Questions tagged [power-supply]

An electronic device which supplies electrical energy to a load. Can be AC or DC input. Typically DC output.

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311 votes
6 answers
401k views

Choosing power supply, how to get the voltage and current ratings?

Power supplies are available in a wide range of voltage and current ratings. If I have a device that has specific voltage and current ratings, how do those relate to the power ratings I need to ...
109 votes
6 answers
71k views

My linear voltage regulator is overheating very fast

I'm using a 5 V / 2 A voltage regulator (L78S05) without a heatsink. I'm testing the circuit with a microcontroller (PIC18FXXXX), a few LEDs and a 1 mA piezzo buzzer. The input voltage is aprox. 24 ...
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89 votes
12 answers
430k views

How can I tell the size of a barrel power connector?

Spring cleaning, and I'm trying to get power supplies for all my devices with missing power supplies. They're all the typical barrel power connector, and I'm having a dickens of a time trying to ...
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69 votes
6 answers
117k views

Why do many laptops run on 19 volts?

Typically mobile devices that have a mains-powered supply will accept voltage that is multiple of some single battery voltage. For example, 4.5 volts is 1.5 volts (AA primary battery) 3 times and 36 ...
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67 votes
3 answers
93k views

What is the ideal way to handle data pins D+ and D- on a USB power adapter to be compatible with fast charging on devices?

I have found that many USB wall chargers use a resistive voltage divider to set the D+ and D- pins to a specific voltage, usually between 2 and 3 volts. Other USB wall chargers short the D+ and D- ...
66 votes
7 answers
8k views

Why is my circuit so incredibly sensitive to electric fluctuation?

I recently finished building a circuit showcased in a beginner’s electronics book. I have included the picture of my creation below because I think it may become relevant to the question. At the ...
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66 votes
4 answers
102k views

What's the purpose of two capacitors in parallel?

What's the purpose of the two capacitors in parallel on each side of the regulator in this power supply circuit I've seen similar setups in other similar circuits and can guess that it's related to ...
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 ...
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63 votes
8 answers
39k views

Why do we need an isolation transformer to connect an oscilloscope?

My professor always insists that I provide power to an oscilloscope thorough an isolation transformer. What is the necessity of this? What is the risk if I don't connect it?
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57 votes
1 answer
7k views

What does this symbol mean on my Samsung travel adapter?

The charger was bought in Austria two years ago, and I know about all symbols/labels except this one. Google reverse image search did not help.
54 votes
13 answers
13k views

What kind of components in a computer power supply can explode loudly?

Today I heard a loud bang that tripped the circuit breaker in my server room. It must have been really loud because I could hear it 2 rooms away through 2 heavy doors and it was like a firecracker ...
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51 votes
3 answers
9k 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 ...
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49 votes
4 answers
9k views

Why do fire and security systems run at 27.6 V?

I have been looking at buying power supply units for a project, and I keep seeing supplies that output 27.6 V. It's an unusually specific number, so what they are used for, and why is it important ...
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48 votes
3 answers
45k views

When a battery is your power source, what is ground?

For the sake of not incorrectly connecting my power supply and damaging my board I'm going to ask a relatively dumb question. Is ground on my board the negative terminal on my battery? Explicitly ...
47 votes
4 answers
44k views

Why don't we use wireless electricity today?

I was reading about Nikola Tesla today (via the Oatmeal) and read about the Wardenclyffe Tower which (among other things) was intended to transmit electricity wirelessly. Forgive the naivety of the ...
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45 votes
11 answers
164k views

What would happen if I connect two different DC voltage sources in parallel?

I have a computer power supply that I'm hacking together as a bench supply. For this particular model to power on, I need a minimum load across both +5V and +12V. "Easy," I thought, "I'll just ...
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42 votes
6 answers
45k views

Can an audio circuit be powered by a switched-mode power supply?

