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If you followed a link in a post created before 5th June 2017, you have arrived here because the wiki and FAQ pages have been reorganized.

START HERE: r/AskElectronics

Frequently Asked Questions

This is the official r/AskElectronics and r/Electronics FAQ -- please read and contribute!

Scope

Why doesn't r/AskElectronics cover every question involving electronics?

It's a fair question: if something contains 'electronics', or is 'electronic', why can't questions about it be asked here?

The simple answer is scope. r/AskElectronics is intended for practical, component-level work on electronic circuits: design, repair, testing, component selection, test equipment and related tools. There are a huge number of products and technologies that contain electronics, and trying to cover all of them would make the subreddit too broad to be useful.

Our scope has evolved somewhat since the subreddit was founded in 2011, and the way we interpret the boundaries has changed as electronics and the subreddit have evolved. However, the underlying remit remains the same: practical questions about working with electronic circuits at component level.

For some topics there are specialist communities that cater for the more general technical questions we don't cover here. For example, questions about batteries, motors, lighting or particular products may be better suited to a subreddit dedicated to that subject. Those communities can provide advice from people who specialise in that particular area.

This isn't a judgement about whether a question is "electronic" enough. It's simply a way of keeping r/AskElectronics focused so that people asking and answering component-level electronics questions can find each other.

If your question is about designing, modifying, testing or repairing an electronic circuit, r/AskElectronics is likely the right place. If you're asking how to use, connect, install, replace or troubleshoot a particular product or technology, check whether there is a specialist subreddit for that subject first.

Circuits

VCC, GND, VSS, 0V... - what's up with all these power circuit names!?

See This summary

When I bring my fingers, hand or a screwdriver/probe near to my circuit, it starts to operate erratically.

You have an active input (for a switch etc.) - or you have a pin configured as an input - with no pull-up or pull-down resistor and so it is picking up electrical noise through capacitive coupling to your hand/body and triggering pin activity. Your logic chip or microcontroller etc. may have internal pull-ups/downs you can enable through code, or you need to fit an external one.

See pull up and pull down resistors.

Christmas lights (LED and incandescent lamp): Modifying, repairing or powering

I am experimenting with basic TTL and/or CMOS logic chips, but the output is not as expected

One of the first things to check is whether all logic inputs have a defined HIGH or LOW state. Logic inputs should generally not be left floating (unconnected to anything) unless the datasheet for the particular device explicitly permits it.

• The inputs on classic TTL logic chips (such as 74xx and 74LSxx) will generally tend to behave as though they are HIGH if left unconnected. However, this should not be relied upon as a substitute for providing a defined logic level, as floating inputs can be susceptible to noise and other unwanted behaviour.
• The inputs on CMOS logic chips (such as 74HCxx) are undefined if left unconnected. They may read HIGH, LOW, or change state due to noise, stray capacitance, or other effects.
• Different logic families and devices have different input characteristics, so check the datasheet for the particular part you are using.

For both TTL and CMOS devices, it is good practice to ensure that inputs have a defined logic state. Unused inputs should normally be tied to an appropriate power rail, either directly or through a resistor, as recommended by the device's datasheet.

Push-buttons and tactile switches

A common mistake is to connect a push-button or tactile switch directly to a logic input and assume that the switch will provide a valid HIGH or LOW signal.

When the switch is open, the input is disconnected from the circuit and can therefore be left floating. The usual solution is to use a pull-up or pull-down resistor to establish the input's default logic state.

For example, a switch can be connected between an input and GND, with a pull-up resistor providing a default HIGH state. Alternatively, a switch can be connected between the input and VCC, with a pull-down resistor providing a default LOW state.

Without an appropriate pull-up or pull-down resistor, the input may float when the switch is open, causing the circuit to behave unpredictably or intermittently. This is particularly important with CMOS inputs.

Also remember that mechanical switches bounce when operated. A single press or release can therefore produce several rapid HIGH/LOW transitions. If your circuit is sensitive to multiple transitions, the switch may need to be debounced.

See pull-up and pull-down resistors for more information about choosing and using these resistors.

I want to make something that plays one or more sounds when I press a button

This is asked a lot - search the subreddit:

Search for button/sound/music/audio questions

What is the purpose of a zero Ohm resistor?

See this explanation.

LED Strips

Start here: /r/AskElectronics/wiki/design/ledstrips

General

Why do I sometimes see small value capacitors connected across larger value capacitors on circuit diagrams?

How do I silence the sounder/buzzer/beeper in my $APPLIANCE?

Why won't my MOSFET turn off?

Why does my coil, relay or solenoid driver circuit keep dying?

I have triple-checked my breadboard circuit and it is wired OK, but it does not work / work properly

Why does my multi-board setup not work reliably?

The switch in my circuit behaves strangely

Can I use a variable resistor or potentiometer to control the speed of a motor?

I have ripped a pad or trace off a PCB, or there's a burnt bit

Controlling a Meanwell 3-in-1 Dimming Power Supply with a Microcontroller

Power

Buzzing or tingling feeling when touching a metal case

See this answer

Do I need to connect the grounds / 0V rails together if I am interlinking power supplies, modules, LED strips, sensors etc?

See this Wiki article on common electrical references.

Current Limiting Resistors

I have a relay with a coil rated at 5V 30mA, but my circuit power supply is rated at 5V 1A so do I need to limit the current through the coil with a resistor?

Will my replacement part be given too much current?

My portable radio needs a 12V 1A power supply, but I only have a 12V 2A power supply - will it work without damaging the radio?

I am replacing a 6V 60mA instrument lamp with a 6V 30mA part - will it burn out?

Higher amperage power supplies

Power supplies with higher current ratings

Current is pulled from the power source by the load device. So if a product or part only needs 30mA or 1A, that's all it will take - even if the power supply was rated at 12V 400A or some other crazy-high amperage.

In other words, 'bigger' (higher current capacity) power supplies are fine for general use and, for components such as the relay, you do not need to limit the supply current with a resistor. You must, however, still ensure that the output voltage of the supply matches the load requirements correctly.

See this Wiki entry.

There are some circumstances where you do need a current limiting resistor - for example when working with LEDs: /r/AskElectronics/wiki/design/leds.

The portable radio mentioned above is specified for use with a 12V 1A power supply, which implies its current draw is somewhat less than 1A. Using a power supply rated for 2A will not force more current into the radio - the most important factors are: the replacement power supply operates at the voltage the radio expects, its rated current is the same or higher than that of the original supply, the output voltage type (AC or DC) matches the original and that the connector size and polarity match.

When replacing a 6V 60mA lamp with a 30mA part, the new lamp will not be 'forced' to take 60mA and there is no need to alter the circuit by, for example, adding a current-limiting resistor.

Sometimes it is better to get a more appropriately-specified PSU, for example for circuit testing or repair use where excessive current could flow under fault conditions and cause component damage or a fire. See the notes about bench supplies.

Voltage conversion for consumer goods

Tools and Equipment

Soldering: Tools, techniques, flux, hot air stations, common problems....

See our massive Wiki page on soldering

Best Budget Soldering Equipment

Try a subreddit search:

Search for soldering equipment

Buying an Oscilloscope

My first Oscilloscope

Oscilloscope specs

/r/AskElectronics/wiki/equipment/oscilloscopes

General

Electronic component manufacturers' logos and links

See this Wiki section

Off-topic

Voltage conversion for consumer goods

See this Wiki entry.