Safety Note
Working with mains voltage (120VAC, 60Hz) is inherently dangerous. Please ensure you have the knowledge and tools to stay safe.
Load Switching Capacity
Note that the switching capacity of mechanical relays varies depending on the type of current being switched. For example, our relay (U5, ALQ3F12) switches:
- 5A maximum when switching DC current at 30V or less
- 10A maximum when switching AC current at 125 VAC
- 5A maximum when switching AC current at 250 VAC
Consult the data sheet for your relay to determine its switching limitations.
Prevent Contact Welding — Implement a Snubber Across Relay Contacts
Imagine a relay is closed and significant current (several amps) are being transmitted through the contacts.
When the relay is opened, a spark will be drawn between the two contacts before current flow stops. This spark has the potential to weld the relay contacts together — making the relay permanently inoperable.
To greatly reduce the probability of relay welding, place an RC filter (called a snubber) across the relay contacts.
The snubber acts to transmit high frequency energy generated by the spark around the relay contacts, thus protecting them.
Very important safety note: if you intend to switch mains voltage (120VAC, 60Hz); you need to carefully select the components used in the snubber.
- The capacitor needs to be Y2 rated (consult the capacitor data sheet). This means that in the case the capacitor fails — it will fail open, meaning 120V will not be placed directly across the snubber resistor. Note that generic ceramic capacitors generally fail closed, making them unsafe to use in a mains power snubber application.
- Select a resistor for the snubber that can dissipate enough power. Our resistor, R15, can dissipate 0.75W. Additionally, consult the data sheet to ensure that the resistor can withstand having mains voltage placed across it. Our resistor is rated for 400V.
Note that in an AC voltage application, implementing a snubber will mean that a small amount of current ‘leaks’ through the circuit when the relay is open. In our design, if 240VAC was applied, the maximum leakage current would be 350uA.
In DC applications, the leakage current is limited to what ever the capacitor leakage current is (see the capacitor data sheet). Generally, this leakage current is extremely low.
Creepage and Clearance
When designing the relay circuit layout on your PCB, you will need to provide clearance between the tracks carrying mains voltage, and all other components and tracks.
The amount of clearance is dictated by the safety standard relevant to the product you are designing. The EN 60950 standard often is the basis for creepage and clearance requirements.
For example, the M&A Board meets the 60950 mains basic insulation requirement, pollution degree 2, material group 2, for 250Vrms applied as 1.8mm clearance is provided between mains voltage carrying tracks and all other components and tracks on the PCB.
- Please note, for even greater safety margin, the data sheet for the M&A Board de-rates its switching capacity to 160VAC.
You can find more information on clearance requirements here.
PCB Track Width
You must make the track routed to your relay wide enough to accommodate the maximum current switched by the relay.
The width of the track is determined by the increase in track temperature you are willing to accept while the track is transmitting the maximum current.
For example, a 20C temp rise will occur when 1oz copper (normal PCB copper weight), track of width 4.84mm is transmitting 10A.
A chart describing various temperature vs. current vs. track widths can be found here.
PCB routing tip: You can place the same track on both the top and bottom side of the PCB to ‘double’ the width of the track while saving space.
Need Some Help?
We’re always happy to share our knowledge — feel free to email us at:
- support@magicdaq.com
Working Model
Want to play with a working model? You can find this circuit on the M&A Board at MagicDAQ.com
