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Weird results from air-core transformer

Weird results from air-core transformer

Weird results from air-core transformer

(OP)
I'm seeing some strange results from an air-core transformer - basically two coils of 30ga wire.  Perhaps someone can shed some light ?

Primary coil is 170 turns of 30ga wire that was wound on a 2" dowel.  The secondary is 100 turns the same way.  The two coils are sitting on the desk, the secondary on top of the primary with about a 1/4" gap.

The primary is driven by an IRF540 - the gate goes via 200ohm resistor to a micro generating a 15kHz 50% duty cycle PWM signal.  The Source goes to GND.  The coil is across the Drain and VCC.  The secondary goes through a bridge rectifier.

I'm seeing about 23Vdc across the secondary.  I was expecting around 8Vdc or so.  100/170 * 14 right ?  Hrmm.  The 23Vdc is what works out from 170/100 * 14, so it's as if the primary and secondary are backwards.

But if I reverse them, put the 100 turns as primary and the 170 turns as secondary, it puts out around 90Vdc.

The other oddity is that I can increase the 14V up to around 30V, and the output only rises from 23V to about 27V or so.

Dean.

RE: Weird results from air-core transformer

I'm not 100% sure but don't transformer calculations only work for sine waves? You are applying a pulse so the main effect will be one of the back-emf generated when the voltage drops (if I had a memory I would quote the formula for the voltage generated, it's something to do with the rate of change of voltage and your pulse has a steep falling edge, so lots of volts!)

RE: Weird results from air-core transformer

Hi, a few things here. 1 you dont say what the drive voltage is. 2 you cant get a dc across the secondary. 3 use a scope to measure your voltages, you are probably getting ringing which is being rectified.

RE: Weird results from air-core transformer

Sounds like you built a flyback converter.
What is your load?
You may be rectifing the ringing depending on your load.
If its ringing alot try a r-c snubber across primary and
mayby one across the secondary.
 Check your primary and secondary wave forms you should
see a corrilation with your turns ratio.
 You will probably see more voltage on the primary then
what you expect.

RE: Weird results from air-core transformer

200 ohms is an awfully big gate resistor.  You may not be turning the FET on to saturation.  What is the threshold voltage of the FET you are using?  What is the output voltage being supplied to the gate?

You really want a gate resistor of about 33 ohms, and you want a gate drive voltage of not less than 10 volts.

RE: Weird results from air-core transformer

Don't forget the coefficient of coupling between the coils in your calculations.  And as stated in other posts - look at the output with an oscilloscope.  Make sure you have sufficient primary inductance for the desired waveform.

RE: Weird results from air-core transformer

(OP)
Cool, some responses pretty quick :)  These forums are great.

dhwilliams - I think you may be right.  Have to dig up my old books, as I was just going on the memory of transformer ratios.  I was only scoping the gate drive to make sure it was a 5V 50% signal, which it was.

cbarn - drive voltage across the primary is 14Vdc, switched by the IRF540 mosfet.  The secondary feeds a typical bridge rectifier to spit out ugly peaky DC.  I'll put a couple of caps across it to smooth it out.

Madcow - hrm - no load at the moment, just the multimeter.  I was looking to see that the output voltage was within expectations.  Never built a flyback converter before, at least not deliberatly.  The meter shows 14V across the primary.

Lewish - the IRF540 barely gets warm at all.  If it wasn't turning on completely, wouldn't it heat up ?  Gate threshold voltage is 2-4V, being driven by an output pin on the AVR micro at 5V.  The 200ohm resistor is to make sure that the current doesn't exceed the pin max - I actually thought it should be higher ...

  http://www.fairchildsemi.com/ds/IR/IRF540.pdf

Dean.

RE: Weird results from air-core transformer

Take a look at the "Gate Charge vs Gate-to-Source Voltage" curve on the datasheet.  It shows that it takes about 5.5 volts to fully turn this part on.  So, I think you are definitely not getting it turned on.  Since it is not turning on more than just a very small amount, NO, it will not be getting warm.
When I need to use a FET as a switch driven by a microcontroller, I use a sensitive-gate FET which has a threshold voltage of 0.9 to 1.2 volts.  Even then, I seem to have trouble with some devices not fully turning on.  The lower your Source to Drain voltage, the harder the FETs will be to turn on.

