Electrical Engineering Stack Exchange
Electrical Engineering Stack Exchange

Questions tagged [relay]

A relay is an electrically controlled switch. Electromechanical relays use an electromagnet to activate mechanical contacts, solid-state relays use semiconductor switches.

A latching relay stays in its current state when power is removed. A non-latching relay reverts to a known state when power is removed.

Relay contacts come in several varieties. The most common are:

  • A: normally open contact (NO)
  • B: normally closed contact (NC)
  • C: one normally-open and one normally-closed contact, both actuated by a single coil
  • X: normally open, double make
  • Y: normally closed, double break
  • Z: one X and one Y, both actuated by a single coil
3117 questions
88
votes
5 answers

Relay vs. Transistor?

Rather basic, I'm afraid, but when would you use a relay, and when would you use a transistor? In a relay the contacts wear out, so why are relays used at all?
hauntsaninja
  • 983
  • 1
  • 6
  • 5
66
votes
4 answers

How to choose a flyback diode for a relay?

A diode is put in parallel with a relay coil (with opposite polarity) to prevent damage to other components when the relay is turned off. Here's an example schematic I found online: I'm planning on using a relay with a coil voltage of 5V and…
Nate
  • 4,496
  • 11
  • 56
  • 75
65
votes
8 answers

Why are there so many relays used in a car, instead of transistors?

Recently I did some work on my car electric circuits. I see many relays are used in car circuits. These relays are used for simple switching, and I wonder why these circuits are based on relays and not on transistors or other electronic components…
Al Kepp
  • 2,971
  • 7
  • 26
  • 32
55
votes
5 answers

Triac versus Relay

What are the scenarios where Triacs can not replace a relay or vice-versa?
Daniel Grillo
  • 7,659
  • 18
  • 51
  • 69
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?
Apostle
  • 571
  • 2
  • 5
  • 9
45
votes
8 answers

Why are relays so frequently driven by optocouplers?

Since the advent of the many microcontroller development boards, like Arduino, there have been a number of relay modules sold to drive mains AC loads. A lot of these seem to use an optocoupler, driver transistor and a relay to drive the load…
Cybergibbons
  • 1,934
  • 1
  • 16
  • 21
41
votes
6 answers

What are some ways to use relays more efficiently?

We often seem to use microcontrollers to control relays, and a 5 V microcontroller is often used with 12 V relays. A relay may need several times more power than the microcontroller. Not a problem if you can use an SSR, which you can drive at a few…
stevenvh
  • 145,145
  • 21
  • 455
  • 667
30
votes
6 answers

Why does a relay have a minimum applicable load?

I am trying to find relays for my application and I read a data sheet which looks fine but specifies Minimum applicable load: 10mV 10µA In my circuit, it is expected that the relay closes but no voltage and current is applied. You can think of it…
Thomas Weller
  • 1,008
  • 1
  • 11
  • 25
25
votes
5 answers

Why are relays still used in electric ovens?

I bought a new electric fan oven recently. It has a digital thermostat and control system. Yet much to my surprise, I can hear a relay clicking on and off inside it to control the power to its heating element. The oven is rated at 4kW (230V). I…
nigel222
  • 641
  • 5
  • 10
25
votes
5 answers

Why don't relays incorporate flyback diodes?

Warning: this may be an extremely naive question (if so, please enlighten me). Many applications of relays require a flyback diode to protect against inductive voltage. I'm unable to find any relay that incorporates a flyback diode. Since it's such…
dss539
  • 353
  • 1
  • 3
  • 6
24
votes
5 answers

When/why would you use a Zener diode as a flywheel diode (on the coil of a relay)?

I have just been cogitating on the tutorial at http://www.electronics-tutorials.ws/io/io_5.html, and in the discussion of flywheel diodes it includes this sentence without further elaboration: As well as using flywheel Diodes for protection of…
Kaelin Colclasure
  • 2,669
  • 7
  • 27
  • 37
23
votes
4 answers

What does RU (UR) label on DC power relays mean?

I'm trying to figure out what does RU or maybe UR label mean on the body of many many relays. It's labeled almost on all (I've seen so far) DC relays which make me very curious :). Any suggestions?
Timur Fayzrakhmanov
  • 361
  • 2
  • 4
  • 9
22
votes
7 answers

Is there a downside to using multiple relays simultaneously to increase relay capacity

I want to install a relay to my fusebox which is controlling whole basement with machinery. Relays that I have have got 2A limit each, and I have 32 relays. Can I just use all of them for the same line and assume the relay's limit is 64A ? Is there…
user2102266
  • 343
  • 2
  • 6
19
votes
4 answers

Equivalent circuit of a solid state relay

I want to switch 50V AC voltage. Maximum drained current will be 5A. Frequency is 50Hz. Switching speed is not important, can be real slow, it is not a problem in my application. I wanted to use solid state relay at first for this purpose. But as…
hkBattousai
  • 13,913
  • 31
  • 114
  • 190
18
votes
5 answers

Are BJTs suitable for level-shifters? It seems FETs are more common,how do they compare?

I'm a hobbyist, and never got past the datasheets/tutorials for FET transistors; I'm a BJT man. I never found discussions dealing with BJT vs. FET and specific applications best suited for each type. My projects are very simple switching and logic…
user160409
1
2 3
…
99 100