There are plenty of electronic components which were once ubiquitous but once the niche which led to their existence has passed, they fade away to remain a junkbox curio. The DIAC is the subject of a recent ElectronicsNotes video, and while it might not quite yet have slid into total obscurity yet it’s definitely not the most common of parts in 2023.
If you’ve encountered one it will almost certainly be in the trigger circuit of a lighting dimmer or motor controller, where its bidirectional breakover makes for symmetrical control of a triac gate. This extremely simple circuit allows for perfect control of AC-powered devices, and could once be found everywhere. Its demise over recent years tells an interesting story of our changing use of electricity, as not only have other devices such as smart lights and brushless motors appeared which preclude traditional dimmers, but also we now demand better RF performance from our lighting controls.
The DIAC is still a handy part to know about, and you can take a look at the video below the break. We would normally try to link to another Hackaday story using a DIAC, but is it telling that we couldn’t find one? If you can, link it in the comments!
240VAC scary! Me use 3.3V
240V is still low voltage, though not ELV. Similarly, 3V3 is quite high voltage.
Perhaps not scary, but unnecessarily high for the vast majority of things. Virtually everything “mains powered” thesedays takes a mains input of 240V AC, then down converts it to 3.3V, 5V, 12V or 24V DC for use in the actual stuff the device does.
Fridge, washing machine, coffee machine,lathe,drill press,ceiling fans, stove, sewing machine, microwave, heat pump, freezer, hot water ….
Yup definitely.
Lights, computer, phone.. light in fridge, cpu in washing machine.
But yes, any higher wattage items should still use 230v, or higher.
It’s strange that 3phase appliances are so rare today
It isn’t unnecessarily high at all. For devices that need a lot of power it is necessary (in the average house there are plenty of devices that need a lot of power) and for devices that don’t need the power it is relatively small, easy and cheap to convert down to the needed voltage.
Using lower voltages directly just isn’t practical and having multiple voltages is also impractical. It’s far better to just have a single supply that can power anything you want to plug in.
Trigger coil for xenon flash tube. Remember those?
For anyone else who doesn’t find video an appropriate medium for describing a component: https://en.m.wikipedia.org/wiki/DIAC
Thank you!
@BT said: “For anyone else who doesn’t find video an appropriate medium for describing a component:”
https://en.m.wikipedia.org/wiki/DIAC
And for those of us who have moved beyond highly restrictive, over-priced, insecure, mobile devices that constantly spy on us for money and political control:
https://en.wikipedia.org/wiki/DIAC
And a bit more – related:
Thyristor
https://en.wikipedia.org/wiki/Thyristor
TRIAC
https://en.wikipedia.org/wiki/TRIAC
Silicon controlled rectifier
https://en.wikipedia.org/wiki/Silicon_controlled_rectifier
@Drone said: “And for those of us who have moved beyond highly restrictive, over-priced, insecure, mobile devices that constantly spy on us for money and political control”
Only if you choose the wrong mobile device.
Sorry, but as of 2024 there’s not a single one mobile device that does not contain at least one closed driver or binary blob that runs at privileges higher than root, which makes the best place to put spyware inside. Unfortunately we mostly won the application war, but device drivers are another thing, and we won’t have a fully trustworthy mobile device unless hardware manufacturers start documenting their chips, which is not happening anytime soon.
I mean… I don’t really want to disagree with you here but kind of feel that I must; actually the smartphone is probably the least overpriced computer-form-factor.
A good last gen refurb is like 200€.
The same for a laptop is along the lines of 1500€, it’s just mental.
And if you try to save money and “just upgrade parts” of your desktop, good luck since even a single medium performance part quickly approaches 500€.
In this light, a smartphone is pretty cheap. It’s got a small display, for sure. But pretty much every other thing it will be almost on par with any other systems.
Diacs are used in mains-powered resonant inverters for fluorescent lights. There’s a half-bridge which transistors are controlled by a feedback transformer from LC oscillator, but which one will close first? There’s a capacitor charging through a resistor and when it reaches 32V, a diac starts conducting, giving one of transistors a kick which starts oscillations.
