Forums

oscillator for driving a transformer

Started by panfilero August 21, 2012
Hello,

I'd like to drive a small high frequency transformer at around 100kHz-300kH=
z with a 24Vpk signal, I tried building up an astable multivibrator with a =
couple of caps and transistors but i couldn't get it to drive the transform=
er, it wouldn't even make anything close to a nice square wave.... anyone k=
now of a better simple oscillator circuit I could use to do this?

thanks!
Yeh, what's *on* the transformer?  Not much point in driving a transformer 
unless ya want it for something...

If you have/can put a CT on the transformer (or use a CT'd inductor), you 
can use the classic chopper circuit.  Even better, if you can add a 
feedback winding (if you have to do it with a CT'd inductor, it still 
works), you don't need as many resistors and as much bias current.

No caps, if you try making it resonant (or quasi-) you need a choke in 
series with the supply voltage.  Lack of one causes crappy waveforms.

Tim

-- 
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

"panfilero" <panfilero@gmail.com> wrote in message 
news:d4d20e3c-816c-4172-acdb-a44fb6f85c45@googlegroups.com...
Hello,

I'd like to drive a small high frequency transformer at around 
100kHz-300kHz with a 24Vpk signal, I tried building up an astable 
multivibrator with a couple of caps and transistors but i couldn't get it 
to drive the transformer, it wouldn't even make anything close to a nice 
square wave.... anyone know of a better simple oscillator circuit I could 
use to do this?

thanks! 


On Mon, 20 Aug 2012 21:01:28 -0700 (PDT), panfilero
<panfilero@gmail.com> wrote:

>Hello, > >I'd like to drive a small high frequency transformer at around 100kHz-300kHz with a 24Vpk signal, I tried building up an astable multivibrator with a couple of caps and transistors but i couldn't get it to drive the transformer, it wouldn't even make anything close to a nice square wave.... anyone know of a better simple oscillator circuit I could use to do this? > >thanks!
How much current do you want? This worked pretty well: https://dl.dropbox.com/u/53724080/Circuits/ESM/ESM_power_B.pdf
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message 
news:jt563851ku95ttpuljjj2usmepguamgie5@4ax.com...
> On Mon, 20 Aug 2012 21:01:28 -0700 (PDT), panfilero > <panfilero@gmail.com> wrote: > >>Hello, >> >>I'd like to drive a small high frequency transformer at around >>100kHz-300kHz with a 24Vpk signal, I tried building up an astable >>multivibrator with a couple of caps and transistors but i couldn't get it >>to drive the transformer, it wouldn't even make anything close to a nice >>square wave.... anyone know of a better simple oscillator circuit I could >>use to do this? >> >>thanks! > > How much current do you want? > > This worked pretty well: > > https://dl.dropbox.com/u/53724080/Circuits/ESM/ESM_power_B.pdf > >
Half bridge drivers are ideal for this, they can handle beanpole voltages up to the hundreds, at frequencies in the 100s of KHz, they can be driven with a 5V logic signal, and (at least with the device that I built using them) they avoid problems from simultaneous turnon of the two power devices. For a simple oscillator, you can't go past a schmitt CMOS inverter, R from output to input nad C from input to ground. If duty cycle has to be exactly 50%, just put it through a flipflop divider.
On Aug 21, 6:01=A0am, panfilero <panfil...@gmail.com> wrote:
> Hello, > > I'd like to drive a small high frequency transformer at around 100kHz-300=
kHz with a 24Vpk signal, I tried building up an astable multivibrator with = a couple of caps and transistors but I couldn't get it to drive the transfo= rmer, it wouldn't even make anything close to a nice square wave.... anyone= know of a better simple oscillator circuit I could use to do this? Peter Baxandall's class-D oscillator produces a tolerable nice sine wave, typically with about 1% harmonic content if you can keep the Q of the tank circuit above about 5, and it's happy as long as the frequency of oscillation is below the self-resonant frequency of the transformer (which can easily be less than 100kHz if you aren't careful). http://home.planet.nl/~sloma000/0344_001_Baxandal.pdf http://home.planet.nl/~sloma000/Baxandall%20parallel-resonant%20Class-D%20o= scillator1.htm As you can see, it can be a pretty simple circuit - one centre-tapped transformer, one capacitor, one inductor and two MOS-FET's. If you really want a square wave, a Royer inverter can be even simpler - no inductor - but at 300kHz the ringing on each switching-edge cans be nasty. -- Bill Sloman, Nijmegen
On 08/21/2012 01:10 AM, John Larkin wrote:
> On Mon, 20 Aug 2012 21:01:28 -0700 (PDT), panfilero > <panfilero@gmail.com> wrote: > >> Hello, >> >> I'd like to drive a small high frequency transformer at around 100kHz-300kHz with a 24Vpk signal, I tried building up an astable multivibrator with a couple of caps and transistors but i couldn't get it to drive the transformer, it wouldn't even make anything close to a nice square wave.... anyone know of a better simple oscillator circuit I could use to do this? >> >> thanks! > > How much current do you want? > > This worked pretty well: > > https://dl.dropbox.com/u/53724080/Circuits/ESM/ESM_power_B.pdf > > >
Nice. You get a lot of mileage out of those little ISDN transformers. Is ISDN a good long term bet, though? I'd be starting to worry about them going away. (Also zeners D6-D8 are mislabelled.) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
On Tue, 21 Aug 2012 09:26:59 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 08/21/2012 01:10 AM, John Larkin wrote: >> On Mon, 20 Aug 2012 21:01:28 -0700 (PDT), panfilero >> <panfilero@gmail.com> wrote: >> >>> Hello, >>> >>> I'd like to drive a small high frequency transformer at around 100kHz-300kHz with a 24Vpk signal, I tried building up an astable multivibrator with a couple of caps and transistors but i couldn't get it to drive the transformer, it wouldn't even make anything close to a nice square wave.... anyone know of a better simple oscillator circuit I could use to do this? >>> >>> thanks! >> >> How much current do you want? >> >> This worked pretty well: >> >> https://dl.dropbox.com/u/53724080/Circuits/ESM/ESM_power_B.pdf >> >> >> >Nice. You get a lot of mileage out of those little ISDN transformers. >Is ISDN a good long term bet, though? I'd be starting to worry about >them going away. (Also zeners D6-D8 are mislabelled.) >
The original design had three 25 volt zeners, so the energy sensor photodiode could be selected to run at 25, 50, or 75 volts, which made sense from the data sheet. The customer claims that they will get breakdown in the sensor wiring (1 torr hydrogen atmosphere) so made me drop the selections to 5 and 10 volts. Of course, the photodiode capacitance is nanofarads at that voltage, slowing down production at what may be gigabuck consequences. As a concession, they allowed me to include the 15 volt range, under the condition that it require a 32-bit password to enable it, and that they not know the password. So far, no problem getting ISDN transformers. We have some custom made for us, less winding capacitance, and they don't cost much more than standard ones. There will probably be zillions of ISDNs available forever, if just overstock. They are handy little transformers. Does anyone still use ISDN? European telephones?
On Monday, August 20, 2012 11:01:28 PM UTC-5, panfilero wrote:
> Hello, >=20 >=20 >=20 > I'd like to drive a small high frequency transformer at around 100kHz-300=
kHz with a 24Vpk signal, I tried building up an astable multivibrator with = a couple of caps and transistors but i couldn't get it to drive the transfo= rmer, it wouldn't even make anything close to a nice square wave.... anyone= know of a better simple oscillator circuit I could use to do this?
>=20 >=20 >=20 > thanks!
Thanks for the replies, I'm basically trying to turn 24V into 72V using a v= oltage tripler, then I was gonna send that into a voltage regulator to clea= n it up, the load is only going to draw maybe like.... 5mA.... so I was thi= nking, take the 24Vdc, oscillate it somehow, feed it to a transformer and t= hen triple it...
On Tue, 21 Aug 2012 10:26:37 -0700 (PDT), panfilero
<panfilero@gmail.com> wrote:

