An transconductance (using voltage to control current) electronic component used for switching and amplification. Acronym for Metal-Oxide-Semiconductor Field-Effect Transistor. (from http://en.wikipedia.org/wiki/Transistor)
14
votes
3answers
12k 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 ...
11
votes
4answers
2k views
How can I sense the motor's current?
I need to drive a DC motor @ 24V, 6A with a MOSFET. How can I sense the current that the motor is drawing with a microcontroller? I have to know when the motor is stalled.
10
votes
4answers
1k views
Unnecessary Transistor and FET resistors?
I commonly see weak pull down resistors at the base of NPN transistors. Many electronic sites even recommend doing such things, usually specifying the value as something like 10x the base current ...
4
votes
3answers
636 views
High power three phase ac motor controller
I have come across this question a lot in the design of a special 3 phase motor controller I plan to build this summer and I think I need to ask it directly to solve it.
I need to be able to ...
13
votes
4answers
4k views
When is a MOSFET more appropriate as a switch than a BJT?
In my experimentation, I've used only BJTs as switches (for turning on and off things like LEDs and such) for my MCU outputs. I've been repeatedly told, however, that N-channel enhancement-mode ...
9
votes
3answers
5k views
Is it safe to drive a MOSFET from an output pin of a microcontroller?
I have used commonly-available BJTs such as the 2N2222 and 2N3904 as switches by operating them in "saturation mode" from my MCU. I believe, however, that for these sorts of applications, a MOSFET is ...
4
votes
4answers
2k views
Dissipating 1W on a TO-220 without heatsink?
Can a TO-220 without a heatsink dissipate 1W in still air?
Or, a different way of asking the question is: Assuming an ambient temperature of 25C, how can I go about calculating the maximum power I ...
6
votes
4answers
1k views
Is this a good design for MOSFET H-Bridge?
I have been looking around trying to design a simple but working H-Bridge for an RC car motor (12V and 2~3A).
This bridge will be driven from a microcontroller and need to be fast to support PWM. So ...
4
votes
2answers
113 views
Driving LED strip from microcontroller
I want to drive a strip of LEDs from a microcontroller using PWM to control the brightness. The strip I have takes about 1.5A at 12V. I'm only familiar with purely low power digital electronics so ...
4
votes
2answers
152 views
Signal volume LED Array
I am done a google search on this and have come up blank. What I have is a signal/sound being fed through a wire that ranges from (0-9V). When the volume is low, and i need 1 or 2 leds to light up, ...
1
vote
2answers
348 views
Will the constant voltage appear across the capacitor?
Could you please help me with this question? (Because I'm new here, it seems that I couldn't post an image.)
The circuit diagram is the first picture from this lecture
Voltage Vg is the node voltage ...
0
votes
1answer
781 views
Critique my Soft-start circuit
I've been working on a soft-start circuit to solve an inrush current problem that is causing some sparking when I hotplug my power supply into my board. I played around with different solutions and ...
12
votes
5answers
13k views
MOSFET as a Switch - When is it in Saturation?
I have the following circuit hooked up on a breadboard.
I vary the gate voltage using a potentiometer. Here is what confuses me: according to wikipedia, the MOSFET is in saturation when V(GS) > ...
20
votes
2answers
733 views
When to use what transistor
So there are several types of transistors:
BJT
JFET
MOSFET
Combine all that with the various flavors of each (NPN, PNP, enhancement mode, depletion mode, HEXFET, etc) and you've got a wide array ...
13
votes
4answers
2k views
Why is a MOSFET triggered by Vgs and not Vgd?
Looking carefully at this diagram of one type of MOSFET:
(found in this application note)
We can see the device is virtually symmetrical. What makes the gate reference itself to the source and not ...
9
votes
3answers
2k views
Equivalent circuit of a solid state relay
I want to switch 50V AC voltage. Maximum drained current will be 5A. Frequency is 50Hz. Switching speed is not important, can be real slow, it is not a problem in my application.
I wanted to use ...
12
votes
2answers
1k views
Selecting a MOSFET for driving load from logic
I'm looking to drive a magnetic door lock from an Arduino. I've found a question about driving a solenoid from an Arduino, which includes a circuit that looks perfect for this kind of situtation:
...
