Understanding DIACs

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In this video I explain the basics of the DIAC - what it does, how it works, how it can be used, and I show how it is used in a circuit with a TRIAC.
The actual word DIAC stands for DIode for Alternating Current and this gives a clue about what these electronic components do in an electronic circuit, often assisting in a switch circuit
A DIAC is a two terminal device with its circuit symbol looking a bit like two parallel diodes in opposite orientations closely placed and merged together The electrodes are designated MT, standing for main terminal that's MT1 and MT2 - they may also be called Anode 1 and Anode 2 on some occasions.
There are two types of structure used for DIACs, the three layer structure and a five layer version. There are some differences between the two, but essentially they perform the same function in a circuit.
In terms of its operation the DIAC is not like an ordinary semiconductor diode that allows current through in only one direction, but instead the DIAC allows current through after a certain breakdown voltage is reached - often it is called a DIAC switch for this reason. This is known as the breakover voltage - typically this is between about 25 and 48 volts. The DB3 is a popular DIAC - the DB3 DIAC has a breakover voltage of around 32 volts.
The most common use for DIACs is with TRIACs. TRIACS are widely used for AC mains control - one of the very popular applications is within light dimmers. However TRIACS are notorious because their switching characteristics are different in different halves of the cycle and that's not good.
To overcome this, a DIAC can be placed in the TRIAC circuit in the gate circuit. As DIACs have a nice defined breakover voltage in both directions, it means that when the DIAC reaches its breakover voltage, the TRIAC will be fired and it will switch.
To find out more about DIACs check the following link: www.electronics-notes.com/art...
Website homepage: www.electronics-notes.com
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19 май 2024

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Комментарии : 95

@captainboing 4 месяца назад

When I was a telephone engineer, I used diacs as a privacy device where multiple telephones were on the same pstn circuit. Put one in the line path on each telephone. The line voltage across the diac wil be around 50v so you get dial tone when you pick up, the line voltage falls to around 12v but the diac continues to conduct. Anyone else picking up a phone on that circuit gets a dead line because the 12v is below the break over voltage. Simple single component privacy.

@ElectronicsNotes 4 месяца назад

Yes they had a good number of different uses in s variety of circuits.

@clownhands 4 месяца назад

Genius use of a diac!

@jamescollier3 4 месяца назад

That's like the most intelligent comment in socal media lol

@rex-up9ln 4 месяца назад

@@jamescollier3cool

@gustavotasquer7389 3 месяца назад

Brillant

@headpox5817 4 месяца назад

Thank you for this no-nonsense description of a DIAC.

@ElectronicsNotes 4 месяца назад

Glad it was helpful!

@dl5244 4 месяца назад

Despite the symbol appearing as two diodes with opposite orientation in parallel, it seems that a DIAC behaves very similar to two Zener diodes in series with opposite orientations. This arrangement of two zeners in series would block current flow in both directions until Vbr+Vf is exceeded, just like a DIAC

@Vincent_Sullivan 4 месяца назад

Not so much... Notice in the DIAC V/I curve shown that when the DIAC breakover voltage is reached the voltage across the DIAC falls to a very low voltage (Vf is much less than Vbo) whereas the arrangement of 2 opposite polarity Zeners in series will clamp the voltage across them to the Zener voltage plus the forward voltage of the conducting diode as you state - so the overall effect in a circuit may be quite different.

@SimEon-jt3sr 4 месяца назад

Was just wondering lol like why block it both ways, the zener make a lot more sense but now the symbol still makes sense

@SimEon-jt3sr 4 месяца назад

@@Vincent_Sullivan Oh..... Well luckily I probably won't ever need to design my own circuit with one...

@deantiquisetnovis 4 месяца назад

Short, crisp, clear. In other words: brilliant!

@ElectronicsNotes 4 месяца назад

I’m really glad you found it useful.

@peterbulanyi1708 4 месяца назад

Very nice description of the Diac. I remember seeing these devices on the control terminal of Triacs in simple electric fence energisers when I used to repair them in the early 90s.

@ElectronicsNotes 4 месяца назад

Thank you very much for your comment. Yes, they were very common. I remember them from their use in light dimmers. But the application you found them used in shows how widely they were used.

@stupossibleify 3 месяца назад

Hackaday brought me here, and I'm so glad it did. Thank you.

@ElectronicsNotes 3 месяца назад

I'm really glad you arrived on the channel - please check out some more videos. Thanks for your comment.

@louco2 4 месяца назад

Learned something new today! Thank you so much for taking the time to do these videos.

@ElectronicsNotes 4 месяца назад

Glad it was helpful! It is so good to hear when someone finds these videos useful.

