Relay opening is breaking an inductive current...is it a problem?

[MrAl]

Hello,

Did you say 2 amps through the inductor possibly? That could give rise to an 800v peak, but that peak then goes positive and then negative again, etc., until the inductor runs out of energy, and that dies slowly. So the contacts could actually arc over several times before the relay contacts open completely.

Seriously a cheap diode cant hurt anyone's budget can it?

A snubber will also work yes, but the cost is higher. A 3 amp rectifier diode is 25 cents or less.

 

[Flyback]

the current into it (in the inductor) is 350mA

 

[MrAl]

Hi,

Ok well the theoretical circuit works out to about 140v peak but those peaks occur quite often as the wave damps out, and that happens while the relay is opening and it takes a while for a relay to open it does not open instantaneously. If it takes 50ms to open and the oscillation frequency is 1kHz, that's 50 cycles before it opens completely.

The contacts look like a very low resistance resistor when they are closed, and as they open the resistance starts to rise but current still flows. That develops power heating in the contact surface which heats up the contact, but also there could be migration of material from one contact to the other which could cause erosion even though it is an AC current not DC at that point. The wave damps out very slowly too so it exists for as long as the energy in the coil exists. At 1ms it's still at least 50 percent of what it started out to be.

You can also look across the contacts with a scope in the real life circuit and see what is happening. You should open and close the contacts several times to get an idea how the contacts respond.

Cant you afford a single cheap rectifier diode to settle this problem once and for all?

 

[Flyback]

I now believe we should use the diode........I am just surprised the designer didn't already put one in.

 

[MrAl]

Hi,

Could it be that they were worried about the inductor powering the circuit while it dissipated it's energy? But that's life.

What does this thing NORMALLY power anyway?

 

[Flyback]

rear fog and reverse lights (LED)

 

[ronv]

I always thought it took about 327 volts to initiate an arc in air. So I understand the initial arc as the contacts separate but don't understand how they start again on the next cycle since the voltage is below that. Thought that was why the ac rating on a switch is higher than dc because the arc extinguishes after 1 cycle.
https://en.wikipedia.org/wiki/Paschen's_law

 

[Flyback]

the relay contacts bounce on make and so they are repeatedly connecting/ disconnecting multiple times...so even though it extinguishes quick, it happens again and again....at least I think so.....?

 

[MrAl]

Hello again ronv and Flyback,

Well you are both right to a point...

When the contacts first open, the arc can occur as soon as they are just barely open. Once the wave goes through zero, the arc might stop. But by the time the wave goes high in the other direction the contacts may have only moved 1um or less, so the arc can start again. This can happen many times and depends on the speed at which the contacts open. If they bounce (and they will at least a little) then there is another mechanism for starting up the arc again. But even after the bouncing is complete the contacts may still be close enough together to allow another arc to form. It's all about the voltage breakdown of air over a distance. The smaller the distance the less voltage it takes to create a current flow, so they dont have to be touching for this to happen repeatedly. It's only once the arc is broken AND the contacts have moved far enough apart that the arcing will stop completely.

It's also interesting that the model for an arc in air is a resistance that starts out relatively high then decreases as the arc is formed. So the resistance between two partially open contacts may read high, but once they arc over the resistance goes low even over the same distance between contacts.

The material movement depends a lot on the level of current and this current is relatively low, but then again we dont want the small contact surface areas to start to pit because that makes the closed resistance higher and higher.

This kind of action can really be a problem sometimes with relays that have to switch something between two sources. If the arc lasts long enough one source can actually power the other source even though the contacts are not wired for that kind of operation. It's nuts when this happens because things blow up (like 120vac inverters).

 

[Flyback]

thanks,
there was also previous discussion here
https://www.electro-tech-online.com...n-is-mechanical-switches.136036/#post-1140905