Forums

Bandaxall Oscillator

Started by Kooner April 14, 2012
Hello,

I made a Bandaxall Class D Oscillator with a 11mm x 7mm pot core
transformer and it works quite well. The input is 8VDC and the
secondary output is 800V peak to peak.

The transformer is very difficult (for me) to wind because the
secondary uses #46 awg wire and it breaks easily. Also, I would like
to use a shorter transformer, around 5mm tall. There are smaller pot
cores but there is no way I could wind them.

I'm looking for a SMT off the shelf transformer to use in this
circuit.

A tried a CCFL transformer made by Coilcraft (FL2810-1L)

http://www.coilcraft.com/misc/ccflp.html

but it didn't work. I believe the winding capacitance is too low. The
secondary is wound on a segmented bobbin as opposed to having entire
layers wound on top of each other.

I tried adding a small cap across the secondary, but it wouldn't
oscillate, I'm guessing the winding resistance on the secondary (300
ohms) is too high which prevents the circuit from oscillating.

I tried adding a cap across the primary, the circuit did oscillate but
the input current seemed too high and the current waveform didn't look
right.

Does anyone know where I can get a center tapped SMT transformer with
a high winding capacitance for this circuit? I've spent some time
searching but can't find anything.

Alternatively, is there a way to make this circuit work with a SMT
transformer similar to the FL2810 (low winding capacitance) ?

