How do I stop getting belted by back EMF? - Page 1
Forum section: electronics / beginners

author: Pulseit
author_rank: Contributor
timestamp: September 07, 2023, 05:31:01 pm
content: Hello,Using the following,12v Battery -> Inline switch -> 12v to 230v inverter -> Flyback -> 20kV outputUsing the switch gives me a fair jolt when turning it off, I've looked for ways of preventing this mainly focussed around the search term "snubber circuit". There isn't much available as a complete module other than thishttps://michiel.vanderwulp.be/domotica/Modules/RcAbsorptionModule/which is all over the likes of ebay, aliexpress etc. Would this when placed parallel to the primary input to the flyback deaden the EMF? or is there no suitable generic solution to this? Would it have an effect on the output?

author: tggzzz
author_rank: Super Contributor
timestamp: September 07, 2023, 05:34:08 pm
content: Show a schematic containing all the components, and a photo showing your construction.Those details matter.

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 07, 2023, 05:35:08 pm
content: Probably not backemf but arc by current.Try placing a RC snubber in parallel with the switch.

author: Zero999
author_rank: Super Contributor
timestamp: September 07, 2023, 05:40:48 pm
content: It could be many things.Is the output from the flyback isolated from the input? Perhaps it's coupling across? Is it earthed?

author: Pulseit
author_rank: Contributor
timestamp: September 07, 2023, 06:08:13 pm
content: Quote from: tggzzz on September 07, 2023, 05:34:08 pmIt's just off the shelf parts, a 12v car battery wired through switch then to a 12 to 240v automotive inverter then into a flyback out of a laser power supply. It's no more complex than that. No visible circuitry, I'll attach picture.Quote from: DavidAlfa on September 07, 2023, 05:35:08 pmI'm not sure what you mean by arc by current? I can't say I have taken many shocks to be an expert in this field but it's close to taking hit from a car ignition coil.

author: Pulseit
author_rank: Contributor
timestamp: September 07, 2023, 06:31:00 pm
content: I measured between the inverter and switch with the flyback with an open secondary and got nearly 1kV AC.

author: Manul
author_rank: Super Contributor
timestamp: September 07, 2023, 07:30:42 pm
content: I see less than a volt. Where is 1kV?Do you get a shock at the moment of switch turn-off or when you touch it?I think this circuit may give you shocks even if everything is working perfectly. It is isolated(aka floating), so high voltage arcs or leakage to ground or even corona discharge to air will charge the whole circuit.It can even be real dangerous: if your body is grounded while you touch the circuit and high voltage arcs to ground, you essentially become part of return path.

author: Pulseit
author_rank: Contributor
timestamp: September 07, 2023, 08:12:52 pm
content: It's being measured through a 1:1000 probe, so its showing 944V. To be honest he last two times I turned it on I didn't feel anything.I guess the question isn't really specific to this setup anyway, as for example running a switchmode power supply into a 555 pulser into an ignition coil can make the power supply go haywire, I was assuming this is a similar back EMF effect and asking if there is a generic way to mitigate it.

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 07, 2023, 08:19:27 pm
content: The inverter will have beefy capacitors and snubbers to protect the transistors.So when you remove the power, I see no way of getting 1KV emf.That would only happen if it was an inductive load, like a relay or a motor.However DC is much harder to break than AC, so it will arc briefly over the switch contacts when opening the circuit under load.

author: Manul
author_rank: Super Contributor
timestamp: September 07, 2023, 08:31:11 pm
content: The inverter might have EMI filter at it's input, including some inductance. It is possible that you would feel some back EMF if you turn off the switchwhile having your fingers touch the contacts of the switch(you should not do it anyway). But this shock would feel like a shock through the fingers. If you are not touching the contact side of your switch and you feel a shock going through your arm and / or body - that's most likely not back EMF.As I said, you have high voltage flying on the other end which seeks for a return path. If it discharges into your floor or some objects separate from your circuit, you may get a shock while standing on the floor and touching the circuit.

