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311 votes
6 answers
401k views

Choosing power supply, how to get the voltage and current ratings?

Power supplies are available in a wide range of voltage and current ratings. If I have a device that has specific voltage and current ratings, how do those relate to the power ratings I need to ...
316 votes
8 answers
157k views

Rules and guidelines for drawing good schematics

There are a lot of poorly drawn schematics here. A few times people have actually asked for critiques of their schematics. This question is intended as a single repository on schematic drawing rules ...
69 votes
8 answers
22k views

Reasons not to use a 741 op-amp?

Simple enough question. Why not use a 741 op-amp in a target circuit or anyone's target circuit? What are the reasons not to use it? What might be the reasons to still choose this part?
  • 396k
35 votes
2 answers
13k views

Problems with L293, L298 and SN754410 H-bridge drivers on a low voltage supply

This question is about the following three integrated H-bridge drivers: - L293 or L293D (D = protection diodes added) SN754410 (protection diodes included) L298 (no protection diodes) Time after ...
  • 396k
47 votes
4 answers
43k views

When would I use a voltage regulator vs voltage divider?

When would you use a voltage regulator vs a resistor voltage divider? Are there any uses for which a resistor divider is particularly bad?
  • 850
247 votes
9 answers
89k views

Decoupling caps, PCB layout

I guess I've been somewhat ignorant when it comes to the finer details of pcb layout. Lately I've read a couple of books that try their best to lead me on the straight and narrow. Here is a couple of ...
  • 4,583
214 votes
17 answers
182k views

What is a decoupling capacitor and how do I know if I need one?

What is a decoupling capacitor (or smoothing capacitor as referred to in the link below)? How do I know if I need one and if so, what size and where it needs to go? This question mentions many ...
  • 4,275
61 votes
3 answers
136k views

How do I identify SMD components? (or how do I identify any component)

How do I identify the markings on an SMT component and match it up with a part number so I can be a good designer and actually look at a datasheet (and read the whole thing)? Or identify a part to ...
  • 65.2k
154 votes
6 answers
107k views

Why exactly can't a single resistor be used for many parallel LEDs?

Why can't you use a single resistor for a number of LEDs in parallel instead of one each?
10 votes
1 answer
896 views

What to check for when buying an electronic component or module

This has been raised as a canonical Q and A for future reference How can I acquire electronic parts/items/modules and ensure that there is minimal risk of them being unsuitable for my project or ...
  • 396k
152 votes
4 answers
74k views

How can I efficiently drive an LED?

I understand that I can not connect an LED directly to a battery because it will draw too much current. Thus, there must be something else in the circuit to limit the current. What options are there? ...
  • 55.7k
109 votes
6 answers
71k views

My linear voltage regulator is overheating very fast

I'm using a 5 V / 2 A voltage regulator (L78S05) without a heatsink. I'm testing the circuit with a microcontroller (PIC18FXXXX), a few LEDs and a 1 mA piezzo buzzer. The input voltage is aprox. 24 ...
  • 10.5k
11 votes
1 answer
7k views

Why is Vbc absent from bjt equations?

In BJT biasing the voltages Vbe and Vce are always present in calculations. Is the voltage Vbc absent from any equation because one of the collector, base and emitter is grounded or any other reason ...
  • 113
291 votes
38 answers
118k views

Basic Electronics Book [closed]

I need basic electronics books (diodes,transistors,current.. etc) as I am just starting out with electronics and want to have something to read over the holiday. Any suggestions of good beginners' ...
10 votes
2 answers
2k views

How to calculate the tolerance of this constant current circuit?

I am using the circuit below to provide constant current to th LEDs. I request your suggestion on how to calculate the tolerance of current (minimum and maximum variation of set current) due to ...
  • 333
238 votes
37 answers
105k views

Good tools for drawing schematics

I was going to add a bit of information to my post on a previous day using schematics and some instructions. What programs are being employed for this purpose? I mostly want to see what others are ...
94 votes
10 answers
172k views

How to get more than 100mA from a USB port

I heard that the current limit for a USB port is 100mA. However, I also heard that some devices can get up to 1.8A from a port. How do you get past the 100mA limit?
36 votes
7 answers
9k views

How were schematics drawn before CAD?