RE: Weird results from air-core transformer

(OP)
Lewish - Hmmm I see what you mean.  OK, I'll use a 2n3906 or similar to switch the 14V into the IRF540 gate.  That way we'll get the full 14V into the gate which should turn it on fully.

From microcontroller pin through resistor (1k or so?) to Base of 2n3906.  Collector to +14V, Emitter through resistor (what value ?) to IRF540 Gate.  +14V through primary coil to Drain, and Source to GND.

That should do it I think ?

Dean.

RE: Weird results from air-core transformer

A 1k to 2.7kohm resistor would be good for the base.  Emitter resistor needs to be sized to limit the current thru the 2n3906 to the allowable DC current limit of the 2n3906.

That should do it for you.

RE: Weird results from air-core transformer

It looks like the transformer's primary and secodary are looslely coupled and there is some resonance in the secondary. The loosle coupling would explain why the output voltage doesn't track the input (VCC) voltage changes, and the resonance coupled with the loose coupling results in the secondary ringing up to a higher frequency.

At 15 kHz, you really should be using a ferrite core or equivalent.


Dick Cappels

RE: Weird results from air-core transformer

(OP)
I'll futz with it tonight more if I get time.  Got a sick daughter though ...

This is for transferring power to a rotating circuit, so no way really to use a core.  The rotating part needs about 400mA at 4V min available to be regulated down to 3.3V.

Would the coupling improve with a higher or lower frequency ?

Dean.

RE: Weird results from air-core transformer

The loose coupling results in high leakage inductance, so as the frequency goes up, the regulation problem will get worse. If you aren't transferring very much power, a pair of back-to-back zeners across the secondary would help stabilize the voltage.

On apporach to transfering power to something on a rotating shaft that a fellow I know said he  use once was to take a a pot core and put it in the middle of the rotating shaft. One half of the pot core rotated on the shaft, and the other half stayed fixed. There was an air gap between the two halves. I suspect that made the project a mechanical problem, but at least they were able to transfer plenty of power this way.

Good luck.

Dick

RE: Weird results from air-core transformer

(OP)
Heh, mechanicals there must have been "interesting".  I've been looking around for a slip-ring too, and have found a few, but they're all too big, and bloody expensive.

My basic requirements are pretty simple.  Given a source of around 12-14V, I need to get around 4-5V at around 400mA across to a rotating circuit.  The basic air-core transformer looks like a reasonable way to do it, and experiments above show that it will transfer energy pretty well.

Actually, very well.  I was able to hold the secondary coil above the primary by about 6" and still see around 5V no load on it by the multimeter.  I have a little field strength meter (uncalibrated) that pegs the scale at around 12-18" from the coils, while a typical monitor pegs it when it's about 10" in front of the screen.  So there's plenty of energy there ...

Had some time to scope it briefly last night - very briefly.  The +14V point on the primary coil appears to be seeing a 100V (wow!) spike on it.  More like an exaggerated square pulse really.  

So, to smooth this out, and get rid of those 100V spikes the primary should have a smoother waveform applied to it than the general square wave ...  I can see a couple of ways to do this, none perfect.

The mosfet wants to be a switch.  If it's not fully on or off, it heats up, right ?  So that's probably not the best switching device to use, should probably look for a power transistor.

Apply some capacitance across the gate/base connection to GND, to smooth out that square wave from the micro PWM output.  Apply some more capacitance across the primary coil to smooth out the resultant waveform being sent to the secondary.  But how much ?

For grins I stuck a 470uF cap across the primary before I had to run back upstairs.  Didn't see any appreciable change in the waveform.

So what would be the best way to do this ?

Thanks -

Dean.

RE: Weird results from air-core transformer

OK, a couple of thoughts.  Don't worry about the voltage being a square wave.  If you look at the current you will find that it is a triangle wave.  Power Factor Correction circuits work by chopping the incoming AC voltage into very small slices that are in effect square waves.

Keep the MOSFET.  It is the best switching device for this application.  Just make sure you saturate it so that it doesn't over heat.  But, that probably won't happen as the part you listed is good for 28 Amps continuous DC.

