Electric arc welding power supply

In a fault protection circuit for the main SCR's in a transformer SCR electric arc welding type power source, the SCR's having a known time vs temperature and I.sup.2 t overload curve, said fault protection circuit having a response curve generally parallel to said I.sup.2 t overload curve comprising:

• a. first gain means providing a first signal proportional to said weld current,
• b. a plurality of first switching means which go `on` in sequence as said first signal progressively increases above a predetermined level,
• c. second switching means for de-energizing the input voltage to said power source including a first input which turns said second switching means `on` when energized to a predetermined voltage level and a second input which changes said predetermined voltage level,
• d. said first switching means energizing said first input of said second switching means at progressively faster rates as said first switching means progressively go `on`,
• e. second gain means producing a second signal inversely proportional to said first signal on said second switching means second input, some of said first switching means when `on` providing a signal on said second gain means to vary the gain of said second gain means.

In an arc welding power source fault protecting circuit,

• a. means producing a signal proportional to the source output current,
• b. a first opamp having a first and second input, means supplying an adjustable reference voltage to said second input, means supplying said signal to said first input,
• c. a PUT having a gate and an anode and a cathode, means supplying said gate with a voltage normally above said anode voltage, a capacitor from said anode to said cathode, said capacitor when charged to said gate voltage moving said PUT to `on`
• d. a normally `off` SCR which when `on` deenergizes said power source
• e. means when said PUT is `on` arranged to turn said SCR `on,`
• f. a plurality of triggers each normally`off` and energized from the output of said first opamp, each of said triggers arranged to charge said capacitor at a predetermined rate, said triggers being arranged to go `on` sequentially as the output voltage of said first opamp increases with the number of the triggers which are `on` whereby to decrease the time for said PUT to go `on`,
• g. a second opamp having a first input from a reference voltage less than saturation and a second input from the output of said first opamp, the output of said second opamp lowering the gate voltage on said PUT whereby said PUT fires at a lower charging level of said capacitor,
• h. some of said triggers when `on` raising the voltage of said second opamp first input above said reference voltage to saturate said second opamp.

In an A.C. transformer SCR type arc welding power source including:

• a. a plurality of SCR's which when forward biased and their gates energized, conduct to supply a source output voltage, said SCR's having a known time-current overload curve,
• b. means for de-energizing said source before said overload curve is exceeded, said means including:
• c. means producing a voltage proportional to the power source output current.
• d. an amplifier for said voltage,
• e. a plurality of switches, the first being arranged to go `on` when the output voltage of said amplifier exceeds a predetermined amount proportional to the maximum steady state current of said SCR's and the remainder going `on` sequentially with progressively higher output voltages,
• f. switch means which when `on` de-energize said power source and having a pair of input terminals, the energization of the first of which determines the voltage level on the second terminal which will cause said switch means to go `on,`
• g. said switches when on acting on energize said second input terminal to said level in decreasing amounts of time proportional to the number of switches being `on,`
• h. a second amplifier for the first amplifier output voltage and arranged to energize said first input terminal to lower said level, whereby the current-time curve for actuation of said switch means is a smooth curve corresponding generally to the SCR time-current overload curve.

In an A.C. transformer SCR type arc welding source including:

• a. a plurality of SCR's which when forward biased and their gates energized conduct to supply a source output voltage,
• b. a firing circuit for the SCR's having a nominal input voltage at which said gates are energized at 0.degree. phaseback,
• c. a control circuit for said firing circuit having an input voltage proportional to the current output of said source,
• d. said control circuit including a plurality of amplifiers for said input voltages,
• e. means limiting the amplifier output voltage when the current proportional voltage is zero to not more than said firing circuit nominal input voltage,
• f. a manually adjustable reference voltage for determining the desired output current of said source,
• g. said amplifiers comparing the current proportional voltage with said adjusted reference voltage and when the amplified current proportional voltage exceeds said reference voltage, increasing their output voltage to said firing circuit to phaseback said SCR's,
• h. the gain of said amplifiers being such as to vary the output voltage of the source by phasing back the SCR's to hold the output current constant,
• i. the improvement which comprises means comparing the source output voltage with a fixed reference voltage and when said output voltage drops to less than a predetermined value due to short circuits on said source, changing said manually adjusted reference voltage to decrease the phaseback of the SCR's and increase the current beyond that determined by the manually adjusted reference voltage.

In an A.C. transformer SCR type arc welding source including:

• a. a plurality of SCR's which when forward biased and their gates energized conduct to supply a source output voltage,
• b. a firing circuit for the SCR's having a nominal input voltage at which said gates are energized at 0.degree. phaseback,
• c. a control circuit for said firing circuit having an input voltage proportional to the current output of said source,
• d. said control circuit including a plurality of amplifiers for said input voltages,
• e. means limiting the amplifier output voltage when the current proportional voltage is zero to not more than said firing circuit nominal input voltage,
• f. a manually adjustable reference voltage for determining the desired output current of said source,
• g. said amplifiers comparing the current proportional voltage with said adjusted reference voltage and when the amplified current proportional voltage exceeds said references voltage, increasing their output voltage to said firing circuit to phaseback said SCR's,
• h. the gain of said amplifiers being such as to vary the output voltage of the source by phasing back the SCR's to hold the output current constant,
• i. the improvement which comprises means simultaneously feeding a portion of said source output voltage to said amplifiers whereby to obtain a small increase in the output current with drops in source output voltage to compensate for line voltage variations.

In an A.C. transformer SCR type arc welding source including:

• a. a plurality of SCR's which when forward biased and their gates energized conduct to supply a source output voltage,
• b. a firing circuit for the SCR's having a nominal input voltage at which said gates are energized at 0.degree. phaseback,
• c. a control circuit for said firing circuit having an input voltage proportional to the output of said source,
• d. said control circuit including a plurality of amplifiers for said input voltages,
• e. means limiting the amplifier output voltage when the current proportional voltage is zero to not more than said firing circuit nominal input voltage,
• f. a manually adjustable reference voltage for determining the desired output of said source,
• g. said amplifiers comparing the proportional voltage with said adjusted reference voltage and when the amplified proportional voltage exceeds said reference voltage, increasing their output voltage to said firing circuit to phaseback said SCR's,
• h. the gain of said amplifiers being such as to vary the output of the source by phasing back the SCR's to hold the output constant,
• i. the improvement which comprises a first zener diode which limits the minimum voltage from said amplifiers to said firing circuit said nominal voltage, and the difference between the voltage of said first zener diode and a second zener diode limits the maximum voltage to said firing circuit and thus the maximum phaseback of said SCR's.

In an A.C. transformer SCR type arc welding source including:

• a. a plurality of SCR's which when forward biased and their gates energized conduct to supply a source output voltage,
• b. a firing circuit for the SCR's having a nominal input voltage at which said gates are energized at 0.degree. phaseback,
• c. a control circuit for said firing circuit having an input voltage proportional to the output of said source,
• d. said control circuit including a plurality of amplifiers for said input voltages,
• e. means limiting the amplifier output voltage when the current proportional voltage is zero to not more than said firing circuit nominal input voltage.
• f. a manually adjustable reference voltage for determining the desired output of said source,
• g. said amplifiers comparing the proportional voltage with said adjusted reference voltage and when the amplified proportional voltage exceeds said reference voltage, increasing their output voltage to said firing circuit to phaseback said SCR's,
• h. the gain of said amplifiers being such as to vary the output of the source by phasing back the SCR's to hold the output constant,
• i. the improvement which comprises said amplifiers having frequency sensitive feedback circuits effective to reduce the gain thereof to rapid voltage changes.