# Questions tagged [power]

Where power is a primary concern for the design under discussion. Use the "low-power" tag when that applies.

4,211 questions
250k views

### What is the difference between $V_{CC}$, $V_{DD}$, $V_{EE}$, $V_{SS}$

I've seen lots of schematics use $V_{CC}$ and $V_{DD}$ interchangeably. I know $V_{CC}$ and $V_{DD}$ are for positive voltage, and $V_{SS}$ and $V_{EE}$ are for ground, but what is the ...
48k views

### Decoupling capacitors: what size and how many?

Lots of chips nowadays require smoothing capacitors between VCC and GND for proper function. Given that my projects run at all sorts of different voltage and current levels, I was wondering if anyone ...
7k views

### What exactly fries the chip when you invert power supply?

From my own experience, burning microcontrollers is quite easy. Put the 5V at ground, GND at VCC and in an instant your chip is burned. What exactly goes on internally that causes it to stop ...
17k views

### What is the practical difference between watts and VA (volt-amps)?

I have seen one inverter rated at 300W, and another at 300VA. What is the practical difference between them, if any?
58k views

### Can I replace all electrolytic capacitors with ceramic ones?

I am designing the power supply circuit for a system that needs several supplies, my questions are: Is it possible to replace all electrolytic caps (Mostly 100uF) by ceramic ones? What are the ...
159k views

### How much current can I draw from the Arduino's pins?

What's the maximum amount of current which I can draw from each of the Arduino's pins without tripping any of the internal fuses? Is there a limit per pin as well as an overall limit for the whole ...
96k views

### Can an Android tablet serve as USB Host and be charged simultaneously through a single port?

A number of Android tablets are powered through the Micro B USB port but also provide USB Host support through the same port (e.g. Motorola Xoom 2, Acer A510/700). I need to be able to use USB Host ...
6k views

### What's the reason to make power supplies external?

Many years ago most electronic devices had internal power supplies only - there was a mains voltage cable running into the unit where mains AC would be converted and distributed for consumption. That ...
5k views

### Why aren’t the main conductors in this underground power cable made from copper?

I’ve found a piece of electrical cable left by the electricity company that supplies the area. They did a repair recently underground. I’m curious why the three big triangular conductors aren’t made ...
14k views

### Why do CPUs need so much current?

I know that a simple CPU (like Intel or AMD) can consume 45-140 W and that many CPUs operate at 1.2 V, 1.25 V, etc. So, assuming a CPU operating at 1.25 V and having TDP of 80 W... it uses 64 Amps (a ...
28k views

### How is wristwatch with 10 years battery life possible?

Turns out Casio offers a handful of wristwatches with "10 year battery life". The claim is that thanks to "an advanced technology" the battery life in those watches is extended to ten years. Now if ...
5k views

### Why does power dip during a lightning storm?

If lightning were to strike power lines, wouldn't you expect it to increase the power, and hence your room lights get brighter? Why do they go dim for 1 second at a time?
7k views

### Why don't we use low voltage Power Sources for high wattage applications?

Super nooby question involving Ohm's law, but this has been on mind this morning. Say I have a 60W device, and I want to power it. Usually this calls for a 120V source or something. However, why not ...
12k views

### How to calculate battery life

How do I calculate how long a battery operated product will run? Here's what I've got: 2 AA, 1.5V, 2700mAH batteries Voltage Regulator with a Iq of 25 uA Voltage Regulator Eff = 80% Active Current = ...
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### How much power is really wasted by a wall wart?

The title pretty much sums it up... Hyperbole aside, how much actual power is consumed by a typical wall wart when the device it's attached to isn't powered on? And are there designs for wall warts ...
4k views

### Can I pedal power a laptop device while the device is being used?

I want to research how easy it would be to use a small laptop device while powering it by pedaling on a stationary frame. (For example, using a tablet on an exercise bike, or using a netbook while ...
3k views

### Spacecraft Power Supply Frequency

I know that aircraft sometimes employ a 400Hz AC system to save weight on transformers, so I'd imagine that spacecraft might employ a similar technique. I'm wondering if anyone in the aerospace field ...
8k views

### Why is Ethernet So Power Hungry?

I was just perusing the Microchip ENC28J60 Ethernet Controller datasheet and saw that the operating current can be as high as 180mA while transmitting and 120 mA typical while active but not ...
35k views

### How to carry high current on PCB

I need to pass high current on some part of my circuit. I used an online PCB track width calculator to see that required track width is about 5mm and minimum clearance is 1mm, which makes it about 7mm ...
3k views

### Why do high-voltage transformers need oil?

Someone told me that high-voltage transformers need oil because it prevents arcing. However, in terms of dielectric breakdown, isn't air the strongest? I remember from classes in college that as the ...
63k views

### Arduino Uno R3: Directly supply regulated 5V to 5V pin?

I'd like to use an Arduino Uno R3 battery powered a datalogger. I want do supply it directly with a Step-Up regulated 5V rechargable battery powersource at power ...
9k views

### Why do many electronics operate on 5 AND 3.3 Volts?

Many modern appliances use a 5V Power connection. Internally they are working with 3.3V. Wouldn't it be easier to have 5V everywhere? Examples are many USB devices or Routers (they use 5V for power ...
5k views