There’s also (Diac + Triac = Quadrac) as well.
https://www.powerelectronictips.com/what-are-triacs-diacs-and-quadracs-good-for-faq/
I’m glad to see someone mentioned the Quadrac. Decades ago, I had a spark generator that used one to control the coil. It was a very basic circuit that I probably wouldn’t recommend today. It had a Ford ignition coil, Quadrac, ballast resistor, resistor, and capacitor. It also plugged directly into the mains via a 2-prong plug.
If you wish to obtain a Diac for some experimentation a readily available source is a worn out or faulty Compact fluorescent lamp (CFL). A Diac is often used in the resonating invertor starting circuit. The Diac can be easily identified. It looks like a glass diode but all the diodes on the PCB will be epoxy encapsulated type so any small glass component found will be a Diac.
There should be no safety issue created in disassembling a CFL. Instead of breaking the tube or smashing the base you simply saw around the plastic part of the base to expose the PCB. The lead wires to the tube and the metallic part of the base can then be cut and you are left with the PCB.
The electronic of CFL are generally robust and failures are often due to the fluorescent tube itself wearing out (tell tale blackening at ends of the tube) so the components on the PCB will probably all be OK.
With the PCB in hand some other components besides the Diac could be harvested. There will be the two high voltage power transistors, MJE15003 or equivalent in the resonating inverter and the DC filter capacitor, a good quality 4.7-10uF 400VW, 135⁰C electrolytic.
Probably nothing else is worth the trouble of de-soldering.
A quick check on Ebay identifies 400+ sources so obscure but not yet rare.
Still widely used in vintage sewing machine replacement speed control in a triac circuit.
Properly designed circuit is not an RF problem.
Oh, link please! This is too replace the foot pedal, variable compressed carbon pellet varistor style control, right? That gets hot with extended use?
I have a rotary tool with a dimmer-like circuit, a full bridge rectifier and a DC motor. What would the modern replacement for such speed controller be like?
The correct answer to (one interpretation of) your question is actually probably in the article; a brushless motor.
Otherwise, if you want an actual replacement for a burned controller, Id guess you can probably get some pwm-solution etc.. but the performance of the original circuit is probably at least as good.
so what is the modern replacement for the shown circuit? I mean what are the alternatives? also ,what is the current so common solution for switching on and off a AC load these days? Damn I feel old!
There are a few options, normal relays, Solid State Relays (SSRs), a number of semiconductor devices or in a lot of cases the AC is just converted to DC and then you don’t need to worry about switching AC.
“WHY NOT *TRY* A DIAC?”
Very funny! I like it!
There are a lot of opto isolated relays/switches that should be used for 240a AC projects. Then there are to smart switching devices.
Wow! I haven’t even heard the word diac since high school back in the early 80’s. In our electronics class we studied and built dimmers and various switching circuits which often used a disc. I can remember in the spring the prom committee would come to us with their theme and ask us what we could do. The woodshop would build the cabinets etc and we would install the lighting. Each student has to layout a small PWB with etch resistant ink tape etc, etch the board then build it. The board had a reed relay, pads for a pot, a diac, resistor and triac. We each would test these (isolation transformer + variac) then these modules were used control the lights in the prom props.
Last time I was drafting schematics the symbol for a variable resistor only has two pins with an arrow through the lines or box (newer style symbol) not three pins which is a reostat or potentiometer.
Note that one can use an optotriac (essentially working as optodiac) to safely drive a high power triac using much lower voltage and current, such as for example a pwm out from a microcontroller or other digital circuit.
Look at the MOC3023 data sheet, for example.
Reading about DIAC and not seeing it’s bigger brother SIDAC being mentioned is sad.
I have seen DIAC used in telephone parallel to the speaker to eliminate “clicks” from the rotary dial short-circuiting and opening the line.