# How to connect this Bench power supply to 220V AC step-down transformer to 12V AC output?

#### cmpx

Joined Oct 23, 2021
22
How to connect this Bench power supply to 220V AC step-down transformer to 12V AC output?

This circuit works normaly on +12V DC power supply:
Blueberry PSB400
Yellow wire: +12V, 12.0A, 144.00 W

The Incandescent Bulb, 6V/0.1A is connected to the Bench power supply output.

How to connect this Bench power supply to 220V AC step-down transformer to 12V
AC output?

When it is connected onto step-down transformer AC output 10-12V~ the R2 trimmer resistor resistor
is set to 10K.
The Bench power supply circuit is open and I just measure the voltage output
value.

When it is connected onto step-down transformer AC output 10-12V~ the R2 trimmer resistor resistor
burns up if I try to set R2 from 10K to 0R.

This does not happen if I connect this Bench power supply circuit into the
regulary DC voltage power supply +12V:
Blueberry PSB400
Yellow wire: +12V, 12.0A = 144.00 W

The Bench power supply circuit works normaly.
The Incandescent Bulb, 6V/0.1A is connected to the Bench power supply output.

DC example works normaly.
AC example - R2 trimmer resistor burns up.

The circuit diagram is taken from:

#### sghioto

Joined Dec 31, 2017
5,102
You have to include a rectifier and filter capacitor on the output of the transformer before connecting to the bench supply.
That's why it works using the DC power supply.

Last edited:

#### BobTPH

Joined Jun 5, 2013
8,108
I think he is trying to tale a DC voltage regulator and use it to control an AC voltage. Not hap’nin.

#### sghioto

Joined Dec 31, 2017
5,102
I don't think so but the TS didn't read the third paragraph about the description of that bench supply

#### cmpx

Joined Oct 23, 2021
22
You have to include a rectifier and filter capacitor on the output of the transformer before connecting to the bench supply.
That's why it works using the DC power supply.
View attachment 300279
What a resistor value calculation should be used for R1 and R2 if I decide to connect it directly into wall outlet phase 220V AC?

#### cmpx

Joined Oct 23, 2021
22
What a resistor value calculation should be used for R1 and R2 if I decide to connect it directly into wall outlet phase 220V AC?
I think I should use 1N4007 rectifer diode?

#### sghioto

Joined Dec 31, 2017
5,102
What a resistor value calculation should be used for R1 and R2 if I decide to connect it directly into wall outlet phase 220V AC?
Doesn't work that way! Major explosion!
Didn't you read my post #2?
Required circuit:

Last edited:

#### BobTPH

Joined Jun 5, 2013
8,108
What a resistor value calculation should be used for R1 and R2 if I decide to connect it directly into wall outlet phase 220V AC?
As I guessed. He thinks he can power that circuit with AC.

#### cmpx

Joined Oct 23, 2021
22
Doesn't work that way! Major explosion!
Didn't you read my post #2?
Required circuit:
View attachment 300286