Temperature control circuit for a hand held soldering tool

In an electrical soldering iron in which a soldering tip is heatable to a predetermined operating temperature to perform a soldering operation by means of an electrical heater element and in which electrical heating and control circuits are provided to supply electric current to the heater element in response to predetermined changes in temperature of said soldering tip, the improvement which comprises:

• a. an electrically conducting sensing element operatively connected with said soldering tip having a resistance to current flow which increases with a rise in its temperature whereby its temperature will rise and fall and its electrical resistance will increase and decrease as the temperature of the soldering tip rises and falls;
• b. said heating circuit having a controlled rectifier as a switch therein;
• c. said control circuit being adapted to regulate said controlled rectifier to open and close the said heating circuit in response to a predetermined change in the resistance of said sensing element, said control circuit comprising a balanced bridge circuit as a first arm of which includes said sensing element, a second arm of said bridge circuit includes a variable resistance whereby the sensing element temperatures at which the heating circuit is opened and closed may be predetermined, third and fourth arms of said circuit including resistances, the gate of said controlled rectifier being connected to the junction of the second and third arms of the cathode of said controlled rectifier being connected to the junction of said first and fourth arms whereby in response to a sufficient electrical potential imbalance across the bridge the controlled rectifier is triggered into conduction;
• and
• d. a bias compensating diode is connected between said first and second arms, the cathode thereof being connected to the first arm and to said heating circuit, the anode of said bias compensating diode being connected to said second arm and through a current limiting diode to the cathode of said controlled rectifier thereby to limit the current and to balance the bias of said controlled rectifier upon said bridge, said fourth arm including a diode to balance the effect upon the bridge of the bias compensating diode whereby heating current flowing through the controlled rectifier, bias compensating diode and current limiting diode will heat the bias compensating diode thereby lowering its voltage and providing an electrical imbalance to said bridge to lower the predetermined temperature at which the heating circuit will be opened.