Power Supplies

What kind of power supplies will work? How many does it need? How do you get both positive and negative voltages out of two identical power packs?

Power Requirements

This levitator works well with small d.c. wall adapters. These power packs are common and inexpensive, and they ensure no dangerous voltages are exposed. You can find them at Radio Shack for under $15, and other places for somewhat less.

You need three power packs. The voltages and minimum current requirements are:

• +12 vdc at 0.5 amp (for the +15v supply)
• +12 vdc at 0.1 amp (for the -15v supply)
• + 9 vdc at 0.8 amp (for the coil supply)

You can use any power pack that has these voltages and AT LEAST this much current rating. It is fine to use supplies with higher current (at same voltage) ratings. The higher current capability might allow it to last longer or run cooler.

It is probably easiest (and the same price) to just get two identical 12vdc supplies and one 9v supply.

Coil Driver Supply

The power to the coil deserves a bit more discussion. Why use a separate supply? Why not save money by sharing the +12v supply for the electronics?

The reason we use 9v on the coil is that it reduces the coil's heat build-up. It's possible to use a 12v supply on the coil but it doesn't seem to work any better and just makes the coil hotter sooner.

A separate power supply also provides isolation. My assumption is that power comes from a cheap wall wart, and the coil's high current demands from a shared and poorly regulated supply would cause problems. If you have a good quality regulated bench supply then you can probably share the power and have no problems. But you'll want to run a separate and heavier power wire from the supply terminals to the coil driver. Otherwise the ohmic drop in the power wire can insert spurious signals into the electronics.

A separate power supply is also more flexible. You may want to change the voltage to the coil driver because different size/shape/gauge coils may need more or less current. A separate supply is an easy way to make changes without affecting the op-amps.

You're have the greatest chance of success by not sharing the +12v power supply. The hardest thing of all to debug is to detect that the power supply is forming a sneak feedback path. You're better off first making it work with three power packs, and later on checking to see if you can share one pack with the coil driver. Now don't say I didn't warn you!

Connecting Power Packs for Plus and Minus Voltages

Suppose you ran out and obediently bought some power packs, and now you're wondering how to take two +12vdc packs and get both positive and negative voltages out of them. Do you just take one and plug it in backwards? No! You connect the positive wire of one pack to the negative wire of the other.

These power packs are good isolation transformers, as well as providing d.c. voltages. They will happily provide their 12vdc at different offset voltages from ground.

A third power pack will give us +9v for the lifting coil.

The reason we use 12v power packs (when the specs call for 15v supplies) is that the voltage is not critical. The '714 op-amps run happily anywhere from 9v to almost 20v, I think. Also, these little power packs are stamped "12vdc" but the output is unregulated has a higher voltage when lightly loaded, and only at full rated current does their voltage droop down to 12v. The control electronics don't demand very much current so if you measure the power rails you will probably see 14v or 15v or maybe even 16v coming out. Cool!

There! Now you probably know more than you wanted about power supplies!