Weld monitor

Claim 1

A weld monitor for utilization with a welding apparatus having an arc torch and provided with a meter having a current indicating section including a dial and a voltage indicating section including a dial to permit an operator to check the output of the welding apparatus by himself; a power supply for the welding apparatus; a voltage averaging circuit electrically connected to receive an instantaneous arc voltage from the power supply as supplied to the arc torch, said voltage averaging circuit being operable to integrate the instantaneous arc voltage and provide an average voltage signal; circuit means electrically connecting said voltage averaging circuit to the voltage indicating section of said meter; a current averaging circuit electrically connected to receive an instantaneous arc current signal, said instantaneous arc current being obtained from a shunt connected across the torch, said current averging circuit being operable to integrate the instantaneous arc current and provide an average current signal; circuit means electrically connecting said current averaging circuit to the current indicating section of the meter; an average arc current memory circuit and an average arc voltage memory circuit electrically connected to receive the average arc current signals and average voltage signals from their respective averaging circuits, said memory circuits being identical and each comprising a capacitor electrically connected to receive the average signal, a field effect transistor interposed between the capacitor and the average signal input thereto, said transistor serving as a switch to connect and disconnect the capacitor from the averaging circuit; a measurement circuit including an operational amplifier electrically connected to permit the charge on said capacitor to be measured without the capacitor being discharged; and switch means to activate said memory circuits including a relay having a normally closed contact in circuit with said field-effect transistor; said relay when energized by the closing of said switch operating to open its normally closed contact to thereby interrupt the reactive bias voltage on the field-effect transistor so that the field-effect transistor switches to an open state, said capacitor in said memory circuit remaining charged at the voltage which existed at the moment that the field effect transistor of said memory circuit was switched to an open state and said operational amplifier provides a voltage output which is equal to the voltage on the capacitor, and a feed back loop in the memory circuit operable to cause the amplifier to have a high input impedance so that voltage on the capcitor is not discharged when the amplifier is measuring the voltage on the capacitor to thereby maintain the memory.