# DC Questions and my understanding

Okay, we had a seriously bad science teacher this year, and long story short, we hardly understood anything relating to DC current(she was fired a few days ago).

I understand Ohm's Law:

Working out current from voltage/resistance

Working out resistance from voltage/current

Working out voltage from current*resistance

I understand what current is(I=q/t, flow of electrons in a circuit)

I understand what voltage is(potential difference)

I understand what resistance is(how much a resistor decreases the current)

I understand what wattage is (voltage*current)

I don't fully understand batteries:

Battery x is rated at 5volts, 2500mAH.

Device y draws 5 volts, 5mA. This means that the battery would last 500 hours correct?

Now, device z draws 5 volts, 3000mA. This means the battery would last under an hour correct?

Now circuit d draws 5 volts, 2500mA. The circuit would last 1 hour.

Now take a AC -> DC adapter, it is rated at 12 volts, 1A.

What does that amp in this circumstance refer to?

Is it to work out the maximum resistance that a device plugged in could have?

Would this adapter be working as the battery in a circuit?

Is it refering to the amperage of the full circuit when a device is plugged in?

So, batteries don't output amperage. It changes depending on the voltage/resistance of the device.

Now wattage. Why are some devices rated in watts?

For example my PSU is 600watts.

So it outputs 600watts total over a bunch of 12v, 5 volt and 3 volt cables? It's input is 220V, so the amperage it draws is: 2.7Amps?

Or is that the amperage it outputs?

Thank you if anyone answers these question, I taught myself all of this from countless days of research and questions.

• Good on you! +1 for persistence and the balls to ask about what you don't know. :) – EM Fields Dec 6 '14 at 21:14
• Mandatory EEVBlog youtube link: battery capacity tutorial – Lorenzo Donati Dec 6 '14 at 22:29

Battery x is rated at 5volts, 2500mAH.

Device y draws 5 volts, 5mA. This means that the battery would last 500 hours correct?

Correct.

Now, device z draws 5 volts, 3000mA. This means the battery would last under an hour correct?

Correct.

Now circuit d draws 5 volts, 2500mA. The circuit would last 1 hour.

Correct

Now take a AC -> DC adapter, it is rated at 12 volts, 1A.

What does that amp in this circumstance refer to?

That's the maximum the supply is capable of providing before it overheats and melts.

Is it to work out the maximum resistance that a device plugged in could have?

No, it's purely a maximum rating.

Would this adapter be working as the battery in a circuit?

In effect, yes. It can be seen as a fixed voltage source.

So, batteries don't output amperage. It changes depending on the voltage/resistance of the device.

That is correct. A battery is a (reasonably) fixed voltage, but the current is dependant on the load (Ohm's Law). The mAh of a battery is how much charge is stored in it. A power supply doesn't have this concept as the charge is essentially infinite. A battery, at 2500mAh, has enough charge to supply 2500mA for 1 hour, or for other currents, an approximation:

$$t = \frac{mAh}{mA}$$

As @PeterBennet mentions, it's rated at a pre-defined discharge time. Discharging faster than that rated time can result in less apparent capacity.

Now wattage. Why are some devices rated in watts?

For example my PSU is 600watts.

So it outputs 600watts total over a bunch of 12v, 5 volt and 3 volt cables? It's input is 220V, so the amperage it draws is: 2.7Amps?

Or is that the amperage it outputs?

Again, it's the upper limit. It can output 600 watts in total over the different outputs before it overheats. For a supply with multiple voltages it's often easier to specify the total limit using a voltage-agnostic value, like Watts. If you just specify current, then you have to specify it for each voltage, and if some current may be shared between voltages (e.g., cascading 3.3V off 5V) then it gets more confusing. Simpler just to give a total power rating.

• Ampere-hour ratings are based on discharging the battery over some time. If you discharge the battery faster than the rated time, you will get fewer total Amp-hours than the spec shows. – Peter Bennett Dec 6 '14 at 21:25

Now wattage. Why are some devices rated in watts? For example my PSU is 600watts. So it outputs 600watts total over a bunch of 12v, 5 volt and 3 volt cables? It's input is 220V, so the amperage it draws is: 2.7Amps? Or is that the amperage it outputs?

There are cases (and this is one of them) where wattage is more of a marketing figure than a technical spec. (Another example is the power output of audio amplifiers.) Tom's Hardware has a good article on PC power supply specs. The short version is that the 600W figure is a "typical" usage number, which means it's based on some manufacturer-defined conditions. This figure is less than the sum of (voltage * max current) for all of the outputs, since that is not a reasonable situation. The maximum currents themselves are more important. (They should be listed on a sticker on the side of the supply.) You also have to look at things like whether there's one big 12V supply (better) or multiple smaller 12V supplies (worse).

Another important spec is efficiency. You need to know that to calculate the input power. Efficiency is usually given for a "typical" load of 50% of max power. (That's the best case, usually.) The 80-Plus standards also specify efficiency at lower and higher loads.

The upshot of all this is that two 600W power supplies are not necessarily interchangeable. This sort of thing is pretty common when looking at electrical specs. You have to understand the operating conditions in order to understand the spec. Fortunately, you don't need to be a master of specmanship to understand circuit theory or E&M physics. :-)

Note: AC voltage and current numbers are actually root-mean-square average values. You can often pretend that these are DC for power calculations. But in general, for AC power you have to consider both the magnitude and the phase of the voltages and currents.

I'll attack the AC adaptor/power supply questions.

An AC->DC adaptor rated at 12 volts, 1 amp will deliver 12 volts and up to 1 amp - the actual current delivered by the supply will be determined by the load - if you have a 12 volt light bulb that draws 100 mA, then the supply will only deliver 100 mA to that bulb.

A DC power supply acts something like a battery, and can replace a battery of the same voltage, as long as the supply's current rating is great enough to supply the current demanded by the load.

The current your 600 watt computer power supply draws from 220 V can be up to 2.7 Amps. The actual power consumed will be the total of the powers drawn from each DC output, plus a little more to account for losses in the power supply itself.

• And as long as the device doesn't depend on other properties of the battery for its operation, e.g. its internal resistance. – Ignacio Vazquez-Abrams Dec 6 '14 at 21:00
• @IgnacioVazquez-Abrams You mean like running an LED direct from a coin cell with no current limiting resistor? ;) – Majenko Dec 6 '14 at 21:22