A Q of about 100, is a reasonable number for an air coil. (You still have to tell us what the DC resistance is!) Whether it's good or bad depends on what you are trying to do. (Well, you usually want a high Q, 'cause you can always 'spoil the Q' by adding some resistance.) So what are you doing? Are you a student?
As you say, each cycle the system losses about 1/Qth of the energy. The amount of energy lost clearly depends on how much energy is in the system. Q is just a ratio.
George H.
Hi, The DC resistance of the each coil is 0.3 ohm measured with ohm meter. Each inductor value is 50uH.
jess
Q = (2*PI*f*L)/R = Xl/R = Q = TheRatioBetween Reactance and Resistance.
Of course let us not forget the basics.
Z = sqrt(R^+X^);
Jamie
Hi,
Tim wrote: You realize that as soon as you put your cat or the neighbor's noisy kid or whatever it is in there that you want to subject to a 100kHz magnetic field, that the resonant frequency, the Q, and everything else is going to change -- yes?
Why will it change? I am testing the field with a reception or secondary coil that also resonate @ 100Khz. Seems to be working working fine. Except some noise at the output.
jess
For #4 maybe your capacitors suck at the frequencies you're working with. What are you using now?
Back when I was into making tesla coils, the loses in the caps tended to exceed that of the coils themselves. The capacitors used in induction heaters are designed for extremely low ESR and losses. The big crazy ones have tubing running though them for water cooling.
Although the values were pretty meaningless, I measured Q and (and k factors) by attaching a scope to the coil+cap which was in a way both in series and parallel at the same time and then driving that coil with another coil connected to a signal generator. I think disconnecting the cap itself would allow you to measure the k factor.
no load resonant frequencies were really easy to measure this way, but all the values were thrown way way of once you move from 1 volt signals to hundreds of kV where even the air breaks down and conducts.
Excellent. So what's the ratio of impedances at 100kHz?
George H.
as
The frequency won't shift too much, unless you reallly load it down.
We use Helmholtz coils for DC fields where there's some resonance that depends on the B field. So you want a nice uniform field to keep all the spins going at the same frequency. I don't know of an application where you care that the AC magnetic field is uniform. Perhaps Jess will tell us.
George H.
ttdesign.com
I'd be surprised by that, few coils have a coil over 1000 (it takes special winds and finely divided litz to achieve that at most frequencies), let alone over 100. Caps are commonly over 1000, good polypropylenes (or C0G ceramics) over 10k.
If you were using shitty dipped polypropylene caps, those can have pretty high ESR, let alone polyester, which are pretty lossy no matter what. I've seen that in my own research.
But I'm spoiled. We mostly use the bolt mounted polypropylene types, rated up to, you know, 600A or so. The steel bathtub ones are rated up to a few kiloamperes and as many volts.
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
You can make your own crude current probe very easely with a current transformer like this:
With a 10 Ohm load resistor it will produce 100mV/A.
-- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico@nctdevpuntnl (punt=.) --------------------------------------------------------------
If I have gathered the correct pertinent info from you over the last few weeks, it is as follows:
You are trying to drive a series-resonant circuit. The bridge will see the resistance of the coils as the current limiting factor. I have forgotten the duty cycle you mentioned weeks ago, but, if your duty cycle is above about 30%, you will see large currents. It will be due to loss in the coils and capacitor.
Drop the duty cycle so that you have about 1us or less on time. Then
*slowly* increase the duty cycle until you get what you want.Hope this helps. Cheers,
John
It's surprising:
They don't even run at crazy high frequencies. These things make utility company power factor correction caps look like childrens' toys.
I've used small induction heater caps with polypropylene dielectric and they still had 3/4" tinned copper braid for internal connections. PP is low loss compared to other stuff, but not a magic material. As you move into the high voltages, corona becomes a really big deal and your capacitors start to get really large to have enough sections. The DC rating of these caps is mostly irrelevant as well for AC.
I used to wind my own caps for fun. My favorite were what I think was polystyrene as the hysteresis was beyond preposterous. You'd short the damn things things out over and over and they're still recover upto thousands of volts seconds later. For AC they were completly unusable. I did many experiments with cap grade polypropylene film and it was interesting to actually see the film oxidize when you'd start to get corona, even at 60Hz.
Capacitors are far more complex than they might appear at first, and alot more though goes into them than you might expect for a roll of film and dielectric with some leads hanging off the ends.