@rohnkd4hct260 4 месяца назад

Thanks for the refresher lesson! Electronics School was llllllloooonnnnggg time ago 😆

@ElectronicsNotes 4 месяца назад

You're welcome! Glad you found it useful.

@Necrocidal 3 месяца назад

Really nice and to-the-point, thanks!

@ElectronicsNotes 3 месяца назад

Glad you found the video useful. Thanks for your comment.

@wisteela 4 месяца назад

I didn't know these existed.

@SlyerFox666 4 месяца назад

Such a good video, could have mentioned the 5 layer as a SIDACs as that's a component that hardly anyone's heard of. 👍

@ElectronicsNotes 4 месяца назад

Glad you liked the video. Yes, I suppose I could have mentioned the SIDAC but I wanted to focus on the DIAC. Thanks for the idea though - maybe another video sometime.

@arthurmoore9488 4 месяца назад

@@ElectronicsNotes Please. I've used DIACs before, but never even heard of a SIDAC.

@ElectronicsNotes 4 месяца назад

@@arthurmoore9488 I thought I had a description of them on my website, but could not find it. However, you can take a look here: en.wikipedia.org/wiki/DIAC#:~:text=A%20silicon%20diode%20for%20alternating,member%20of%20the%20thyristor%20family.

@bretfuzz925 4 месяца назад

I used a SIDACtor to protect the TX side of a DS1 (T1) line interface module from CMOS latch up from transient voltages. Very handy device indeed as it would reset on the zero voltage crossing after a transient event. Much better solution than PTC thermistors.

@SlyerFox666 4 месяца назад

@@bretfuzz925 Well you just taught me a few things there, nice example 👍

@Doom2pro 4 месяца назад

I always thought of them as a solid state spark gap of sorts.

@xanataph 4 месяца назад

Yeah definitely, or I would also say, like a solid state replacement for a neon bulb in such triggering circuits.

@arthurmoore9488 4 месяца назад

Randomly was suggested this video. I have designed a voltage to frequency converter for an isolated high voltage power supply before. They're extremely useful wherever a trigger threshold voltage is needed. Now I'm wondering if I can make a DC mains voltage 555 timer.... On the other hand, Mains AC has such a consistent frequency I could just make a clock.

@borisj4054 4 месяца назад

Very basic but good for beginners

@ElectronicsNotes 4 месяца назад

It is aimed at providing the basics for those who are not familiar with DIACs. I hope you managed to take something away froth e video. Thanks for the comment.

@MrAvant123 4 месяца назад

Also, I would say using a DIAC to switch a Triac cuts right down on the nasty crossing RF interference you get from a raw switched Triac.......

@ElectronicsNotes 4 месяца назад

Indeed it does.

@s.hutton2100 4 месяца назад

Excellent video thank you. Have subscribed. Regards Stephen.

@ElectronicsNotes 4 месяца назад

Glad you liked the video and thanks for subscribing.

@AdrianHiggins83 4 месяца назад

Very good explanation

@ElectronicsNotes 4 месяца назад

Thanks for your comment. I really appreciate it.

@maxwang2537 3 месяца назад

Short and sweet!

@ElectronicsNotes 3 месяца назад

Hope you found it useful.

@maxwang2537 3 месяца назад

@@ElectronicsNotes yes very helpful. Recently I cam across one on a modern sewing machine foot pedal (motor speed controller), which I mistook as a diode.

@jonathanwilhelm8891 4 месяца назад

Very nice video ❤ great work

@ElectronicsNotes 3 месяца назад

Glad you appreciated it. Thanks for the comment.

@xDR1TeK 3 месяца назад

I see the RU-vid algorithm is working well at last. I liked its suggestion quite well. One question though, seeing how the diac is a diode for ac, but works as two zeners back to back, shouldn't it be called a zener for ac instead? Ziac or something like that? Just wondering. Thank you for the wonderful presentation.

@ElectronicsNotes 3 месяца назад

I love your idea. A DiAC actually works in a different way to two Zeners back to back, in terms of the underlying physics, but it does appear to give a similar effect in the surface. Great idea!!

@MrMulleteer 4 месяца назад

Triac/diac dimmers are getting pretty rare because they cant reliably work with the light loads like leds. IC controlled triacs are becoming the norm.

@ElectronicsNotes 4 месяца назад

Indeed, but the DIACs are still used wit them in some instances and also they can be used for other applications such as transient limiting. But yes, with modern LED lamps which require the cycle to be cut on the trailing edge, the traditional circuits are not as widely used.

@maxwang2537 3 месяца назад

2:43 I have difficulty understanding triac. Some questions please. Are the two main terminals of a triac symmetric with reference to the gate. In other words, can we apply a trigger voltage between the gate and either terminal 1 or 2 to switch it on? In the schematic here, which voltage is working as the trigger, the voltage across the pot or the cap, or both? Any help is appreciated!