any help appreciated,

Jason Kooner



On Apr 14, 11:35=A0pm, Kooner <jjkoo...@gmail.com> wrote:
> Hello, > > I made a Bandaxall Class D Oscillator with a 11mm x 7mm pot core > transformer and it works quite well. The input is 8VDC and the > secondary output is 800V peak to peak. > > The transformer is very difficult (for me) to wind because the > secondary uses #46 awg wire and it breaks easily.
I could never do better than 0.1mm OD wire - 42 awg. Professional coil winders can do a lot better - down to 0.2mm wire, around 52 awg.
> Also, I would like > to use a shorter transformer, around 5mm tall. There are smaller pot > cores but there is no way I could wind them.
Again, sub-contract the job to a professional. It's not a business that lends itself to automation, so get one-off jobs down isn't too difficult.
> > I'm looking for a SMT off the shelf transformer to use in this > circuit.
> A tried a CCFL transformer made by Coilcraft (FL2810-1L) > > http://www.coilcraft.com/misc/ccflp.html
There do seem to offer quite a few variants on that theme. http://www.coilcraft.com/search/us_search_results.cfm
> but it didn't work. I believe the winding capacitance is too low. The > secondary is wound on a segmented bobbin as opposed to having entire > layers wound on top of each other.
That's not likely to be a correct explanation of why it didn't work.
> I tried adding a small cap across the secondary, but it wouldn't > oscillate, I'm guessing the winding resistance on the secondary (300 > ohms) is too high which prevents the circuit from oscillating.
It would have needed to be a very small capacitor with a very high voltage rating
> I tried adding a cap across the primary, the circuit did oscillate but > the input current seemed too high and the current waveform didn't look > right.
A high winding capacitance is unlikely to be all that helpful. If you haven't already read J Williams application notes on the subject (AN45, AN49, AN51, AN55, AN61, AN65 on the Linear Technology web-site) you should do so. Jim Williams never called the circuit a Baxandall class -D oscillator but that's what he's describing.
> any help appreciated.
I've got some thoughts on the subject on my web-site http://home.planet.nl/~sloma000/Baxandall%20parallel-resonant%20Class-D%20o= scillator1.htm along with a copy of Peter Baxandalls's 1959 paper http://home.planet.nl/~sloma000/0344_001_Baxandal.pdf which wasn't easy to get hold of. I ended up buying a copy from the UK IEE (and got their permission to put it on my web-site). -- Bill Sloman, Nijmegen
Kooner wrote:
> Hello, > > I made a Bandaxall Class D Oscillator with a 11mm x 7mm pot core > transformer and it works quite well. The input is 8VDC and the > secondary output is 800V peak to peak. > > The transformer is very difficult (for me) to wind because the > secondary uses #46 awg wire and it breaks easily. Also, I would like > to use a shorter transformer, around 5mm tall. There are smaller pot > cores but there is no way I could wind them. > > I'm looking for a SMT off the shelf transformer to use in this > circuit. > > A tried a CCFL transformer made by Coilcraft (FL2810-1L) > > http://www.coilcraft.com/misc/ccflp.html > > but it didn't work. I believe the winding capacitance is too low. The > secondary is wound on a segmented bobbin as opposed to having entire > layers wound on top of each other. > > I tried adding a small cap across the secondary, but it wouldn't > oscillate, I'm guessing the winding resistance on the secondary (300 > ohms) is too high which prevents the circuit from oscillating. > > I tried adding a cap across the primary, the circuit did oscillate but > the input current seemed too high and the current waveform didn't look > right. > > Does anyone know where I can get a center tapped SMT transformer with > a high winding capacitance for this circuit? I've spent some time > searching but can't find anything. > > Alternatively, is there a way to make this circuit work with a SMT > transformer similar to the FL2810 (low winding capacitance) ? > > any help appreciated, > > Jason Kooner > > >
Impossible to use a CFL transformer for that..they are designed to act like a neon display (tube) transformer: high initial voltage for the strike, and constant current for the light (arc).
