| Electronics > Beginners |
| 1-2mH 20-30A peak inductor - how would you make that? |
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| mvs:
--- Quote from: not1xor1 on January 23, 2019, 08:09:55 am ---Since I never saw such a beast I wonder if that would be just too expensive compared to other possible alternatives (ordinary switching preregulator or multitap transformer). --- End quote --- Agilent U8002A power supply has preregulator with 900µH inductor. It is small "transformer" screwed to the case at photo below. http://caxapa.ru/thumbs/324496/TopView.JPG |
| not1xor1:
thanks everybody for all your replies :) BravoV: --- Quote ---I guess you should be looking for Siemens's stuff, as scored these recently at local junk yard (electronic recycler) --- End quote --- Provided that it would work, that would make hard to replicate a possible project. BTW Yes, I already know (and will probably build) Jay_Diddy_B's Inductor Saturation Tester. :) Berni: --- Quote ---But in terms of being a preregulator fed by mains its easier to get a transformer with secondary taps and then use relays(or tiacs if you prefer solid state) to switch those taps into the rectifier as needed. It requires no extra heavy iron core device , is run by very simple control electronics and adds no efficiency loss. --- End quote --- I know about multitap transformer solution, but, apart the fact that they are more difficult to source (unless you stack multiple transformer with increase of cost and space), they are much less efficient when the output voltage is close to the lower limit before tap switch. T3sl4co1l: --- Quote ---Well, 2mH at 30A is 0.9J(!!), and assuming the frequency is low enough for laminated iron (mains frequency + harmonics?), that means Bmax ~ 1.2T is reasonable. 0.9J / (1.2T)^2 * 2*(1.26uH/m) = 1.57u m^3 or 1.57 cm^3 of air gap. At a typical aspect ratio of about 25:25:1, you'd need a core of 1.36mm gap and 34 x 34mm cross section. Which isn't terrifyingly big, about 200VA transformer size I think. --- End quote --- The peak current would be inversely proportional to the inductance, so unless one needs 10A DC output, for 3-5A DC and 2mH 20A peak should be enough. Besides that we are not dealing with DC current, 20-30A would be the peak of few ms pulses... Anyway considering the energy of the load, e.g. 20V*3A*10ms = 0.6J, we are in that order of magnitude. :-// Doctorandus_P --- Quote ---Several years ago I built an "emcie" / "elmcie" compatible LC meter with a LM393 comparator and a uC. primary coils of E-core transformers measured as a few Henries, and Toroidal transformers measured in the tens of Henries, but the measurement errors were probably very high. "Wide bandwith" is sometimes quoted as a disadvantage of toroidal transformers, but good for you :) So if I assume a 10H primary inductance for a 230Vac toroidal transformer and you need 1mH then the secundary winding would be around 230/sqrt(10000:1) = 2,3Vac, and you want it to be able to handle 30A, so the size would be for a 70VA transformer. So if you have a 70VA or larger toroidal transformer in the closet and you put a handfull of windings from thick enough wire around it you have an inductor of the size you want. --- End quote --- Your calculations agree with what I got applying the formula in the page linked by Berni: Imax =Φmax*N/L to a 40VA transformer I got about 1.4mH from 26 turns and calculated 0.6mWb as Φ and so the result was about 11A of max current. BTW I already know about those inductor saturation tester projects and the advantages of a real switching PSU, but was just wondering about the feasibility of this different solution. mvs: --- Quote ---Agilent U8002A power supply has preregulator with 900µH inductor. It is small "transformer" screwed to the case at photo below. --- End quote --- I found the schematic in a Chinese forum, but I think it is likely wrong as there are many parts which do not make any sense. |
| not1xor1:
I noticed there are various relatively cheap common mode choke rated for tenths of Amps. e.g.: on mouser How can that be possible? Is that due to the fact that current flows in opposite directions and so the total magnetic flux is just the difference (i.e. close to null)? They also sell a 1mH 100A inductor. That is on E-I core and is really cheap... just about 264€ >:D BTW I have a few large common mode choke I probably salvaged from old broken ATX PSUs. Does anybody know if they can be re-winded and used for a switching PSU? Is the core suitable for that? |
| Berni:
Common mode chokes indeed use the opposing current trick to get to the impressive current ratings. They always have two identical coils around the same core, but when it is used as a common mode choke all the current flowing forwards trough one coil is returning in reverse on the other coil. This causes the coils to produce the same field strength but in opposite directions so they cancel out and create near zero field in the actual core. As a result the core can be smaller or made out of a high permeability material to get more inductance for a given form factor. The cores might also be a ferrite blend that is designed to have high core losses so that the differential mode current that it is meant to stop is swallowed by the losses rather than being fed back out from the stored energy in the core. Yeah ATX PSU power factor correction inductors are a common cheap way to improve the power factor of them. Id say go ahead and try measuring one to see what you get from it. |
| Psi:
i made 6mH 10A dummy load for 12VDC so i could exactly simulate and test a product that was to drive a 10A automotive solenoid. Used four 'Apple' sized electromagnets wired in parallel. This came out to around 6.3mH total or something close enough to 6mH for my application. However i didn't need 10A continuous. Just short bursts of a minute or so. Even my large dummy load can't handle continuous use at 10A which is 120W of heat. I once left it on for 10min by accident and I started to smell burning enameled wire :-DD After that experience I thermal epoxied a 20A 60degC thermostat on to it for safety. It's been going good for 4 years now |
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