Back emf question - Page 1
Forum section: electronics / beginners

author: Felix64
author_rank: Newbie
timestamp: April 16, 2022, 05:38:04 am
content: Hi everyone.Can the back emf from an inductor destroy the inductor itself if the back emf is too high and the wire insulation of the inductor is not strong enough?Like for example when you switch an inductor.I mean when you use an inductor in a switching circuit without any anti back emf components like diodes.Thank you.

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 16, 2022, 12:08:27 pm
content: I do use clamping diode to clamp the back EMF but that to prevent damage to the circuit that switch the inductor on/off. I don't think the back EMF would damage the inductor itself.

author: Ian.M
author_rank: Super Contributor
timestamp: April 16, 2022, 12:25:05 pm
content: Its certainly possible, but more so for tapped inductors (i.e. transformers). Generally, if there is any load on the coil it keeps the voltage low enough to prevent insulation breakdown, but if the output lead of the coil gets disconnected, it can fail that way. This failure mode is relatively common in older small engine magneto coils using breaker points, as simply cranking with the plug lead off can push old insulation weakened by age and micro-cracked over the edge to breakdown.

author: strawberry
author_rank: Super Contributor
timestamp: April 16, 2022, 01:15:06 pm
content: potential between turns is low1turn/1V = 1000turns/1kVisolation between each turn must withstand 1Visolation between each 1000turns or layer/section must withstand 1..2kVcolor CRT TV flyback coil work easy with 20..30kV

author: CaptDon
author_rank: Super Contributor
timestamp: April 16, 2022, 02:19:04 pm
content: It would depend on several factors. One in particular being the spacing of the start-of-winding lead vs. finish-of-winding lead and their proximity. The example given by another poster of high voltage transformers and flybacks doesn't exactly apply to your question since those transformers were specifically designed for the high voltage application and as such can be layered and insulated differently than a run-of-the-mill inductor. Another factor is total inductance and amount of energy stored in the magnetic field. We had a 10 Henry inductor from a radar unit that could pull 1/4 inch sparks when excited by a 1.5 volt D-cell. It had very low resistance and could easily pull about 2 amps out of the battery. In many cases the voltage will be limited by the arc length of the opening contacts (which is a bad thing for the contacts). If I tried hard enough I could probably ruin an inductor but it would take some effort and would not represent the manner in which a good circuit would be designed. Some soft modulators using inductors for small radars are purposely designed to deliver a 10KV @ 10amp inductive kick voltage into the magnetron producing a 12KW output pulse at 9Ghz. Of course those inductors are well insulated and purpose designed.

author: Benta
author_rank: Super Contributor
timestamp: April 16, 2022, 06:34:10 pm
content: Yes, it can happen.One thing that will alleviate the issue is inter-winding capacitance (which is parasitic, but not always unwanted).The other is the winding technique: as an example, if you have a two-layer winding, winding it from one end to the other and back again (which is the easiest way to make an inductor) will result in very high tension at the input and output. Winding it in one direction, making a "flyback" and winding in the same direction again will halve the stress.As always with inductors, it's extremely application/implementation dependent.

author: Felix64
author_rank: Newbie
timestamp: April 19, 2022, 07:13:36 am
content: Thank you everyone for the helpful answers. But there is still one thing I dont understand.What would happen if you would use a coil with a bifilar winding? Would that somehow cancel the effect of increased maximum voltage with each turn or would it still work the same way?Quote from: Benta on April 16, 2022, 06:34:10 pm