Kooner wrote:
> Hello, > > I made a Bandaxall Class D Oscillator with a 11mm x 7mm pot core > transformer and it works quite well. The input is 8VDC and the > secondary output is 800V peak to peak. > > The transformer is very difficult (for me) to wind because the > secondary uses #46 awg wire and it breaks easily. Also, I would like > to use a shorter transformer, around 5mm tall. There are smaller pot > cores but there is no way I could wind them. > > I'm looking for a SMT off the shelf transformer to use in this > circuit. > > A tried a CCFL transformer made by Coilcraft (FL2810-1L) > > http://www.coilcraft.com/misc/ccflp.html > > but it didn't work. I believe the winding capacitance is too low. The > secondary is wound on a segmented bobbin as opposed to having entire > layers wound on top of each other. > > I tried adding a small cap across the secondary, but it wouldn't > oscillate, I'm guessing the winding resistance on the secondary (300 > ohms) is too high which prevents the circuit from oscillating. > > I tried adding a cap across the primary, the circuit did oscillate but > the input current seemed too high and the current waveform didn't look > right. > > Does anyone know where I can get a center tapped SMT transformer with > a high winding capacitance for this circuit? I've spent some time > searching but can't find anything. > > Alternatively, is there a way to make this circuit work with a SMT > transformer similar to the FL2810 (low winding capacitance) ? > > any help appreciated, > > Jason Kooner > > >
I did a little snooping, SMT only (by way of the baby bird: goo-gull): Mini-circuits have RF transformers, balanced (CT) to unbalanced up to 1:16 turns ratio. Or Coilcraft S5499,looks like 1:10 turns ratio. There are a number of thru-hole xfmrs, some with multiple windings allowing a wide truns ratio that maybe worth looking at.
On Apr 15, 6:47=A0am, Robert Baer <robertb...@localnet.com> wrote:
> Kooner wrote: > > Hello, > > > I made a Bandaxall Class D Oscillator with a 11mm x 7mm pot core > > transformer and it works quite well. The input is 8VDC and the > > secondary output is 800V peak to peak. > > > The transformer is very difficult (for me) to wind because the > > secondary uses #46 awg wire and it breaks easily. Also, I would like > > to use a shorter transformer, around 5mm tall. There are smaller pot > > cores but there is no way I could wind them. > > > I'm looking for a SMT off the shelf transformer to use in this > > circuit. > > > A tried a CCFL transformer made by Coilcraft (FL2810-1L) > > >http://www.coilcraft.com/misc/ccflp.html > > > but it didn't work. I believe the winding capacitance is too low. The > > secondary is wound on a segmented bobbin as opposed to having entire > > layers wound on top of each other. > > > I tried adding a small cap across the secondary, but it wouldn't > > oscillate, I'm guessing the winding resistance on the secondary (300 > > ohms) is too high which prevents the circuit from oscillating. > > > I tried adding a cap across the primary, the circuit did oscillate but > > the input current seemed too high and the current waveform didn't look > > right. > > > Does anyone know where I can get a center tapped SMT transformer with > > a high winding capacitance for this circuit? I've spent some time > > searching but can't find anything. > > > Alternatively, is there a way to make this circuit work with a SMT > > transformer similar to the FL2810 (low winding capacitance) ? > > > any help appreciated, > > > Jason Kooner > > =A0 =A0Impossible to use a CFL transformer for that..they are designed to > act like a neon display (tube) transformer: high initial voltage for the > strike, and constant current for the light (arc).
Read Jim Williams' Linear Technology application notes on the subject - AN45, AN49, AN51, AN55, AN61, and AN65. Cold cathode fluorescent tubes are a slightly different breed of cat from neon display tubes, and Jim Williams' version of the Baxanadall oscillator seems to have driven the back-lighting in a whole lot of lap-top computers. -- Bill Sloman, Nijmegen
Robert Baer <robertbaer@localnet.com> writes:

