Can i use an npn bjt as a triac , like can i apply an ac 220v 50hz ?

 

Can i use an npn bjt as a triac , like can i apply an ac 220v 50hz ?

No. BJTs and TRIACs are two completely different things.

 

Here is a relationship with Mosfet and BJT.
Max.

 

No, you can use a Thyristor and bridge rectifier instead, but why bother.

 

No. BJTs and TRIACs are two completely different things.

Yes i know bro but can i apply ac across the collecto

No, you can use a Thyristor and bridge rectifier instead, but why bother.

i mean can i apply ac at its gate and across the collector and emitter , logicaly it will work at the positive half cycle but at the negative one it doesnt so it may work as a half wave rectifier knowing that i can use a diode for that but just wondering is it correct ?

 

Here is a relationship with Mosfet and BJT.
Max.

i mean can i apply ac at its gate and across the collector and emitter ,logicaly it will work at the positive half cycle but at the negative one it doesnt so it may work as a half wave rectifier knowing that i can use a diode for that but just wondering is it correct ?

 

With no base current (open-base), an NPN transistor will block the positive half-cycle (if within its voltage rating) but will conduct on the negative half-cycle when the base-emitter junction breaks down at about 5V or so.
If any significant current flows under this reverse voltage condition, the transistor will likely be zapped.

Note that a BJT doesn't have a gate, it has a base.

 

With no base current (open-base), an NPN transistor will block the positive half-cycle (if within its voltage rating) but will conduct on the negative half-cycle when the base-emitter junction breaks down at about 5V or so.
If any significant current flows under this reverse voltage condition, the transistor will likely be zapped.

Note that a BJT doesn't have a gate, it has a base.

So

With no base current (open-base), an NPN transistor will block the positive half-cycle (if within its voltage rating) but will conduct on the negative half-cycle when the base-emitter junction breaks down at about 5V or so.
If any significant current flows under this reverse voltage condition, the transistor will likely be zapped.

Note that a BJT doesn't have a gate, it has a base.

I tried to simulate it as u said positive cycle is blocked when no current at the base but when the base is biased both negative and positive pass and it acts like a triac and i used a resistive load of 10k connected to the collector so no significant current passes it is just about few milli amps ... what about that isit correct ... thank you

 

The simulator likely doesn't properly simulate the reverse breakdown of the base-emitter junction.

It does not act like a triac which stays on, even with no gate current, after it is triggered on.

 

The simulator likely doesn't properly simulate the reverse breakdown of the base-emitter junction.

It does not act like a triac which stays on, even with no gate current, after it is triggered on.

So right , thank you for the info ...

 

So right , thank you for the info ...

But as i remember its the scr wich stays on after trigger not the triac ... correct ?

 

Both a triac and an SCR will remain on once triggered until the zero crossing. The main difference is that an SCR can only conduct when the anode is positive with respect to the cathode. A triac can conduct regardless of the polarity of the applied voltage.

This circuit uses an NPN to switch AC. If you can get a triac that is probably a better solutioin.

 

Both a triac and an SCR will remain on once triggered until the zero crossing. The main difference is that an SCR can only conduct when the anode is positive with respect to the cathode. A triac can conduct regardless of the polarity of the applied voltage.
View attachment 129906
This circuit uses an NPN to switch AC. If you can get a triac that is probably a better solutioin.

thank you mr. dick

 

Hi,

Transistors also have a problem when they turn off when there is some inductance in the circuit. There could be a kickback that causes a high spike to appear. It's harder to deal with in an AC circuit. Triac's do not have this problem because theoretically the current must be zero when they turn off. You can trick a triac with a transistor, but then you're back to the transistor spike problem.

 

Hi,

Transistors also have a problem when they turn off when there is some inductance in the circuit. There could be a kickback that causes a high spike to appear. It's harder to deal with in an AC circuit. Triac's do not have this problem because theoretically the current must be zero when they turn off. You can trick a triac with a transistor, but then you're back to the transistor spike problem.

thank you mral