The circuit typology you have described is a "flyback converter".  That means it is going to need a snubber circuit on the primary side, or else a lot of you power will go into waste voltage.  That is the 100 volts you are seeing.

Take a look at the Linear Tech app. notes on flyback converters for a good tutorial on what you are doing.

RE: Weird results from air-core transformer

I think you will do well to take a look at some applications notes on switchmode power supply transformer design since this is what you are trying to design. None of the app notes I've seen deal with air cores, but the basic principles are the same - reduce leakage fields by increasing coupling thereby improving regulation.  With air you will not have to worry about saturating the core, though!

Switching from an FET to a bioplar transistor will not solve the basic problem of poor coupling in the transformer.  That you have high and likely variable leakage inductances suggests that an FET is a better choice since it will not avelanche.

To transfer power efficently you need to get the best coupling between the primary and secondary you can, and eliminate losses wherever you can - fast switching times (another reason to stay with an FET), minimize gate (or base) drive, minimize rectifier losses. Yes, a snubber is an excellent idea, by the way.

Take a look at resonant topologies. Maybe you can make the leakage inductance work in your favor by using it to tune the secondary, then you can achieve regulation by varying the switching frequency -but you'll need to invent a feedback scheme that can bridge the gap.

This looks like a challenging project. Good luck with it.

RE: Weird results from air-core transformer

(OP)
Heh.  This always seems to happen.  Something simple on the surface always reveals hiddens depths and layers ...

It sounds like the principle of using a PNP to switch the gate drive to the mosfet is the right approach.  Guarantees proper saturation, and only needs the extra pnp and base resistor.

In case anyone is interested, I found an excellent description of flyback converters here :

http://ece-www.colorado.edu/~ecen4517/course_material/Exp6/flyback.pdf

General idea seems sound - get at least 4-5V usable at 400mA across the gap.  The rotating part has it's own regulation (LDO linear regulator - LM1086-3.3), so the actual values aren't critical.  

In talking to colleagues at work, it's been suggested to tune the RC circuit that the primary coil and caps make to resonate at the drive frequency (15KHz) to increase the coupling efficiency and reduce the drive requirements.

Dean.

RE: Weird results from air-core transformer

Be careful making the circuit resonant as your flyback primary voltage will GREATLY increase.  Make sure your snubber can handle that voltage level.

RE: Weird results from air-core transformer

(OP)
Yup, definitely have to be careful.  Actually, I don't really think that should be a criteria, since efficiency isn't really drastically necessary (apart from the purist ideal).

This is wall-wart powered, and so long as it gets that 400mA at 4-5V across, well, that's fine.  Ugliness in *how* it does that while aesthetically nasty, doesn't really affect things in the long run.

Now, to choose the right components and design for the snubber ...

Dean.

RE: Weird results from air-core transformer

(OP)
Madcow -

Excellent - thanks very much for the link.  I do wish it was easier to embed images here - I would like to show the schematic as I have it.

Futzed a bit with it tonight.  Let me see if I can describe the schematic.

NPN 2n3904 to act as a gate-drive switch for the mosfet.  Base has 15kHz PWM square wave to it via 1k resistor.  This is fed from the microcontroller as a 0-5V signal.  Collector goes to VCC/14V.  Emitter has 1k resistor to LED then to GND.  This is to show it's on/switching and also to bias the mosfet gate to GND.

Emitter also goes via 200ohm to gate of IRF540.  Source goes to GND.  Drain goes to primary coil, then to VCC/14V.  So power goes from VCC through coil, through switch/drain to GND/source.

The snubber consists of a diode (anode) going from the drain/coil junction to (cathode) a parallel resistor/capacitor, which then go to VCC/14V.  This seems to be the classic snubber circuit for a flyback from my reading.

Heh, not too many had examples of *values* though, but for one.  1uF cap, 1k resistor, which I tried.  It helped some, but not as much as I thought it would - it did remove the spikes, but the peak-peak was still pretty high.

22nF = 62Vp-p ugly spiky waveform
1uF = 41Vp-p clean square wave, rounded on the bottom
10uF = 33Vp-p clean square wave

1K resistor got hot.  I mean it got HOT.  I put a ~60ohm load across the secondary so it would pull some current.

