Slow Rise AC

Thread Starter


Joined Jan 6, 2018
I need a circuit that will make the AC automatically rise slowly when I plug it in the wall (2-3 seconds to maximum) for the primary of my transformer. The primary is the AC wall voltage of 120 volts. With the attached circuit R1 and R2 should be one resistor so I need to know the value of that resistor and the value of the capacitor C1 (can it be polarized?)triac-circuit.gif and what type of DIAC and TRIAC to use.
Last edited:

Deleted member 440916

Joined Dec 31, 1969
This is a typical phase controlled dimmer so is designed to provide a fixed proportion sine wave controlled by the value of R2.
From your description you require a soft start circuit, that is a different animal from this.
Could you describe the load in more detail, like what is the power rating of the transformer and what is the nature of the load (is it a power supply with capacitors or something else) ?

Thread Starter


Joined Jan 6, 2018
It is a switching power supply adapter. The output is 12V dc. I might use a transformer with discrete parts for the 12 volt output instead of the adapter. I would like at least 200 milliamps at 12 volts. I want the primary voltage to the adapter or transformer to rise slowly when I plug it in the wall. The input is house voltage of 120ac


Joined Aug 21, 2008
To allow C1 to be polarized the circuit would have to be made considerably more complex. The voltage in that part of the circuit (indeed the whole circuit) is AC.


Joined Nov 6, 2012
(Why is this a problem with every posting ?)
Please state, in detail, the exact problem you are trying to solve.

You state that you only need 200ma.
How much output current is your Power Supply rated for ?
Do you need tight Voltage Regulation ?
Using a Triac into a SMPS can cause all sorts of weirdness, and might even destroy the SMPS.
The SMPS can probably provide full Output Voltage with only 30vac input.
I can't imagine that you think that you have a problem with "In-rush Current", so
you must want the Output Voltage to ramp-up smoothly for some reason,
although what that reason is, is a mystery.
.Reverse Polarity Protection Soft Start  .PNG