I was looking at the schematic of an old HP power supply I bought. It can be found here, around page 60-61. This schematic was drawn long before CAD was a tool used by engineers. Things were still ...
  • 12.2k
73 votes
10 answers
419k views

How much voltage/current is "dangerous"?

Related: Safe current/voltage limit for human contact? From what I've heard: 110 V (or 220 V; household voltage pretty much) is dangerous (i.e. can kill you) I think there's consensus on this, ...
  • 1,114
27 votes
1 answer
26k views

Understanding an 'ideal' diode made from a p-channel MOSFET and PNP transistors

The Raspberry Pi B+ models have a protection circuit between the USB connector and the 5V net on the board. They recommend putting a similar protection circuity on a Pi HAT before 'backpowering' the ...
42 votes
2 answers
21k views

Correct formula for LED current-limiting resistor?

I'm trying to work out what value resistor to use in a LED circuit. The equation I'd use to do this is: $$ R = \frac{V_{cc} - V_f}{I_f} $$ Seems logical, and makes complete sense. The answers to the ...
  • 1,092
35 votes
5 answers
447k views

how to reduce DC voltage using resistors?

How would one go about using a 12 V DC power source to power something which needs 4.5 V DC using resistors? Is there a way to determine how much adding a resistor would drop the voltage?
  • 497
75 votes
4 answers
94k views

What is ground and what does it do?

I'm a bit confused about the concept of ground, and perhaps voltage as well, particularly when trying to analyze a circuit. When I learned about Ohm's law in grade school, I learned how to apply the ...
  • 1,310
13 votes
1 answer
28k views

How to properly use a relay module with JD-VCC from Arduino/Raspberry?

I got two 4-relay-modules and I am probably doing something wrong because none of them seems to work: My first try was using a breadboard 5v power supply, connecting VCC and In1 to 5V and GND to GND. ...
336 votes
6 answers
84k views

Why is there so much fear surrounding LiPo batteries?

I've been trying to design a charging system for a small robot powered by a 2S 20C lithium polymer (LiPo) battery. Were I to trust everything I read online, I would believe that the LiPo will kill me ...
  • 4,339
183 votes
10 answers
113k views

How much does it cost to have a custom ASIC made?

I have browsed several ASIC manufacturer's webs, but I haven't found an actual number. I assume there would be a fixed cost associated with creating masks and such and then there will be a cost per ...
  • 2,944
174 votes
12 answers
17k views

How does the current know how much to flow, before having seen the resistor?

With the following circuits as examples : and How will the current I know how much to flow? Would any other wave travel first in the circuit and then come back ...
68 votes
3 answers
40k views

What does the Y capacitor in a SMPS do?

It seems that a well-designed SMPS has a capacitor connecting the ground planes of the primary and secondary sides of the transformer, such as the C13 capacitor here. What is the purpose of this ...
  • 845
14 votes
1 answer
8k views

Analog & Power Ground Planes Doubts

Working on a new board, I have to face some requirements to ensure the performance of the whole system once is manufactured. PCB is a 4-layer one, and it has both analog and digital components. But ...
  • 357
27 votes
6 answers
4k views

Is the NE555 the IC I need, and if not, what do I replace it with?

I've noticed that whilst I have a well-known solution using an NE555, that IC doesn't fulfill my needs perfectly. Others have told me that the NE555 is very rarely an appropriate solution to one of ...
25 votes
2 answers
70k views

Level shifting a +/- 2.5V signal to 0 - 5V

I have a front end module that generates an (ECG) signal that varies from +/-2.5 V. I want to shift this signal to 0 - 5V. What is the best way to do this? Would a summing amplifier like the below ...
  • 393
46 votes
6 answers
184k views

Why is there a diode connected in parallel to a relay coil?

In most electrical circuits with a relay, a diode is connected in parallel to the coil of the relay. Why? Is it always a good practice?
  • 571
5 votes
3 answers
5k views

How to fix this LTspice error

Below is an H bridge inverter circuit model I came across in net: I'm getting Singular matrix Iteration error when I try to run this. If I delete the circuit on the right things work. Or if I ground ...
  • 11.4k
5 votes
2 answers
2k views

How to use motor drivers with H-bridge and PWM input, to control direction and speed of DC motors?