No not the big caps, we work with those. Those aren't surprising, just expensive. :) I meant the losses in the TC -- of course, if you're a budget 'coiler, you'll want to cut corners and get the caps as hot as possible without melting them completely, thus getting the best for your budget. :)
EMI / across-the-line caps can get ridiculously cheap. They really use only as much metallization as they can get away with. I once bought a few hundred 0.1uF 275VAC from Allied. They work for low power induction heating (under a kW maybe), but even that makes them heat up too much.
EPCOS makes some pulse rated types that do a wonderful job, but they make so many types that they're constantly coming and going from stock.
One factor I've noticed that isn't always picked up on: skin effect. And I don't mean on the surface of, like, the foil; I mean over the entire winding. When you make a noninductive capcacitor and short together all the sides of one electrode, it looks like a solid bar again, because it's carrying current, which generates magnetic field, which forces current to the outside -- the outer layers of the winding. At least that's my supposition.
We regularly use those can-with-flange stud mount caps, these things:
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
When I dropped out of coiling, the rage was using dozens to hundreds of extended foil caps with radial leads soldered together. There was some big deal series from Cornell-Dublier folks got all excited over. The idea was you can blow a string of caps and not lose all your money or something like that. When I started, doorknob caps were a big deal, but some of those weren't good for AC use and would just short out, burn or explode. I was never able to kill any caps from Ceramite up in in Wisconsin. I finally ended up with a custom made oil filled cap, which eventually sprung a leak. I still have to ship it back for a "top off" as the manufacturer called it.
I've toured some HV cap factories and they make a big deal about case inductances and how stuff is wired internally for stuff in pulse duty applications. Even though it's easier to wind completely round caps, I noticed a these places seem to like to flatten the sections out and then wire them up, maybe because they pack into the cans better that way over cylinders. There's actually a company (a stupid one too) that makes huge computer grade looking caps that are just huge dry film/foil caps. you really can't tell the difference by picking one up. They must be interesting to watch as they fail testing. RF-wise (which they're not rated for) those things must act like giant bars as you said.
haha, those things look like starter solenoids.
Most common CDE "pulse rated" series is 935 and 940C, both of which are a little on the pricey side for most of my purposes. They also aren't much good over 100kHz -- fine for snubbers in the 10s of kHz, but you don't get as much past there.
We have some ceramic and mica caps laying around, ranging from door knobs to those big, suggestively shaped pot style ones. There are still tube oscillators out in the field, and we still have one unit on the shop floor, so if a customer wants to run a different part, we can run it ourselves and do the QA for them.
Not just "a stupid [company]", lots of people have 'Filmlytics' these days; CDE, AVX I think, ECI Capacitors.. Some of those 'starter solenoid' type caps also come with threaded inserts vs. studs, so aside from the mounting, they're sort of 'lytic' style.
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
Because cats and children, being ugly bags of mostly water, are conductive. The coils will induce current in the salty water, the water will absorb energy from the magnetic field, (see "inductive heating"). The fact that your neighbor's cat (or child) is absorbing energy from the field will, necessarily, lower the coil's Q.
Have you _told_ your neighbor what happened to his cat?
You certainly haven't told us what your ultimate goal is. We're not free, you know -- this advice gets paid for by satisfying our pointless curiosity about what you're doing out there in USENET land.
-- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
What do you think should happen to 'this' cat? ;-)
-- You can't have a sense of humor, if you have no sense.
Hi,
I am trying to charge cell phone batteries wirelessly. My questions are
jess
You need to determine on which end your problem is? Is it the receiving end or the transmitting end?
Are you putting a non sinusoidal wave into the TX coil?
I think you need to use your scope and determine which side is going into oscillation.
P.S. Lowering the Q of coils helps in many cases to remove many nuisance problems.
Jamie
Ughh, Why Helmholtz coils? You are putting a 'chargeing coil' in the center of them? With both circuits tuned?
You need to look up how the resonant frequency shifts with Q.
You'd have to give more information about the system. What's the 20% duty cycle about? Not a square wave or sine wave drive?
Ahh, you can have an series LC or a parallel LC and still use Hlemholtz coils for the L. But don't you have two coils? One for driving and the other for pick-up?
I would try a series LC for driving, (you tell me why.) and parallel for the reciever. (but other here may have a better idea.) You've got some diode on the receiver coils to rectify your 100kHz?
Do they also get out of
Yup! As I said you need to do some serious reading about LC circuits.
(maybe Terman's "Radio Engineering". Who was just elected to the EE hall of fame, according to this months Electronic Design)
George H.
And coupling.
...Jim Thompson
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