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In article <809@kaos.MATH.UCLA.EDU> hgw@math.ucla.edu (Harold Wong) writes:
>
>Could someone explain the uses of toroids?  Specifically,
>	For what purpose should they be used for.

Inductors and transformers at higher frequencies.

>	Where should they be used?

Anywhere you want to minimize coupling/radiation between/from inductors,
and for high-frequency (ie, >= 1 Mhz) transformers. Be careful using
them in high-current circuits because ferrite saturates at a comparatively
lower flux density than iron. Yup, I seen a smokin' toroidal xfrmr in an
experimental power converter. They can even growl.

>	How should they be used?

As with any inductor, put them in series with the signal path you want 
to low-pass filter.

>	What are the differences between the donut vs bar version?

The main advantage of toroids over bar-inductors is that the flux is
confined to the core, which reduces coupling between adjacent inductors.

I also think toroids are more mechanically secure than bar inductors.

The one thing I hate about toroids is that they can be a real pain-in-the-butt
to wind.

Gee, they look neat too ! Especially with multi-colored windings.

>	How do you determine the size you need?

	    uN^2 W^2
Toroid: L = --------
	    6.28 R

                    N^2 r^2
Bar:    L = 3.14*K*u-------- 
		        l

Where N = # turns
      u = permeability
      W = thickness of square-cross-sectional toroid
      R = mean radius of toroid
      r = radius of bar (in millimeters)
      l = length of bar (in millimeters)
      K = constant determined by r/l.
      L = inductance in millihenries

	   r/l       K
	   0         1
	   0.5      0.7
	   1.0      0.5
	   2.0      0.4
	   10       0.1

>	Should I wrap all my wires around toroids?

Depends. Inductors (of any kind) are helpful for filtering power supply
lines in analog circuitry. Anywhere you want to block high frequencies
and pass low-frequencies is a good place for an inductor. You certainly
wouldn't want to wrap, say, the signal line from your UHF antenna around a
toroid.

If you want to reduce interference from a device, you can pass the input &
output signals through a toroid (or other inductor). You can 'cancel'
common-mode signals with a transformer.

For light dimmers, inductors (of any kind) serve two useful purposes:

	(a) They limit di/dt during turn-on. If di/dt is too high, you get
	    hot-spots which shorten the life of the triac/SCR. Theoretically,
	    a resisitive load (ie light bulb) has infinite di/dt if the
	    scr/triac is triggered while forward biased.

        (b) (Related to a) They help reduce EMI.