Do all relays need a diode in parallel with the coil?

Question

I often see circuits with relays and diodes like this:

Note the diode D1 in parallel with RLY1, at reverse polarity to the driving voltage V1. Per my research, the reason is that the relay coil is basically an inductor, and when an inductor is deenergized it wants to continue forcing current along the same path regardless of external circumstances, and if such path isn't available bad things can happen due to a phenomenon called flyback voltage spike.

I built a circuit (below) which functions as desired without any diodes. I can see no voltage spikes appearing around either relay coil in the simulator. But I'd like to know, do either of the relays need such a diode? If yes, do all relay circuits need such diodes? If no, in what circumstances can designers get by without such diodes?

Answer 1

Yeah, pretty much. If you're switching a coil off, there will be a reverse voltage (flyback). You use either a diode, R-C snubber or some other means to absorb it. The best placement for the diode or other suppressor is right at the coil itself to get maximum effectiveness.

Any coil that is suddenly de-energized will make a fly-back reverse voltage spike as its magnetic flux collapses. The flyback voltage can be estimated as follows:

• \$V_{ind} = L_{ind}\frac{dI} {dt}\$

A typical switch turning off a coil will result in a near-instant \$dt\$, resulting in a good-sized spike even for a smallish, low-current relay.

Let's work an example. This 5V relay: https://www.farnell.com/datasheets/16770.pdf

• L = 65mH
• Coil resistance (current @5V) = 130 ohms (38.5mA)

So now we chop the current from 38.5mA to nothing (\$dI\$) in 1ms (\$dt\$):

• V = 65mH * -38.5mA/1ms = -38.4V

Note that at 1ms I'm being generous with \$dt\$ here. It would be on the order of microseconds, which could result in hundreds, if not thousands of volts at the coil - well above what your switching components can stand (FET, BJT, etc.) Even switch contacts, such as from a push button or another relay, can be damaged by flyback-induced arcing and eventually fail, not to mention the spike making its way back to the power supply and nerfing other parts of your system.

(Flyback diodes. Cheap insurance.)

If your coil drive voltage is AC, a diode won't work. You can use an RC snubber, back-to-back Zener diodes, or a purpose-built device called a TVS (transient voltage suppressor).

Regardless of type of spike-catcher you use, the best placement for the suppressor is right at the coil itself to get maximum effectiveness.

As for your proposed circuit, the first relay certainly needs a diode. The second relay probably isn't an issue as the current should have decayed to zero by the time the contacts swing. However, tf that time is short (say, if the relay contacts or the switch chatters) you could still have a problem.

Answer 2

The main purpose of a flyback diode is to protect the switching transistor from inductive high voltage spikes that result when the transistor switches off.

If you are using mechanical switches, they may be more tolerant of such spikes. However, if you are trying to design a long-lasting reliable product, you should consider that high voltage transients still create a small arc when a mechanical switch opens. Repeated arcs will create pitting and start to erode the contacts of the switch, eventually leading to a phenomenon of a "sticky" switch where sometimes contact isn't made or broken as expected (the operator might toggle the switch again and it works fine the second or third time, etc.).

So the answer to your question, "In what circumstances can designers get by without such diodes?" is "When it is a temporary or hobby design where longevity isn't a concern."

Answer 3

Do all relays need a diode,

No, not all, only about 90% of them do.

In your circuit if you build it for real you'll see sparking in the switch. a diode on the first relay would help that, and make the switch last for longer. Simulated parts are indestructible, real parts wear out and break.

The second relay will be mostly switched with the coil unpowered so no diode, and probably no snubber even needed,

As the power to this relay's coil is bi-directional a simple diode won't work, you could power it through a bridge rectifier, or go with a resistor-capacitor snubber instead. or just ignore it because mostly the circuit will be switched with no current through the relay.

Answer 4

It's not always necessary, and not always desirable.

Machine controls that have relays switching relays typically don't have diodes (especially if the coils are AC) or snubbers. The lack of such causes a bit more sparking on opening, but the contacts are generally robust and that is not limiting, even with 24/7 repetitive operation.

The lack of a diode or snubber can lengthen the life of the relay that is being switched since it allows the contacts to open faster and thus arc less. That can be a more important consideration than the small amount of sparking from a mA level coil current. One might note that relay life (in the relay datasheet) is typically specified without a diode across the coil.

The real situation with the open coil after the circuit has been opened is not that the voltage would become enormous anyway, if you measure it, you'll find "ringing" (a damped sinusoidal waveform) as the coil inductance resonates with the distributed capacitance of the coil. That limits the voltage so it's hardly worse than switching a small current at mains voltage. If memory serves, the resonant frequency is typically in the AM radio band, give or take, depending on coil construction etc.

When you have fragile silicon junctions switching the coil rather than macroscopic chunks of metal, a few hundred volts at mA can lead to permanent damage so diodes or similar means (zener with diode in series, for example) are often used. One clever method used in some power shift registers is to fabricate a zener diode on the chip that partially turns on the MOSFET with over-voltage, but that absorbs the energy in the chip.

Answer 5

I found this on TE Connectivity, in the definitions section of "Automotive, General Purpose and Signal Relays" pdf 05.2018:

Coil suppression circuit

Circuit to reduce the inductive switch off voltage peak of the relay coil (EMC protection, switch off voltage peak). Most of such circuits reduce the armature release speed, which may decrease the relay lifetime depending on the application load. Especially diodes or any PN-junction of the electronic control system in parallel to the coil will significantly reduce the electrical lifetime. Recommended that relay performance testings to be evaluated with the suppression that will be used.

Note: unless otherwise specified the indicated relay data refers to coils without any components in parallel or in series to the coil.

I can warmly recommend also the Panasonic "Automotive relay user guide". It is a treasure.

On DC loads they advice using zener diodes twice the supply / load voltage, either across the coil, with one diode and a zener in series or from low side of coil to ground when the coil is supplied directly from supply or no harm can be done in the supply end of the coil.

Google: "Panasonic automotive relay user guide" and you are good.

There are an extensive elaboration and mostly warnings about slow release times with subsequent arcing and not using caps because of increased arcing when loading the cap.