A spot welder can be anything between very simple to very complex. People have made them using transformers hacked from Microwave ovens on a simple scale.

Among the problems with using a comparator to measure or compare an AC signal is the AC is constantly crossing a zero reference so the comparator Out will just be a pulse train as the input varies above and below a reference. Even if we use a uC like an Arduino to detect an AC signal using an analog input channel the same problem will persist. Normally AC voltage measurement can be carried out by converting AC voltage into proportional DC Voltage using rectifier and filter circuits. For low AC voltage (mili volts) measurement precision rectifier is used as diode knee voltage is 0.7 Volt. I don't know of a simpler way to go about it.

Ron

 

I also have two taps on the secondary to measure the voltage during a weld to calculate the Weld current. All these are initiated at the press of a button which am hoping to automate when the electrodes touch a conductive material like here, nickel strip.

A bit late to the party, but normally when spot welding the electrode or contact pressure is measured or maintained before any current is turned on, if this is not done, arc splashing exists and results in a poor weld condition as well as appearance.
The power is not normally applied, until this pressure is present.
Max.

 

A bit late to the party, but normally when spot welding the electrode or contact pressure is measured or maintained before any current is turned on, if this is not done, arc splashing exists and results in a poor weld condition as well as appearance.
The power is not normally applied, until this pressure is present.
Max.

While I have to strongly agree Max, this has all been a little less than conventional. This being a second thread on the subject.

Ron

 

The idea here is when the secondary is shorted there would be a voltage drop at the primary and when this voltage drops that would disable the optocoupler and trigger the thyristors via MUC like an arduino UNO R3.

The voltage at the inverting pin is around1.05Vac and when a voltage drop happens it would come down to 0.5Vac. That would make a difference of 0.55Vac, so hat does this value mean for the opamp ? Basically i would just need the output to read HIGH or LOW.

 

Well i checked with a precision rectifier that was suggested by LT but when i tested it out on the LTspice it did not work as i expected it to, in the sense when i change the AC voltage the output does not seems to change. Is that how it would be in reality ?