Hot answers tagged

68

I am not sure why this wasn't the first thing pointed out by any of the earlier answers, but it is because as transistors are made smaller to increase speed, increase density, and reduce power consumption, the gate oxide layer is made thinner (which also increases leakage currents). A thin gate oxide layer can't withstand very high voltages so you end up ...


35

Short answer, yes you are right. The Ri of a battery limits the current it can supply, but the Ri is not the real cause, more a symptom. The design and characteristics of the electrodes, chemical processes, temperature, etc. all kinds of internal and external parameters interact when current is "requested" and Ri is just your way to put all these ...


32

200mA is lethal (actually much much less is lethal) and can kill, but only it if gets inside you. For it to get into you, it needs enough voltage behind it to push into you. If there is enough voltage to push a lethal level of current into you, and enough of that current is actually available to push then it is lethal. Think about a bullet. It's a little ...


29

Direct current is more difficult to interrupt because it is continuous. Alternating current alternates polarity so it crosses zero on its own which helps to extinguish the arc produced across the breaker contacts when they open. The contacts doesn't get this help if it's DC so the contacts have to be hardier. Interrupting DC with an AC relay/breaker could ...


27

tl; dr version: The transistor's Vce(on) is lower than the LED's Vf, so when the transistor is on the LED is well below its Vf threshold and so doesn't conduct. LEDs, Vf and Hue The LED, like all diodes, has a forward anode-to-cathode voltage, Vf. The LED will not conduct until the Vf threshold is reached, after which current climbs rapidly. This Vf ...


27

The power required to switch a capacitance from logic 0 to logic 1 (or vice versa) is proportional to the clock frequency times the supply voltage squared. In CMOS digital circuits the logic gate inputs look like capacitors, so charging and discharging capacitances uses most of the power in these circuits. As you mention, the \$I^2R\$ losses in conductors ...


24

You certainly can use Square D "QO" panels/breakers on DC And the reason you can do that is Underwriter's Laboratories (UL) listed the breakers for DC service, and the labeling and instructions (which are approved by UL) state that explicitly. This was backed by testing in UL's lab. This is a condition of using them in mains power (NEC 110.2). Naturally,...


20

The mathematical truth is that the current will actually split between the LED and the transistor, but the current that flows through the LED will be effectively zero. The LED only starts to draw significant current when the voltage across it gets relatively close to the normal operating Vf (typically around 2-3V for visible light LEDs). The transistor ...


20

Acting as a closed or open switch is just an extension of it acting as an amplifier at its limits. Imagine you use your weak little fingers to push some buttons to control a massive flood gate. Anything in between full closed and fully opened is throttling the flow of water somehow, but when fully open or fully close it's just acting as a switch to block or ...


17

Well, there's no little resistor inside the battery – instead, "internal resistance" is our model for the proportional-to-current draw voltage drop you'll see with any real-world power source in some current region. So, as schnedan said (go and upvote their answer!), "internal resistance" is the symptom, not the cause. Of course, actual ...


17

In a series circuit, the current in each component of the series will be the same. This is true no matter what the component may be: resistor, capacitor, inductor, diode, battery, etc.


16

You are mixing up steady-state solutions with transient solutions. The steady state solution where sinusoidal voltage leads sinusoidal current exists only after the sine wave input has been there for a very long time. That means that there was voltage just before you measure the current. If you have an ideal inductor (or a non-ideal one made with ...


16

simulate this circuit – Schematic created using CircuitLab We know that for a BJT the collector current increases with the increase in base current. For example, a 0.01mA increase in base current has caused a 10mA increase in collector current. Now, let's assume, you have connected the collector and emitter through a copper wire (i.e. short-...


15

'Path of Least Resistance' is a phrase that only really applies where you have alternative paths for, for instance, a walker who chooses to go through a gap in the wall next to the gate, rather than open the gate. Wikipedia says this: In physics, the "path of least resistance" is a heuristic from folk physics that can sometimes, in very simple situations, ...


14

In-rush current is a killer for contacts in relay (and elsewhere). A short current spike during operation, when the relay is properly closed might be tolerable (ask the manufacturer). In-rush current on the other hand is much worse: It starts flowing at the very first instant the contacts barely close. Usually contacts bounce for a short moment, causing a ...


