Relays and diodes

[OzzyDaveO]

The voltage is induced the same way as an ignition coil. When power is removed from the relay coil the magnetic field collapses just like in an igniton coi.
Ericl

Correct - dampens the back emf from the collapsing magnetic field with power is removed from the windings.
otherwise there could easily be 2 times the voltage present - electronics dont like that and the 'smoke gets out'

 

[dkmc]

Correct - dampens the back emf from the collapsing magnetic field with power is removed from the windings.
otherwise there could easily be 2 times the voltage present - electronics dont like that and the 'smoke gets out'

2 times? I would think 10 or 20 times the voltage could be produced. That's what does the damage.

 

[charlessenf]

Diode Connected in Parallel to a Relay Coil – Relay diode - Electronics Area

The purpose of a diode connected in parallel to a relay coil (flywheel diode or freewheeling diode) is to avoid damaging some nearby components sensitive to high voltage.

electronicsarea.com

Thanks for the post! Electronics Area dot com is a good site to know about.

 

[charlessenf]

"Fear The Government That Wants To Take Your Guns" - Thomas Jefferson.

He also did not believe women should have rights (vote, chose a husband, refuse copulation with the master, etc etc etc.) He lived in a feudal world most of us know very little about - save 'Important Dates in History' and the like.
Today we pick and choose from among the millions of words written by those who, in relative ignorance, proceeded us upon this earth to, most often, justify our actions and philosophy as against the objections raised by others in our republic.
Do any of us have 'the' answer?
History would indicate to an objective reader that such is seldom the case.

 

[John_Mc]

Yes that's what the diode across relay coil is for, to dampen reverse or back emf. No diode means noise or a voltage spike happens along the supply line which can cause damage.
Sometimes you get lucky without it, but better safe than sorry.
I'm a retired electronic technician 43 years.

Yep. They are often referred to as a "Flyback Diode":

Flyback diode - Wikipedia

en.wikipedia.org

 

[jgmo]

Another thread slowly degenerating into goolbideegook. Thanks so much.

 

[aircommuter]

19 has the full description and they are primarily for protecting the switch that activates the relay from arc when the switch is opened.

 

[rv7charlie]

19 has the full description and they are primarily for protecting the switch that activates the relay from arc when the switch is opened.

Ding!
A lot of the prior posts properly describe the function of the diode (to shunt the very high voltage pulse that results when the coil's field collapses), but the primary *reason* you want it is to protect the contacts in the switch that controls the relay. Without the diode, that high voltage pulse will generate an arc across the switch contacts each time they open, eventually burning them to the point that they can no longer make contact. For an analogy, think of the 'condenser' in the distributor of an old Kettering ignition system. Without the condenser, the points quickly burn up.

edit: If 'nearby components' (other than the switch) are damaged by the relay coil, somebody did a really poor job of designing the system.

 

[topgun05]

To simplify the wiring polarity issues and in the case of a 4 terminal relay that could be installed either direction, a resistor is used instead of a diode to perform same job and is not affected then by which leg of the coil has B+.
I would guess most are this way, whether stated or not, and would also venture almost impossible to buy one without any clamping built-in.

 

[DutchInNZ]

I've been studying the circuit diagrams and operation of various electrical functions in the service manual for the Kioti DK 35 45 50 tractor.

I see a number of relays (not all, but some, including the start relay) that have a diode placed in a loop around/between the terminals for power and ground on the coil circuit. Having dealt with this style relay for years, I don't understand the need for a diode in this application. Maybe someone can give me a "nutshell" explanation for why that might be needed?

I'm not looking for long winded breakdown on the theory of diodes/rectifiers, atoms and molecules, protons and neutrons or any of that. I know what a diode does. I just don't see why there would be one here.

When a coil is switched off, due to its induction it develops a spike in reverse polarity; the plus becomes a strong negative voltage. Transistors can not have that and break down; therefore a diode is connected over the coil that isolates that reversal voltage to the coil itself. Most newer switching transistors and computer outputs, and sometimes the relay itself, nowadays have an in-built diode, so an external one is not necessary anymore. A mechanical on-off switch doesn't care about voltage reversal, so that is why some claim that they had no problem with replacing a relay.

Ignition coils for spark plugs are made to use this high reversal peak for sparking, so there it is enhanced to somewhere around 15,000 Volts. Your electric fence also is an ignition coil but with an extremely low current so that it does not do damage to the body.