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Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
Here is something I have been working on for the last few days. I got the idea after looking at Tesladownunder's site, and I decided to design my own circuit using the same basic idea. The outputs of the two hall effect sensors are subtracted from each other, then the difference is amplified and put through a lead compensation circuit for stability. The signal is then amplified again due to the loss in the lead compensation circuit, and is then fed into the DTC pin of a tl494 PWM chip set for around 30khz operation. The switching is done with rfp50n06 MOSFET. The coil is made of 18AWG magnet wire and is wound around a 5/8" solid steel core and plastic form. The spike generated by the coil when it is switched off is clamped by an ultrafast diode. The device will suspend up to around 5oz, and can support 1oz at about 1" away from the coil while drawing about 15w. The levitated object is farily stable due to the lead compensation circuit, even with no damper under it. Soon I want to use the remaining opamp in the lm324 to turn off the circuit when the levitated object falls or is pulled away.
Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Nice work! The lead compensation (resistor/capacitor) is the key to the stability and was not present in the circuit I got this from originally. I had thought about doing a retrofit of my science museum maglev setup but it has been going fine for 18 months 8 hours a day 6 days a week so I am reluctant to mess with it. It remains a popular display and still no-one has pinched the magnet.
Your circuit is much simpler than Rick Hoadley's which I always thought was overly complicated.
I had plans for a PIC controlled setup with the original intention of controlling the instability but also to allow unusual actions such as the ability to place and leave a magnet in space.
Does your solid steel core get warm? You may have losses here compared with a laminated core but at 15 W it probably is not important. I have to have a very hefty fan in my setup to cope with the 75 - 300 W load in an enclosed magnet.
Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
I was worried about losses, but nothing gets more than a little warm during normal operation. I am now rebuilding it to look better, so I should be posting some pictures soon. Maybe because the coil is not really AC but pulsed DC causes it to stay cool?
Right now it is levitating a .9oz stack of magnets 1" away from the coil and drawing .4a at 12v, so I guess it is a little more efficient than I thought.
Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Would you be more efficient at a lower frequency? I only run 1kHz vs your 30kHz. Less back EMF pulses to be lost per second. On the other hand it is above the frequency response of the Hall device and may smooth the effect of the applied field from the electromagenet.
The addition of an alarm to sound when the magnet is removed is very useful for a public show. Particularly if this is graded in intensity as the magnet is pulled away.
I wonder if playing with the lead compensation resistor (ie making it variable) will allow you to tune for stability with different loads.
Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
I did try using differnt frequencies down to about 1khz but I found it made an annoying sound at whatever frequency I had it runing at. Maybe this was just my power suppy(a moded ATX PSU) and not the levitation circuit. Anyway, in my final version which I am building now, I will put a large pot on the 494 so the frequency is adjustable from 30khz to the low khz range. It does seem to run cooler at lower frequencies. Changing the value of the lead compensation resistor didn't seem to do much, but it is hard to tell seeing as I am also adjusting gain at the same time. Right now I am just trying to cram it all on a small circuit board.
Registered Member #65
Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155
These are neat, IIRC some Hall effect devices performance is only rated at around 20KHz. I use them as no wear sensors when optical is not suitable.
I like the 3pin UGN3503 linear sensor with built in preamp (about 20KHz at +-900G). They will tie really nicely into an A/D unit (or th CV pin on a 555 )
I often use them for switches (cheap) -- but they will work for analog stuff too.
Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
It is now rebuilt including a new board. It seems to run more effieciently this way, and maybe even a little cooler. The only problem I have now is that I seem to have lost some stablilty by rebuilding it. It still can be made more or less stable, but the gain has to be set just right, otherwise the magnet will either fall or stick to the core. Anyone have any ideas? The only thing I can think of is that maybe I put the in the lead compensation cap backwards(I am using a tantalum).
Things to do: - Make it turn off when magnet is removed - Solve stability problem - Build a cool stand for it so I can sit it on my desk
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