> Kooner wrote: >> Hello, >> >> I made a Bandaxall Class D Oscillator with a 11mm x 7mm pot core >> transformer and it works quite well. The input is 8VDC and the >> secondary output is 800V peak to peak. >> >> The transformer is very difficult (for me) to wind because the >> secondary uses #46 awg wire and it breaks easily. Also, I would like >> to use a shorter transformer, around 5mm tall. There are smaller pot >> cores but there is no way I could wind them. >> >> I'm looking for a SMT off the shelf transformer to use in this >> circuit. >> >> A tried a CCFL transformer made by Coilcraft (FL2810-1L) >> >> http://www.coilcraft.com/misc/ccflp.html >> >> but it didn't work. I believe the winding capacitance is too low. The >> secondary is wound on a segmented bobbin as opposed to having entire >> layers wound on top of each other. >> >> I tried adding a small cap across the secondary, but it wouldn't >> oscillate, I'm guessing the winding resistance on the secondary (300 >> ohms) is too high which prevents the circuit from oscillating. >> >> I tried adding a cap across the primary, the circuit did oscillate but >> the input current seemed too high and the current waveform didn't look >> right. >> >> Does anyone know where I can get a center tapped SMT transformer with >> a high winding capacitance for this circuit? I've spent some time >> searching but can't find anything. >> >> Alternatively, is there a way to make this circuit work with a SMT >> transformer similar to the FL2810 (low winding capacitance) ? >> >> any help appreciated, >> >> Jason Kooner >> >> >> > Impossible to use a CFL transformer for that..they are designed to > act like a neon display (tube) transformer: high initial voltage for > the strike, and constant current for the light (arc).
I thought this was achieved more by the series capacitor than by the transformer design. That is, the transformer runs at a more-or-less constant voltage (a couple of kV perhaps). And the series capacitor creates a "constant current" drive. So when the CCFL is initially high impedance it sees a high voltage, and after the arc strikes it experiences a low voltage and a constant current drive. That is how it seemed to behave when I have made these circuits. (I have made several using various CCFL transformers and had no trouble getting them to oscillate, so I think the OPs problem is yet to be found). -- John Devereux
"John Devereux" <john@devereux.me.uk> wrote in message 
news:87y5pxcqlz.fsf@devereux.me.uk...
> I thought this was achieved more by the series capacitor than by the > transformer design. That is, the transformer runs at a more-or-less > constant voltage (a couple of kV perhaps). And the series capacitor > creates a "constant current" drive. So when the CCFL is initially high > impedance it sees a high voltage, and after the arc strikes it > experiences a low voltage and a constant current drive.
It's both -- the voltage on the primary will be constant (assuming saturated switching and constant supply through a series inductor), but the secondary voltage in general will be different due to leakage inductance. CCFL transformers are usually constructed in a way which gives low capacitance (bank wound secondary) and high leakage inductance (one bank for primary, relatively distant from the secondaries). The equivalent circuit model looks like a series inductor (the LL) feeding an LC parallel resonant tank (secondary LL / self L working into parasitic capacitance). This allows the secondary voltage to ramp up more than turns ratio suggests. When the tube fires, the series cap acts to limit current and maintain resonance. Q is lower so the secondary terminal voltage is probably below turns ratio, but this depends on how much -- a short circuit puts the lamp C in parallel with winding C and Q and voltage go back up. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
On Apr 14, 6:47=A0pm, Bill Sloman <bill.slo...@ieee.org> wrote:
> On Apr 14, 11:35=A0pm,Kooner<jjkoo...@gmail.com> wrote: > > > Hello, > > > I made aBandaxallClass D Oscillator with a 11mm x 7mm pot core > > transformer and it works quite well. The input is 8VDC and the > > secondary output is 800V peak to peak. > > > The transformer is very difficult (for me) to wind because the > > secondary uses #46 awg wire and it breaks easily. > > I could never do better than 0.1mm OD wire - 42 awg. Professional coil > winders can do a lot better - down to 0.2mm wire, around 52 awg. > > > Also, I would like > > to use a shorter transformer, around 5mm tall. There are smaller pot > > cores but there is no way I could wind them. > > Again, sub-contract the job to a professional. It's not a business > that lends itself to automation, so get one-off jobs down isn't too > difficult. > > > > > I'm looking for a SMT off the shelf transformer to use in this > > circuit. > > A tried a CCFL transformer made by Coilcraft (FL2810-1L) > > >http://www.coilcraft.com/misc/ccflp.html > > There do seem to offer quite a few variants on that theme. > > http://www.coilcraft.com/search/us_search_results.cfm > > > but it didn't work. I believe the winding capacitance is too low. The > > secondary is wound on a segmented bobbin as opposed to having entire > > layers wound on top of each other. > > That's not likely to be a correct explanation of why it didn't work.
