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What is the function of the two diodes and the resistor? Why does not the SET coil have resistor?

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The diagram shown is for an AC-coil (or AC-DC) latching relay. The series diodes are there to rectify the AC. They're typically rated at something like 1kV PIV.

The resistor is for "ampere-turn compensation". Generally the reset coil needs much less current than the set coil.

For higher voltage relays (>50VAC) it may be present inside the relay. For lower coil voltage relays it may not be present (just a coil).

It's more of a problem with high voltage relays because the wire would have to be very fine to get the optimal resistance so it's easier to just throw away some of the power in a resistor.

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  • \$\begingroup\$ Why do I need to rectify the AC? \$\endgroup\$ – Blue_Electronx Oct 9 '16 at 0:40
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    \$\begingroup\$ The latching depends on a permanent magnet, which has polarity. If you want to oppose or aid the magnet your field must have the appropriate polarity. So AC won't work. \$\endgroup\$ – Spehro Pefhany Oct 9 '16 at 0:44
  • \$\begingroup\$ Xavier, allow me to explain... You may choose either an AC or a DC Relay . The Diode & R is included in 50Vdc and above DC Coils, so that AC can also be used on the DC Relay \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 9 '16 at 2:16
  • \$\begingroup\$ Thanks, I almost get it. I just try to understand properly how the manufacturer achieve to latch the Set coil by using one pulse. \$\endgroup\$ – Blue_Electronx Oct 9 '16 at 22:45
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Partial answer: The diodes are required to make sure that the "set" and "reset" coils perform only their labeled functions.

It's likely that if you were to push reverse current through the "set" coil, it would actually reset the relay, and vice versa. The diodes prevent this from happening.

Pure guess: The resistor in the "reset" lead is there to make sure that if you pulse both coils simultaneously (with the same voltage), the "set" function "wins".

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schematic

I added an RC snubber to what appears to be a half wave rectified latching relay coil.

  • R is a resistor for ampere-turn correction. This resistor is included in models for 50 VAC or 48 VDC or higher.
  • For DC models, check the coil polarity for both the set and reset coils and wire all connections correctly. If the connections are not correct, unintended operation may occur.
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