-1
\$\begingroup\$

These are something that has confused me for quite some time. Just like a lot of you, I'm sure, I'm a tinkerer. I like to take things apart, salvage parts, etc. A lot of stuff I take apart have a triac for AC control (a good example is a broken heater fan I'm trying to fix, but the triac isn't the issue there).

I know (or I think I know) that a triac is used for phase control of AC circuits, but I always see them used with a diac in schematics, and that's where the main confusion comes in. In almost everything I've taken apart with a triac in it, there's no specific component that comes up as a diac (when I look up part numbers).

My understanding is that a diac can simply be two regular schottky diodes in parallel with reverse polarities, is this true? I'm not overly fond of the idea of trying it with live AC without understanding it a little better. I'd like to phase-control a 120V resistive heating element (or more than one, if possible) and I just want to be more comfortable with what I'm doing. The internet keeps contradicting itself with the information I find.

\$\endgroup\$
  • 3
    \$\begingroup\$ This is like a monologue, I'm already fed up at the 3rd line. Split your question into paragraphs to make it easier to read. Focus on your question instead of mentioning useless and irrelevant information. \$\endgroup\$ – Bimpelrekkie Dec 7 '16 at 10:11
  • \$\begingroup\$ I agree with the above, strip out the useless words and focus on the question. \$\endgroup\$ – Andy aka Dec 7 '16 at 10:13
  • \$\begingroup\$ Also: first read: en.wikipedia.org/wiki/Thyristor then en.wikipedia.org/wiki/TRIAC and notice how 2 Thyristors make a TRIAC. Then read en.wikipedia.org/wiki/DIAC The DIAC is only related to a TRIAC in that it is often used in TRIAC based circuits. \$\endgroup\$ – Bimpelrekkie Dec 7 '16 at 10:15
  • \$\begingroup\$ Large amounts on web on this. | A DIAC is a device which "brfeaks down" / conducts non destructively when voltage acrossit reaches a set level. If used in the path to a TRIAC or SCR gate it allows eg a capacitor to charge to the trigger voltage and then "dumps" the capacitor into the gate, allowing faster and more positive triggering. || A DIAC (CANNOT be properly replaced with Schottky diodes. They DO NOT do the same job. | Search for phase controlled lamp dimmers to see how a lower voltage heating element can be controlled. \$\endgroup\$ – Russell McMahon Dec 7 '16 at 11:33
  • \$\begingroup\$ Apologies, I wrote the question after a long day at work. My confusion lies with multitudes of information on the internet, some of which contradicts some others. Like here where it says "two Shockley diodes may be joined in parallel facing different directions to form a new kind of thyristor, the DIAC". It's easy to identify a TRIAC on a circuit board (where it's used for phase control), but none of the other parts on the board are called a DIAC when I search part numbers. \$\endgroup\$ – HaLo2FrEeEk Dec 7 '16 at 14:44
1
\$\begingroup\$

There are many diacs (I don't believe it should be capitalized) available on the market, most currently sold are variations on the DB3 part number.

Older numbers included the ST2 originated by General Electric.

A diac is a bidirectional switch that breaks over at a controlled voltage in each direction. It does not behave like two zener diodes- it has a negative resistance characteristic like a thyristor- the voltage across it drops (assuming the current is limited) and it will continue to conduct until the current drops below the holding current.

A similar functionality is provided by a SIDAC device.

You could replace a diac with four BJTs (2 NPN/2 PNP) and a couple zeners and a couple resistors, but a couple ordinary diodes are not going to behave the same.

\$\endgroup\$
  • \$\begingroup\$ Thank you for the information. After digging through my parts bins I found an EL 3063 (datasheet) and a BCR 16PM (datasheet). Am I correct in assuming that the EL 3063 is, essentially, a "photo-diac", just one that isolates the control circuit from the AC circuit? \$\endgroup\$ – HaLo2FrEeEk Dec 7 '16 at 17:07
  • \$\begingroup\$ It is a photo triac with a zero-crossing circuit so it will not turn on if the voltage across the triac is more than a certain amount (20V maximum)- mostly in order to reduce EMI. Yes, the purpose is to isolate AC circuit from control. \$\endgroup\$ – Spehro Pefhany Dec 7 '16 at 20:08
  • \$\begingroup\$ Could you explain why it would be useful to have it not turn on above a certain voltage? And I'm guessing that it wouldn't be super useful to what I'm trying to do. Also I can't figure out why, out of all of the devices that I've taken apart that contain a triac, none seem to contain a diac. Even the controller board for a washing machine, into which the motor directly plugged in. \$\endgroup\$ – HaLo2FrEeEk Dec 7 '16 at 20:25
  • \$\begingroup\$ Zero crossing is as I said above "...mostly in order to reduce EMI". Diacs are used mostly in light dimmers and sometimes in fluorescent lights. Digikey has a couple hundred thousand in stock so they're not in short supply. \$\endgroup\$ – Spehro Pefhany Dec 7 '16 at 20:32
  • \$\begingroup\$ It's more that I'm in short supply of money. I have to move in a week or two and I can't justify spending money on something like this, even if it's very little money. Add to that that the person I'm living with now isn't delivering me my mail. I'm trying to make the best out of what I have. \$\endgroup\$ – HaLo2FrEeEk Dec 7 '16 at 20:40
0
\$\begingroup\$

There is large amounts on web on all of this.

A DIAC is a device which "breaks down" / conducts non destructively when voltage across it reaches a set level. If used in the path to a TRIAC or SCR gate it allows eg a capacitor to charge to the trigger voltage and then "dumps" the capacitor into the gate, allowing faster and more positive triggering.

A DIAC CANNOT be properly replaced with Schottky diodes.
They DO NOT do the same job.

Search for phase controlled lamp dimmers to see how a lower voltage heating element can be powered from AC mains.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.