There's a matter with the Diac

[Fahime]

Hi,
I'm using a Diac (DB3) in a simple circuit to study its behavior. The schematic is attached. The output is not what I expect! The break over voltage of this Diac is 34 volts.
The time the input sin wave is less than 34 Volts, The diac is off and the output is the same as input. when the intput voltage exceeds 34 volts, then the diac turns on and so the output should be zero!!!! but it's not in practice.
What I see on the scope is the constant 34 volts while the diac is on. Why's that? I can't figure out!!!!
Looking forward for your assists.

 

[CheapSlider]

Relaxation Oscillator

You should have a capacitor across the diac.

If the supply is DC
The voltage across the capacitor will rise (as it's charged up through the resistor) untill it reaches the turn on voltage of the diac.
When the diac turns on the capacitor will discharge.
The process will then start again.


If you have a supply of over 100V DC then you can use a neon bulb instead of the diac. The neon bulb has a turn on (break down) voltage of about 90V


The circuit can be used as a simple way to determine the value of an unknown capacitor.

Edit:
If the capacitor is too big you may 'zap' the diac
.

 

[Fahime]

You should have a capacitor across the diac.
If the supply is DC
.

My supply is 48Vac not DC. Do I need a capacitor any more?

 

[Nigel Goodwin]

Last time I looked it was 2007?, why are you playing with such old obselete technology?, which was never even very popular back in the 1970's!.

 

[Fahime]

Last time I looked it was 2007?, why are you playing with such old obselete technology?, which was never even very popular back in the 1970's!.

I plan to built a light dimmer. I need a Teriac to control voltage. and in order to control the gate of Teriac I need a Diac. So I have to practice it to know how it works. Then I can put it properly in the circuit.
Please tell me what is the matter with this circuit.

I also tried a SCR.It's behavior was unexpected too. I had the above circuit for the SCR and a series potantiometer and normal diod from Anode to the Gate of SCR . What's wrong with it?

 

[ericgibbs]

hi fahime,

Have a look at this site.

https://www.epanorama.net/documents/lights/lightdimmer.html

Lots of light dimmers, some with the 'olde worlde' diac!

IMO there are better ways to get the job done.

I suspect that the 220K resistor is current starving the diac, so it cannot go into a avalanche state, also whats the output
resistance of the voltage source?
Usually the diac has a low value capacitor across it, and it 'dumps' the charge on the capacitor into a load, or triac gate.

Regards

 

[CheapSlider]

In a lamp dimmer, its very important to get the resistor and capacitance values correct as the diac has to trigger the triac within the half cycle of the AC supply. The values will be dependant on both the voltage and frequency of your AC supply.

It's best to build a circuit that works then you can experiment.
A voltmeter is unlikely to be of much help in this application.

The dimmer circuit needs a minimum load otherwise the triac will not conduct



Edit: Relaxation Oscillator
This 'old' circuit is used to kick start SMPS or CFL electronic ballasts

 

[Hero999]

Last time I looked it was 2007?, why are you playing with such old obselete technology?, which was never even very popular back in the 1970's!.

DIACs are still widely used today in lamp dimmers and motor controllers, I wouldn't consider them to be obsolete.

 

[Nigel Goodwin]

DIACs are still widely used today in lamp dimmers and motor controllers, I wouldn't consider them to be obsolete.

His original post said he was "studying their behaviour", which I suggested was pretty pointless for such an obscure and ancient component? - he later altered his question to trying to make a dimmer work.

Are diacs used in modern dimmers anyway?, going back probably 20 years or so most didn't use them anymore.

 

[Fahime]

Are diacs used in modern dimmers anyway?, going back probably 20 years or so most didn't use them anymore.

Then what's new in dimmers?

In the responce to other firends, as you said my problem was the capacitor and the resistors that were somehow large. Thanks for all for your assistance.

 

[Nigel Goodwin]

Then what's new in dimmers?

They used IC's instead.

 

[Hero999]

Are diacs used in modern dimmers anyway?, going back probably 20 years or so most didn't use them anymore.

Yes.

They used IC's instead.

I know there are ICs (no apostrophe is required here, it's plural no possessive ) around but I've never seen any dimmers that actually use ICs. All of the dimmers I've opened up use discrete components.