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 07, 2023, 08:46:15 pm
content: Totally right!

author: Zero999
author_rank: Super Contributor
timestamp: September 07, 2023, 08:57:16 pm
content: Quote from: Pulseit on September 07, 2023, 06:08:13 pmNo, that's not the right way to drive a fly-back transformer.The inverter will output 50Hz, whilst a fly-back transformer needs tens of thousands of Hz. The inverter's over load protection circuitry will be kicking in. You're lucky it's giving 1kV and not smoking.

author: tggzzz
author_rank: Super Contributor
timestamp: September 07, 2023, 08:59:56 pm
content: What is the specification of all the insulation on the wires, including any heatsink or similar?What is the specification of the modules you have used?Have you followed the instructions in the datasheets? Components are designed to be used in specified ways, and using them in other ways can lead to surprises. Details matter.Finally, a schematic showing the module types, the names on the module's connections, and how they are wired together would help.Details matter.

author: Pulseit
author_rank: Contributor
timestamp: September 08, 2023, 03:08:25 am
content: Quote from: Manul on September 07, 2023, 08:31:11 pmOk I cannot dispute that, I'll try get better measurements tomorrow. The only thing I can see that may give erroneous measurement is the probe is limited to 60hz AC whereas it looked wildly staticky through the oscilloscope function of the multimeter.Quote from: Zero999 on September 07, 2023, 08:57:16 pmIt does sound like a cicada when the circuit is open. The HV out gives over 20kV though. At first I used a 18kHz PWM dimmer but I believe it drew too much power and fried the mosfet, it did smoke and die after a few seconds.

author: Swainster
author_rank: Frequent Contributor
timestamp: September 08, 2023, 05:27:44 am
content: Ooh, that's extremely nasty. Zero999 has hit the nail on the head. That is not how you drive a flyback transformer and what you are doing is extremely sketchy.Build or buy yourself a matching driver board for the flyback transformer. Here is a discussion of such a driverhttps://forum.makerforums.info/t/if-anyone-can-help-im-having-issues-with-my-psu/14709. If you are not driving a laser then this can be very much simplified, however you still need something to convert the AC mains to something suitable to drive the flyback. If I was going to build something from scratch (rather than buying an off the shelf module), then I'd skip the whole AC mains inverter step and build a driver that can take 12V input.Regarding getting zapped by the switch, you could try mounting everything on a plastic chopping/food preparation board to properly isolate it from your floor. Fix the switch to the chopping board and when you switch it off make sure that you are only touching the switch and no other part of the circuit. i.e. not holding anything steady while operating the switch. Make sure no cables enter or leave the chopping board when in use i.e. totally floating.

author: coppercone2
author_rank: Super Contributor
timestamp: September 08, 2023, 06:02:04 am
content: you can hang a plastic rod on a plastic chain from the instrument to push the switch with.or for this application, built a insulating box that the switch is mounted in, with some kind of push rod thingWell, isent that desperate? putting a switch in a box with some mechanical contraption? No, not really. plane parts are built with switches like that mounted to rupe goldberg mechanisms (like air liners)As to if its proper to drive the flyback transformer this way, maybe, maybe not. probobly not great######Adding a snubber is good too, but this basically mean that your switch is not up to the job you put it through. The safety of a switch should NOT depend on a RC circuit functioning. I.e. if a RC circuit breaks, and this results in you getting shocked, you have a bad design. I can see a broken solder joint or something causing an accident where you hurt yourself because you got scared when using that equipment you put together for some more complicated experiment It should still be safe to touch if all the parts breakI think you learned something valuable about the switch you chosen in the type of system that you made. Realizing that this can happen in a hV system with the hardware that you chose for user interface is a extremely valuable lesson, way more valuable then a ideally functioning circuitI would take the improperly driven circuit with a good switch that can't shock you over a properly driven circuit that can shock you if some random electrical sub component goes bad. Because one makes smoke the other can shock you. I would rather have smoke. one is safe and the other is well behaved and avoids scrutiny if the parts are working. This is howbadaccidents happen.If something is belting you, and the proposed solutions involve 1) thin bits of wire would on ceramic, 2) thin foil wrapped in a spiral, 3) semiconductors , then you need to totally ignore those solutions, because for safety that is silly. Those parts all break, sometimes for no reason. pretend they can all break. Feeling apprehensive about dealing with something because a metalized plastic joint on some tiny little cylinder you bought on ebay failed? that means you need a redesign.

author: Zero999
author_rank: Super Contributor
timestamp: September 08, 2023, 10:07:43 pm
content: Quote from: Swainster on September 08, 2023, 05:27:44 amIt's possible I might be wrong and that could be a module, with a built-in mains powered driver, but I stand by what I said, unless the OP provides a part number/specification/data sheet.

author: RJSV
author_rank: Super Contributor
timestamp: September 09, 2023, 01:00:26 am
content: As per Mehdi (Electroboom videos):If you are holding, say an insulated screwdriver handle, you can receive an ARC jumping to your fingers. The path is to arc through the air, to the metal of the tool, and conducted down to the (insulated) handle.Then, if voltage is sufficient high, you get a second arc from the tool, jumps to your hand. If you aren't grounded it discharges just to your capacitance, even without any sensible further path, to ground.That's not an expert opinion, for safety, but maybe a rough idea about your item.