3
votes
3answers
4k views
Level Shifting 1.8V to 5V with N-channel FET
I am using BeagleBoard-xM GPIO outputs to drive some DC motors with the help of L293D IC. The problem is that there is a difference between voltage levels. The GPIO outputs only supply 1.8V while ...
8
votes
3answers
10k views
How to drive a MOSFET with an optocoupler?
What's the proper schematic for driving this MOSFET from a microcontroller pin through this or this optocoupler?
The MOSFET will drive a motor @ 24V, 6A.
10
votes
2answers
917 views
How to choose a switch?
There seem to be about 4 major flavors of transistors, and then there are NPN/PNP versions. There are also relays, SCRs, and TRIACs.
When I need an microcontroller-controlled switch, what rules of ...
10
votes
4answers
3k views
What is the purpose of “MOSFET driver” IC's
There are dedicated "MOSFET driver" IC's available (ICL7667, Max622/626, TD340, IXD*404).
Some also control IGBTs. What is the practical purpose of these? Is it all about maximizing the switching ...
7
votes
2answers
483 views
Calculating the pulldown resistance for a given MOSFET's gate
I searched and read many similar questions, but did not find a specific answer for how to calculate the correct value for a pulldown resistorn for a MOSFET's floating gate. Seems like everyone dodges ...
3
votes
2answers
219 views
BJTs versus (MOS)FET to switch a load from a microcontroller
I would like to know what are the reasons for choosing bipolar junction transistor (BJT) versus a field-effect transistor (FET) (MOSFET or a JFET) to switch a load from a microcontroller. In our ...
12
votes
4answers
2k views
Are discrete MOSFETs ESD sensitive?
CMOS inputs on microcontrollers and other ICs can be damaged by ESD discharges. Can the gate of a big discrete MOSFET (2N7000, IRF9530, etc.) be damaged by ESD discharges?
8
votes
5answers
673 views
PWM & output voltage
while 555 contest is long gone, I am still debugging my device, where I already abandoned 555 itself :-)
At the moment, I am driving PC fan from an PWM (30kHz) signal from atmel uC.
I am powering ...
5
votes
1answer
3k views
Basic p type MOSFET question
With an N type MOSFET you let the current flow from the drain through the source when a high enough voltage is applied to the gate. With a P type MOSFET which direction is the current meant to flow? ...
4
votes
3answers
482 views
Design considerations for micro dc to three phase ac AVR based (ESC) for miniature UAV applications
This is a huge question, I plan to present the problem in detail and my current plan of action for solving it. I'm hoping to get pointers on what to and not to do.
I was involved with a recent ...
3
votes
2answers
516 views
Simple circuit to limit current through a FET
I am looking for a simple circuit which can be used to limit the current flowing through a FET. There is a bus line which is pulled up to the battery voltage (12V) by a 510\$\Omega\$ resistor. A FET ...
2
votes
2answers
276 views
Square Wave Voltage Level Shifting
I have a square wave signal of 300 KHz which is 0.2 V to 1 V. It is the output of FP5138 Non-Sync PWM Controller IC. I want to drive a PMOS with this square wave but with the levels shifted to ...
1
vote
2answers
117 views
Mosfets as a switch for batteries in series
I am contemplating a way to build a new battery pack for a project that I am working on. However I think that I have run into a problem, but I am not sure.
I have a battery pack with about 27 ...
1
vote
2answers
397 views
What's the transconductance of a PMOS?
The transconductance of an NMOS is
$$ g_m = \frac{\partial i_d}{\partial V_{gs}} $$
Is the transconductance of a PMOS the same?
5
votes
1answer
5k views
How to control a 12V solenoid valve with a mosfet?
I'm trying to control a 12V DC solenoid valve through a MOSFET (BS170), which receives its control signal (5V) from an Arduino microcontroller.
This is the basic schematic:
When I test the MOSFET, ...
5
votes
4answers
3k views
Source vs Drain polarity for MOSFETS
When using MOSTEFS as a switch I always see the drain connected to the higher potential and and the load and the Source is always connected to ground. Can you switch those so that the Source pin ...