Here's a simulation of the circuit I've been working with. http://www.flickr.com/photos/58722598@N07/7083763563/in/photostream http://www.flickr.com/photos/58722598@N07/6937689758/in/photostream/ So far, I haven't had to add a capacitor to the primary to get it to work because there is enough reflected capacitance (15pF from 1000 turns on a pot core transformer). In the simulation I removed the 15pF on the secondary and added a 4.2nF across the primary and got the same results. I don't know why the circuit didn't work in the lab using the FL2180 (low winding capacitance) with a capacitance added to the primary as shown on Bill's page : http://home.planet.nl/~sloma000/Baxandall%20parallel-resonant%20Class-D%20o= scillator1.htm but I will keep trying.....
> > > I tried adding a small cap across the secondary, but it wouldn't > > oscillate, I'm guessing the winding resistance on the secondary (300 > > ohms) is too high which prevents the circuit from oscillating. > > It would have needed to be a very small capacitor with a very high > voltage rating > > > I tried adding a cap across the primary, the circuit did oscillate but > > the input current seemed too high and the current waveform didn't look > > right. > > A high winding capacitance is unlikely to be all that helpful. > > If you haven't already read J Williams application notes on the > subject (AN45, AN49, AN51, AN55, AN61, AN65 on the Linear Technology > web-site) you should do so. Jim Williams never called the circuit a > Baxandall class -D oscillator but that's what he's describing.
Bill, I looked through those notes....there is an extra winding on the transformer to drive the bipolar transistors, is this circuit the same principle as the Bandaxall osc? Also, I've been using mosfets because I thought this was necessary since they need to conduct both positive and negative current ? - Jason Kooner
> > > any help appreciated. > > I've got some thoughts on the subject on my web-site > > http://home.planet.nl/~sloma000/Baxandall%20parallel-resonant%20Class... > > along with a copy of Peter Baxandalls's 1959 paper > > http://home.planet.nl/~sloma000/0344_001_Baxandal.pdf > > which wasn't easy to get hold of. I ended up buying a copy from the UK > IEE (and got their permission to put it on my web-site). > > -- > Bill Sloman, Nijmegen
Kooner wrote:
> On Apr 14, 6:47 pm, Bill Sloman<bill.slo...@ieee.org> wrote: >> On Apr 14, 11:35 pm,Kooner<jjkoo...@gmail.com> wrote: >> >>> Hello, >> >>> I made aBandaxallClass D Oscillator with a 11mm x 7mm pot core >>> transformer and it works quite well. The input is 8VDC and the >>> secondary output is 800V peak to peak. >> >>> The transformer is very difficult (for me) to wind because the >>> secondary uses #46 awg wire and it breaks easily. >> >> I could never do better than 0.1mm OD wire - 42 awg. Professional coil >> winders can do a lot better - down to 0.2mm wire, around 52 awg. >> >>> Also, I would like >>> to use a shorter transformer, around 5mm tall. There are smaller pot >>> cores but there is no way I could wind them. >> >> Again, sub-contract the job to a professional. It's not a business >> that lends itself to automation, so get one-off jobs down isn't too >> difficult. >> >> >> >>> I'm looking for a SMT off the shelf transformer to use in this >>> circuit. >>> A tried a CCFL transformer made by Coilcraft (FL2810-1L) >> >>> http://www.coilcraft.com/misc/ccflp.html >> >> There do seem to offer quite a few variants on that theme. >> >> http://www.coilcraft.com/search/us_search_results.cfm >> >>> but it didn't work. I believe the winding capacitance is too low. The >>> secondary is wound on a segmented bobbin as opposed to having entire >>> layers wound on top of each other. >> >> That's not likely to be a correct explanation of why it didn't work. > > > Here's a simulation of the circuit I've been working with. > > http://www.flickr.com/photos/58722598@N07/7083763563/in/photostream > > http://www.flickr.com/photos/58722598@N07/6937689758/in/photostream/ > > So far, I haven't had to add a capacitor to the primary to get it to > work because there is enough reflected capacitance (15pF from 1000 > turns on a pot core transformer). > > In the simulation I removed the 15pF on the secondary and added a > 4.2nF across the primary and got the same results. > > I don't know why the circuit didn't work in the lab using the FL2180 > (low winding capacitance) with a capacitance added to the primary as > shown on Bill's page : > > http://home.planet.nl/~sloma000/Baxandall%20parallel-resonant%20Class-D%20oscillator1.htm > > but I will keep trying..... > > > > >> >>> I tried adding a small cap across the secondary, but it wouldn't >>> oscillate, I'm guessing the winding resistance on the secondary (300 >>> ohms) is too high which prevents the circuit from oscillating. >> >> It would have needed to be a very small capacitor with a very high >> voltage rating >> >>> I tried adding a cap across the primary, the circuit did oscillate but >>> the input current seemed too high and the current waveform didn't look >>> right. >> >> A high winding capacitance is unlikely to be all that helpful. >> >> If you haven't already read J Williams application notes on the >> subject (AN45, AN49, AN51, AN55, AN61, AN65 on the Linear Technology >> web-site) you should do so. Jim Williams never called the circuit a >> Baxandall class -D oscillator but that's what he's describing. > > > Bill, I looked through those notes....there is an extra winding on the > transformer to drive the bipolar transistors, is this circuit the same > principle as the Bandaxall osc? Also, I've been using mosfets because > I thought this was necessary since they need to conduct both positive > and negative current ? > > - Jason Kooner > >> >>> any help appreciated. >> >> I've got some thoughts on the subject on my web-site >> >> http://home.planet.nl/~sloma000/Baxandall%20parallel-resonant%20Class... >> >> along with a copy of Peter Baxandalls's 1959 paper >> >> http://home.planet.nl/~sloma000/0344_001_Baxandal.pdf >> >> which wasn't easy to get hold of. I ended up buying a copy from the UK >> IEE (and got their permission to put it on my web-site). >> >> -- >> Bill Sloman, Nijmegen >
