Abstract: The present system includes a bridge circuit (with four legs) with one leg including a heating element. The mid-points of the bridge circuit are connected to a silicon controlled rectifier (SCR) which is further connected to the control element of a monostable multivibrator. Further connected to the power input elements of the monostable multivibrator is a D.C. power source. The monostable multivibrator is connected to a transistor which has a light emitting diode (LED) in its cathode circuit. The LED is part of a circuit which further includes a light sensitive triac. The light sensitive triac is further connected through a pulse generating circuit to the control element to a second triac which in turn is connected to supply heavy electrical current to the heating element.

Abstract: The present device includes a bridge circuit with a first intermediate terminal and a second intermediate terminal and with one leg of the bridge circuit being a heating element. Connected across the power lines of the bridge circuit is a main bidirectional switching circuit (having a gate element) which is turned on bidirectionally by control signals developed by an R-C circuit connected to the gate element. The resistors of the R-C circuit are for the most part integral components of two photoconductor isolator devices. These last mentioned resistors provide a low resistance after their associated LED's are turned on and the one of them whose associated LED turns off at the zero crossover remains at a low resistance value for a period of time after its associated LED's has turned off. In addition the present device includes a circuit to bypass the control signal which ordinarily would go to the main capacitor of the R-C circuit in the event that an overcurrent situation (e.g.

Abstract: The present device provides an air duct means disposed between the top end of the heat exchanger of a furnace and the entry port into the flue pipe of the furnace. An electrical blower means is coupled to said duct means to draw hot air therethrough which would otherwise go out to the atmosphere through the flue. The hot air drawn by the blower is redirected to heat, or assist in heating, the building to which the furnace is directing heat, or heat some other facility. An electrical control circuit is connected to the blower means and controls the blower means such that if the furnace is burning fuel, the blower cannot be turned on. The blower can only be turned on after a predetermined time has elapsed from the time that the fuel burning has terminated, so that combustion gases which otherwise would pass through the flue and which might contain harmful ingredients therein, will not be redirected to be used as a source of heat.

Abstract: The present circuit controls the high current delivered to a load from a direct current (d.c.) power supply through a pair of power silicon controlled rectifiers (SCRS). For each power SCR there is also employed an associated current conduction control SCR. Each of the current conduction control SCRS acts, when triggered, to terminate the current conduction of its associated power SCR. Each current conduction control SCR is controlled by the provision of current thereto during conduction and non-conduction half cycles of an alternating current source, which operates independently of the d.c. power supply. Accordingly, the occurrence of a current (load) transient, even though it may provide an abundance of current to either of said power SCRS, will not prevent the termination of the current conduction of said power SCR for an excessive period after it should have been turned off.