Magnetic amplifier made from common 12 volt transformers and magnetic cores - Page 1
Forum section: electronics / projects

author: eneuro
author_rank: Super Contributor
timestamp: July 08, 2014, 09:25:24 am
content: Hello,I've found sometime ago magnetic amplifier and magnetic switches used in military applications in fighter aircraftsOn magnetic amplifers in aitcraft applicationWhat I'm interested in now in tree-phase application projectModeling of a three-phase application of a magnetic amplifierHere it is interesting article aboutMagnetic amplifier made from common 12 volt transformersIt looks like 9V battery and 1k potentiometer does the trick using common transformersHowever, in this schematics from this article I'm not sure whether voltage created in controll windings (it looks like primary with higher number of turns is used? ) will cancel each otherIdea behind using two cores in magnetic switch is to cancel this load inducted voltage in controll wires , so everything is ok at above magnetic amplifier circuit?I DO NOT see there exchanged wires in those two transformers used on primary or secondaryBelow it is correct schematics with two cores from professional papers linked at the begining of this thread:Did you have expierience with magnetic amplifiers and could take a look at this 12V transformers schematics to verify this, please?My guess is this schematics is wrong and danger high voltage could apear in DC controll circuitMaybe those rectifier diodes do some trick with higher gains, but I think those transformers should be connected to cancel induced voltage in DC controll loop...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 08, 2014, 09:37:35 am
content: Looks right to me. The dot phasing is same-same and the winding connection is same-opposite.

author: Andy Watson
author_rank: Super Contributor
timestamp: July 08, 2014, 09:50:41 am
content: Quote from: eneuro on July 08, 2014, 09:25:24 amWhy should they be swapped? The two transformers are performing the same action for different parts of the cycle - as determined by the diodes.

author: eneuro
author_rank: Super Contributor
timestamp: July 08, 2014, 04:57:26 pm
content: Thank you for hints.Quote from: Andy Watson on July 08, 2014, 09:50:41 amIn magnetic amplifier with reactance coils in series they are swapped and it does not matter if we swap transformers/toroids controll wires or load wiresSo, in this case they have to be swapped and it is easy to understand why induced voltage in controll wires will cancel when two cores are used in this case.When magnetic amplifier with reactance coils in parallel (with or without diodes) are used it looks like they are connected without swapping and this configuration is more trickyI did not notice it before, so its time to follow AC curents paths and try to simulate this circuit with diodes in brain again and understand what is going on when reactance coils are in parallel

author: IanB
author_rank: Super Contributor
timestamp: July 08, 2014, 08:30:49 pm
content: Quote from: eneuro on July 08, 2014, 09:25:24 amLet's consider the diagram above and call the windings on the left the control windings, and the windings on the right the load windings.Now the DC control current, being DC, can only flow through the control windings in one direction. Therefore the load current must flow in the same direction through both load windings also, so that the magnetic fields have the same polarity. This is what the diodes are for.First, consider the case when there is no control current. The active load transformer has a high self-inductance and a high impedance, and little current flows in the load. However, a significant voltage may be induced in the control winding. However, the other transformer also has a high impedance and so it will tend to block and absorb this voltage, being in series with the first.Second, consider the case when there is a large control current. The active load transformer will have a saturated core, a low self-inductance and a low impedance, so a large current can flow in the load. However, due to the low inductance there will be little voltage induced in the control winding.Either way, the battery and control potentiometer are relatively safe.

author: Zepnat
author_rank: Regular Contributor
timestamp: July 08, 2014, 09:05:47 pm
content: Wow fascinating. Trying hard to understand the principles of operation but sleep depravation isn't helping. Where might something like this be used, would the average person (me!) have unknowingly used something with this inside?

author: tom66
author_rank: Super Contributor
timestamp: July 08, 2014, 09:43:59 pm
content: A magamp regulator is found inside many computer ATX power supplies. It is used to regulate the 3.3V rail.Most ATX power supplies have a regulated 5V rail, and a quasi-regulated 12V rail. The 12V rail is regulated via a multi-winding inductor as about 1V variation overall is tolerable (11.4V to 12.6V.) But the 3.3V is usually regulated by a magamp given its smaller tolerance of 300mV (3.15V to 3.45V.)The main transformer produces an AC square wave, and a driver circuit is used to control the saturation of an inductor to regulate the output voltage. With this, power transistor isn't necessary to switch the full load current (only needs to switch a fraction of it, for the magamp control voltage) and the magamp inductor is cheaper than a proper buck converter inductor (since it is designed to saturate, whereas a buck converter never wants a saturated inductor) the circuit costs less than using a buck converter.

