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Questions tagged [dc]

DC stands for Direct Current, which means the flow of electric charge in a single direction. Examples of a DC source are batteries, solar panels, dynamos.

64
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9answers
231k views

How does a capacitor block DC?

I am confused with this! How does a capacitor block DC? I have seen many circuits using capacitors powered by a DC supply. So, if capacitor blocks DC, why should it be used in such circuits? Also, ...
43
votes
7answers
174k views

230V AC to 5V DC converter, lossless

Is there any IC which converts 230V AC to 5V DC? As lossless as possible. I want to connect my microcontroller to an ordinary electrical socket and I don't have enough space available. Thanks.
39
votes
3answers
51k views

Switching DC with MOSFET: p-Channel or n-Channel; Low Side Load or High Side Load?

I think, it is time I understand working principle of MOSFET transistors... Suppose that; I want to switch voltage on a resistive load by a MOSFET transistor. Any control signal between -500V and +...
38
votes
11answers
141k 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 ...
28
votes
8answers
8k views

Can I reduce voltage from 3.3V to 1.8V using a 1.5V battery?

I am interfacing to an EEPROM that works via SPI at 1.8V; unfortunately I do not have a 1.8V power source around and being this a weekend project, I wanted to complete it without going to the shop. I ...
27
votes
8answers
35k views

Why does a capacitor block DC and not AC? [duplicate]

If someone can explain why a capacitor blocks DC, but not AC, with some math, I will understand all of that much better. I know that there are picture animations illustrating this, but I really want ...
25
votes
10answers
11k views

How do I power/wire something that is rotating constantly WITHOUT extra batteries?

So I want to make a Lazy Susan that is rotating constantly in one direction. I also want to have some other things on top such as neon signs, moving things, and etc. Problem is I want this to all run ...
24
votes
1answer
20k 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 plugshttp://media.digikey.com/photos/CUI%20Photos/MFG_PJ-037B_tmb.jpg and found that most of them have three ...
23
votes
7answers
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 ...
23
votes
2answers
4k views

Why does inverter synchronization with the grid take 5 minutes?

We had some solar panels & an inverter installed recently. Every time the inverter is turned on (after maintenance or with the first light of the day) it has to synchronize its generation phase ...
22
votes
3answers
25k 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 ...
19
votes
6answers
17k views

Why are DC signals bad for loud speakers?

I googled and found out through some forum that: DC has a constant amplitude which overheats and destroys the voice coil of the speaker. Could someone clarify if this answer is complete and ...
18
votes
3answers
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 ...
18
votes
5answers
8k views

Circuit for measuring high voltage DC voltage (up to 1000V)

I am a final year E&E student and I am trying to built a power meter that must be able to measure quite high DC voltages, up to 1000 V DC. I am measuring with a simple 12-bit ADC that has an ...
17
votes
6answers
10k views

What are the highest DC and AC voltages or currents that can be considered as safe?

By safe I mean safe for us when we touch a wire that is not insulated. I have heard that human body resistance is normally 50k Ohm when dry and 20K Ohm when wet. Therefore, these voltages or currents ...
17
votes
6answers
56k views

Why is feedback required in op-amp circuits?

I understand that, in order for an op-amp to function correctly, a DC feedback loop from the output to either the inverting or the non-inverting input (depending on the external circuitry) is required....
17
votes
3answers
57k views

Wiring a DC socket - why three pins?

I have this DC socket: Which I intend to wire up to a circuit to provide power. I have a DC power supply which has a positive tip polarity. I'm good to go and wire up the socket but I am a little ...
17
votes
3answers
26k views

DC biasing audio signal

I have seen a few different ways of adding DC bias to an audio signal. I have simulated them and they all give me similar results, but I cant Figure out is why choose A over B or C. My audio source ...
17
votes
4answers
11k views

Cellphone charger has no transformer?

I opened up a really tiny mobile phone charger I have, to see how it is designed. The entire "charger" is integrated into a small 2-pin mains plug, 1 x 1.25 x 0.5 inch, that has an USB socket for the ...
17
votes
3answers
45k views

Why is PWM used to control DC motor speed instead of using a variable resistance?

My question is at the end (to change the speed) we are controlling the input voltage of a DC motor in both PWM (pulse width modulation) and variable resistance cases. Is the only reason for choosing ...
16
votes
12answers
31k views

Ways to determine polarity (dc) without a meter

I came across this question about how to figure out the polarity of a cell phone charger without a meter. This made me think of what methods you could use to figure out the polarity other than using a ...
15
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6answers
3k views

RF Energy Harvesting

Why can energy invested in RF radiation not be harvested fully? What are the main factors that affect this? Is there any significant research nowadays that fully describes this process? I stand on the ...
14
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5answers
2k views

Why don't more devices incorporate full-wave rectifiers as reverse polarity protection?

Recently, I was introduced to the idea of using a full-wave rectifier, in order to protect against reverse polarity damage in DC devices. I hadn't even considered using a rectifier in an already DC ...
14
votes
7answers
9k views

Why are rectifiers-inverters used to drive AC motors instead of using the rectified current to drive DC motors?

I understand the advantages of using a rectifier-inverter system to drive an AC motor instead of simply plugging it into mains power, as it allows much better control of its speed and performance; but ...
14
votes
4answers
8k views

What is the role of capacitors in DC circuits like motherboard, graphic card etc?

I was just reading in Capacitors in DC Circuits that "Capacitors do not play an important role in DC circuits because it is impossible for a steady current to flow across a capacitor". I think it ...
13
votes
14answers
24k 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 ...
13
votes
4answers
38k views

Is frequency for dc zero Hz?

