be lower than others which creates problems for electrons wanting to flow through them.

AC power sources such as mains will still be connected across resistances, but at the same time, they will also be connected directly to loads which means that current flowing through those resistances will not affect circuit powering other components as it doesn't get distributed among them.

This is because higher electric resistance indicates a high voltage drop between terminal and load since all electrons have more obstacles on their way from power supplies to load resulting in voltage being higher than if there was no component with high resistance within its path.  

Therefore even though some of the parts are responsible for passing energy into load others aren't so current would still focus on taking energy from the source without sharing it over the entire circuit like when there was no resistor present (ie connecting lamp directly across the power source or to AC mains).

That's why adding a resistor into the circuit makes it work differently because now both voltage drops on different components are not equal thus resulting in the current changing its path over time.

Because electrons flowing into load via a lower resistance path more energy gets supplied to it than what it would receive if there was no resistor in the circuit.  

If we had a current generator within our power source then it would pump out a larger amount of energy than lamps can handle causing them to burn out faster, so in order for resistance not to affect the amount of energy that reaches load there has to be some way of controlling how much is being pumped out by sources and how much goes into the lamps otherwise they will burn before their time.

That's why we need a switch because when current flows through one terminal it starts positive charge pile up on one side moving towards another terminal where negative charges are concentrated resulting in electric fields between two terminals until eventually equilibrium forms between them after some time.  

In general, all this means that increasing resistance within the circuit reduces current flow through it which is the opposite of what happens with voltages which increase as more electrons flow through a component.

This also explains why we need to connect resistors in series since then only a higher one will produce a noticeable voltage drop when the load gets attached to both (assuming they're identical). Surprisingly enough lowering resistance can have a similar effect of raising voltage if more open circuit paths are created for electricity to travel thus making lower resistance bad for circuits as well.

That's not all though, there are few things that I haven't covered yet like the effects of coils and capacitors but those will be explained later on so stay tuned for the next installment!

Just remember that DC power current is only happening when electrons move through loads, but AC current happens even when no load is present because electricity gets distributed among components thus making it harder for any of them to take in as much current as possible unless you use a resistor.

Can AC DC Cables Run Together?

AC DC cable can run together. I run both 12 and 18 gauge together all the time behind my racks in my home theater. As long as you are not using them for two different circuits of power or anything like that, there should be no problem running them together.

AC is 120 volt (or 240) alternating current and DC power supply is direct current at a single voltage; it doesn't alternate.

AC power (alternating current) and DC power (direct current) are different things. AC is used in most electrical power systems because the voltage varies over time as the generator turns at 60 cycles per second. Voltage is the push of electricity while current is how much electricity flows through something in one second. Well known example: if your battery has 12 volts on it, then you need 12 volts x 1 amp = 12 watts to get a watt of power into something electrically.

AC will cause AC power current to flow through a DC power supply because the electricity is pushing against the circuit components and making them move in a manner that causes alternating current with polarity having been reversed.

A diode can be used to keep the electricity from flowing one way, but it will allow electricity to flow in the other direction although at reduced strength (due to losses). If you have taken your car battery and hooked it up without any diodes or regulators or anything like that then that would create AC voltage on your wires.

Direct current can also vary over time if there is some sort of varying load on it as well. But when we think about typical power systems with lights and computer equipment it is all AC. An example of this would be a window air conditioner or a water pump that is connected to your 120 VAC line.

If you put a voltmeter across the wires going down to your breaker box, you will see that there are in fact "brain waves" from the electricity being generated at 60 cycles per second by the power plant miles away. I am not kidding about using meters on your house wiring!

Can AC power and DC power Currents be Combined?

 Yes. Can't say it can't be done, but I wouldn't recommend it. Remember that Current=Amps*Volts, so you're doubling the Amps if you use both AC power and DC power. If you're going to run both currents down a wire then make sure your wires are rated for at least twice of the current they are designed for.

Yes, 600v DC power supply and 120V AC power can be combined (even higher voltages) in a single wire as long as you don't exceed the wire's maximum ampere rating. However, I wouldn't do this unless it is absolutely necessary (have to power something with both).

