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57 votes

When is a MOSFET more appropriate as a switch than a BJT?

When is a MOSFET more appropriate as a switch than a BJT? Answer: 1) a MOSFET is better than a BJT when: When you need really low power. MOSFETs are voltage-controlled. So, you can just charge ...
Gabriel Staples's user avatar
34 votes
Accepted

Is it good practice to run large amounts of current through a MOSFET?

Why can a thick copper wire handle a large current? Because it has a low resistance. As long as you keep the resistance low (switch the MOSFET fully on, for example use Vgs = 10 V as in the datasheet ...
Bimpelrekkie's user avatar
32 votes

I need to energize a 25 watt incandescent bulb. When that bulb burns out another 25 watt incandescent bulb will energize

The lifetime of an incandescent bulb goes roughly inversely as the 14th power of the voltage (was a 'fact' I remembered from somewhere in my dark engineering past). If you use two bulbs in series, so ...
Neil_UK's user avatar
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30 votes
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Why do BLDC motor (1 kW) controllers have so many MOSFETs?

The reason to use multiple MOSFETs is to lower power dissipation resulting in a cheaper design. Yes one MOSFET can handle the current but it will dissipate some power as it does have some resistance, ...
Bimpelrekkie's user avatar
28 votes
Accepted

Why to disconnect the negative/ground terminal, not positive?

There is no real inherent distinction between breaking one side of a loop or the other side- it's all in series so breaking the negative or the positive side of the supply keeps electrons from flowing....
Spehro Pefhany's user avatar
27 votes
Accepted

Help me understand this circuit - unexpected LED behaviour

The blue line shows how the current flows when the switch is open and the motor is off: When the switch is open, the electrons can only flow by going through the LED. Very little current goes through ...
JRE's user avatar
  • 71.8k
25 votes

In which region should a MOSFET be operated as a switch?

When your article says this (wrongly): - MOSFET in saturation region is preferred to make it work as a switch. It's because it's written by someone who thinks that the name of the equivalent ...
Andy aka's user avatar
  • 460k
21 votes

When does a transistor act as a switch, and when as an amplifier?

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. ...
DKNguyen's user avatar
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20 votes
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Is this a sensbile way to switch a 12V line with 3.3V logic

Lots of problems with your circuit: T1 is used as a emitter follower. Its output will therefore be less than the 3.3 V input. Figure 700 mV for the B-E drop, and the maximum the gate of Q1 is ...
Olin Lathrop's user avatar
19 votes

Help me understand this circuit - unexpected LED behaviour

You need to re-draw the circuit to be more readable. Something like this: simulate this circuit – Schematic created using CircuitLab This has been simplified to match the diagram in the ...
MCG's user avatar
  • 12k
19 votes

I need to energize a 25 watt incandescent bulb. When that bulb burns out another 25 watt incandescent bulb will energize

Rather than a lightbulb, there are better solutions. They're probably also cheaper than the extra parts you'd need to build a suitable mains-rated circuit. Low wattage heaters: There are various low-...
Chris H's user avatar
  • 2,611
18 votes

Ribbon Cable Cross Talk - Is there a fix after the fact?

You can leave the board design as-is, but make a short adapter on both ends of the cable, and make the actual cable either as a non-ribbon cable (micro coax, this will be the best), or use proper ...
Ale..chenski's user avatar
  • 40.8k
17 votes
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When does a transistor act as a switch, and when as an amplifier?

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 ...
Sadat Rafi's user avatar
  • 2,531
17 votes
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Art of Electronics 2.5 - Transistor saturation

It's not as complex as you imagine. Let's look at a slightly re-drawn, but equivalent circuit diagram: simulate this circuit – Schematic created using CircuitLab There's no need for the switch, ...
jonk's user avatar
  • 78.1k
16 votes

Why does switching cause power dissipation?

Consider a MOSFET switching. Switching is a transient event, so losses are not usually modeled as "power". Each switching action dissipates a bit of energy. Then you multiply this by the ...
bobflux's user avatar
  • 78.2k
15 votes

Can MOSFET exceed breakdown voltage without being permanently damaged?

Many MOSFETs are characterized to allow overvoltage and the subsequent breakdown of drain to source provided the energy limits in avalanche are not exceeded. Provided you don't exceed these limits the ...
Kevin White's user avatar
  • 33.8k
15 votes
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Do any power loads require both power lines disconnected by the "off" switch?

The American, residential, split-phase power distribution system has 240 V across two live lines and 120 V between each line and neutral. In this system , a single pole switch is sufficient for 120 V ...
vu2nan's user avatar
  • 18.4k
13 votes

Light bulb burns out -- why there is an arc in the switch?

When the lamps fails, it is often the case that the delicate filament collapes on itself, causing a short circuit. This causes a momentary peak of current. So much current that the short almost ...
Ian Bland's user avatar
  • 2,651
13 votes

Reversed DIP Switch

This is what your circuit looks like: simulate this circuit – Schematic created using CircuitLab When you close the DIPSW the LED will be shorted thus the current will flow through the DIPSW ...
Rohat Kılıç's user avatar
13 votes

Device to power either a red LED or a green LED

You mention an IO pin; this is how you could do it with one IO pin if you are using a microcontroller (assuming one that uses 5 V logic): simulate this circuit – Schematic created using ...
ocrdu's user avatar
  • 9,260
12 votes
Accepted

Does a TIP120 transistor have an internal resistance when switched on?

I'm pretty sure the transistor is getting switched on all the way The TIP120 is a Darlington transistor and NEVER can get switched "all the way". It will always "drop" at least 0.5 volts across it ...
Andy aka's user avatar
  • 460k
12 votes

Ribbon Cable Cross Talk - Is there a fix after the fact?

The signals are 1 MHz data signals with no ground wire separating them. This is pretty slow, so first check if there are source termination resistors on the driving side. If there are resistors, you ...
bobflux's user avatar
  • 78.2k
12 votes

Do any power loads require both power lines disconnected by the "off" switch?

While there may be regulatory or safety reasons you would want to switch both power rails, there is no functional or physical reason it would ever be required. The electrons need to go through a ...
Hearth's user avatar
  • 33k
11 votes
Accepted

Is there any pitfall to using resistors in series AND parallel for an LED array?

Your LEDs may or may not work exactly as you expect, depending on what you expect. The change in brightness when operating the switch will vary depending on whether you have 1 or 9 LEDs lit. With 1 ...
Neil_UK's user avatar
  • 167k
11 votes
Accepted

Why not use actual switches instead of Relays

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 ...
pjc50's user avatar
  • 46.8k
11 votes
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Why are bipolar transistors used for switching applications?

1: Bipolars are cheaper So you can do larger experiments with the same budget. 2: Bipolars don't die an instant death from ESD So there's less risk of the user giving up on electronics because "...
Jasen  Слава Україні's user avatar
11 votes

Why does switching cause power dissipation?

Let's focus on CMOS technology, which is the most widely used logic technology nowadays and it's fairly simple to understand. The basic building block of CMOS gates is a structure that works as an ...
LorenzoDonati4Ukraine-OnStrike's user avatar
10 votes

How to increase mosfet switching speed, and decrease switching losses?

Either choose a better MOSFET or use a push-pull driver like this: - Notice that this chip uses identical MOSFETs in the output stage. Here's another using the FAN7842 from Fairchild: - You should ...
Andy aka's user avatar
  • 460k

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