Putting a relay in upside down can cause a variety of issues depending on the type of relay and circuit it is installed in. The most common problems that can occur are the relay failing to activate properly, short circuits, and damage to the relay itself. Understanding what can go wrong and how to avoid these problems is crucial for anyone working with relays.
How Relays Work
A relay is an electrically operated switch that uses an electromagnet to mechanically operate one or more sets of contacts to make or break electrical connections. The relay consists of a coil, an iron core, a movable armature, and one or more sets of contacts.
When voltage is applied to the coil, it generates a magnetic field that activates the armature and causes the relay contacts to change state. The contacts either open or close depending on the design of the relay. The contacts control a secondary circuit that has a much higher current flowing through it than the coil circuit.
The coil circuit typically operates at low voltages, like 5V, 12V, or 24V DC. The contact circuits can switch much higher voltages and currents depending on the relay construction and ratings. For example, a 12V automotive relay may switch 30 amps at 12V, while an industrial control relay may switch 240V AC and have contacts rated for 10 amps or more.
Installing a Relay Upside Down
When installing a relay into a circuit, it is critical that it is oriented correctly. Most standard relays have the coil connections on one side and the contact terminals on the other side. There is always a correct orientation that must be followed for the relay to operate properly.
If the relay is accidentally installed upside down, the coil and contact connections will be reversed. This reversal causes the problems in activating the relay and potential damage.
Relay Failing to Activate
One of the most common issues from reversing a relay is that it may fail to activate at all. With the coil and contact terminals swapped, the coil will not reliably pull in the armature when power is applied.
The coil side of the relay requires proper polarity to magnetize the core and activate the switch. If the input power is connected to the contact side instead, the coil may try to operate reverse biased without magnetizing. This can lead to the armature not fully closing the contacts or not pulling in at all.
Short Circuits
An inverted relay can also lead to potentially dangerous short circuits when power is applied. With the higher voltage contact terminals connected to the low voltage control circuit, a direct short is created when activated.
For example, if a 12V coil was wired to a 120V contact circuit, applying 12V to the relay would instantly create a short between the 120V source through the coil wiring. This could easily damage wiring, blow fuses, or cause harm.
Any time there is a reversal of the higher and lower voltage sides of the relay, these hazardous shorts can occur. Safety mechanisms like fuses may fail to protect the circuit due to the very low impedance path through the relay coil.
Damage to Contacts
Relays installed upside may experience excessive arcing and damage to the contact surfaces as they try to switch load currents reversed. The contacts are designed to handle breaking and making electrical connections in a specific direction of load current flow. Reversing this can increase wear and arcing as the contacts activate.
Higher rated power relays are more susceptible to contact issues from improper polarity. Attempting to break an inductive load circuit by opening the normally closed contacts can greatly increase arcing wear when operated backwards.
Identifying an Upside Down Relay
To prevent reversed installation, it’s important to be able to visually identify the proper relay orientation before wiring it into a circuit. Here are some ways to tell if a relay is upside down or backwards:
- Look for coil and contact terminal labeling on the relay body. Terminals may be labeled +/- for the coil and NO/NC for contacts.
- Coil terminals are typically on the smaller pinned side, while contact terminals are on the blade side.
- There may be an orientation arrow showing the direction of load current flow.
- Relays designed for PCB mounting often have a dot, notch, or indent on the coil side.
- Larger power relays usually have the coils on top when upright and contacts on the bottom.
Referring to relay wiring diagrams, installation instructions, or following labeling and part features are key to avoiding incorrect polarity situations.
Correcting an Upside Down Relay
If a relay has been wired incorrectly, the first step is to safely power down the equipment and disconnect all wiring to the relay. Check for any signs of short circuit damage before proceeding.
Gently pry or unscrew the relay from its socket and re-install it in the proper upright direction. Double check the terminal positions again before reconnecting any wiring. Apply power and test operation before returning the system to normal use.
It’s highly recommended to replace the relay if there are any doubts about potential damage from reversed operation or shorts through the coil. Contacts can become welded or blown open, coils may lose winding insulation, and plastics can be melted. A new relay that is installed properly avoids any uncertainties.
Preventing Upside Down Relays
Here are some tips to help prevent relays being accidentally installed in the wrong orientation:
- Clearly label relay terminals or socket pinouts.
- Use relay bases with shaped or keyed mounts.
- Design PCB relay footprints to match the correctly oriented part.
- Use relay retainer clips that only allow proper insertion.
- Inspect and double check relay direction before applying power.
- Follow relay wiring diagrams and schematics closely.
- Ensure technicians are properly trained on relay symbols and installation.
A little time taken during installation and maintenance to verify relay polarity can prevent expensive downtime or damage. Following component labeling, printed diagrams, and board layouts carefully will help eliminate reversing mistakes.
Conclusion
Installing a relay upside down or backwards is a common mistake than can lead to the relay failing to operate, dangerous short circuits, or excessive contact arcing. Reversed coil voltage polarity prevents the magnetic field from properly activating the contacts.
Swapping the low voltage control and high current circuits creates shorts as soon as power is applied. Damage can also occur over time from contact arcing as the relay tries switching reverse load currents. Identifying proper relay orientation and verifying direction before wiring and testing can avoid these issues.
Taking preventative measures like labeling terminals, shaping mounting hardware, following schematics, and inspector installation work will eliminate accidental reversals. Knowing what to look for and how to identify upside down relays will ensure correct polarity and trouble-free operation.