author: ignator
author_rank: Regular Contributor
timestamp: July 09, 2014, 02:11:56 am
content: Quote from: Zepnat onTodayat 04:05:47 PMThese were used in avionics for analog autopilots, and many other equipments back in the day with TUBES/VALVES, before transistors and OPAMPS became common. Aircraft used 26VAC 400Hz for synchros/resolvers and such that needed amplifiers in the AC realm.This predates me (avionics design engineer) by about 15 years but the old guys (30 years ago) cut their teeth on them at the company I worked for.

author: IanB
author_rank: Super Contributor
timestamp: July 09, 2014, 03:11:53 am
content: It's something I had never heard about before this thread, but now I know of it I'm definitely going to try it out. I think I can use one transformer instead of two, with a full wave bridge on the load side, and I'm wondering if I can use a snubber diode on the control side to catch the voltage spikes?

author: johnwa
author_rank: Frequent Contributor
timestamp: July 09, 2014, 06:48:20 am
content: I had a play around with these circuits a while ago, it was fun being able to control a 50W halogen lamp using nothing more than an LDR! Though I can't remember the exact details of the phasing I used.For the 'self biasing' configuration, you want positive feedback action, so the load current reinforces the control current. I think that this will only happen for one polarity of the input signal - the other will result in negative feedback, and reduced sensitivity. The output polarity is the same in both cases, so the amplifier effectively responds to the absolute value of the input, though with differing gain depending on the polarity of the input.Note that this is a separate effect to the relative phasing of the transformers though. I will have to think about it a bit more, but I believe that what IanB wrote is basically right.On another note, does anyone have any thoughts on an appropriate parameter or procedure for evaluating the performance of these magnetic amplifiers? I did attempt to do some power gain measurements on the circuit that I built up, but realised that the results were somewhat meaningless, depending on how you accounted for the DC bias power, and the power transferred back from the load circuit to the input.

author: mzzj
author_rank: Super Contributor
timestamp: July 09, 2014, 07:55:14 am
content: Quote from: tom66 on July 08, 2014, 09:43:59 pmMaybe so 15 years ago but not anymore. 12v rail has been the regulated work-horse since year 2003 ATX12V 2.0 or so.Nowadays most? atx power supplies also use invidually regulated power supply rails instead of magamps.

author: eneuro
author_rank: Super Contributor
timestamp: July 09, 2014, 08:04:45 am
content: Quote from: tom66 on July 08, 2014, 09:43:59 pmIt is interesting that changeing the saturation we can change conductivity in continous way without disconnecting load from mains if I understand it well.Here it is another article aboutMagnetic Amplifiers: How They Work, What They Dodated to July 1960This magamp schematics from these days shows exactly how transformer wires are connected with rectifier diodes and load in AC circuit to form full-wave magnetic amplifier.I think it is worth spend a while and test this using transformer with known core hysteresis and maybe AC sinus signal generator at higher frequency lets say 10kHz with fast Shotky rectifiers-than much less power turns are needed I guess and it might work, while now it is clear how these two power coils and controll coil in the middle are connectedUpdate: I've found nice (iron?) E core transformer from microwave oven teardownIt has 10.5cm2 outer core area and 21cm2 in the middle.It looks like there is about 144 turns @ its primary 230VAC coil (2.3 Ohm).If I remove high voltage coil by cutting welder tracks at the bottom than I could easy add another power coil and controll in the middle for testing.Just wondering what magnetic relative permeabilityu/u0: 200-1000 of this microwave transformer core (iron?) might be for calculation?BTW: Started read this excelent book"Magnetic Amplifiers - Principles and Applications - 1960"by Paul Mali and it explains a lot.It is time toandthose transformer magampsHowever, it is always worth to see and hear nice looking girl in electronics lab which seams to know howto not to destroy oscilloscope

