Tag Archives: current regulator

The current regulator

The current regulator – this module converts the current setpoint signal (from the position regulator) into a drive current for the magnet coil. This is achieved using a TL494 PWM modulator chip, and two BUZ11A (50 V – 0.045 Ohm – 26 A) MOSFETs, each operated at 50% duty.

levi_current_reg

Brief description:
The current setpoint signal is connected to JP1-9, connected to one of the comparators of IC2, the TL494. The other comparator is used as a current limiter: the sense wires of a 0.05 Ohm shunt in the coil supply line are connected to the input of IC1, and intrumentational amplifier, via JP1-7 and JP1-8. You might have to change the amplifier gain by modification of R5.
Current setpoint is adjusted by trimmer R14. JP1-3 and JP-4 provide the gate drive for the two BUZ11A MOSFETs. The drains are connected to the coil, the sources are joined at the current shunt.
Directly, at the MOSFETs, which are mounted on a heatshink (but don’t actually get hot), together with current shunt, there are several electrolytic capacitors (low ESR type), and a 1 µF PP cap (polypropylene dielectric, for pulse current service) in parallel with the (non-regulated) magnet supply. In combination with a strong and fast Schottky diode (anode at the MOSFETs, cathode a the positive supply), these caps are the best protection against any transient voltages when the coil is de-energized.

The coil, the circuits

Others have explained at length the theorical background of magnetic leviation, and the associated regulator maths. I don’t want to get into this here – feel free to ask, I will explain it to you.

The goal: an apparatus that can hold a sphere of at least 1 kg mass (about 60 mm diameter, 2 1/2″) suspended from the pole of the magnet, in a distance of at last 25 mm/1″. It should be adjustable to hold also other parts, and not restricted to magnets (many of the available hobbyist leviation circuits can only hold magnets).

With the goal set, and after some calculations, related to electric magnet, heat dissipation, allowable maximum temperature for free air operation, time constants – the coil: about 10 kg of copper, of roughly 3 mm diameter wire. I selected a high-temperature type W210 isolated copper wire, this will withstand long term use even if the coil is operated permanently at full current. Don’t worry about the space the 10 kg will take, copper is very dense… Winding it can be a bit tough, as always, ask a friend for help, and best wind it with the help of a lathe (by manually turning it), or some other makeshift fixture. Winding it free-hand is not a good choice, because you won’t get a nice and tight coil.

4838811106_6008111b04_z

For the circuits, these were build in modular style:

(1) The position detector – based on an infrared LED, and infrared photodiode, and fully independent of the lighting conditions. Is uses a modulated emitter, and a frequency-selective detector. It needs to have a reasonably fast response, and low noise/drift, otherwise, the regulation loop will be unstable – the sphere will start oscillating, and drop off.

(2) The position regulator – this is a classical PID regulator, build using a few TL074 opams. It compares the position setpoint and acutal position (from position detector, item 1), and determines the current needed to hold the sphere stable (current setpoint signal).

(3) The current regulator – this circuit converts the current setpoint signal into a drive current for the coil. To avoid undue powder dissipation in the regulator, this is achieved use a classic TL494 PWM regulation scheme. There is also a current limiting circuit, build-in, with a 0.050 Ohm 4-lead sense resistor in the coil current loop, and an AD620 instrumentation amplifier.

(4) Some auxilliary circuits – to switch off the magnets by disabling the current regulator, if there is no object attached. Otherwise, the magnet would run at full current, if the sphere drops off, and dissipate a low of heat for no good reason. Some power supply circuits – to provide 12 V power for the regulators, and unregulated 35 V to drive the magnet.