About Relays

When attaching relay boards to HMS boards, the diodes to prevent EMI/RFI when controlling the relays are built into the HMS board.

When using the relay boards in a 'stand alone environment' (such as with switches or controllers that don't have the diodes in the outputs) use the -SA version of the board. (SA is for 'Stand Alone')

Please inquire about 'Custom Orders' (other than 12 volt relays coils).

Relay Coils:

Why the diodes for the relay coils (often referred to as 'foldback diodes')?  Relay coils can be considered an inductor.  Meaning, that when current is flowing thru the coil, the current has to keep flowing (ramping to zero in a millisecond or 2) when the drive (switch) is opened.  If there is no 'foldback diode' to allow the current to ramp down, then the coil of the relay will generate as much voltage as is needed to keep the current flowing.  This is a small spark if you were using a switch.  Without the 'foldback diodes' the coil would generate up to a thousand (or more) volts to ensure current continues.  This high voltage generation would damage any solid state driver.  This is how a spark coil works in your car!

Relay Outputs:

DC Switching:

If the relay is driving a non-inductive load (such as LED Lighting) then you may not have to be concerned about the output of the relay.  However, it is a very good practice to put a 'foldback' diodes across 'DC' the loads of the Relays.

Note: The -AC versions of the board have snubber networks built into the outputs.  When using the relay board to switch both DC and AC voltage, use the -AC board.  If necessary, you can put foldback diodes across the DC loads.  The Cathode of the diode (line side of the diode) goes to +DC volts (this is 'high' side of the load.  The other side of the diode connects to the 'low' side of the load.

AC Switching:

Order the -AC versions of the relay boards.  The 'AC' versions have snubber networks suitable for 120/240VAC operation.  They have .047 microfarad capacitors rated at 450 volts.

Note: In the unlikely event that the snubber capacitor blows, then the snubber capacitor size is not sufficient for your circuit.  In that case you will have to manually add a snubber network across the contacts of the relay.  Contact HMS Technical support for help with resolving this issue.