Is it easy? What transistor(s) would I need and what to consider?
I simply want to inc/decrease AC voltage (amplitude)
Thanks
AC voltage
That’s too wide open.
What exactly are you trying to do?
Use a Triac
MarkDerbyshire:
Use a Triac
Usually, you switch with that. The OP thinks he wants alter the amplitude of the voltage.
larryd:
That’s too wide open.What exactly are you trying to do?
I want to regulate AC amplitude between 0-120V - from mains - current about 0.5-4A
Basic idea is to very slowly turn on power devices - don't want to use relay
Deous:
I want to regulate AC amplitude between 0-120V - from mains - current about 0.5-4A
Basic idea is to very slowly turn on power devices - don't want to use relay
What kind of devices and why?
A lot of devices might not like that at all, especially when electronics are involved.
Deous:
Basic idea is to very slowly turn on power devices - don't want to use relay
How slowly - 1 sec. or 1 min, why you need that ?
In AC terminology, there are differences among Amplitude, RMS, Average, and Instantaneous.
Probably, the OP wants RMS of a phase controlled sine wave which could be achieved by a TRIAC or a back-to-back connected two SCRs.
A common way to achieve slow turn on of a mains device is to use a negative temperature constant resistor in series with the load.
RS, Farnell, Mauser etc keep loads in various current ratings.
Allan
This is NOT an easy thing to build. You need something similar to an audio amplifier, but higher voltage & current. A modern "digital variac" probably uses a switching design (similar to a class-D audio amplifier).
It might be better to buy a regular-old manual [u]Variac[/u].
Deous:
I want to regulate AC amplitude between 0-120V - from mains - current about 0.5-4A
Basic idea is to very slowly turn on power devices - don't want to use relay
You don’t want to use a relay, then good because a relay will not do what you want.
Now why do you think you need to turn on a load slowly? This has all the hallmarks of an X-Y problem.
Hi,
How many projects have you got going...
Is this thread related to controlling the Air Con motor?
Can you please tell us your electronics, programming, Arduino, hardware experience?
Tom..... 
TomGeorge:
Hi,Is this thread related to controlling the Air Con motor?
Tom.....
Good point, maybe compressor motor ?
good luck with results.
Update: I finished that circuit - fairly simple - 2 n-ch mosfets and 1 npn bipolar with hihg hfe.
Turned out my power mosfets 250v/300A vere too powerful for simple 110V AC and they have too much charge capacity between GDS. For AC mosfets in series are mandatory. I wonder if they make integrated ICs like that
Without circuit is it hard to give the answer.
Without knowing application is it hard to give advice.
There are audio amplifier IC 20W