I'm running series of experiments using a 6V DC motor. I'm using desktop DC power supply to power the motor and regulate its speed and everything works just fine, the setup has been running for months....
  • 53
347 votes
8 answers
344k views

What is the difference between \$V_{CC}\$, \$V_{DD}\$, \$V_{EE}\$, \$V_{SS}\$

I've seen lots of schematics use \$V_{CC}\$ and \$V_{DD}\$ interchangeably. I know \$V_{CC}\$ and \$V_{DD}\$ are for positive voltage, and \$V_{SS}\$ and \$V_{EE}\$ are for ground, but what is the ...
  • 5,814
89 votes
11 answers
72k views

Can you make a non-polar electrolytic capacitor out of two regular electrolytic capacitors?

There was some discussion on this question What are some reasons to connect capacitors in series? What are some reasons to connect capacitors in series? which I don't see as being conclusively ...
  • 28k
204 votes
13 answers
253k views

What's the difference between a microcontroller and a microprocessor?

What's the difference between a microcontroller and a microprocessor?
  • 28k
33 votes
2 answers
25k views

BJT in Reverse Active Mode of Operation

What will happen if for a BJT transistor, it's emitter terminal is treated as collector and collector as emitter in a common emitter amplifier circuit?
22 votes
4 answers
48k views

What does a resistor do?

OK, very basic question here. I read lots of books, searched quite a bit, and every description I read seemed to talk about the flow of electrons and right away go too deep in theory for me to grasp ...
  • 553
5 votes
4 answers
2k views

How to chain transistor logic gates

I've been trying to build some circuits that use logic gates, without using pre-made chips. My main goal is to further my understanding of the subject, but it'd be nice to make something useful in the ...
  • 53
80 votes
9 answers
271k 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, ...
  • 1,205
78 votes
9 answers
14k views

What exactly is voltage?

Bit of a strange question, but what is it? My physics teacher said it was kind of like a "push" that pushes electrons around the circuit. Can I have a more complex explanation? Any help is much ...
  • 1,059
94 votes
4 answers
72k views

What kind of components are black blobs on a PCB?

In low-cost mass-produced items I often run into black blobs of what looks like resin applied directly on top of something on the PCB. What are these things exactly? I suspect this is some kind of ...
  • 3,685
65 votes
5 answers
12k views

What's the use of a decoupling capacitor near a reservoir capacitor?

I've seen some circuits where a decoupling capacitor is used as well as a reservoir capacitor, like this (C4 and C5): I've read about decoupling capacitors and for me it looks as if they are meant to ...
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106 votes
3 answers
99k views

How do I calculate the required value for a pull-up resistor?

There are many tutorials that use a pull-up or pull-down resistor in conjunction with a switch to avoid a floating ground, e.g. http://www.arduino.cc/en/Tutorial/button Many of these projects use ...
67 votes
3 answers
93k views

What is the ideal way to handle data pins D+ and D- on a USB power adapter to be compatible with fast charging on devices?

I have found that many USB wall chargers use a resistive voltage divider to set the D+ and D- pins to a specific voltage, usually between 2 and 3 volts. Other USB wall chargers short the D+ and D- ...
63 votes
8 answers
39k views

Why do we need an isolation transformer to connect an oscilloscope?

My professor always insists that I provide power to an oscilloscope thorough an isolation transformer. What is the necessity of this? What is the risk if I don't connect it?
  • 1,535
33 votes
9 answers
60k views

Is there any bidirectional 5v-3.3v level shifter?

Is there any IC that interfaces any 3.3V Input/Output to 5V Output/Input? I need it primarily for an Arduino Due but if there any bidirectional IC that works like that it would be great. Some people ...
  • 845
0 votes
3 answers
2k views

Op-amp: Virtual ground principle and other doubts [closed]

I can't understand some stuff about op-amps. Here is my list of doubts: If an op-amp amplify the difference between the V+ and the V-, how can it work if for the principle of virtual ground V+ = V-, ...
2 votes
4 answers
1k views

AC Circuit Having Only Capacitor

In my book, it is written that at any instant, PD across the Capacitor Plates = Applied EMF ---------(1) Assuming that it's a pure Capacitor of Capacitance C, I think if Voltage across Capacitor ...

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