Most audio circuits are powered with large, heavy transformers and a small ripple after smoothing. SMPS are smaller and more efficient. EMI can be shielded by a metal enclosure and the output filtered ...
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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 ...
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40 votes
1 answer
2k views

What is / how can I get a student-safe power supply for electronics projects?

What is / how can I get a student-safe power supply for electronics projects? I'm looking for a student-safe power supply for solder less breadboard. I'm developing a course to teach electrical ...
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39 votes
3 answers
8k views

Computer power supplies usually have higher efficiency on 230V than on 115V. Why?

As seen here in the example, this particular power supply (and most others too) has higher efficiency when running on 230V. Given that computer power supplies are usually required to output a ...
  • 529
38 votes
1 answer
31k views

What to do with third contact in DC barrel plug with only two input contacts

My AC-DC adapter gives output of 12V and GND. I tried to search for suitable DC barrel plugs and found that most of them have three terminals So, pin 1 is positive in my case. 1. Which pin should I ...
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38 votes
6 answers
17k views

Switch between 5V power supplies?

I'm designing a board that can be powered either by USB or an ATX power supply. They can both be plugged in at the same time, independently. I'd like the ATX to provide 5V if its plugged in, but ...
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35 votes
12 answers
147k views

Is it really OK to supply more current than what the component is rated for?

In this heavily upvoted answer the answerer states that it is okay to supply a component with more current than what it's rated for. The analogy is that (paraphrasing here) "If Johnny wants to eat two ...
35 votes
7 answers
10k 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 ...
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33 votes
3 answers
10k views

Why would a Intel 8080 chip be destroyed if +12 V is connected before −5 V?

The Intel 8080 is a classic microprocessor released in 1974, fabricated using an enhancement-mode NMOS process, and shows various unique characteristics related to this process, such as the ...
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33 votes
3 answers
16k views

How can I detect a power outage with a microcontroller?

I have the following power supply configuration: AC MAINS -> UPS -> 24V POWER SUPPLY -> 5V VOLTAGE REGULATOR -> PCB (microcontroller). What's the best solution to detect the power outage on the mains ...
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32 votes
6 answers
14k views

Voltage regulator vs. voltage reference

I need a cheap, somewhat accurate (~0.5%) voltage reference for some DACs. At first I was going to use a LDO voltage regulator (a TC1223 specifically) for this, it seems to fit the bill looking at the ...
  • 580
32 votes
7 answers
82k views

When people talk about a device "drawing" current, what do they mean? Why do devices under load "draw" more current?

In my (extremely rudimentary) understanding, the amount of current flowing in a circuit is determined by a) its resistance, and b) the voltage of the power source (voltage from beginning to end), ...
31 votes
7 answers
83k 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 ...
30 votes
7 answers
9k views

Why is root mean square used when calculating average power, and not simply the average of voltage/current?

$$P = I_{\text{eff}}^2 \times R$$ where \$I_{\text{eff}}\$ is the effective current. For power to be average \$I\$ must be average current, so I am surmising that the effective current is the average ...
30 votes
4 answers
4k views

Why put a 10k resistor between VUSB and GND?

I recently came across Yet Another RP2040 Trinket (the Beetle RP2040.) In their linked schematic something caught my attention: It is R10 below - a 10k resistor ...
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29 votes
6 answers
25k views

How to convert AC to DC

I am designing a circuit that needs to output 5VDC @ 1A. I'm trying to use a wall transformer to step the voltage down to 12VAC. The next step is the diode bridge and ripple capacitor. The ripple ...
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29 votes
4 answers
32k views

What is the usage of the negative voltages on a PC motherboard?

What is the usage of theses negative voltages? Are they there only for backward compatibility? In nowadays PC power supplies, we have: +12V +5V +3.3V but also: -12V -5V But the current rating of ...
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28 votes
7 answers
6k views

Why is this power adapter transformerless?