I'll read that snubber design link tomorrow.  Hrm, would be nice to know the inductance of the coils.  They're from 30ga magnet wire wound on a 2" form.  The primary is 170 turns, the secondary is 100 turns.  I know there's a formula out there to calculate it, but I'm too tired right now to search it out :)

More tomorrow.

Dean.

RE: Weird results from air-core transformer

(OP)
Hrm - a thought.  Instead of feeding 14Vdc through the mosfet to the coil, what about regular old AC ?  Use a 12-20Vac wall-wart.  Use the PWM to lop off the tops of the waveform to control the total power feed to the secondary.

Given that this is air-core, and with some distance (5-10mm) between the coils, how badly would the change from 15kHz to 60Hz affect the power transfer ?

Dean.

RE: Weird results from air-core transformer

(OP)
OK - take a look at this :

  http://seawall.areyes.com/PWM-mosfet.pdf

I have this breadboarded up right now.  Works fine, except that there are two "situations" ...

If the 0.1uF bypass cap is in place, then the mosfet gets blisteringly hot.  This is regardless of the presence of the snubber components.  This setup produces the nicest sine-type waveform, so the snubber isn't really needed - no nasty spikes.

If the 0.1uF cap is removed, then the 1k resistor in the snubber gets really hot.  But the mosfet stays nice and cool.  The snubber is required, or we see those 90V spikes on it.

Any reccomendations ?

Dean.

RE: Weird results from air-core transformer

The reason your IRF540 gets so hot during normal operation is because you have essentially shorted power to ground, thru your coil.  Try putting some series impedance before the transistor and see if that cools it down (it should limit the current).

Move D1, R3, and C1 to be connected across the relay coil.  The current location of these devices won't clamp the flyback voltage since Q2 essentially isolated them.  All you have to do is disconnect the anode of D1 and connect it to the Drain of Q2 and everything should run nice and cool.

Good luck and keep us posted!

RE: Weird results from air-core transformer

(OP)
Dang it !  The breadboard is actually correct, the bloody schematic is wrong.  It's late ...

The anode end of the diode is actually connected to the drain of the mosfet Q2.  I just redid the pdf - so it now reflects the breadboard appropriately.  Note - heating effects are still as noted above.

I added in a series resistor in the schematic between the mosfet source and GND.  Haven't done it yet - tomorrow.  Or should it go between the coil and VCC ?  Does it make a difference ?

Dean.

RE: Weird results from air-core transformer

It should be between VCC and the coil.  Otherwise, you will raise the Souce voltage on Q2, which will reduce Vgs, i.e. not turn on the transistor as hard.

RE: Weird results from air-core transformer

Also, eliminate R3.  This will clip the flyback voltage to Vd.

RE: Weird results from air-core transformer

Get rid of C1 as well.

RE: Weird results from air-core transformer

Sorry, last post.  Why do you have C2 across the Drain to Source?  Why don't you put in on the Gate leg, and simply slew the turn on / off time, and generate your shape that way?

One more thing, you might want to place a 10K-100K resistor from Gate to Ground.  This will help evacuate the charge from the Gate of the transistor.

RE: Weird results from air-core transformer

(OP)
Cool - more comments.  No such thing as last post.  Here's the link again.

  http://seawall.areyes.com/PWM-mosfet.pdf

Once I thought about it, putting the resistor between the VCC/coil and the mosfet makes sense.

The C1/R3 combo is the snubber circuit - that clips the spikes down from their 100V high to something more manageable.  At least that's what I read.

C2 is across the drain to source to round off the waveform going into the coil.  The coil power transfer deals better with a more sine-wave form than a square wave, and it also helps to reduce EMI ("fewer" harmonics).  If it's across the gate leg, then the mosfet won't just be switching, it will also be operating in the linear region, so heat up, won't it ?

There is the 1k resistor and LED from the gate to GND - shouldn't that help evacuate the gate charge ?

With the snubber in place, R3 gets hot, dumping the back-emf, and I measure about 150mA through the primary coil.  The waveform is a nice clipped sine wave.  Without the snubber, the mosfet gets cooking hot, and about 100mA through the coil.  The waveform is more ramp-shaped, with much higher peaks.  Those are DC current measurements on the meter.  AC-rms are somewhat lower.  The coil measures 6.7ohms across it.

