How to use flyback diodes to prevent relay burnout?
How to use flyback diodes to prevent relay burnout?
Inductive loads, such as solenoids and contactors, can cause arcing and malfunctions that can stop electromechanical switching equipment, resulting in costly downtime. However, the cost of solving this problem is actually not high.
Imagine a PLC's relay output card is a DC solenoid used to drive a large hydraulic cylinder. But every few months, the card fails, causing equipment downtime and unnecessary costs for replacing the relay card.
What is flyback voltage? Electrically moving switches or plungers use the concept of electromagnetism, where a coil generates a magnetic field while consuming current.
This magnet is strong enough to move metal flaps or even spring-loaded plungers. Larger loads require more current.
When this electromagnet generates a magnetic field, it is stored inside the load. In other words, you use current to generate a magnetic field, and the magnetic field is now stored inside the plunger.
When the coil current is "off," the magnetic field stored inside the plunger still exists, and for a short time, it tries to generate a voltage across the coil.
When is flyback voltage good? In some situations, this generated reverse voltage is a very useful characteristic.
The spark plugs in a car work by using the 12-volt battery voltage to power the coils at precise moments (from the distributor, the electronic coil assembly, and even the coils on each plug).
When the battery voltage is removed, the coils attempt to drive current through the circuit – here, the provided path is precisely across the spark plug gap.
However, 12 volts is insufficient to produce a spark spanning several millimeters. Fortunately, the voltage generated by the coils is many times greater than the original charging voltage, and the current only lasts for a moment until the magnetic field collapses. Polarity Relationship Between a Solenoid Load and a Typical Flyback Diode
The diagram above shows the circuit schematic of the polarity relationship between a solenoid load and a typical flyback diode, a common scenario in this case. A practical wiring example will be shown later.
When is Flyback Voltage Undesirable? Back in an industrial environment, the fact that the reverse voltage is many times greater than the original voltage is concerning.
If the control circuit is 24 volts, the reverse flyback voltage can be well over 100 volts.
Even a small gap in a circuit can be enough to produce a fairly large spark. While useful for igniting fuel with a spark plug, it's dangerous in control systems.
Gap exists in the form of electromechanical control devices, namely switches and mechanical relays.
When a relay or switch contact opens, the gap increases from zero to several milliseconds, providing ample time for an ideal spark gap to produce a visible arc.
I've seen this phenomenon in a control cabinet with the lights off; the visual display of the spark inside a transparent ice cube relay was quite striking.
If the relay is small, such as a relay on a PLC relay output card, the spark can quickly degrade the relay, leading to frequent failures.
If you seem to be experiencing recurring single I/O point failures on a module, but the current is within rated limits, the culprit is likely a DC-powered coil device sparking when powered down.
How to prevent flyback voltage? There are several ways to prevent this problematic phenomenon. Some are simple, inexpensive solutions that cover the symptoms, but others completely prevent the problem.
Buffer Diodes and Circuits These small, inexpensive devices have several names: buffer, flyback, freewheeling, surge, protection, etc.
They can be ordered in bulk from electronic component distributors for a few cents, with the name "rectifier diode" indicating the desired level of robustness.
Many manufacturers have recognized this issue and designed integrated solutions that mount diodes directly into the wiring of a junction box or directly inside coil-based devices.
Simply look for the word "diode" and ensure it is mounted with the correct polarity.
A diode is a solid-state device with only two wires, allowing current to flow in only one direction.
They are mounted in parallel with an inductive load, so when the load is powered from the controller, the diode is reverse-biased, preventing current from bypassing itself.
Red/Black and Blu/WhtBlu are used to indicate the polarity of the load compared to the diode (note the gray stripes on the diode body).
However, the moment the control voltage is removed, the magnetic field attempts to generate a voltage across the coil, which now acts almost like a high-voltage battery.
Instantly, this forward-biased diode allows current to flow in a short loop from the coil to the diode. Diodes can withstand large currents for very short periods, so the magnetic field (and current) dissipates safely in the blink of an eye.
Because diodes keep the voltage below 1 volt, the voltage will never increase to a level that would produce a spark, thus eliminating the fear of danger.
A variation of the diode is the resistor-capacitor (RC) series circuit, which allows the field to discharge at a predictable rate.
This circuit can also be installed in parallel with a control device, allowing current to drain harmlessly around the contacts of a switch or relay.
Solid-state switching devices PLC output modules with DC outputs instead of relay outputs can be purchased.
This can be an effective solution in some situations, but the advantage of relay outputs is the high current rating required by large solenoids. Many DC output cards simply cannot handle such large currents.
DC output modules provide current through transistors that limit the danger of reverse voltage arcing.
With high off-state resistance and high voltage tolerance for a very short time, only extremely high sustained voltage spikes become the terminating condition.
However, the large current required to drive a load is by no means transient and is (and often smells like) a nuisance for normal transistor outputs.
AC-Powered Loads While 24V DC is a common control voltage, flyback occurs with AC voltage because the inductor coil doesn't charge to a constant value and then suddenly discharges when power is cut off.
Many solenoids are available in both 120V AC and 24V AC; the installation and troubleshooting methods remain largely the same, and the relay card still works.
Obviously, changes in voltage sources and equipment replacements may result in undesirable downtime, but in the long run, it may be worth the time.
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