Questions tagged [power]

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

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348 votes
8 answers
344k 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 ...
  • 5,824
80 votes
8 answers
74k 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 ...
  • 4,997
83 votes
5 answers
284k 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 ...
  • 1,849
7 votes
2 answers
14k views

Switch between battery and USB power

Following on an earlier question I am looking for a circuit where it powers from USB when it is available and uses the battery when USB is not plugged in. I want voltage drop to be minimal, zero if ...
  • 2,792
13 votes
14 answers
26k views

Why are the power transmission/distribution systems AC and not DC?

Is there a good reason why we are not in the process of completely converting our electrical transmission system to DC? The main reason for using AC on the grid (no offense Tesla, I love you man) was ...
17 votes
2 answers
15k views

Calculate battery life

I would like to know how to calculate battery life when using an LED strip. The LED strip is powered by 8 AA batteries in series. LED Current Draw = 260 mA. LED power consumption: 3.1 W. LED Operating ...
  • 183
7 votes
1 answer
9k views

How do I tell what gauge wire I need?

I've been working on providing power for high current applications for my project. I'm working on a project with high current (or at least compared to what I'm used to...) What a lot of people point ...
11 votes
3 answers
64k views

Whats this white stuff on my board?

I'm not an engineer, but my monitor stopped working yesterday (it just wouldn't display anything but still gets power) so I figured I'd like to take it apart and see what went wrong. At the very ...
  • 213
34 votes
2 answers
13k 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 = ...
  • 3,212
16 votes
5 answers
14k views

If a PIC MCU provides multiple Vdd/Vss should you provide power to them all?

Take a look at this example schematic: The chip in question is the PIC18F4550 and as you can see power is provided on both sides (with a 100nF cap to smooth noise I guess). Is this strictly required ...
15 votes
4 answers
5k views

Protection against automotive power supply hazards

I'm looking for a way to protect a small circuit which is to be used inside of a car or truck (12V or 24V power system). The circuit consumes about 12-15W. I use an isolated DC/DC converter module ...
11 votes
1 answer
3k views

Separating USB/MCU power buses

There are two power supply sources in the following diagram - USB and 12V. Only one power supply can be connected at the same time. I am trying to disable power to ...
  • 1,539
64 votes
4 answers
70k views

Can I replace all electrolytic capacitors with ceramic?

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 ...
  • 2,577
39 votes
2 answers
66k 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 ...
12 votes
3 answers
9k views

Why too many capacitors in parallel for Vdd supply net? Can't we just add all to replace with one big capacitor?

Here is a schematic of a Basys-2 board's power regulator IC and filters. It is just an example but this is fairly similar to many designs I've seen. Why are there so many capacitors added in parallel ...
  • 1,570
1 vote
2 answers
22k views

Is 20 watts of electricity dangerous?

I have a few circumstances which invlove someone being shocked with 20 watts of electricity and whether it would be deadly. So here they are: Would 1000 volts at 20 miliamperes of AC (2MHz frequency) ...
3 votes
3 answers
5k views

what the best way to calculate Rg gate driver for Mosfet

I need to drive a MOSFET IPW90R120C3 from Infineon here the Specification of MOSFET VDS @ TJ=25°C 900 V Rdson @ TJ=25°C : 0.12ohm Qg = 270nC Specification of ...
6 votes
2 answers
19k views

How is the power factor part involved in wattmeter reading?

In a wattmeter, the current coil helps in measuring current and the potential coil is used for measuring voltage... but the real power that the wattmeter measures is \$V \times I \times \cos(\phi)\$. ...
58 votes
1 answer
19k 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?
  • 31.2k
16 votes
4 answers
30k views

Deciphering a DC jack schematic

I am trying to include a DC jack on one of my PCBs. I am just setting up the footprints now, and I am not quite sure which pin is which. What do the symbols on the drawing mean? I have included a ...
  • 4,997
26 votes
3 answers
36k 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 ...
15 votes
5 answers
5k views

detecting zero cross in ac?

I need to detect zero crossing for a soft starter. A long while ago, I have done it using a 1 Mega ohm resistor directly connected to micro controller at one side and live power at another side. I was ...
  • 507
1 vote
1 answer
4k views

P channel mosfet battery charging/power selection circuit help

I am a hobbyist and new to this, but I am not sure if my circuit it right. The goal is: To power the rest of the circuit from a DC source, a USB, or a battery, in that order. When there is no DC or ...
  • 13
30 votes
7 answers
31k 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 ...
  • 1,381
11 votes
2 answers
17k views

Creating a high-current bus on a PCB

I'm planning a custom power-distribution / fuse-board for in-car application, but that requires carrying high currents (50A+ @12v*) on a PCB trace. Since I'd like to keep the board cost, and physical ...
  • 6,639
3 votes
2 answers
2k views

Automatic switching between external 5V and USB bus power supplies

I'm designing a USB device which can either be powered by USB or an external 5V power supply. When an external 5V power supply is present, the device should be powered solely by this external supply. ...
  • 181
3 votes
3 answers
2k views

Car bluetooth power delay- USB capacitor? [closed]

I just bought a bluetooth adapter for my car. It's input is 5V/0.3A and is powered via a 5V/1A USB power adaptor that is plugged into the 12V/10A/120W max. socket in the center console of my 2004 ...
1 vote
1 answer
3k views

Single phase energy meter measurement

In a single phase meter measurement, the power is being measured by using the instantaneous power which is p(t) = v(t) * i(t). But as we need to sample the voltage and current to get the power, hence ...
  • 13
4 votes
3 answers
2k views

Why 220V and 110V?

