HIO16: Relay ratings

Note: This document is not yet complete. It may contain inaccuracies.

The following is a summary of the relay contact ratings. For full details please see the relay data sheet (type jqx-102f):

• Current rating (steady state); 20A at 240VAC
• Surge rating (motor turn-on): 80A at 240V (0.3 seconds)
• Horse power: 1HP @ 120VAC, 2HP @ 240VAC.

The relays are characterized specifically for AC motor loads. It is generally held that for AC loads a resistive-capacitive snubber will reduce contact damage due to arcing. In our experience with comprehensive tests of driving a large contactor, this is not necessarily true. We found an R-C snubber actually did more harm than good. There is some excellent material here on this and other relay related topics.

If you are driving DC inductive loads (say a 24VDC motor) you can reduce the strain on the relay contacts by connecting a diode across the load, cathode to the positive load terminal. This will prevent a spark from forming at turn-off. On a motor it will also act as a dynamic brake, making it stop faster. You should use a diode with a reverse voltage rating (PIV) greater than the DC supply voltage. For a non-motor load the diode should have a half cycle surge current rating equal to the DC load current. A 1N5404 will cover most applications. For motor loads the diode may need a higher current rating, because there will be stored rotational energy as well as electrical energy dissipated in the diode each time the motor stops. Don't attempt to use diodes with AC loads.