>On Monday, August 20, 2012 11:01:28 PM UTC-5, panfilero wrote: >> Hello, >> >> >> >> I'd like to drive a small high frequency transformer at around 100kHz-300kHz with a 24Vpk signal, I tried building up an astable multivibrator with a couple of caps and transistors but i couldn't get it to drive the transformer, it wouldn't even make anything close to a nice square wave.... anyone know of a better simple oscillator circuit I could use to do this? >> >> >> >> thanks! > >Thanks for the replies, I'm basically trying to turn 24V into 72V using a voltage tripler, then I was gonna send that into a voltage regulator to clean it up, the load is only going to draw maybe like.... 5mA.... so I was thinking, take the 24Vdc, oscillate it somehow, feed it to a transformer and then triple it...
That will work. Or build a simple boost converter. If you don't need isolation, that takes a single inductor. Or buy a couple of SIP dc/dc converters, 24-to-24. Stack their outputs on top if the 24 input and get 72, likely a bit more at light loads. The SIPs are cheap, like $4.
On Monday, August 20, 2012 9:01:28 PM UTC-7, panfilero wrote:

> I'd like to drive a small high frequency transformer at around 100kHz-300kHz with a 24Vpk signal, I tried building up an astable multivibrator with a couple of caps and transistors but ...
Transformers (or any inductor) really, REALLY misbehave if you give them any DC component on their drive. So, a symmetric drive signal is key. Easiest would be to use a SMPS chip (TL494 will almost work, there's LOTS of 'em to choose from) that provides two drive transistors, and connect a center-tap on the transformer primary to your +24V power. The collector voltage (with 24V on the center tap) can rise to 48V on the un-driven leg of the transformer; you'll want that much voltage rating on the drive transistors (the internal transistors of the TL494 max out at 40V, so you'd have to use external transistors).