# All Questions

10,724 questions
342k 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 ...
117k 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 ...
71k 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 ...
10k views

### L293, L298 and SN754410 H-bridge drivers on low voltage power 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 ...
12k 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?
60k 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? ...
156k 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 ...
82k 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?
107k 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' ...
60k 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 ...
42k 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 ...
89k views

### Good tools for drawing schematics [closed]

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 ...
30k 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?
143k 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?
83k 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 ...
19k 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 ...
347k 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, ...
2k 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 ...
82k 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 ...
12k 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 ...
249k 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 ...
7k views

### How to choose a MCU platform? [closed]

There are numerous MCU platforms and once somebody has gotten used to one, they are generally reluctant to switch to another platform. My question is: If one started using a MCU for general purpose ...
32k 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 ...
64k 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 ...
54k 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 ...
330k 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?
5k 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 ...
77k 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 ...
234k 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, ...
83k 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 ...
10k 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 ...
67k views

### How to drive a Peltier element?

Maybe the underlying question is what the Voltage-Current curve looks like. Can I drive it from a voltage source (like you drive a heater) or from a current source (like you drive an LED)? Or even ...
244k views

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

What's the difference between a microcontroller and a microprocessor?
51k 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 ...
45k 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 ...
50k 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 ...
44k views

### What happens if I omit the pullup resistors on I2C lines?

Just now I realized that the I2C data and clock lines (SDA and SCL) must have pullup resistors. Well, I've built a couple of clocks using the DS1307 RTC (see datasheet) according to the schematic ...
17k views

### Why would one drive LEDs with a common emitter?

I've seen tutorials aimed at beginners suggest the way to drive an LED from something without enough current drive is this: (option A) but why not this: (option B) Option B seems to have some ...
146k 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?
18k 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?
15k 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 ...
58k views

### How do I calculate the resistor value for a simple LED circuit?

I have a simple circuit: The max rating (current) of the LED is 30mA. How can I work out what the resistance of the resistor needs to be? By using ohm's law I found it to be $3V/0.03A = 100 \Omega$...
9k 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 ...
94k views

### Is there a correct resistance value for I2C pull-up resistors?

The datasheet of the 24LC256 EEPROM states that: The SDA bus requires a pull-up resistor to VCC (typical 10 kΩ for 100 kHz, 2 kΩ for 400 kHz and 1 MHz). I thought that any resistor with a kΩ value ...
8k views

### Grounded thermocouple vs “grounded” thermocouple

I'm having problems with documentation and what appears to be conflicting use of the word grounded. The way I understand it, a thermocouple can be [grounded, ungrounded, or unsheathed] (http://www....
151k views

### How to become an embedded software developer?

I'd like some tips for those who want to become a good embedded software developer or want to improve in this area. What should I need learn about hardware, software? Which books are most ...
17k views

### What is the reason that the value “47” is so popular in electrical engineering?

We often see component values of 4.7K Ohm, 470uF, or 0.47uH. For example, digikey has millions of 4.7uF ceramic capacitors, and not a single 4.8uF or 4.6uF and only 1 listed for 4.5uF (specialty ...
21k 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 ...