Suppressor circuit [t-head1]

Electronic equipment is today being increasingly used in combination with conventional switching devices such as contactors. This equipment includes programmable logic controllers (PLCs), timing relays and coupling modules, whose operation can be adversely affected by disturbances from interactions between all the components.

One of the disturbance factors occurs when inductive loads, such as coils of electromagnetic switching devices, are switched off. High cut-off induction voltages can be produced when such devices are switched off and, under some circumstances, can destroy adjacent electronic devices or, via capacitive coupling mechanisms, can generate interference voltage pulses and thus cause disruptions in operation.

Since interference-free disconnection is impossible without an accessory, the coils may be connected to a suppressor module, depending on the application. The advantages and disadvantages of the various suppressor circuits are explained in the following table.

Circuit diagram		Load current and voltage responses		Proof against incorrect connection also for AC		Additional dropout delay		Induction voltage limiting defined
				–		Very long		1 V
				–		Medium		UZD
				yes		Short		UVDR
				yes		Short		–