# AC Current Regulation

Discussion in 'General Electronics Chat' started by rainyday101, Feb 9, 2012.

1. ### rainyday101 Thread Starter Member

Sep 24, 2009
50
0
Hi,
I am building an ac/dc power supply for testing some controls that utilizes a variac. The AC output will be 0-135V from the variac at 0-20 amps. Is there any easy way to regulate the current output to a selectable amount? By that, I mean I would like to adjust the ac current limit via a pot knob and a current display. The power supply would then have current limit at that set point. I have spent several hours surfing the net and have not come up with anything. Any help would be appreciated.

2. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,854
968
Fuses can limit current. Better yet would be Breaker switches. They are used for On/Off function as well as current limit. You could select different current levels with a shorting plug to select which Breaker switch power is applied to for your selections.

If you use a Variac to vary the voltage; the current CANNOT stay the same unless the resistance of the load is changing as well. It's all there in simple arithmetic by virtue of Ohms law. Current = Voltage / Resistance. Change voltage and current changes too.

Short answer is there is no easy way to have variable AC with current limit adjusted by knobs and displayed. This is one of those 'buy it' situations if the low tech suggestion cannot be tolerated

3. ### rainyday101 Thread Starter Member

Sep 24, 2009
50
0
Thanks for the answer. I knew resistive current limiting was out of the question due to the power levels I would have. A switch with breaker selection though is a really good alternative. It will accomplish basically what I need to do and still provide levels of protection from a shorted load. Originally we were going to use an Arduino to drive a servo that was hooked to the variac to keep the voltage constant as the load changed. We would have accomplished this by getting conditioned CT feedback to the analog inputs of the Arduino. This would have been setup so that the user could enter his target ac or dc voltage which could then be maintained via the servo with the load changing slightly. We have scratched that due to time and cost considerations and have decided to go with the old manually adjusted variac and voltage displays. Thanks again for the input because I never thought of selectable breakers!