author: eneuro
author_rank: Super Contributor
timestamp: May 24, 2015, 09:47:43 pm
content: Quote from: eneuro on July 09, 2014, 08:04:45 amOk, it is time to try... saturate this microwave oven transformer core using oryginal 3 turns secondary with 230VAC primary connected in series with loadNote: I've disconnected HV secondary wire from transformer core and it was the first thing I've made-of course ensured HV cap is discharged-it wasn't powered up before.Be carefull and remove HV secondary, unless you know what are you doingThose low voltage 3 turns are made with ~0.5mm wire, but probably it is not copper, I guess. In microwave oven it was used to heat up magnetron filnament.As I remember it were 3VAC on those 3 turnsI have some idea to make something like synchronized in time with 230VAC mains saturating this transformer core using 2 phase H-bridge driver and 12VDC with current limit.How do you think-how many DC amperes could be needed to saturate this trafo with those 3 turns?BTW: I can remove HV secondary, so will be ablle to make more than 3 turns if needed, but I wouldn't like to go above 24VAC and use two transformers in this "magamp" experiment.

author: eneuro
author_rank: Super Contributor
timestamp: May 25, 2015, 08:28:53 am
content: Yep, it is time to remove this scary HV secondary windings and put there in place a few 12VAC low current PSUs as well as 2 coils designed to saturate this thing with the help of two mosfets and PC817 in antiparallel to detect mains polaritySomething similar was done in this DIy spot welder to limit current in welder tarsformer:http://www.mike-worth.com/2013/07/31/adding-a-current-limiter-to-my-mot-welder/However, current is limited there by changing inductor core gap on the bottom of cut microwave oven transformer (increasing gap-moving up-will lower down inductance and increase welder current in one of welder transformers)So, it will be interesting if I will be able to do something similar not by changing inductor core gap, but synchronized saturation of its core.Probably using two separate a few turn coils (for each polarity saturation) and two mosfets will be easier than messing with H-bridge there, than 2 MPU pins PWM will control amount of DC current in those coils based on detected mains polarity, so I should be able saturate this core in each AC mains +/- period with low voltage and decent current.Lets see what happends in real experiment

author: eneuro
author_rank: Super Contributor
timestamp: May 25, 2015, 03:07:34 pm
content: Quote from: eneuro on May 25, 2015, 08:28:53 amEverything was fine and I was able to remove HV secondary windings without need to cut MOT at the bottom welder pathsUnfortunatelly, wanted also cut bloody insulation paper close to primary winding above magnetic shunts and.... at this side was hiden slightly inside core and angle grinder slightly touched primary windings and of course cut less than 1mm, but few at the top of wires are damagedProbably onlyMac Gaywerskills could repair this MOT primary windingsBut maybe you have some hints, howto reconnect those top two layers wires?I haven't got such thin copper pipes to make bridges, but while this primary coil will not be moved and some space above available there is a little chance some patent could save this nice MOT primary

author: Zero999
author_rank: Super Contributor
timestamp: May 25, 2015, 05:32:16 pm
content: Have you measured the continuity of the damage primary? If it's still in the 100mOhm range then you could still use it, as long as you use it at a tiny fraction of the rated current.

author: eneuro
author_rank: Super Contributor
timestamp: May 25, 2015, 06:33:30 pm
content: Quote from: Hero999 on May 25, 2015, 05:32:16 pmNope, there is no connection between terminals from half of this damaged windings-a few top were cut.Anyway, will try to find such small copper pipes with 1mm inside diameter, while I need them for other project too and make bridges.Another option is add small copper wires and solder together with reinforced thin copper using spot welder, but need to adjust current while 1mm copper wires disapear at longer spot welder pulsesIt will be used in prototype and I do not expect higher currents than 10A, so maybe will be able "glue" lower layer wires at 5mm bridges and above them another layer 10mm.There is still decent window above for other experimantal windings.If it fails, than will cut bottom and remove this coil and I'll have nice E core for custommagampshown below and above, but it is classic one so here again

author: SeanB
author_rank: Super Contributor
timestamp: May 25, 2015, 07:56:58 pm
content: That wire is copper plated aluminium, and is not going to solder at all. Best is to simply get another donor microwave and try again, as you will not be able to fix this, just rewind with new wire.