Hello..... Did i not say why? The transformer is made to limit the current for constant conduction (think requirements of fluorescent, of CFL); the turns ratio obviously there to make the required high voltage for initializing the arc.
On Apr 17, 9:15=A0am, Robert Baer <robertb...@localnet.com> wrote:
> Kooner wrote: > > On Apr 14, 6:47 pm,BillSloman<bill.slo...@ieee.org> =A0wrote: > >> On Apr 14, 11:35 pm,Kooner<jjkoo...@gmail.com> =A0wrote: > > >>> Hello, > > >>> I made aBandaxallClass D Oscillator with a 11mm x 7mm pot core > >>> transformer and it works quite well. The input is 8VDC and the > >>> secondary output is 800V peak to peak. > > >>> The transformer is very difficult (for me) to wind because the > >>> secondary uses #46 awg wire and it breaks easily. > > >> I could never do better than 0.1mm OD wire - 42 awg. Professional coil > >> winders can do a lot better - down to 0.2mm wire, around 52 awg. > > >>> Also, I would like > >>> to use a shorter transformer, around 5mm tall. There are smaller pot > >>> cores but there is no way I could wind them. > > >> Again, sub-contract the job to a professional. It's not a business > >> that lends itself to automation, so get one-off jobs down isn't too > >> difficult. > > >>> I'm looking for a SMT off the shelf transformer to use in this > >>> circuit. > >>> A tried a CCFL transformer made by Coilcraft (FL2810-1L) > > >>>http://www.coilcraft.com/misc/ccflp.html > > >> There do seem to offer quite a few variants on that theme. > > >>http://www.coilcraft.com/search/us_search_results.cfm > > >>> but it didn't work. I believe the winding capacitance is too low. The > >>> secondary is wound on a segmented bobbin as opposed to having entire > >>> layers wound on top of each other. > > >> That's not likely to be a correct explanation of why it didn't work. > > > Here's a simulation of the circuit I've been working with. > > >http://www.flickr.com/photos/58722598@N07/7083763563/in/photostream > > >http://www.flickr.com/photos/58722598@N07/6937689758/in/photostream/ > > > So far, I haven't had to add a capacitor to the primary to get it to > > work because there is enough reflected capacitance (15pF from 1000 > > turns on a pot core transformer). > > > In the simulation I removed the 15pF on the secondary and added a > > 4.2nF across the primary and got the same results. > > > I don't know why the circuit didn't work in the lab using the FL2180 > > (low winding capacitance) with a capacitance added to the primary as > > shown onBill'spage : > > >http://home.planet.nl/~sloma000/Baxandall%20parallel-resonant%20Class... > > > but I will keep trying..... > > >>> I tried adding a small cap across the secondary, but it wouldn't > >>> oscillate, I'm guessing the winding resistance on the secondary (300 > >>> ohms) is too high which prevents the circuit from oscillating. > > >> It would have needed to be a very small capacitor with a very high > >> voltage rating > > >>> I tried adding a cap across the primary, the circuit did oscillate bu=
t
> >>> the input current seemed too high and the current waveform didn't loo=
k
> >>> right. > > >> A high winding capacitance is unlikely to be all that helpful. > > >> If you haven't already read J Williams application notes on the > >> subject (AN45, AN49, AN51, AN55, AN61, AN65 on the Linear Technology > >> web-site) you should do so. Jim Williams never called the circuit a > >> Baxandall class -D oscillator but that's what he's describing. > > >Bill, I looked through those notes....there is an extra winding on the > > transformer to drive the bipolar transistors, is this circuit the same > > principle as the Bandaxall osc? =A0Also, I've been using mosfets becaus=
e
> > I thought this was necessary since they need to conduct both positive > > and negative current ? > > > - Jason Kooner > > >>> any help appreciated. > > >> I've got some thoughts on the subject on my web-site > > >>http://home.planet.nl/~sloma000/Baxandall%20parallel-resonant%20Class..=
.
> > >> along with a copy of Peter Baxandalls's 1959 paper > > >>http://home.planet.nl/~sloma000/0344_001_Baxandal.pdf > > >> which wasn't easy to get hold of. I ended up buying a copy from the UK > >> IEE (and got their permission to put it on my web-site). > > =A0 =A0Hello..... > =A0 =A0Did i not say why? > =A0 =A0The transformer is made to limit the current for constant conducti=
on
> (think requirements of fluorescent, of CFL); the turns ratio obviously > there to make the required high voltage for initializing the arc.
There's nothing in the Baxandall circuit that explicitly limits the output current. The CoilCraft transformer will obviously have a relatively high resistance secondary (because it is made with lots of turns of fine wire) and will have a highish leakage inductance - which the OP could measure by shorting one or other of the coils and measuring the residual inductance of the other coil - but I wouldn't think that either feature was designed in as an intentional current limit. -- Bill Sloman, Nijmegen