Hot answers tagged

80

That's the Ukrsepro mark. It means the product has passed the certification of UkrSEPRO, the safety certification requirement symbol for trading in the Ukraine region. Scope of products covered by the above regulations include but not limited to the following: Information technology equipment Audio and video equipment Household and similar ...


48

20 amp glass cartridge type slow blow. It has a thermal mass that generates a delay before the solder melts at the left hand end. The spring pulls the mass away from the wire, making sure the two conductors separate and stay separated.


38

Because UL 1741 does all the heavy lifting for you. That's what makes it "so easy". UL 1741 is a complicated spec for "grid-tie solar inverters". Aside from doing their usual inverter thing (itself no small matter), a 1741 inverter also senses the presence of the grid, and obviously syncs its output to the grid. A UL 1741 inverter is intentionally ...


28

It's an overly-general statement. Obviously, with a resistive load, all frequencies transfer power. It's really a statement about rotating machinery specifically (motors and generators). For these devices, energy at frequencies other than the fundamental is just as likely to oppose the work being done as aid it. Also, the energy of high frequencies is often ...


27

So if we have a 10A fuse, and it has some arbitrary resistance such as 100 ohms, ... This typical 10 A fuse has a resistance of 5 mΩ. So your guess was out by a factor of about 20,000. At 10 A the power dissipated is given by \$ P = I^2R = 10^2 \times 5m = 500 \ \text {mW} \$. RESISTANCE: The resistance of a fuse is usually an insignificant part ...


26

BobT has it right - my answer serves only to add information. This class of fuse is commonly known as a "3AG" or "3AB" fuse. The Buss number would be AGC-20 (fast blow) or ABC-20 (Slow Blow). Although they are still available with a clear glass housing, most modern versions now use an opaque ceramic housing for safety reasons (ceramic doesn't shatter or ...


25

You could add two Schottky diodes to the switch, allowing either battery to power the load via a diode. During switchover the voltage will drop 0.35 V (1N5817 @200 mA) below the voltage of the battery with the most charge, and it avoids the current surge caused by adding a capacitor. You could even remove the switch if a 3% power loss is acceptable....


23

Ohmls Law states V = IR. That means when we increase voltage we must also increase the current (I). That is true when feeding a resistor. But transformer increases the current while decreasing the voltage or decreasing the current while increasing the voltage. A transformer is not a resistor so you can't use Ohm's law on it. How does it happen? A ...


20

A little simple maths: In a capacitor charge, Q, and voltage, V, are related by \$ Q = CV \$. Current is the rate of charge flow so, differentiation gives us \$ I = \frac {dQ}{dt} = C \frac {dV}{dt} \$ You want to calculate your voltage drop for the duration of the switch transfer so we'll rearrange as $$ C = \frac {I}{\frac {dv}{dt}} = I \frac {dt}{dv}$...


20

First before answering what I think is truly your question, I will have to beat into you how voltage are relative: An atom has protons in the nucleus which is orbited by electrons. If the numbers are equal, then the positive and negative charges cancel out and the charge is zero. Suppose we start with two terminals. Each terminal has a net charge of zero ...


19

The conductor fails when it reaches a certain temperature. Because the fuse is in thermal contact with ambient, it can dissipate a certain amount of power before it blows. Your 10A fuse is designed to blow at 10A (plus or minus some tolerance). So it should run at 9A all day. But that 10A fuse will take a good long time to blow at 10A, and will blow much ...


18

In addition to Autistic's correct answer, the value of C9 (X rated) is a compromise between consuming excessive AC current at 60 Hz while suppressing the noise voltage spikes by about 3 dB to 6 dB at the transformer primary if power is cut off when the 60 Hz sine wave is at or close to peak current. C9 does NOT behave like a MOV or ...


16

That is true only if the current is being distorted by the load and not due to distortion of the voltage waveform of the AC line. If you multiply the instantaneous values, point by point, of two sine waves of different frequencies, you get a waveform that has an average of zero. You have positive power during some intervals and negative power in other ...


15

This might seem overly simplistic, but I have worked around this in the past with a simple momentary switch across the main toggle switch. Holding it in connects the two 12 V sources together while you flip the main. So at no point is the power interrupted. And with this the two sources are completely isolated during normal operation and cannot be ...


