Safety control circuit

Abstract

A safety control circuit for an electrically operable machine such as a punch press is described. The circuit includes input means for delivering electrical power to the machine and first and second relays, each including switching means and an associated relay coil for operating said switching means. The switching means of the relays are connected in said input means so that electrical power is delivered to the machine only when both relay coils are energized. First and second capacitors are associated respectively with said first and second relay coils and a source of direct current is provided for charging the capacitors. The direct current source is connectable with both capacitors through said switching means of said relays when both relay coils are de-energized, so that the capacitors are both normally charged when the circuit is at rest. First and second machine activating switch means are associated respectively with said first and second capacitors. Each of said switch means is arranged, when operated, to connect the relevant one of said capacitors with the associated relay coil to cause the capacitor to discharge into the coil and operate the relay. The circuit also includes resistor means connectable with either of said capacitors through the switching means of the relays, said switching means being arranged in the event that only one of said machine activating switch means is operated, to connect the capacitor associated with the other switch means to said resistor means, whereby, in use, the relevant capacitor will progressively discharge into the resistor means, preventing energization of the associated relay coil and hence activation of the machine after the level of charge in the capacitor has fallen below that required to energize the associated relay coil.

Dependencies are inferred from the claim text.