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DIACs gone the way of dinosaur?

SeanF

Mar 14, 2012
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Greetings all, and new to the forums.

I have been involved in a circuit design and research for the circuit. Our circuit uses a DIAC, with a breakover voltage of 35-40. Looks like the DIAC is slowly being phased out, as more and more, newer technology emerges in the lighting industry. DIACs were very commonly used to trigger TRIACs in light dimmers, however, with more lighting going towards LED and CFL's, looks like demand for the old world DIAC has died off, and manufacturers are not wanting to make these things. Read that as expensive and difficult to obtain.

I have found a few manufacturers, DIOTEC, STMICRO, and NTE that still make them. Only for who knows how much longer before they send out a obsolescence notice...

So that brings me to my question. What else can take the place of a DIAC... Are the manufacturers just including this part in with the TRIACs, or has the technology changed that much were there just isnt a viable solid replacement for this?

Thanks
 

Sid723

Jan 28, 2010
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If a diac is usually used in conjunction with another device such as a triac, then why not make devices that already have the diac "built-in"?

Some inductive drive transistors already have a diode built-in from collector to emitter, so why not do the same with the diac in other specialized devices?

So, if this can be done, then there will not be much call for a "Lone" diac. Not much demand leads to manufacturers who stop manufacturing the device.

Make sense?
 

john monks

Mar 9, 2012
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DIAC's are normally used with CFL's. They are used to start the oscillation of an oscillator.
 

twister

Feb 12, 2012
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A diac is nothing more than two zener diodes back to back. I have used two 34 volt zener diodes before and they worked good. I don't know what the exact voltage of a diac is. :)
 

duke37

Jan 9, 2011
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A diac is not the same as two zener diodes back to back. When the diac turns on, the voltage across it will drop unlike the zener diode case, thus it can discharge a capacitor not just hold it at a certain voltage. See Wikipedia.
 

twister

Feb 12, 2012
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A diac is not the same as two zener diodes back to back. When the diac turns on, the voltage across it will drop unlike the zener diode case, thus it can discharge a capacitor not just hold it at a certain voltage. See Wikipedia.

Maybe I'm wrong; I only know that I replaced one with zener diodes and the circuit preformed exactly like it did with the diac. Have you ever tested one, to see if wikipedia is correct?
 

duke37

Jan 9, 2011
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Hi Twister,
I have not investigated the characteristics of diacs but I have used them to switch a SCR about once a second.
A capacitor was charged with a very low current and when the diac fired, a large current switched the scr on. If I used a zener in this circuit, I woud only get the input current which is very low and would be insufficient to fire the scr.
 

twister

Feb 12, 2012
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Hi Twister,
I have not investigated the characteristics of diacs but I have used them to switch a SCR about once a second.
A capacitor was charged with a very low current and when the diac fired, a large current switched the scr on. If I used a zener in this circuit, I would only get the input current which is very low and would be insufficient to fire the scr.

I built a simple lite dimmer circuit and I accidental shorted out my diac, so I replaced it with two zeners. Don't know how long it would have lasted but it worked good for testing my circuit. I was trying to vary the current dynamically, but I couldn't get it figured out. I think I have thought of a way now, though.
I just checked Jameco's catalog and they no longer have diacs in there that I could see. I was going to order some. It's a shame when we no longer can buy the parts that we need. I bet they can still get them in China. And even the parts we can get are so high that the Chinese can sell a finished product cheaper than we can buy the parts.
 

Sid723

Jan 28, 2010
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John and Duke are correct. I also used to design high frequency electronic ballasts. The diac was used to start the oscillator circuit.

When power is applied, a capacitor would charge up to around 32volts, then the diac would fire and apply current to the base of a push-pull oscillator circuit and start the oscillator.

When a diac fires, it's resistance becomes very low until a certain low current level is reached, then it goes back to it's non-conductive state.
 

twister

Feb 12, 2012
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I was looking at diodes in wikipedia. It looks like it says that diodes over 5.6 volts are avalanche diodes.
But it is not really clear. ha :)

Well, I just went to the Jameco website, and they say nothing about the higher voltage zeners being avalanche diodes.
I also found that they do sell diacs. So they might be still making them, or maybe it would be wise to stock up while they are still available. I heard that they are going to outlaw 100 watt lite bulbs too, so I stocked up on those.

http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_1386187_-1
 
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