contact sense using opamp

[anishkgt]

Hi All,

Would it possible to use an opamp to sense when two terminals touch each other ?

My idea to use it for my spot welder where the contact sense idea would be used to sense if the electrodes makes a firm contact with the spot to be welded. I've just starting learning about op-amps and this seemed like a good start to learn about them with a practical approach.

Thanks in advance.

[anishkgt]

Forgot mention that the electrodes are connected to the secondary of a MOT. So i doubt if this would be possible.

[anishkgt]

Would this be a correct schematic ? to sense when two electrodes are closed ?

[anishkgt]

at the moment my circuit detects the zero cross on the AC sine wave and triggers two Thyristors in a back-to-back configuration which switches on a MOT(Microwave Oven Transformer) at the peak of the sine, which is the typical way to switch on inductive loads to avoid inrush currents. 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, an nickel strip

Just not sure how to do this with an opamp or is there a better way to accomplish this task.

[David Hess]

The low secondary resistance will make this more difficult but not impossible to do at DC by measuring the change in resistance.  10s of microvolts is sufficient with a precision operational amplifier used as a comparator and I once used an OP-90 in an application like this.

AC excitation would generate a larger signal do to the secondary inductance and could take advantage of synchronous detection to remove noise yielding a much higher sensitivity.  This is a more complicated circuit but would be more reliable.

In both cases, the excitation output circuit and operational amplifier input circuit needs to be protected from the open circuit welding voltage and any voltage spikes which is not difficult using series and shunt protection.

[anishkgt]

sounds near to impossible and added cost for components.

Thought it could be accomplished with an opamp as a comparator but the second input to the comparator would be hard to get.