### Short circuit = no power?

Now I want to build stuff and I'm really interested in learning things (consider I'm starting from scratch). So I'm reading all of this website and the following line in this article got me scratching ...
8k views

### Why do high voltage power lines hiss when it's raining?

I noticed when walking home the other day that the high voltage (200kv I believe) lines running through here were hissing in the rain. What is causing them to hiss?
20k views

### How do I calculate the power rating for zero-ohm resistors?

Based on my earlier question, since there supposed to be zero voltage (V=IR) drop across a 0 Ω resistor, how do we select the power rating of such a component? For example, let's say I were to ...
32k views

### Why are deep cycle batteries rated in amp hours instead of watts hours?

Why are deep cycle batteries rated in amp-hours instead of watt-hours? For example if I see a 85Ah battery, I multiply 85Ah x 12V to get 1020 watts. As household electricity use is measured in watt ...
35k views

### How does a USB C port provide the power to charge laptops?

I've heard about laptops such as the new Chromebooks that are charged via a wall charger that connects to a USB-C port. I'm quite happy that this will supposedly standardize laptop chargers but I'm a ...
19k views

### Should I isolate grounds of an isolated DC/DC converter?

If I use an isolated DC/DC converter, when designing the PCB, should I isolate the ground of the input and the ground of the output as shown below? I've never isolated grounds (except for AGND and ...
135k views

### How to compare mAh and Wh

When looking at consumer electronic devices with batteries I sometimes see the battery capacity listed in Wh (watt hours) and sometimes in mAh (milliamp hours). I would like to be able to compare the ...
4k views

### What can I put a kilowatt into? (Or: help me save my hairdryers)

TL;DR: I need something to discharge roughly 160A at 14V into, or 2.24 kilowatts. Any comments, or answers, with either a) something I can dump a kilowatt into, b) some way I can modify a common item ...
4k views

### Can roles of primary and secondary windings be reversed?

Is there a practical reason that the roles of the primary and secondary of a simple single-phase transformer cannot be swapped? Say you have a transformer (from the old vacuum tube days) that has ...
3k views

### Why is only the fundamental frequency component said to give useful power?

In most books it's said that the harmonic content in an AC line does not transfer power (just the fundamental frequency does), but no explanation. It seems intuitive, but why is that true? Edit: In ...
3k views

### How to combat noise from my circuit polluting my 12V rail?

I made a controller for 12V DC fan. It is basically a buck DC-DC converter controlled by voltage. It regulates voltage for fan from 3V (lowest speed, fan draws 60mA @ 3V) to 12V (full speed, fan draws ...
5k views

### How much of a GPU's electrical usage is probably turned into a increase in air temperature?

I am trying to find a good rough guesstimate of how much of the maximum wattage usage of a graphics card is turned into hot air. Say a graphics card is rated at using X watts at maximum usage. How ...
5k views

### Can you stack SMD resistors in parallel to reduce power dissipation per resistor?

I sent my PCB a couple of days ago for fabrication but just realized a terrible error: I need to send 70mA to an IR LED with a 5V supply so I would need a resistor of about 70ohm, which means the ...
2k views

### Why is it so problematic to have close to zero standby power consumption?

Each electronic device consumes electric power when it is "idle" unless it has a mechanical switch. I can understand that for example a TV with a remote control needs to "be ready" to receive a ...
17k views

### Why do power companies never bother residential customers about power factors?

I recently bought a Kill-a-Watt meter to measure power consumption of my devices. This was just a fun project for me. I put this on my Mac-Mini to see power consumption during various usage states - ...
687 views

### Does this device which claims to reduce reactive energy from a house work?

I've opened a device that states that it can reduce the reactive energy from a house, by plugin it directly to the power outlet/wall socket. I think it is a con, so I've opened it and I saw this black ...
5k views

### How can low voltage, high current (kA) power be dangerous?

Aluminum refineries use electricity to separate aluminum from minerals it naturally occurs in. This electricity typically takes the form of low voltage DC ("low" meaning 4 to 6 volts), at very high ...
51k views

### What happens to excess energy fed into the power grid?

The closest question to this is Linear useage of excess power generation. I'm no engineer so I may not be able to phrase this correctly and would appreciate an answer which assumes minimal ...
9k views

### Can you store energy in an inductor and use it later?

My company uses supercaps to power the device if power is cut. I was wondering if you could do the same thing with an inductor. If you can't, why not?
9k views

### Is there a constant power supply?

A voltage source is a device that produces constant voltage, but allows amperage and power to vary, in accordance with the laws of electricity. A current source is a device that produces constant ...
6k views

### How do I protect SD card against unexpected power failures?

I am working on a device that uses the Microchip MDDFS library to store data to an SD card. The logger will log data at a maximum rate of 1 entry (56bytes) every minute. The problem is the device may ...
37k views

### Why is it bad to mix new and old batteries?

I've seen warnings that it's bad to mix new and old batteries -- why? Is it a matter of a battery's age or a battery's voltage/current (remaining)?
6k views

### Smooth switching between 12 V batteries, with a toggle switch

I have a number of instruments (12 volts, drawing a few hundred milliamperes) that occasionally need to be switched between one of two 12 volt lead acid batteries. The toggle switch is break-before-...