15

C9 snubs the transformer primary inductance. This damps the inductive spike that occurs when the power switch is turned off when current is flowing. This capacitor is often present in audio systems. This can save speakers from a turnoff plop sound that can be annoying or even destructive.


15

Current flow as ‘positive’ to ‘negative’ is a convention that predates charged particle theory. Blame Ben Franklin, for one: https://whyy.org/articles/does-our-confusing-electrical-nomenclature-start-with-ben-franklins-theory/ Some 150 years after Franklin when electron theory came about (JJ Thomson and others), this made things confusing, as clearly the ...


14

A quick look at the Radio Museum pictures and descriptions makes it look like it ought to be fairly easy to hook up. Here is the picture of the connections from the Radio Museum: You provide it with 4V through the jacks on the left side. You could use a battery pack with 3 normal AA or C cells in series. That's the easiest way, and will probably sound ...


14

It is really not that easy. It's easy for you because standards and regulations have been put in place that affect the power company and anyone selling you grid-tie inverters. The system is designed and regulated by law so that you can just buy a shiny box, pay someone to hook it up, and not worry. The grid-tie inverter is pretty complicated. It has to ...


13

THIS CIRCUIT WILL KILL YOU Or your family Or your friends The capacitor MUST be X or Y rated for AC mains use. If it is getting hot you are using the wrong cap. Regardless of what formulae and calculations say, if the resistor is getting too hot you must use a larger wattage one or cool it better. The same applies to the zener diode. This circuit ...


13

First, the power burnt up in a component is equal to the voltage drop of that component times the current going through it. The transistor has a \$V_{CE}\$ of 3V when the collector current is 20A, so that works out to 60W. Second, the power derating curve works by telling you the dissipation you can allow for a given case temperature. You read across ...


12

Google "laptop battery pinout". Typically you'll have: Plus and Minus power terminals (maybe several contacts each for higher current) Some form of communication like I2C Temperature sensing (for example a thermistor) Your typical laptop battery has several cells in series, so it requires balancing which is usually implemented in the battery management ...


11

You are assuming cables and load to be one "unit". But this does not make sense in this case. Typically, your load requires a specific power and for that it needs some specific current (calcualted by Ohms Law). This current now creates additional losses in the cables and these losses are lower when the resistance is smaller. Take, for example, a 500 W ...


11

Normally, a fuse doesn't know what voltage circuit it is used in - it only knows the current that is flowing through it, so that is the only thing that can cause it to blow. Fuses also have a voltage rating because, once the fuse blows, it will have the full circuit voltage across it, so it must be designed to safely handle that voltage without arcing.


11

Your gate drive voltage is too low. That MOSFET needs 10V to turn on completely. 5V just barely clears the 4V threshold when the MOSFET just barely starts to conduct. DO NOT use the Vgsth if you intend to use your MOSFET at a switch. That is the voltage it just barely starts to conduct at. Use a Vgs at least as high as the one used to obtain the given ...


11

That's what a relay is. In both cases you have a set of contacts carrying current. In both cases there is a mechanism for moving the switch: in a relay, this is done by a magnetic coil. All techniques available for switch construction can also be applied to relay contacts. The lifetime issue is only visible on relays because they generally get switched a ...


11

That is just used as a comparator. The sine wave is either above or below the reference and that is the result of the comparison.


11

If you average the product of the current and voltage in this situation, you’d get zero. Think again... I think you need to look at the top left diagram (resistive load). Picture from this answer. See also these answers: - What's the most economical way to digitally measure 240V mains voltage, current and power factor? Average Power Versus Real Power ...


11

If you want to call it PoE, you need to stick to IEEE 802.3af-2003 or IEEE 802.3at-2009. These standards call for isolation. This answer could end here: no, you cannot build a PoE device without isolation. This is also assuming that the data lines are already isolated, so the only potentials without isolation is the power lines. You also will have a ...


10

You are in good company. The maximum power transfer concept frequently causes confusion. Take note of the fact that maximum power transfer does not mean maximum power efficiency. I think this may be the main point that trips people up. In fact, when Rload = Rsource, then the maximum possible efficiency is 50%, since the load and source will be consuming ...


10

Use a MOSFET instead of a NPN, You'll be dissipating a lot less power that way. Using "Rdson" also makes calculating power loss easier. You may also want to consider adding a gate driver at that size of device. Never get a MOSFET rated for X amps when you think you will need X amps. Your part is rated for 20A if you can keep the part cool enough. There is ...


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