Flyback Voltages on AC-Powered Layouts

It is a major issue with electronic circuitry in modern equipment.  Check the forum for many such discussions HERE.

ADCX Rob posted:

It is a major issue with electronic circuitry in modern equipment.  Check the forum for many such discussions HERE.

I probably should have been more specific, as the intent of my question was more related to PW equipment that is sans-electronics.  I can see the problems introduced when DC-powered electronics are involved - but what about equipment that is operating purely off of AC?

Since the link references TVS, is then a solution to AC flyback the insertion of a bi-directional TVS in between appropriate points in an AC circuit (say, between the two contacts on a toggle switch)?  If so, does anyone do this for this purpose?

I think that you mean voltage spikes and not flyback voltages as these are normally had with major high voltage transformers that handle over 5000 volts AC in 3 phase electrical work or 3 phase motor work.

A voltage spike can happen with a short circuit for a brief part of a second and can damage voltage sensitive circuit boards. A TVS unit will handle most voltage spikes if used for the correct voltage, with AC voltage it must be bi-directional.

Lee Fritz

JTrains posted:

ADCX Rob posted:

It is a major issue with electronic circuitry in modern equipment.  Check the forum for many such discussions HERE.

I probably should have been more specific, as the intent of my question was more related to PW equipment that is sans-electronics.  I can see the problems introduced when DC-powered electronics are involved - but what about equipment that is operating purely off of AC?

Since the link references TVS, is then a solution to AC flyback the insertion of a bi-directional TVS in between appropriate points in an AC circuit (say, between the two contacts on a toggle switch)?  If so, does anyone do this for this purpose?

The TVS unit is placed in the circuit like you are shorting it out, one side to hot and the other to the neutral or common side. Don't try to use a TVS like a circuit breaker because it will short out instead.

Lee Fritz

phillyreading posted:

I think that you mean voltage spikes and not flyback voltages as these are normally had with major high voltage transformers that handle over 5000 volts AC in 3 phase electrical work or 3 phase motor work.

 

Thanks for the posts - I did mean the flyback voltages that are produced when current is suddenly interrupted (like in a switch) to an inductor (such as a solenoid).    The collapsing magnetic field creates a reverse potential as the energy stored in the inductor tries to find a place to go.  One can create flyback voltages even at relatively low voltages and current draws.

But, Rob's post had got me thinking that a correctly-sized bi-directional TVS could be used to provide a path for the current to flow in an AC scenario.  Not sure if that would unnecessarily wear a TVS, though, or if there's another problem that I am overlooking.

I vote for a TVS. I have them on all my track power blocks. Yes it's  emf = -n times d phi/d t. 

When I was a kid I always wondered how an auto spark coil could generate 40,000 volts when it only had a turns ratio of 100 to 1. Then I learned about Faraday's law and that the emf generated was proportional to the strength of the collapsing field which was proportional to the current thru it,  and which produced around -400 volts peak. Thinking about it physically, it generates an emf in the direction that tries to maintain the current in the coil that was interrupted. So as a circuit element, it looks like a battery where the current flows from plus to minus toward the formally positive end of the coil.

Thinking about TVSs for a few minutes...I've always assumed that the primary use on them was to handle occasional, unplanned, one-off sorts of transients: mains power fluctuations, static discharges, and the like.  But after re-reading this post, perhaps I've missed their primary usefulness on layouts: the handling of flyback.  I guess in my mental model I never thought of flyback voltages as transients (and thus a TVS being the solution) - as, of course, they are - but rather as "something else".  This is perhaps what I was missing.

From my searching there are few mentions of AC-generated flyback on the Internet - I did find one and it discusses using a TVS to mitigate it. 

The TVS should handle a practically unlimited number of voltage spikes unless it's peak power rating is exceed, or you simply hit it with so many that it overheats.  It's not like an MOV which degrades for every transient that it absorbs.