We know the frequency of a direct current is zero. The reason is that there is no repetitive pattern. But I was stumbled when I noticed, why can't that straight line be cut into smaller pieces, and ...
13
votes
3answers
42k views

Scale 30-50 mV signal to 0-5 V range

I have a CO2 sensor that output signal values 30-50 mV. I need to translate these voltages to 0-5 V for my microcontroller with the highest resolution. I understand that I can amplify the ...
13
votes
5answers
7k views

24VAC/5VDC power supply design

I am planning to create a water valve controller using an MCU and a set of solenoid-controlled valves. The solenoids run on 24VAC (40mA inrush, 20mA holding). The MCU is on a board that draws ~100mA, ...
13
votes
4answers
20k views

12V DC UPS for Network Equipment

My requirement is to keep running supply of low power devices like DSL modems, etc. Most of these devices input 12V/9V DC so i don't see the value in using ordinary UPS which converts DC to AC then ...
12
votes
3answers
3k views

5V DC over Coax

I need a sanity check. My house is wired with a Coax cable that is currently completely useless to me. I want to use the coax wiring to send 5V DC power for small appliances (like Raspberry Pi's) ...
12
votes
6answers
28k views

Very Slow Electric Motor

A customer has asked: I want to slow down a small hobby sized DC motor to a user variable range that runs from slow to zero RPM. I would simply use a wall wart for a power supply and a ...
12
votes
4answers
136k views

How to tell polarity expected of a DC barrel jack?

I'm sure this information is somewhere around the internet, but I can't find it. I have a device that takes a DC barrel jack. I don't know the polarity it expects though. There is a marking though. ...
12
votes
5answers
57k views

Protect DC Circuit from too much voltage

How do I protect a 3v DC circuit from being damaged if someone accidentally plugs in a 12v power supply? The circuit will draw much less than 1 amp.
12
votes
5answers
2k views

Anything special to consider when running a 12v “Bus” throughout my house?

I've recently begun playing a lot with tiny programmable computers/controllers (like the Raspberry Pi and the Arduino), and I'm planning to distribute several throughout my house as various sensors. ...
11
votes
4answers
2k views

Using DC in transformers?

I read in many websites that transformers can only be used, to step voltage up or down, with AC current (which is why AC is preferred for power transmission since they can transfer huge power in the ...
11
votes
2answers
13k views

Does a DC toy motor emit AC when you spin it?

I just watched a video on Facebook by somebody who built a project to run an LED using a 9 volt DC motor scrounged from a battery operated toy. They simply wired the LED to the motor and then used a ...
11
votes
2answers
22k views

DC/AC coupling in an oscilloscope

I would like to know when it's suitable to use AC coupling and when it's not in general? When using an oscilloscope, I want to know in general when it's better to use DC coupling over AC coupling ...
11
votes
7answers
2k views

Is there any difference between transferring DC over one 36 mm² wire or six 6 mm² wires?

I am designing an off-grid DIY solar power site where the distance between the solar panels and inverter is going to be several dozen meters, so I would prefer to put a thicker wire to minimize losses....
11
votes
3answers
2k views

What is the difference between a 30 V power supply and a ±15 V power supply?

I need to get a ±15 V power supply and I am wondering where to find it. Should I get a 30V DC power supply in its place?
11
votes
3answers
590 views

Help me solve this DC circuit problem

The circuit looks like this: All the elements are known, except for \$I_{g2}\$. \$E_1=3V,E_4=10V,E_6=2V,E_8=1V,E_9=4V,I_{g7}=1mA,\$ \$R_1=1k\Omega,R_2=1k\Omega,R_3=1k\Omega,R_4=2k\Omega,R_5=4k\Omega,...
10
votes
12answers
29k views

Avoiding DC voltage drop over long distances

I am trying to power my IP camera over a long distance from the power socket (about 10m). The included power adapter, rated for 5V 2A is about 1.5m long. I cut the cable and extended it using a ...
10
votes
3answers
42k views

What does RU (UR) label on DC power relays mean?

I'm trying to figure out what does RU or maybe UR label mean on the body of many many relays. It's labeled almost on all (I've seen so far) DC relays which make me very curious :). Any suggestions?
10
votes
3answers
1k views

What do I do with the extra “ground chassis” tab on a DC power jack?

Product Link Datasheet I have about fifteen 9v wall warts within specs for the circuit; I'm going to use the one that fits the jack. There are three solder tabs on the jack. One for the positive tip,...
10
votes
4answers
48k views

How do I find the voltage range for an unknown DC motor?

I have a small unmarked motor with a burned out speed controller board. I'm able to figure out that the motor is a DC motor because it has two wires coming out of it and there's a rectifier on the ...
10
votes
3answers
1k views

Locking up a DC motor with constant supply a bad thing?

I am working on a little pen plotter project and for one of my motors (which supports the pen) I have used a small DC motor obtained from an old DVD drive. I have noticed that if I supply a constant ...
9
votes
6answers
2k views

DC adapters: why so few amps?

If I have a device that draws 5 amps at 12 volts, I can use any 12 volt DC adapter that can provide at least 5 amps. Why don't all DC adapters have the capacity to provide loads of amps!? If all DC ...
9
votes
6answers
3k views

Is it necessary to separate DC power cables and data cables?

I have had this discussion with my colleague earlier. A DC power supply is non-alternating, so the magnetic field a DC power wire generates is constant (is it?). Now, I know the rule is to separate ...
9
votes
6answers
3k views

Why are RMS values considered DC equivalent?

RMS is defined as the AC equivalent voltage that produces the same amount of heat or power in a resistor if the same is passed in the form of a DC voltage to the resistor. But shouldn't the power in ...
9
votes
4answers
8k views

What is transimpedance?

I've googled almost 100 times, unable to find what really is transimpedance. Every search displayed results about transimpedance amplifier, but didn't tell about the term transimpedence.