I used a capacitor decade box to arrive at the 0.1uF cap across the mosfet.  That value was the best compromise between waveshape rounding and power transfer.  Smaller values still had large spikes, while larger ones damped it down too much.

I need to get about 400mA at about 5V across to the secondary.

Dean.

RE: Weird results from air-core transformer

- If the diode is placed correctly, you will clamp the flyback to ~0.7V.

- By moving C2, you will definitely cause Q2 to operate in the linear region.  This will translate to heat.

- The led needs about ~2V to turn on.  Therefore, once Vgs drops below ~2V, you will no longer have a path to evacuate the charge (other than the leakage thru the LED).

RE: Weird results from air-core transformer

(OP)
OK - take a look at the link again.  I've updated the schematic to what I will test with tonight.  You may have to refresh your browser.  Changes are :

1) Increase 1k resistor in snubber circuit, to limit the current it's carrying.  This will reduce the amount of clamping it will do, but that should be offset by the rounding effect of C2.  May even be able to remove the snubber components (D1, C1, R3) completely depending on the size of the spikes.

2) Add in R7 at about 10 ohms to start with.  This will limit the current through the primary coil, and hence the mosfet heating.

3) Move C2 low leg to below R7.  Hrm - should it be right at GND, or right across the mosfet ?

I'll also add in the 3.3V LDO linear regulator to see how stable the output voltage is.

Dean.

RE: Weird results from air-core transformer

- Don't put R7 in the Source leg, it must be in the Drain leg of the circuit (for the reasons mentioned above).

- Increase R8 to 10K-100K.  If R8 is too small, you will create too much of a divider, and therefore not bias the gate hard enough.

- Get rid of R3 and C1.  They won't do you much good.

RE: Weird results from air-core transformer

(OP)
Get rid of just R3 and C1, or D1 as well (the whole snubber) ?

Dean.

RE: Weird results from air-core transformer

(OP)
OK.  Another iteration.  The snubber is gone, and it looks like this now :

  http://seawall.areyes.com/PWM-mosfet.pdf

Guess what ?  The mosfet *still* gets hot.  Even with a heatsink on it, after a few minutes it's very hot.  The bench power supply is only giving it around 80mA at 14V, and the meter between the coil and 14V is showing about 70mA consumption through the coil.

So why is the bloody thing heating up so much ?  It should be simple : NPN acting as a switch to 14V to act as a mosfet gate driver.  Mosfet to act as a switch for a coil.  On off on off on off.  0.1uF cap to smooth out the square wave.  Ba da bing ba da boom :)

Dean.

RE: Weird results from air-core transformer

The problem might the transistor that you are using to turn on the FET.  The maximum voltage that you will be able to see should be ~ (5V-0.8V)*1000/(1200) = 3.5V (assuming you use a 220ohm series gate resistor).  By the way, does Q1 get hot?

Is there any way to increase the voltage on your PWM generator, to say, 14V?  I have a sneeky suspicion that Q1 might be the source of your problem.  I think that Q2 isn't properly turning on because Q1 isn't fully active.

Some quick and dirty simulation indicates that you are not sufficiently turning on your FET since you can only hope to achieve 3.5V, and the gate needs time to fully charge / discharge.  Try using the following values for your components:

R8 = 810
R2 = 47
ADD a series resistor of 2K between Q1 collector and 14V (ABSOLUTELY ESSENTIAL!!!)

This should hopefully solve your problem if you can't get the PWM voltage increased.  Also, try decreasing your PWM rate to give Q2 a chance to cool down.

Finally, keep D1 in your circuit.  The extra capacitor and resistor probably won't do you a lot of good, but the diode can REALLY help you.

Good luck and keep us posted!

RE: Weird results from air-core transformer

(OP)
Hrm.  It's certainly acting like the mosfet is still operating in the linear region.  Right now I have a 33ohm gate resistor as you had mentioned earlier.

The AVR is running at 5.1V, and the PWM signal entering the 1k base resistor shows a 5.6Vp-p on the scope.
Q1 gets only very slightly warm.  The scope shows a p-p voltage at Q1-emitter of about 5.6V as well - this is the first time I've actually checked that, I thought it would be higher.

What is the 2k in the Q1 collector for ?

