# Low voltage sense

I've been reading the HIGH and LOW voltage detector circuit mentioned here . I have a similar idea to sense the voltage of a load where the open voltage is 32vac and need to detect a drop in that. The circuit i am trying to achieve. The Load here is a MOT (Microwave Oven Transformer) rewound with 3 turns of 25mm2 copper cable to form a spot welder. The opamp circuit detect the voltage at the primary which is 33vac and when the secondary are shorted the voltage drops to 26vac. I need to detect this drop/change in voltage.

I assume that you are trying to detect change in primary voltage of the MOT. The best way to do it is by rectifying the 33V AC to DC using a full bridge rectifier. The output voltage of the rectifier will be directly proportional the the AC input voltage. Then you can map down the voltage to say 15V using a potential divider circuit and then use two Op-Amps in comparator mode to detect the maximum and minimum voltage.

If you are trying to measure the input AC voltage then you can map it down to 5V and read it using a ADC and calculate the primary voltage of the transformer. However the accuracy will be low here.

Also creating a spot welder with MOT is a brilliant idea, I am curious to know how it turns out so keep us updated on it

Joined August 16, 2016      997
Tuesday at 12:29 AM