4
votes
3answers
121 views
How to Power a Tank Circuit with Mosfet without the Inductor Undesireably Triggering the Mosfet
BACKGROUND: I have this circuit that I asked about in a previous question:
Someone kindly explained that the problem I'm having with the circuit is that the mosfet is being triggered by negative ...
4
votes
2answers
202 views
Question about mosfet gate resistor
I have n mosfet gate connected to 4043 logic and Id is about 100mA. Both 4043 and mosfet has +5v. I have plan to use 2N7000 mosfet
Questions are:
How large gate resistor i need between 4043 and ...
4
votes
2answers
2k views
P-Channel MOSFET high side switch
I am trying to reduce the power dissipation of a P-Channel MOSFET high side switch. So my question is:
is there any way in which this circuit can be modified so that the P-Channel
MOSFET will ...
4
votes
3answers
358 views
Increasing Fall/Rise Times for Signal Integrity
I have a board that tests a wiring harness' continuity by outputting a set of test vectors onto the harness and reading it back. The wires in the harness can be as little as 1 m and as large as 10 m.
...
3
votes
1answer
352 views
driving low side of a mosfet bridge with 3.3V
I'm trying to drive a the low side of a n-channel half bridge with a 3.3V signal that is not enough to switch the MOSFET I think (sometimes it does switch and sometimes it doesn't).
I did this: ...
3
votes
1answer
307 views
Is this a good circuit to control LED?
I'm taking an intro electronics course, and we recently covered MOSFETs. Would the circuit below work for controlling a 3W LED? Here is the part info. The LED wants about 1A of current. I'm thinking ...
3
votes
2answers
954 views
USB / Battery Switchover Circuit
Background: I am a non-EE (an 0x11? :^) thrust into the world of embedded systems design. I started this project with a only college physics-level understanding of circuits, but I'm gradually ...
3
votes
1answer
452 views
How do I treat a PMOS compared to an NMOS in small-signal analysis?
What exhaustive list can I use to determine the differences between NMOS and PMOS devices when doing small-signal analysis?
Equivalently, what are the fundamental differences between NMOS and PMOS ...
3
votes
4answers
623 views
Recomendation for a digital inverter made of discrete components
I have a digital circuit in which I need only one logic inverter, and both PCB size and power consumption are serious constrains. So I thought about using a pair of complementary SMD MOSFETs (the ...
2
votes
1answer
691 views
Power MOSFETs and Gate drivers
I'm trying to finally apply what I learned at university in the course of power electronics.
However I find that both that course and most of the books / online references are lacking on some points.
...
2
votes
3answers
418 views
AC heater, bridge and MOSFET, possible?
In my project I already have all the circuits for a PWM driving some high power MOSFET. Now the hardware guys are coming with this AC heater, and I've been breaking my head around Triacs and phase ...
2
votes
3answers
269 views
Do I need to duplicate the 4th “substrate” connection when building CMOS gates out of discrete transistors?
All CMOS digital integrated circuits I've ever seen connect all the nFET substrates together to GND.
In particular, the IC CMOS NAND gate has one nFET that has its substrate connected to GND, but its ...
1
vote
1answer
52 views
Designing stable PMOS voltage regulator
As a follow-on to the question "Using forward voltage drop of diodes with linear regulator" I am looking into designing PMOS voltage regulators.
General Topology
A common problem with the intuitive ...
1
vote
2answers
948 views
Low Threshold N-channel MOSFET
Got a quick question about a simple circuit design I'm working on. I have a circuit which does something not important but results in an output is normally low, and then on activation climbs to a ...
1
vote
2answers
2k views
W/L Ratio of a MOSFET
In the schematic builder that calculates voltages/currents/transient analysis that we have been provided for the electrical engineering course, MOSFETs have the parameter "W/L Ratio".
I figured that ...
1
vote
2answers
412 views
Designing a simple automatic USB switch
I'm trying to come up with a simple way of sharing a USB peripheral between two USB hosts (computers) so that the host that is currently turned on controls the peripheral. I'm targeting low-speed ...
1
vote
1answer
240 views
Using BSS84 P-channel MOSFET
I am using a P-channel MOSFET as a switch in my design & I have connected the drain to the load. The load needs 0.8mA of current. How can I decide whether the MOSFET is conducting that much ...
