reference will now be made in detail to the present preferred embodiment of the invention , an example of which is illustrated in the accompanying drawing . the exemplary embodiment of the relay circuit of the present invention is shown in fig2 and is designated generally by the reference numeral 100 . relay circuit 100 includes a transformer 120 having four terminals including terminals 121 and 122 of a secondary winding 120b and terminals 123 and 124 of a primary winding 120a , a relay switch 130 , a resistor 140 , and a switch 180 having three terminals 181 , 182 , and 183 . a relay coil 135 of relay switch 130 and resistor 140 are connected in parallel with one common node connected to the first secondary winding terminal 121 of transformer 120 , and the other common node connected to a movable contact terminal 181 of switch 180 . terminal 183 of switch 180 is connected to second secondary winding terminal 122 of transformer 120 . relay circuit 100 is preferably provided on a circuit board and terminals 123 and 124 are provided on the circuit board to connect the primary winding 120a of transformer 120 to a power supply , such as a commercial 120 v ac power supply . terminals 154a , 154b , 155a , and 155b are provided for connection to a power supply , which is preferably the same commercial power supply that supplies the primary winding of transformer 120 . terminals 150 , 151 , 152a , 152b , 153a , and 153b are provided on the circuit board to allow external components to be connected . for example , a first load device 160a , such as a hot water heater , heating / air conditioning equipment , circulator , oil burner , gas valve , or any other line voltage load , may be connected to load terminals 152a and 153a , while a control device 170 , such as a thermostat , may be connected to control terminals 150 and 151 . control terminal 150 provides a connection to second secondary winding terminal 122 of transformer 120 , and control terminal 151 is connected to terminal 182 of switch 180 . load terminals 152a and 152b are connected to terminals 132a and 132b , respectively , of relay switch 130 , and load terminals 153a and 153b are directly connected to power supply terminals 154a and 154b , respectively . terminals 156a and 156b , which are respectively connected to terminals 133a and 133b of relay switch 130 , may optionally be provided to connect additional load devices that operate whenever first load device 160a and an optional and second load device 160b are not operating . relay switch 130 may be a commercially available relay , such as a nldx2 motor controller component type scl - dpdt manufactured by song chuan precision company , ltd . a commercially available transformer 120 suitable for use in the present invention is a class 2 transformer , type be110910gbb manufactured by basler electric . resistor 140 preferably has a resistance between 430 and 510 ohms and is rated at 5 w . in normal operation , control device 170 will provide a closed circuit across control terminals 150 and 151 thereby causing normally open relay switch 130 to close across terminals 131a and 132a ( and across terminals 131b and 132b ) and causing power to be delivered to energize first load device 160a while switch 180 is in its illustrated position . similarly , control device 170 will provide an open circuit between control terminals 150 and 151 , thereby causing relay switch 130 to interrupt the power supplied to first load device 160a . switch 180 preferably comprises a single - pole double - throw switch provided on the circuit board to selectively connect the second common node of resistor 140 and relay switch 130 with either control terminal 151 , via terminal 182 , or with second secondary winding terminal 122 of transformer 120 , via terminal 183 . when switch 180 connects the second common node to control terminal 151 , relay circuit 100 operates in the manner described above . however , when switch 180 connects the second common node to second secondary winding terminal 122 of transformer 120 , relay circuit 100 becomes a closed circuit causing the load device ( s ) to turn on . at the same time , switch 180 disconnects control device 170 from relay circuit 100 . by providing switch 180 in relay circuit 100 , one may test relay circuit 100 in isolation from control device 170 , making trouble shooting easier . further , by automatically disconnecting control device 170 from relay circuit 100 during testing , the present invention prevents damage to control device 170 caused by backfeeding . as stated above , electronic thermostats are particularly prone to damage resulting from backfeeding . moreover , backfeeding also affects conventional thermostats by damaging the heating anticipator circuit . thus , the present invention is particularly advantageous when utilized with an electronic or conventional thermostat or other control devices particularly prone to damage resulting from backfeeding . although the preferred embodiment of the present invention has been described as controlling a single load device when the relay switch is closed , a second load device 160b may also be controllably turned on simultaneously with first load device 160a . such an arrangement may be desirable when first load device 160a is a circulator of a hot water heater and second load device 160b is a burner of the same hot water heater . the foregoing description of the preferred embodiment of the invention has been presented for purposes of illustration and description . it is not intended to be exhaustive or to limit the invention to the precise form disclosed , and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention . the embodiment was chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated . it is intended that the scope of the invention be defined by the claims appended hereto , and their equivalents .