14

If you want a canned solution you can use a "current sense amplifier". This is the name of the category you should dig in at your favorite supplier. These amps are characterized by the usual parameters: offset, bandwidth, acceptable common mode range, etc. Make sure you check. But they have extra features on top of that. They work from a low impedance ...


13

To make it easy: 1- Car battery 12V (hundreds of Amps) will not kill you. Laptop charger 20V 7A will not kill you. Therefore, the 5V 2A will not kill you also. Most of power sources you will see are voltage source. Therefore, the current will follow the voltage (by other way the current is dependent on the voltage ) and the impedance (your impedance here) ...


13

If you "forget about" internal resistance, then the maximum current is infinite. An "ideal" component, non-existent in the real world, can provide mathematically "pure" infinite or zero amounts of resistance, voltage, current, and all the rest. Different battery compositions will have different amounts of real-world "impure" limitations. Internal resistance,...


13

It is the other way round. For the sake of simplicity, we model the battery ability to deliver high power with something called "internal resistance". The model is pretty much basic and in particular really bad for high currents. The "internal resistance" of an electrochemical cell is non-linear. It consist of more-or-less ohmic behavior ...


12

First what you need to understand is that the inductor always follows this equation: $$V_L = L\times\frac{\mathrm{d}I }{\mathrm{d}t}$$ This equation indicates that inductance voltage depends not on the current which actually flows through the inductance, but on its rate of change. This means that to produce the voltage across an inductance, the applied ...


11

Huh, it never occurred to me that anyone would interpret it as (a). I believe it is (b). 300us tells you the on-time, 1% implies the period (how frequent you can do it). This is because it says pulse DURATION, and a pulse exists only while it is on.


11

The power source you are looking into is a voltage source (like 99% of the power sources you interact with in your daily life). This means that 2A is the maximum it is designed to supply, but it will try to keep a steady 5V. Thus, the current is determined by the electrical resistance between the terminals. It can't kill you because your body electrical ...


11

Contact rating is almost always limited by the switching current, not the steady state. It is at switching that the arcing and damage will occur. Photo 1. (Left) Pristine contacts from a relay (Right) The nearly destroyed contacts from a relay operated under power for nearly 100,000 cycles. Image source: Arc suppression on Wikipedia. While the damage in ...


11

'Short Circuit' gets used in two different ways. In the context of a battery (or any power source), we usually mean it to be a load that is far too large for the source. Any battery, whether a high voltage or low voltage battery, will be 'short-circuited' by putting a low or zero resistance load on it. A short circuit usually produces damaging conditions ...


11

In the first circuit, you have one (single) voltage source and one (single) resistor. This one (single) resistor is connected directly across the voltage source terminals ( terminals \$A\$ and \$B\$). Thus, from point \$B\$ to \$\$A the voltage is equal to the battery terminal voltage \$V_B\$ and because our single resistor is also connected directly ...


11

But what does it mean when one says,"It is a capacitive load" It means the load behaves like a capacitor. You have to deliver charge to it before the voltage across it will change. why output impedance should be low for capacitive loads? Because if you want to change the voltage across this load, you'll have to deliver a significant current. You ...


11

The "plate" is the relay armature. Figure 1. Image source: D&T Online. When the armature is pulled in by the coil it closes the magnetic circuit. This results in an increase in coil inducatance and since the impedance (resistance to current flow) is given by \$ Z = 2\pi fL \$ it should be clear that for a given frequency the impedance will ...


10

This is the one: So typically 5 mA, maximum 8 mA How to quickly find that: I know it's a current so I look for any parameter specified in A (or mA, uA) in the UNIT column. Then I look at each and see if that could be the one. There will be a small increase with increasing load current, see the line under the red rectangle.


10

simulate this circuit – Schematic created using CircuitLab Figure 1. Test circuit. L1 has 1 Ω ESR. SW1 is set to close at t = 1 ms. Figure 2. The resultant voltage and current curves at the top of L1. So, my question is that (physically speaking) how can a current exist at t=0 in such a circuit even without a voltage? You can see from the ...


9

EVERYONE who is not aware of how series-fed constant current runway lighting systems work should read the excellent article cited by Kevin White. Series Lighting Circuitry: Why Use It? Many airport lighting systems did and still do use series feed - but, ... They are vastly more sophisticated systems than are used in eg Christmas Tree lights. 10's of kW,...


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