• Relay contact protection
• IES5541A
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• IES5541A datasheet
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Fig. 1 Waveform of unprotected relay contacts switching resistive load at mains voltage showing relay make at the mains peak voltage and significant arcing at relay break.

Fig. 2 Waveform of protected relay contacts switching resistive loads at mains voltage showing relay make at mains zero crossing with no significant arcing at relay break.

The series of waveform shown in figure 3 are step-to-step guide in describing the operation of IES5541A in providing relay contact protection. This mains relay is switching a resistive load at mains voltage.

1. After input signal ON received, SCR is triggered for every mains positive half cycle to drive the relay.
   
2. At the same positive half cycle when the SCR is triggered, the IES5541A also supply a negative trigger pulse to switch the triac. This occurs at every mains zero crossing for 8 half mains cycles.
   
3. Triac then conducts from the first triac trigger pulse to conduct any arcing that might occur during relay ‘make’.
   
4. Further arcing at relay contacts during relay bounce before relay fully closes.
   
5. IES5541A detects the additional arcing during contact bounce. IES5541A generates additional negative trigger pulse to the triac.
   
6. After input signal OFF received, the SCR is no longer triggered. As the relay attempt to break, arcing occurs between the contacts.
   
7. Detection of arcing by IES5541A during relay break generate a triac negative trigger pulse.
   
8. The triac then conducts for the remaining positive half cycle whilst the relay contact open.
   
9. This method of relay contact protection minimises the arcing at the relay contacts during relay make - break and contact bounce.

NOTE: The delay until the relay contact opens is dependant on the relay type and the associated circuitry around the relay.

Figure 3

Fig. 4 Waveform of protected relay contacts during
relay ‘MAKE’ of an inductive load at mains voltage

Fig 5 Waveform of protected relay contacts during
relay ‘BREAK’ of an inductive load at mains voltage

At relay ‘make’ with inductive loads, IES5541A behaves similarly as when it is switching resistive loads.

1. When the relay make, negative trigger pulse is generated by IES5541A to trigger the triac.
   
2. In addition to the negative trigger pulse at every mains zero crossing for 8 half mains cycle, additional pulses is generated when arcing is detected during contact bounce.
   
3. The triac absorbs any arcing caused during relay make, break and bounce.

At relay ‘break’, the IES5541A behaves differently than when it is switching resistive loads off.

1. Each time the inductive load is switched off, a transient is generated as the relay breaks.
   
2. The transient detected will retrigger the transient detection circuit of IES5541A. IES5541A will then generate negative pulse to trigger the triac.
   
3. This will occur within the 8 half-mains cycle time frame after which the load is fully turned off.