THE RELAYS USED in the standard alarms have contact voltage and current ratings that are adequate for most low-level signaling applications. Table 6a shows contact ratings for the most common alarms:
These ratings are usually high enough for signal-level remote annunciation, but you may need a higher rating – for example, to operate a local indicator light, or an external user-supplied contactor.
You can order auxiliary relays with higher contact ratings. Octal-based plug-in relays are available with contact ratings of 5.0 A at 120 Vac, and 0.5 A at 130 Vdc. Need higher ratings? Relays with ratings up to 10 A at 150 Vdc are available even if they’re not listed in your manufacturer’s catalog.
HANDLING INDUCTIVE DC LOADS
If you want to drive an external dc relay, using the standard on-board alarm relay (in the CASM, e.g.), you need to handle the inductive energy from the external relay coil when the CASM contact opens. The inductive “kick” caused by the external relay turning off causes arcing in the CASM relay contact, which at a minimum will shorten the relay’s life. In the extreme, it could weld the contacts together and the external relay would never de-energize.
Yes, there’s an app for that. The instructions for adding a freewheeling diode to handle the inductive energy are in HindlePower Application Note JD5011-00, available on the company’s web site.
Where does that “inductive kick” come from?
When you first energize the external relay, current flows in the relay coil, and that sets up a magnetic field in the relay. It’s the magnetic field that does the work of pulling the relay contact closed. Magnetic fields, though, don’t go quietly into the night. When you want to release the relay contact, you need to interrupt the current in the coil, but the magnetic field sees things differently – it wants the coil current to continue. So it starts raising the circuit voltage to try to keep the current flowing. When the voltage gets high enough, it causes arcing in the CASM relay. The solution is to provide another path for the coil current, and that’s what JD5011-00 does.