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316 votes
6 answers

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 ...
325 votes
8 answers

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 ...
70 votes
8 answers

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?
  • 419k
35 votes
2 answers

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 ...
  • 419k
55 votes
4 answers

When would I use a voltage regulator vs a 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?
  • 930
248 votes
9 answers

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 ...
  • 4,613
216 votes
17 answers

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 ...
  • 4,305
10 votes
1 answer

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 ...
  • 419k
156 votes
6 answers

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?
64 votes
3 answers

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 ...
  • 72.5k
110 votes
6 answers

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 ...
  • 10.6k
11 votes
1 answer

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
152 votes
4 answers

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? ...
  • 56.3k
293 votes
38 answers

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' ...
12 votes
2 answers

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 ...
  • 363
35 votes
7 answers

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 ...
  • 12.3k
241 votes
37 answers

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

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?
28 votes
1 answer

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 ...
73 votes
10 answers

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
39 votes
6 answers

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?
  • 537
42 votes
2 answers

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
14 votes
1 answer

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. ...
75 votes
4 answers

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
346 votes
6 answers

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 ...
  • 4,439
28 votes
6 answers

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 ...
69 votes
3 answers

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 ...
  • 855
5 votes
4 answers

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
187 votes
11 answers

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 ...
  • 2,994
173 votes
12 answers

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 ...
14 votes
1 answer

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 ...
  • 367
5 votes
3 answers

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 ...
  • 11.7k
362 votes
8 answers

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 ...
  • 5,984
91 votes
11 answers

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 ...
  • 28.3k
46 votes
6 answers

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
26 votes
2 answers

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 ...
  • 403
5 votes
2 answers

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
37 votes
2 answers

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?
206 votes
13 answers

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

What's the difference between a microcontroller and a microprocessor?
  • 28.3k
95 votes
4 answers

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 ...
  • 3,715
22 votes
4 answers

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
0 votes
3 answers

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-, ...
111 votes
9 answers

Why does a resistor need to be on the anode of an LED?

Please be kind, I am an electronics nub. This is in reference to getting an LED to emit photons. From what I read (Getting Started in Electronics - Forrest Mims III and Make: Electronics) electrons ...
  • 1,213
80 votes
9 answers

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
79 votes
9 answers

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 ...
  • 1,079
65 votes
5 answers

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 ...
user avatar
64 votes
8 answers

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?
  • 1,545
129 votes
6 answers

Should chassis ground be attached to digital ground?

I'm working on a PCB that has shielded RJ45 (ethernet), RS232, and USB connectors, and is powered by a 12V AC/DC brick power adapter (I do the 5V and 3.3V step down on board). The entire design is ...
  • 1,633
106 votes
3 answers

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. Many of these projects use ...
72 votes
3 answers

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- ...

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