Why did the industry choose these seemingly unintuitive values of 110V and 220V for the mains AC? Is there a physical reason why they aren't simply 100V and 200V?
  • 381
0 votes
2 answers
544 views

Ordinary Differential Relay Problem

The answer given in my workbook to the above problem is A. The things I was able to point out secondary c-phase current = 300A and other secondary phase currents = 0A since the entire current ...
58 votes
8 answers
108k 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 ...
29 votes
7 answers
45k 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 ...
  • 13.6k
12 votes
6 answers
25k views

Automatically switching from 9V battery to DC wall adapter on insertion

I have a simple circuit that runs off of a 9V battery. I'm re-designing it so that it can also run off of an external 12V DC source (ie: a wall adapter). I want to design the circuit so that if both ...
  • 820
11 votes
7 answers
40k views

Can you harvest electrical energy from the air?

I found a website recently about harvesting energy from the air, and I am wondering if someone could tell me why the following wouldn't work? Their "generator" is supposed to make electricity from ...
41 votes
3 answers
39k 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 ...
  • 12.2k
10 votes
1 answer
205k views

How to calculate voltage drop over and power loss in wires

How do I calculate the voltage drop over wires given a supply voltage and a current? How do I anticipate on voltage drop so that the final load has the correct supply voltage? What will be the power ...
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26 votes
3 answers
13k 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 ...
  • 367
12 votes
7 answers
2k views

Power grids : AC vs DC

We know that we have now 50/60Hz in our walls due to mainly historical reasons - back 100 years ago there were no ways to up/down scale DC voltage. These days we just have problems due to that - ...
  • 3,231
9 votes
4 answers
6k views

Can you split power between two voltage regulators?

In order to supply more power to a circuit. can you split the power between 2 voltage regulators, in parallel? Can this overpower one of the Voltage Regulators?
28 votes
5 answers
10k views

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 ...
6 votes
4 answers
11k views

120VAC 60Hz Zero Cross Detector

I'm designing a digital phase-controlled light dimmer, and I need a zero-crossing detector circuit. From what I've read, it's a bad idea to wire the AC main directly to the microcontroller through a ...
3 votes
1 answer
3k views

Arduino Battery Over-Discharge Protection (O-DP)

I am getting an Arduino Uno soon, and am planning to run off a 8-cell NiMH battery pack (through the power jack). Assuming a .9V cut-off per cell, that'll mean a 7.2 cut-off voltage. How would I wire ...
15 votes
2 answers
2k views

Getting rid of AC/DC adapters

Many of us have lots of devices that uses AC mains (110V/220V) to DC (typically 12VDC, 9VDC and sometimes 15VDC) adapters. There's my Wi-Fi router, my powered speakers, printer, scanner, etc. So I ...
  • 767
6 votes
2 answers
936 views

Reducing volume for a split-phase 230VAC@60Hz sealed power supply providing 5VDC@50mA

I'm a hobbyist interested in re-designing and implementing a DC power supply providing a \$+5V_{DC}\pm 200mV\$ output with a current compliance of \$50mA\$ (I could live with \$30mA\$ successfully) ...
  • 72.8k
5 votes
4 answers
2k views

Is it ok to feed the output of one LDO regulator to the input of another?

Sorry if this is a stupid question, but I am mainly working with 5V PIC circuits and now want to integrate a component (nRF24l01+) that needs a 3.3V supply. As I have a box full of ~7V wall-warts, I ...
4 votes
3 answers
6k views

Choosing the right power regulator for Battery powered designs

I've come across this problem a number of times in my hobby projects, but I am sick of making an educated guess. I often do microcontroller projects and often want them to be powered by a 3.7V Lipo ...
  • 173
1 vote
1 answer
3k views

battery backup supply switch over design

I have an application with two AA/AAA batteries (so total of 3V or less) powering a unit with a micro controller. I want to design a low cost switch over circuit that can disconnect the battery supply ...
10 votes
7 answers
3k views

Power for POV display

I want to build a small POV display, but how is the power (and if possible a communication line) transmitted to the spinning part? I've looked at some POV project, but they usually don't explain that ...
2 votes
3 answers
910 views

Transformer for Flyback power supply

I am designing an isolated DC-DC converter with an input of 12V and an output of 5V@12A, at the moment, a flyback seems like the best option. I have read several books regarding the subject and I am ...
5 votes
4 answers
12k views

External power to STM32F4 Discovery board

I've recently finished programming the STM32F429ZI DISC1 board. I'm fairly new to this and now want to move away from connecting it to the PC via USB and power it up via a power supply. At present I ...
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