Right now the whole snubber circuit is out (D1, C1, R3).  The 0.1uF cap across the mosfet rounds off the wave nicely - no spikes.  So it looks like the cap isn't needed.  I'll try a smaller cap (more spikes), and add back in D1 to shunt the spikes.

R7 - the coil current limiter, is 27ohms.  It gets pretty warm.  I would expect that dissipation to reduce when there's a real load on the secondary, as more power will be getting transferred over rather than being dissipated in R7.

Getting there.  Supposed to be simple, never is, but getting there, and I'm learning, which is key :)

Dean.

RE: Weird results from air-core transformer

The 2K is required since you are running from 2 supplies.  Your PWM can only get as high as 5.6V, but you want to drive the gate with 14V.  The problem is that with the NPN, the voltages around the device must sum to 0, otherwise, things don't work.  That 2K drops enough voltage so Q1 can operate properly.

RE: Weird results from air-core transformer

(OP)
Hokay - I think we got it.  Woo hoo.  Check out the links again :

  http://seawall.areyes.com/Air-core-xformer.pdf

I finally broke down and ran it through spice, playing with values.  This was SwitcherCAD from Linear Tech.  I used close-fit parts for the mosfet and the diode.

  http://seawall.areyes.com/Air-core-xformer.asc

In practice, it's really close.  The gate voltage is actually around 11.5V rather than the 9.5V predicted.  Mosfet stays nice and cool, as does the 2n3904.  The drain shows a nice 16V square wave - it would be nice to round that off a little.  What gets hot now is the coils !

To be expected I suppose - 170 turns of 30ga on a 2" form.  Pushing 400mA through gets it nice and toasty.  I may have to redo it with 28ga instead.

The secondary coil is 100 turns, the same.  It only shows about 2.5Vp-p on the scope.  The coil ends are going through a pot set to about 20ohms.  One leg has a schottky diode on it instead of the bridge.

Which means the efficiency is lousy.  Might swap the coils ...

Dean.

RE: Weird results from air-core transformer

Change R3 -> 1K and R2 -> 100K.  This should help limit the current through Q1 (1K ~14mA, while 470 ~28mA), and turn on M1 harder ([14V-0.1V]*100K/[100K+1K]).  Otherwise, congratulation on getting it working.

RE: Weird results from air-core transformer

(OP)
Hey all -

I found another version of the air-core transformer here :

http://www.geocities.com/tjacodesign/propclock/propclock.html

My version looks like this :

http://seawall.areyes.com/Air-core-xformer.pdf

He's got only 20 turns for the primary, I have 170.  He has 25 for the secondary, I have 100.  Since he's actually stepping up slightly, I'm assuming that the 470uF cap will be soaking up most of the spikes before the 7805 regulator sees them ?

The 4.7nF cap across the primary coil is the same as the one is the propclock project.  This is actually the best value, as determined with a capacitor decade box.

Well, been playing with this for a bit.  It does work which is cool.  But I can't seem to get any decent power through it, which isn't.

For instance, the secondary coil feeds a bridge rectifier, which then feeds an LM1086-3.3 regulator.  There's a 220uF cap across the regulator input, and a 1uF cap across the output.  I see a nice clean 3.3V output, while the secondary coil sees around 18V AC p-p.

This is unloaded though.  As soon as I put a load (several LEDs) across the LM1086, everything drops.  The secondary drops to around 5V or so.  If I add on a bunch of LEDs, the clean 3.3V drops down to around 1.8V or so - each additional one dims it a bit more.

The bench power supply feeding the PWM circuit at 14V only shows a current draw of around 55mA unleaded, and about 40mA with any number of LEDs on.

Q1 is turning on nice and hard, and the drain waveform looks pretty good.  Nothing is getting hot, not the mosfet nor the primary coil.

If I change C1 from 4.7nF up to 1uF, the current drawn according to the bench PS jumps to about 1A.  Up to 10uF and it jumps to 1.5A.  Things start to get hot.

OK, wait a minute here.  Something just went weird.  CUrrent draw is now 2A with the 4.7nF cap across the primary.  The drain is now showing no waveform at all.  The gate has anice clean 14V square-wave from the Q1 driver just fine, but the drain which used to show a nice ramp waveform, now just sits ...  Hmmm, just tried another IRF540 - same thing.

What ??

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