author: eneuro
author_rank: Super Contributor
timestamp: May 25, 2015, 10:25:15 pm
content: Quote from: SeanB on May 25, 2015, 07:56:58 pmYep, it is copper plated aluminium and any attempt to press it harder destroys itAnyway, identified those 13-14 damaged wires and it doesn't look good.However, realized that I don't need transformer with exact numbers of oryginal turns, while it will be used mainly as saturable inductor, so there is some hope, that I will be able take those identified damaged wires back and connect with another endm than try to use spot welder to melt joined aluminium wires which should be possible, since copper wires at such gage basicly disapeared at spot welder maximum power with noise similar to gun shotThis way I'll loose those few turns (13) but it is still only maybe ~10%, so no problem-only minimum current whill be slightly higher than if this primary coil were oryginalWe'll see if I will be able to melt those aluminium wires joints to create strong connected alumium wires.

author: eneuro
author_rank: Super Contributor
timestamp: May 26, 2015, 12:05:07 pm
content: It could be epic MOT primary repair too, while managed to move all cut wires back from its power terminals and 30% connections foundHowever, have to find a way to weld those bloody coppered aluminium wires, since it looks like the best idea try to spot them together when connection order puzle will be solved.Holy crap! Do not turn on microwave ovens- take them apart

author: jolshefsky
author_rank: Regular Contributor
timestamp: May 26, 2015, 12:38:38 pm
content: Quote from: eneuro on May 26, 2015, 12:05:07 pmCan you use crimps?

author: paulie
author_rank: Frequent Contributor
timestamp: May 26, 2015, 02:44:32 pm
content: I loooooove Jeri Ellsworth. Jeri Ellsworth is my favorite girl. I want to marry Jeri Ellsworth when I grow up.

author: eneuro
author_rank: Super Contributor
timestamp: May 26, 2015, 05:35:47 pm
content: Quote from: paulie on May 26, 2015, 02:44:32 pmCircuit Girl

author: eneuro
author_rank: Super Contributor
timestamp: May 26, 2015, 05:56:16 pm
content: Quote from: jolshefsky on May 26, 2015, 12:38:38 pmProbably this is the only option, while no way attempt to spot weld those coppered aluminium wires leads to nowhere and thinking about something like this PCB but with 5mm copper vias, put two wires srew them and reinforce with copper wires and solder everything to bottom PCb copper-of course aluminium will not solder, but it will hold those wires in PCb, than by adding hot glue or epoxy will insulate everything.I need 8 such holders on one pCB with additional 9th free if needed do the same trick in the future during experimentsRemoved few useless bloody aluminium wires, no longer needed since do not creates any loops no more:Epic reconnected wires now shows decent resistance lowered from 2.3 Ohm to 2.1 Ohm, so it makes senseHowever, it was difficult operation, while one bloody aluminium wire cracked close to windings when tried to push it from the coil (its is very weak mechanicaly) and hopefully after removing a few other cut wires was able catch it and unwind backIt is never easy, but it looks like I will be able experiment with magamp using this inductor or add classic magamp two other coils using of course copper wires and this crappy aluminium coil will be used as control wires to saturate magamp

author: TerraHertz
author_rank: Super Contributor
timestamp: May 27, 2015, 01:17:45 pm
content: Quote from: eneuro on May 26, 2015, 05:56:16 pmWhat on Earth?! Attempting to splice aluminum wires?Why are you putting so much effort into a lost-cause damaged MOT? Just go find another one. Or ten. Every single pile of discarded junk anywhere will have multiple old microwave ovens. (OK, exaggeration, but not by much.) I made myself stop picking them up, as enough is enough.And next time, when angle grinding a transformer, shove in something solid as a protection on the winding you want to keep. Iron is good, as you can see by the sparks when you hit it. Or ceramic.

author: eneuro
author_rank: Super Contributor
timestamp: May 27, 2015, 04:15:01 pm
content: Quote from: TerraHertz on May 27, 2015, 01:17:45 pmAlumiConn InstallationBut I haven't got such aluminum to copper conectors, so joining two srewed aluminum wires and pressing them on PCB with solder and insulating from air to avoid any oxidation should work in this prototype saturable inductor.