The reason being that there will be only a small portion of the voltage available for anything directly connected to that wire. This could cause some issues with sensitive electronics during peak load times, etc.. If you had an air conditioner running on 110 volts alternating then its motors would kick up 60 times per second so what was once 120 volts would act closer to around 10-13 volts half the time (or less if your wiring is long, say around 150 feet).

While it would still cool the room it would be much less efficient. This normally wouldn't be a big deal but if you're trying to run sensitive electronics off of the same power supply then you could have problems with them running too hot or getting some sort of interference on low frequencies that might cause them to reset occasionally.

Really there's no safe way I know of to put AC power and DC power in one wire without having them somehow electrically isolated so they won't mix up. If you do choose to try this then just make sure your wires are rated for at least twice more than normal amperage because if you hook both up at once and get an overload (which isn't hard) then things will probably catch fire or start melting. Don't say I didn't warn you...

Can AC and DC voltage be combined onto a single wire?

Yes they can, but it's not recommended because of the low efficiency and high risk for fire mishaps. If you really want to do it then all you have to do is connect the two wires in parallel with each other (AC hot going one way and DC power going another) within a connector box so that they stay isolated from each other. It doesn't matter if AC wires are connected directly to each other at different points along their length as long as there aren't any junctions between these different sections that allow AC current into the DC power supply branch. 

What happens when you combine an oscillating voltage (AC) with a steady DC voltage?

The alternating current would cause the battery to charge and discharge 60 times per second regardless of what was needed. I wouldn't recommend using AC power for anything that needs DC power unless you take great care in keeping them isolated from each other.

This is not recommended as it may very well kill your equipment by overcharging it or sending it interference on low frequencies. If you must do it then be sure to only use wire rated for at least twice the amperage and make sure all junctions between wires are done properly so they stay isolated from each other. 

Can AC/DC current be combined ?

Yes AC power and DC power can be combined, but it is not really worth doing. You effectively halve the voltage on anything that is in series with both of them, and what you gain in power savings, you lose to the efficiency penalty. If it's really necessary to run AC then put a full-wave rectifier circuit on one/both of the supply wires so that they can't accidentally be fed from both sources.

Can AC devices run on DC power ?

Yes. In fact, lots of electronic devices (like desk lamps and fans) are designed with a "DC" input and can be plugged right into an adapter that changes the 120 VAC coming from the wall plug to 12-15 VDC for these small units. But most appliances require AC power to operate properly, so it's not recommended unless it's absolutely necessary (for example you want to power an air conditioner or refrigerator with batteries). 

If you allow current to flow through a transformer in both directions at one time, won't that jam up the circuitry?  

No, as long as there is some form of a control circuit that keeps the current flowing in one direction at all times. The energy lost needs to go somewhere so it is converted into heat within the core.

An object that produces electricity when placed in an electromagnetic field is called a: Generator or Electromagnet or Inductor.

A device that changes alternating current to direct current is called a rectifier. It uses four diodes (1N4007) and has an adjustment pot to fine tune the output voltage of 12 VDC with about 90% efficiency.

A set of two filters consisting of 1000 uF capacitors followed by 0.33 uF capacitors are used to smooth out the ripple from this crude rectifier circuit but I found it still generated quite a bit of hum even with them on there...If you're wondering what kind of crappy rectifier I used to make this power supply then its just a simple diode bridge made by wiring four 1N4007s together on the same heatsink.

How does an ac/dc adapter work?  

An AC-DC adapter is just a transformer that changes 120 VAC from the wall into about 12 VDC (more or less).

The higher voltage electricity will flow through the primary coil of wire inside and generate a magnetic field which then induces a current in the secondary coil composed of copper windings. This secondary output can be used for low voltage DC power supplies in devices like laptop computers and cell phone chargers that require more than 6 VDC but I wouldn't recommend using it for anything else due to poor efficiency and quality.

If you were to measure the output voltage of a wall-wart you'd find its at about half the value advertised (which is why they burst into flames when you accidentally short them out).

How does a car cigarette lighter plug work?

It's just an AC-DC adapter with some extra circuitry inside to convert it from 120 VAC to 12 VDC.

You'll notice that your mobile devices run much slower on battery power than when plugged into a wall socket. The converter has to boost the 60 Hz AC from the alternator up above 15 kHz so that it can drive the power semiconductor rectifier before converting it back down to DC power again.

If enough current is drawn then this top diode will get hot and either burn out or stop working.

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