# Litz wire Q

Discussion in 'Electronic Design' started by Tim Williams, Mar 24, 2013.

1. ### Tim WilliamsGuest

30MHz? I hear 3MHz or so is usually where it loses value. There's
probably something about capacitance between strands, too -- the eddy
voltage (i.e., the voltage between strands, which would've caused eddy
currents in a solid conductor) just keeps going up.

Going up much higher requires extremely fine wire to remain valuable.
Finest litz I've heard of has strands around #46, which is finer than
human hair. I don't think they make much finer than that.

Tim

2. ### Jamie MGuest

Hi,

http://www.litz-wire.com/technical.html shows 2.8MHz max recommended
for 48AWG litz wire. I guess litz is probably used for a lot higher
than that, but the Q will go down. For 30MHz the skin depth is
13.9um! I think 48AWG wire is about 30.48um diameter (0.0012") so you
would need some higher gauge litz wire maybe if it is made!

cheers,
Jamie

3. ### Robert BaerGuest

Nominally,what is the highest frequency that litz wire been used at?
I am guessing 30Mhz is the top end.

4. ### Guest

For local oscillator or front ends tuned by a typical 500 pF variable
capacitor, the inductance required for the long wave band is about
2000 uH, for medium wave band bout 200 uH and for some HF band 2 uH.
Since the inductance is proportional to the square of number of turns,
so for similar cores, the MW coil requires 10 times the number of
turns than the HF coil and th LF coil 30 times the number of turns
than HF. The linear length and hence losses are proportional to the
number of turns. If you try to squeeze 10-30 times the number of turns
on a coil former, you have to use thinner wires, increasing losses.

Thus the Litz wire is critical for LF/MF, but for HF/VHF, the low
number of turns makes it possible to use a thick conductor, but
actually only use the surface due to skin effect.

5. ### Tim WilliamsGuest

I think of it this way: although the skin is getting thinner, it goes as
1/sqrt(F), whereas reactance goes as F. So at higher frequencies, Q is
simply high enough not to care.

You can still get some amazing Qs with litz. I've heard of Q over 1000
for a spider wound type with the finest litz. That's up there with
polypropylene capacitors, maybe even air variables. I think that was
LF/MF, don't recall if there are measurements and comparisons at HF+ for
anything.

I don't think this works for transformers though. Transformers are about
two things, flux density and current density. Even for optimal core
materials, flux density drops (loss limited instead of saturation
limited), but it's more like 1/sqrt(F), while flux goes as 1/F. So the
core gets smaller, and length of turn gets smaller. But copper is still
copper, and if you can only divide it so far, you're actually going to
need the same or more!

Tim

6. ### Robert BaerGuest

None specifically; just a randumb idea that was partly linked to Litz
wire.
Much stronger link to surface smoothness VS frequency - as seen in
Rogers (and other PCBs).
Am thinking,all else being kept constant, that a better surface
smoothness gives a better frequency frequency response/capability.
A little hard to see why the loss at a given high frequency wold be less.

7. ### Robert BaerGuest

Somehow what you wrote cot completely lost, so with apologies i copy
some of it here to give a more coherent response:
"
Burst (gated, instant-on) oscillators are fun.

https://dl.dropbox.com/u/53724080/Circuits/Oscillators/Burst_Osc.jpg

The trick is to have every edge, especially the first few, happen just
on time.
And to have the prop delay from gate to the first edge be minimal.
"

I must commend you,the turn-on is virtually perfect; only the first
half-cycle is slightly wider.
This turn-on response is not easy to achieve; i lucked out in trying
that when i tried an ECL (NOR?) gate where one input was the on/off
control and the other was for the feedback for oscillation.
Been over 30 years, but i think that yours is better.

8. ### amdxGuest

Some may be interested in this thread on Rap N Tap, a crystal radio
forum. Claim is high Q in the AMBCB using three pieces of 640/46 litz.

Or, watch the wrap,

10. ### amdxGuest

When I pasted the link it showed as 4 lines of text,
so I pasted it as a quotation and it was one line.
Obviously it fixed itself when I mailed it.

But what about the coil with a Q of 2000?
Mikek

11. ### JamieGuest

That's just in case you were not watching for the wrap when you looked
it the first time! (

Actually, we've tested litz wire before, because we use it in a 100khz
application for a transformer coupling a voltage multiplier in a SF6
filled vessel. If memory serves, I think 5Mhz was the tops before it
got hairy.. But then again, there are different constructions of this
material.

You got spiral twist, braided and straight bunch with a thread wrap
to hold it together, we use the bunched style.

Jamie