I saw a battery charger that converts 220V AC to 6V DC without a transformer. Now I am wondering why many (if not all) power adapters are using a transformer, is it about efficiency or about ...
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28 votes
7 answers
37k views

How to find the ESR of a capacitor

I am building a power supply circuit, and the switching regulator (L4963) calls for a low-ESR output capacitor. The capacitor in question is C3 of the evaluation board circuit. What does "low" mean? ...
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27 votes
4 answers
63k 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)?
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27 votes
4 answers
33k views

Considerations in a bench/lab power supply?

I'm looking at adding a low-cost variable power supply to my electronics hobby bench. After years of cobbling together LM317's and LM7805's, it's about time... Considerations: I've found a few ...
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26 votes
4 answers
9k views

How does Windows know it's on battery power?

Laptop DC-DC regulator boards are somehow able to communicate with Windows to see how much battery power is left, but my research finds nothing. Is there a data connection from the proprietary ...
  • 397
26 votes
7 answers
35k views

How efficient is a capacitive power supply?

Something like this Falstad sim version of it (I'm tired, I keep making mistakes, so please excuse me for the second time.) Now these are not very safe PSU's, due to the lack of isolation. But in ...
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26 votes
5 answers
2k views

Help with homemade electromagnet

Just for fun, I decided to build what I was hoping would be a relatively powerful electromagnet. I used this copper wire (20 AWG, entire 1 pound spool) and this soft iron core (0.5 in diameter, 6 in ...
26 votes
3 answers
195k views

Powering Arduino Nano 12volts

I know that the Arduino Nano can handle 12 volts, and it says it is recommended to power it between 7 and 12 volts. So I am wondering do I just hack together a 12 volt adapter to a Mini-B or is it ...
  • 403
26 votes
3 answers
29k views

How do the tiny AC->USB power supplies work?

The iPhone and Kindle both come with tiny power supplies that take in 120v AC and output around 5V at 0.85 to 1 amp. They seem to be too small and light to have a transformer, and generate very little ...
25 votes
6 answers
30k views

Can the noise of any switching PSU be attenuated if I put linear regulator before the output?

A friend told me that noise of any switching PSU can be attenuated if I put linear regulator before output. Is that true? For example, if I want to power a +-12 V op-amp for an amplifier, I can ...
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25 votes
5 answers
4k views

Are all linear regulators bad at filtering input ripple? (or really as bad as Dave suggests)?

Background In EEVBlog #1116, Dave discusses a method to remove power supply ripple, and goes on to show (see 5:17 to 6:15) that you cannot count on linear regulators to remove your input ripple. He ...
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25 votes
6 answers
11k views

Normal capacitor vs. audio capacitor

I'm building a power supply for old Schoeps CMT30F microphones. Standard condenser mic are 48V-powered nowadays, but these are 60s/70s Schoeps mic from RadioFrance/ORTF, customized at that time to be -...
  • 1,700
25 votes
1 answer
2k views

What is this transformer-like component, and why is it not really connected?

Recently, a power surge took out a few components in my PC's ATX power supply. While repairing (I hope), one component in the power drew my attention. I'm an electronics engineer, but I'm confused ...
  • 1,035
24 votes
3 answers
27k 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 ...
  • 2,567
24 votes
3 answers
17k views

How to limit inrush current?

I'm designing a device powered from USB. The device uses FTDI FT2232 chip for USB connection. Upon a command from a computer FT2232 chip should enable power via a MOSFET switch to rest of the circuit. ...
  • 303
23 votes
6 answers
23k views

Benchtop power supplies: linear vs switching? [closed]

I'm looking into getting a decent benchtop power supply ($150-300) for hobby-level electronic design and testing. I don't want to modify or use a computer ATX supply because I want: Adjustable ...
  • 28.3k
23 votes
4 answers
21k views

How do I select the accompanying components for an optocoupler?

I'm using an optocoupler (MOC3021) to sense the On/Off state of an electrial appliance using a microcontroller ATmega16L. How do i go about doing this? My mains supply specs are 230V, 50Hz. How do I ...
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