@xanataph 4 месяца назад

You see in a lot of older schematics for light dimmers, drill speed controllers etc they used neon bulbs to do a similar job to the DIAC due to their breakdown characteristics. But I would presume the DIAC's characteristics are sharper than that of a neon? (And of course exhibit the breakdown at a lower voltage)

@ElectronicsNotes 3 месяца назад

I confess I've never thought of using a neon, but good point.

@enggdiplomabook7076 3 месяца назад

Very nice video❤

@ElectronicsNotes 3 месяца назад

Thank you!

@maxwang2537 3 месяца назад

2:32 a question here. I suppose what’s shown here is the behaviour of an ideal triac we would expect. How far off are the actual characteristics of a real triac from this?

@peetduplessis7796 4 месяца назад

Like your AVO Model 8/9. Still have a 9 Mk II.

@ElectronicsNotes 4 месяца назад

Yes, it’s an AVO 8 and it still works well.

@peetduplessis7796 4 месяца назад

Proper equipment that is well cared for will work for many years! @@ElectronicsNotes

@Accumulator1 3 месяца назад

On those light dimmers, what component is likely to fail on them? Would it perhaps be the triac or diac? I have several that stopped working, I like to repair.

@ElectronicsNotes 3 месяца назад

My guess is that kit is most likely the Triac, but I've no great experience on fixing light dimmers.

@kamalhamdi6739 3 месяца назад

Thank you for information khalid Morocco Casablanca

@ElectronicsNotes 3 месяца назад

I’m glad you found the video useful.

@qzorn4440 3 месяца назад

very nice. Thanks.

@ElectronicsNotes 3 месяца назад

Most welcome - glad you appreciated it. Thanks for the comment.

@bob_mosavo 4 месяца назад

Thanks 👍

@ElectronicsNotes 4 месяца назад

Thank you too!

@tenlittleindians 4 месяца назад

So how do you test these components when found in a circuit?

@ElectronicsNotes 3 месяца назад

Not easy to test, and especially when in circuit.

@henryj.8528 4 месяца назад

What was that four-layer device Shockley was always working on?

@ElectronicsNotes 4 месяца назад

He was working on a four layer device, and it probably developed into what we know today as a Thyristor.

@StephanBuchin 4 месяца назад

2:33 Is the conduction waveform presented here corrected by the diac? Because it looks pretty symetrical to me.

@ElectronicsNotes 4 месяца назад

I used the perfectly switched waveform and not one that you might find from a triac on its own. I was trying to illustrate the way the triac controlled the waveform.

@StephanBuchin 4 месяца назад

@@ElectronicsNotes Thank you for answering my stupid question, you relieved my anxiety of not understanding. I remember i was already this tiring when I was a student 🤓

@ElectronicsNotes 4 месяца назад

@@StephanBuchin You question was perfectly valid and not stupid in anyway. We all learn by asking questions, and often the 'stupid' questions are the ones that everyone else wants to ask, but nobody dares. Thank you.

@user-lp3cf5yn5b 2 месяца назад

So basically it acts like a wall until the breakdown voltage is exceeded then it conducts like a diode until the voltage drops below breakdown voltage again?

@ElectronicsNotes 2 месяца назад

That is a sort of descriptor it. The characteristic is a little more complicated, but it acts like a “wall” until a break over voltage is reached. Then the voltage falls and slightly rises with increasing current.

@ECHQ23 3 месяца назад

you could expand on this and compare to a "modern" bidirectional TVS diode.

@ElectronicsNotes 3 месяца назад

That is certainly a possibility.

@johnaweiss 3 месяца назад

I wonder if it could be used as a dead-time generator.

@ElectronicsNotes 3 месяца назад

I had never thought of that - may be it could.

@johnaweiss 3 месяца назад

@@ElectronicsNotes i guess maybe you'd have to feed it a sine wave... mains is a sine wave.

@stanleybest8833 3 месяца назад

Don't advocate dimmer switches. They generate a heap of radio noise.

@billfargo9616 3 месяца назад

LEDs are dimmed using pulse width modulation, not voltage control. .

@ElectronicsNotes 3 месяца назад

I wanted to say that the light dimmer I was showing was for incandescent lamps as LEDs use a different approach.

@rickhunt3183 4 месяца назад

A diac never met a triac it didn't like.

@user-xi5oh9yc4c 4 месяца назад

I do not understand how you read that plot at 1:45

@ElectronicsNotes 3 месяца назад

Looking at the top right quarter, imagine the voltage rising and little current is passed, when it reaches its breaker point, the current increases and the voltage across it falls, and then this voltage is increases slightly as the current increases much more.