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  • Electromagnetic Chuck

    I procured a microwave transformer, and cut it in half. I wound a second coil. It has a 10 ohm 35 watt resistor, and a 19v 3.3a power supply. Do I connect the two in series or parallel?

  • #2
    Originally posted by John Buffum View Post
    It has a 10 ohm 35 watt resistor, and a 19v 3.3a power supply. Do I connect the two in series or parallel?
    Series. Watch out for over currents on the chuck. I have a 24" walker and its 1amp max for the table. JR

    My old yahoo group. Bridgeport Mill Group

    https://groups.yahoo.com/neo/groups/...port_mill/info

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    • #3
      Did you keep the primary or secondary? How many turns on the one you wound? What is the resistance of each of them?

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      • #4
        A microwave oven transformer should have maybe a hundred ohms on the secondary, and maybe 1 ohm on the primary. There is another winding which is two or three turns and separate from the others. That should read virtually zero ohms. There is nothing there to give you 19 volts. Do you mean the smaller one that lives on the front panel circuit board? And where does the resistor come in?

        A few more details would help us know what you're dealing with.
        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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        • #5
          Magnetic field strength is a function of ampere-turns. A 1000 watt microwave transformer can draw about 10 amps on the primary, and will probably have 1-2 ohms resistance. The secondary (about 1500 volts) will carry about 1 ampere and may have about 100-200 ohms resistance. The 19V supply would drive about 5-10 amps directly into the primary. The 10 ohm resistor would limit the current to less than 2 amps through the primary. You could possibly just rectify the 120 VAC to get about 150 VDC and that might drive 0.75 to 1.5 amps through the secondary. That would give you a lot more ampere-turns and more clamping power. As long as you don't exceed 100-200 watts of power in the transformer it should be OK. You could always add an SCR/TRIAC dimmer control to reduce the current.
          http://pauleschoen.com/pix/PM08_P76_P54.png
          Paul , P S Technology, Inc. and MrTibbs
          USA Maryland 21030

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          • #6
            Originally posted by PStechPaul View Post
            You could always add an SCR/TRIAC dimmer control to reduce the current.
            Thats basically all I did with my mag chuck.

            It didnt come with a power supply so I cobbled up one with a variable transformer and some other stuffs for DE-gaussing the table. She can get sticky even with the power off. Push of a button and all is relieved. JR

            My old yahoo group. Bridgeport Mill Group

            https://groups.yahoo.com/neo/groups/...port_mill/info

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            • #7
              Using the primary coil for one half, and have similar size wire for the second half. Today, I made the form for winding the second coil with similar size wire.

              Roger, I copy series. Makes sense. With 19V and 10 ohm + .5 ohm + .5 ohm allows 1.72 amp, giving 33 watt. The resistor is rated at 35 watt. As everything electrical is +- 10% unless stated otherwise, I'm still safe. I'd LOVE to put rectified 110V through it, but there's heat dissipation. 1.1KW. LOTS of heat. Just put your hand near an old 100W light bulb.

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              • #8
                I don't like to rain on a parade, but ... I think that you would be better off using the secondary. I just did that and putting 70v (full wave rectified) on it produces a grip of 600 lbs (measured with crane scale).

                I got the 70v by figuring that it could handle 500ma and adjusting the voltage to give that. The 500ma came from assuming a 1000w oven and 2000v on the secondary. That was verified by the coil only getting mildly warm at 500ma, but too warm at 600. I'm using a variac to get the 70v, but it could be dropped from 120v with a 100R (50v/.5A), 25w (50v*.5A) resistor.

                I'm using it as a stop on the drill press and it works great.

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                • #9
                  Just wondering...

                  In the early years of microwave ovens, the transformers were massive beasts; nowadays they appear puny by comparison. I assume that the oven manufacturers have replaced the old brute force (60 Hz/sec) approach with switching power supplies that enables much smaller transformers. Can this be compensated by more transformers = more iron? Or...?

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                  • #10
                    The 'old' transformers were iron- the 'new' switching ones are ferrite. You would not use ferrite for a magnetic chuck.

                    My thinking in this case is if you're going to wind one coil anyway, you might as well wind two- and make them of a suitable gauge of wire so the resistance matches the power supply you want to use. If it's 19 volts and capable of 3 amps, then that's about 60 watts. What does a regular electromagnetic chuck draw?

                    Assuming that's the power level you want, then wire both coils in series. Each coil should then be about 3 to 4 ohms. That probably means using wire of about 20 gauge. You might have a choice when you buy the wire- what's available to you? Any gauge from 19 to 22 would probably be just fine. Go with the finer wire if you can- you'll still be drawing 35 watts or more, and that will give you lots of holding power, and will minimize heating. Remember- heating changes dimensions. You don't want to find that after several minutes of machining your workpiece has moved up a few thou. You might find that you should pre-heat the chuck until the temperature stabilizes- but then too that heat will go into the table and change things there.

                    I haven't looked into it, but I don't think you need 60 watts- half that is probably lots.
                    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                    • #11
                      Ooops! What I meant to say is if you cobble some stuff together be carefuf to not OPEN the coil in the chuck with an over current!!

                      IF YOU DO THT THE CHUCK MIGHT BE TOAST ) i GOT CAP LOCKED. NOT FIXING IT FOR PRINCIPLE.

                      Listen up. If you cook the very fine copper line in the Chuck you will ruin it. JR
                      My old yahoo group. Bridgeport Mill Group

                      https://groups.yahoo.com/neo/groups/...port_mill/info

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                      • #12
                        I dont understand what the resistor is for. Just buy a 12v 2 amp wall wart and add a switch

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                        • #13
                          Originally posted by darryl View Post
                          The 'old' transformers were iron- the 'new' switching ones are ferrite. You would not use ferrite for a magnetic chuck.
                          This sounds spot on. The new microwave ovens use what they call "inverter technology". Why do I know this? 2020 has been my year of all appliances dying.

                          My micro went out, hence the pic of a tube Lew didnt like (so I took it down). My old one was also inverter type, I bought the same model, other one lasted 8 years. Thing is the big counter top units are maxing out the house hold circuit.

                          The one I have is a beast. Unfortunalty I tossed the transformer because there didnt look to be enough wire to salvage (and I am a scrounger). And I couldnt figure out a use for it. I woulda sent it to you to play with.

                          So yeah, small X-frmer in size and weight. Super powerful. Panasonic.

                          Shows the tube degrade. This new one, same type really bangs out the RF. Nice and active. Oooops...

                          Sorry... JR

                          My old yahoo group. Bridgeport Mill Group

                          https://groups.yahoo.com/neo/groups/...port_mill/info

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                          • #14
                            Bummer!

                            Laptop power supply. No resistor. Two in series. Didn't overdraw without the resistor. 19V 3.37A rating. Voltage verified. 120 turns of THICK, solid copper wire on each magnet. 16 ga wire connections.

                            BTW, the wooden form chucked in my lathe worked like a champ in winding the wire. 5 minute epoxy on each layer. Held its form perfectly when dismounting and mounting on the iron frame.

                            The results were severely disappointing. Alnico magnets hold better. Nowhere near neodymium level. Bummer.

                            In desperation, tomorrow, I'll try parallel. Solderless connectors make experimenting so much easier. If I burn out the power supply, I'm out only $10.

                            Should I be looking at a bridge rectifier and house current?

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                            • #15
                              Tried the power supply with one magnet. SLIGHTLY stronger pull, but then, it started clicking on and off. Guess I exceeded the power supply's capability. Need a LOT more voltage. Suggestions?

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