All Questions
16,990
questions
326
votes
6
answers
416k
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 ...
- 312k
331
votes
8
answers
167k
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 ...
75
votes
8
answers
25k
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?
- 443k
58
votes
6
answers
46k
views
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?
- 1,002
35
votes
2
answers
14k
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 ...
- 443k
249
votes
9
answers
92k
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 ...
- 4,653
218
votes
17
answers
185k
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 ...
- 4,345
64
votes
3
answers
153k
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 ...
- 78k
157
votes
4
answers
77k
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? ...
- 57k
10
votes
1
answer
1k
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 ...
- 443k
158
votes
6
answers
110k
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?
- 1,745
11
votes
1
answer
8k
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
111
votes
6
answers
73k
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 ...
- 10.7k
12
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 ...
- 353
293
votes
38
answers
120k
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' ...
37
votes
7
answers
10k
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 ...
- 12.4k
244
votes
37
answers
108k
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
179k
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?
- 1,594
77
votes
10
answers
449k
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, ...
- 1,164
28
votes
2
answers
31k
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 ...
- 383
40
votes
6
answers
482k
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?
- 547
30
votes
7
answers
6k
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 ...
- 90.3k
42
votes
2
answers
22k
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
18
votes
1
answer
47k
views
How to properly use a relay module with JD-VCC from Arduino/Raspberry?
[EDIT]
A quick benefit of JD-VCC, is that allows you to control the relays that are 5V with 3.3V GPIOs. Check the full answer to understand why :)
[Original question]
I got two 4-relay-modules and I ...
- 1,184
77
votes
4
answers
96k
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 ...
- 1,350
348
votes
6
answers
88k
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 ...
- 4,469
175
votes
12
answers
18k
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
...
- 1,835
74
votes
3
answers
46k
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 ...
- 915
5
votes
4
answers
3k
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
190
votes
11
answers
121k
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 ...
- 3,044
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 ...
- 367
5
votes
3
answers
6k
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 ...
- 11.9k
372
votes
8
answers
376k
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 ...
- 6,084
100
votes
4
answers
79k
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 ...
- 3,765
91
votes
11
answers
75k
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 ...
- 28.7k
46
votes
6
answers
190k
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
26
votes
2
answers
77k
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 ...
- 403
5
votes
2
answers
3k
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
38
votes
2
answers
27k
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?
- 555
206
votes
13
answers
255k
views
What's the difference between a microcontroller and a microprocessor?
What's the difference between a microcontroller and a microprocessor?
- 28.7k
67
votes
8
answers
43k
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?
- 1,585
22
votes
4
answers
50k
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
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-, ...
- 49
111
votes
9
answers
116k
views
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
109
votes
4
answers
105k
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 ...
- 10.6k
83
votes
9
answers
277k
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, ...
- 1,235
81
votes
9
answers
16k
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 ...
- 1,099
65
votes
5
answers
13k
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 ...
user17592
23
votes
5
answers
27k
views
OR-ing power supplies (diode or mosfet)
I have got two power supply options on my board:
USB 5.0V
DC Power Jack of 5.0V
My intention is to design a or-ing power supply option, as either of them will be used at a time. Possible solutions ...
- 2,293
134
votes
6
answers
63k
views
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 ...
- 1,703