author: iXod
author_rank: Frequent Contributor
timestamp: May 28, 2015, 02:07:23 pm
content: Quote from: eneuro on July 08, 2014, 09:25:24 amI've known this as a "saturable reactor".Dave

author: eneuro
author_rank: Super Contributor
timestamp: September 06, 2015, 06:17:41 pm
content: Finishing this research project too and I'm very close to final magamp PCBs like this 1inch2:Etching this prototype tomorow and testing, than half smaller PCBs (similar to this one scaled down to size 12.5mm x 12.5mm -half inch ) for magamp switches rated at HF low currents will be madeWe'll see if it works maybe even with common small cheap bead ferrite cores

author: AF6LJ
author_rank: Unknown
timestamp: September 06, 2015, 07:14:59 pm
content: Oh Boy does this bring back memories.The days when industrial controls were large hunks of iron instead of fist sized chunks of silicon.

author: eneuro
author_rank: Super Contributor
timestamp: September 06, 2015, 08:58:05 pm
content: Quote from: AF6LJ on September 06, 2015, 07:14:59 pmNowadays we have better materials for toroid coresAnyway, such magamp has some advantages-there is no way to break this thing by overvoltage if insulation holds, so no worry about VDS, VGS, transients, etcI've soldered my prototype low current HF magamp, but in slightly different configuration... as you might see this (sorry, not a great photo in artificial light conditions) without control wires, since my plan is saturate this thing using... neodymium magnetMaybe, someone have idea what could be saturation magnetic flux density B of such ferrite bead?Trying to estimate inductanceApproximate Inductance of a Toroidin this thing to choose switching frequency to get decent impedance at not saturated state.Lets try figure out, if I will be able saturate this thing with strong magnet, so it could be nice to know expected saturation levels of ferrite beads, but unfortunatelly I haven't got any datasheet of this core-it was salvaged from.. CRT monitor flyback filterNo chance to get such detailed information about this random ferrite core, like we have for thisPC90 material: Low loss and high saturation magnetic flux density power ferritewith close to 0.5T magnetic flux saturationI've found for NiZn ferrite 0.33T and in this publicationChoosing Core for Flyback Transformer and Choke Designsthey says about 0.36T saturation magnetix flux and 1.1T for iron powder, so probably something like 0.3T or less 0.2T might be decent estimation for those unknown ferrite beads?Anyway, can't wait to experiment with this new magamp concept -we'll see what happends soon

author: calexanian
author_rank: Super Contributor
timestamp: September 07, 2015, 04:15:06 am
content: Was it already covered that these systems are not really amplifiers but just saturate the transformer core constricting the amount of AC flux that is passed along to the load therefore reducing power?? Look up an Amplidine. Those were widely used in gun turrets until modern servo amplifiers were figured out.

author: AF6LJ
author_rank: Unknown
timestamp: September 07, 2015, 04:34:04 am
content: Quote from: eneuro on September 06, 2015, 08:58:05 pmI am not so sure you could get ferrites to do the same thing. My experience with ferrites suggests that if you saturate them with enough current (in my case RF) they get hot and their magical powers go away pertinently.

author: calexanian
author_rank: Super Contributor
timestamp: September 07, 2015, 05:29:53 am
content: I agree. I would first experiment with small traditional transformers. A split primary split secondary transformer will give you access to all the windings. Much like a small flat pack PCB mount transformer, say in the 20VA or so range should serve as a good test platform.

author: calexanian
author_rank: Super Contributor
timestamp: September 07, 2015, 05:52:24 am
content: Oh. and I forgot to mention. Be careful of the voltage being generated by the AC side of the transformer back on the DC control side. Don't want to blow up your lab supply with high voltage AC being thrown back at it. You should have proper protection via blocking diodes, and a filter network of some kind to keep things safe.

author: eneuro
author_rank: Super Contributor
timestamp: September 07, 2015, 08:15:19 am
content: Quote from: AF6LJ on September 07, 2015, 04:34:04 amYep, but RF frequency is orders of magnitude higher than switching frequency <<1MHz and those ferrite bead are designed to have low losses at this lower frequency and filter high frequency crapp and dispate this energy as heat, so no supprise for meIn this setup ferrire bead will not transfer high currents, but low current and magamp will work as a switch detecting presence of strong magnetic field from neodymium magnets.I've checked datasheets of a few cheap small neodymium magnets and they have easy 0.4T at 0.7mm distance from magnet.

