Couple of Questions about a DC electromagnet & cap - Page 1
Forum section: electronics / beginners

author: THATguy
author_rank: Regular Contributor
timestamp: September 24, 2018, 01:03:29 am
content: I want to build a DC electromagnet (unsure if I will add ferrous core yet).I'm basing it upon a microwave capacitor - see attached image.I might make some custom wire that is water cooled, if there is too much heat.However since I just charge up the cap, shut off the power & blam! the electromagnet gets a sudden blast of electricity from the cap. Since it is a very short blast of DC will I need to worry too much about heat/melting???(I have some 10 gauge wire that I can test with)

author: ArthurDent
author_rank: Super Contributor
timestamp: September 24, 2018, 01:55:22 am
content: You do not want to try this. Until you know what you're doing you stand a good chance of getting hurt or electrocuted.Just leave the microwave and its high voltages alone.

author: THATguy
author_rank: Regular Contributor
timestamp: September 24, 2018, 02:15:04 am
content: I've already fully dismantled the microwave for scrapping.I'm well aware of the dangers of capacitors, thanks for your concern.I also know the dangers of a large magnetic force.My current concern is of a melted electromagnet, do you have any useful info regarding my question?

author: ArthurDent
author_rank: Super Contributor
timestamp: September 24, 2018, 02:25:16 am
content: The information I gave you was very useful and should be followed.

author: THATguy
author_rank: Regular Contributor
timestamp: September 24, 2018, 10:31:54 pm
content: I've learned a lot from this video, & will be refining my design. Electromagnets have a much weaker range than I was expecting.https://youtu.be/XM6CZIBMMg0

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 24, 2018, 10:48:18 pm
content: Well if you want useful information put the following to work.Energy stored in capactor is (C*V^2)/2.Limiting current is given by Ohms law. I=V/R.rate of change of current is di/dt=V/L. This equation tells you you need to solve a differential equation to determine voltage over time.You are calling your device an electromagnet and are therefore probably interested in either field strength or pull in force. There are too many different impacts from configuration to give a simple answer. In any case the field will be rapidly time varying and reversing in direction.If you can't put those three equations to work, the caution you were given is warranted. I wouldn't say you shouldn't proceed, but there are a number of opportunities for learning the old way. Be careful.

author: THATguy
author_rank: Regular Contributor
timestamp: September 26, 2018, 07:42:53 am
content: Because it is DC I can use a permanent magnet core as Cody did.I want to wrap it in some MU metal to help concentrate the magnetic fields too.I do not mind if there is a repulsion force after the initial blast of attraction from the capacitor.To reduce eddy losses I need the thinnest wire possible - but worried about it melting from heat...I understand more turns increases magnetic force, however if I wind backwards down the shaft, & then up again in multiple layers would this have counteractive effects?A low resistance wire is good eg copper, iron would carry more magnetic force how ever has higher resistance, copper is diamagnetic so creates an opposing magnetic force, alumium is paramagnetic, good conductivity (heck it's even used in some permanent magnets), though I've heard it can heat up quickly - reaching it's currie point & being blasted away with great force, if it was semi moulten that would not be good.I've some smaller capacitors that I will start off with, not even sure if my microwave cap is functional.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 26, 2018, 08:20:18 am
content: You are using words attached to advanced magnetic concepts that you clearly do not understand. The most efficient way to get this knowledge is reading and thinking. A good text on magnetism, which fully explains paramagnetism, diamagnetism and the other forms of magnetism is by Taguchi. I don't recall the title right now, but Google is your friend. I will warn you that substantial math is required to follow the book.You can also learn by continuing your experiments. Be both very patient and very careful. You will be disappointed by most of your results. There are no easy paths to very powerful magnets. High currents and high energies are involved when you get there, so the consequences of error become large. One broad area you may not have thought of is that Newton's old laws apply, every action has a counter reaction. Containing very large magnetic fields has much in common with designing a pressure vessel.

author: THATguy
author_rank: Regular Contributor
timestamp: September 26, 2018, 11:02:19 am
content: Yes I understand the risks of high voltage/amperage & metal being attracted, even magnetic forces acting directly upon the human body & brain. I'll be taking a cautionary approach to my experiments. I enjoy having 10 fingers, two eyes & being alive!Found this image regarding many layered coil winding:http://freeenergy2000.tripod.com/selectron.gif

author: vk6zgo
author_rank: Super Contributor
timestamp: September 26, 2018, 01:27:21 pm
content: Youdorealise that is really quite a small value of capacitance, even though it is physically large?Basically, you are putting a near short across a smallish capacitor, so you will get a very short transient, with a lot more relationship to ac than DC, & with miniscule energy levels.As an electromagnet it will pretty much suck!

author: THATguy
author_rank: Regular Contributor
timestamp: September 27, 2018, 11:22:58 am
content: Right so I should get a lot of my smaller capacitors then.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 27, 2018, 02:51:52 pm
content: Some calculations will save you a lot of work. Simple stuff like ampere turns and impedance.

author: THATguy
author_rank: Regular Contributor
timestamp: October 10, 2018, 02:02:48 am
content: I've tried using inductance calculations in the past to build metal detecting coils, but found the explanations of width & such to be confusing (before I had these forums).I'll be doing a lot of testing of prototype coil designs with small caps first.Inadvertently it seems I'm creating a EMP blaster!Just found out the large microwave caps have a built in discharge resistor, now thats annoying! Hope I can dig it out.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 10, 2018, 03:02:49 am
content: Coil equations can be very confusing. Not least because they are approximations. But even misapplied they should get you close enough to tell if you are getting in the ballpark you wish. Amp turns will tell you the magnetic field in air. If you have a core the B=mu H equation finishes that. Knowing how many hundred or thousand amps you need to get the field you want then lets you test feasibility using the resistance and voltage. At these levels you can't ignore wires or the internal resistance of your capacitors. On your first pass you probably won't like your answers and you can start playing with number of turns. But as turns go up size gets out of hand and you want to reduce wire size, increasing resistance. Inductance also goes up limiting how rapidly current can rise. If you look at all of the possible equations you might use for inductance (at least in the air core versions), you will find there isn't an order of magnitude difference in the answers. Which is all the closer you need to be at this step in the process. It is a race between whether current can get to the level you want before your capacitors drop voltage too much to drive that current.A few simple equations in a spreadsheet can let you do a lot of experimenting before you get your hands dirty.

author: Brumby
author_rank: Unknown
timestamp: October 10, 2018, 03:46:55 am
content: I wish you well in your experiments - but I, too, have some concerns.Let me just say that, before you get up into some really high energy territory that you can wrangle some maths and understand the physical implications of that math in something you want to test.There are two reasons for this. The first would be for your safety (sorry if this is sounding like a broken record - but I'm sure you know where I'm coming from). The second is for optimising your experimental efforts. If the math doesn't point to a particular experiment as being worthwhile, then why spend the time building it and running a series of tests? It's like trying to head-butt your way through a wall instead of taking the time to find the doorway.