author: eneuro
author_rank: Super Contributor
timestamp: September 07, 2015, 08:22:16 am
content: Quote from: calexanian on September 07, 2015, 05:29:53 amMaybe title of this thread is somehow talking about transformers used for magamp, but I'd rather like to use custom magnetic cores with far less number of turns, thanks to higher switching frequency (but of course no a few MHz RF frequency).Anyway, in this latest experiment with ferrite bead we need NOT ONLY access to windings, but also to... its core, since there is no third control DC coil there, my idea is use pure neodymium magnets to saturate this thingBTW: I've updated thread topic and added magnetic cores, while of ocurse not only 12V transformers now are in my interest

author: eneuro
author_rank: Super Contributor
timestamp: September 07, 2015, 11:02:24 am
content: Yep, thanks to lectures by prof. Walter Lewin8.02x - Lect 16 - Electromagnetic Induction, Faraday's Law, Lenz Law, SUPER DEMOhttps://www.youtube.com/watch?feature=player_detailpage&v=nGQbA2jwkWI#t=1103physics works and my magamp increases load current since induction falls in those ferrite bread magamp windings when strong magnet creates decent bias magnetic field inside toroidSimilar effect in this simple full-wave SMPS driven at the same 400kHz frequency and low current-when strong magnet is applied to its secondary ferrite bead, induction collapses and induced EMF is very low, not able provide even fraction of mA to power up white diodeOf course saturating core in classic SMPS converter is not a best idea since its primary inductance will collapse too, while those windings are usually well magnetically coupled, but in the case of magamp made even using ferrite bead it looks like there is no difference if this core were saturated by third DC bias current applied in the middle of this thing (as shown a few posts above in classic magamp with diodes to improove sensitivity) and ... strong magnet, which can have very usefull practical applicationsPreparing for more spctacular demo with more power full-wave DC-DC converter

author: eneuro
author_rank: Super Contributor
timestamp: September 07, 2015, 02:21:21 pm
content: Quote from: calexanian on September 07, 2015, 05:29:53 amI was able saturate this bigger (choke I guess) E trafo core, while I've one coil was in series with those leds above (anti-parallel configured to detect AC, not DC only), however problem with those E cores is that coils a in the middle and we can only try expose this thing to external magnetic field from both sides, which means to be able apply symetric external field for each half of AC waveform, we have to put two magnets from opposite sides and.. we have a problem-magnetic field in the middle of this E core will cancel if we use the same magnets, so only part of the coil will see strong magnetic field.In the case of, when those two magamp coils are not in the middle, when external field is apllied in the middle (by magnets or DC voltage) than magnetic flux is divided and flows to each side of magamp, so when additional diodes are present it makes symetric saturation easy at the same level of additional magnetic flux in whole AC period, so it looks like there is advantage when those (transformer) coils are not in the middle of E core.Anyway this choke has much higher inductance when not saturated, while this simple ferrite bead one side has probably less than 100uH, since it blocked 100kHZ AC at similar way like this known SMD 0.1 Ohm 100uH inductor at 100kHz.So, yep this choke (each coil has) much higher inductance than SMD inductor and ferrite bead tested, but magnetic flux inside this thing is asymetric (the same single SMD inductor), when applied strong magnetic filed to ONLY one side of this transformer (SMD inductor), so probably we should be able to see that one of those LEds make more bright light, since those inductors will saturate asymetrically when we'll be close to its maximum magnetic flux density (saturation)Low power magamp controled by strong magnet looks quite interesting, however we have to know where apply external magnetic field, since in one direction it will be added to internal magetic flux created by its coils, but otherwise they will try to cancel each other, so we can still get this magamp core saturation effect when magnet field will be very strong, but when those magnetic fields are in correct direction, then the same effect can be achived using smaller magnet.It was interesting experiment-I need to order a few stronger neodymium magnets to experiment more with this thing, but early results are very good and this thing can be usable even with ferrite bead