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Hi.

I've googled for hours now and can't find an answer, i hope someone can help me :)

Is there a simple circuit that can take a DC input and flip it's polarity at a specific frequency ?


Thanks,

Gary.

depends, how much voltage and currant at what frequency.
try looking up an H bridge.

DeepCut,
The remarks by Jesus and others re the 555 timer chip circuit and polarity switching in the Lee Crock thread might be of help:
http://www.overunity.com/index.php?topic=9123.msg260166#msg260166
Bob

Also depends on what you consider "simple".

The H-bridge is very versatile and can be used from DC on up to many megaHertz depending on components and construction.

My TinselKoil uses 2 H-bridges: one made with 2 NPN and 2 PNP silicon RF power transistors, to take the low-current positive pulse output of the TL494 controller chip and boost the current while flipping polarities. The output of this stage drives 2 toroidal phase transformers, which make oppositely-phased outputs to drive the gates of the second H-bridge.The second bridge is made of all N-channel MOSFETS, which take the higher input voltage to the coil primary and chop it into sharp positive and negative pulses to produce the fast dB/dt that makes the secondary swing like a quarter-wave whip. IGBTs can also be used here.

The high-power H-bridge is a bit more complicated as it requires diodes for surge and overvoltage protection for the transistors.

In this video I am driving the H-bridges at about 800 kHz and the high-power section is switching about 6 amps at that frequency.  This is about the top of the frequency range for the kind of construction I am using: busswire and breadboard. To go higher in frequency or power, a well-designed circuit board with low-inductance current paths is necessary.

http://www.youtube.com/watch?v=3DjcRPSljB4 (http://www.youtube.com/watch?v=3DjcRPSljB4)

Thanks very much chaps, that's plenty to be getting on with.

Cheers,

Gary :)

Please excuse what is most probably my ignorance but i don't think the 555 or h-bridge solutions are what i need.

I'm trying to do a solid-state version of Jo Newman's 'special commutator'.

I've got a PWM that will pulse the coil at 24 Hz.

I want the polarity of the output of the PWM to reverse at 2 Hz.

So, every 12 pulses the polarity should reverse.

Any help greatly appreciated :)


Gary.

Hi DeepCut,

Is the attached picture the kind of signal you are trying to generate?

This could be saved as a 32 bit floating point .wav file. If you have a small MP3 player in loop mode or computer audio you could feed the signal out to a power mosfet to drive your circuit.

I attached the code so you can change the settings.

I coded it in Octave which is a open source Matlab like clone.
http://www.gnu.org/software/octave/

For a circuit what I see is a 1Hz square wave signal with a 24Hz square wave. Feed the 1Hz wave into the + of a OpAmp and the 24Hz into the negative. When the 1Hz wave is positive the 24hz wave will be positive. When the 1Hz wave goes negative the 24Hz wave will be negative. So you should get a 12Hz +/- waveform out.

Thanks DTB :)

I've always loved your name now i love you too ;+}

What a novel approach !

Thanks again,

Gary.

Although now i think about it, it would still be a positive signal coming out of the MP3 player :(

OCTAVE looks really, really interesting, a bit like BASIC but specialised.


Gary.

QuoteAlthough now i think about it, it would still be a positive signal coming out of the MP3 player.

Still positive? How negative do you need them?  ;) I have a MP3 player with multiple test signals(sine, square, etc) 30hz, 50hz, 60hz, 120hz files and they look fine on the scope.

Octave is a really nice language for math processing. I've programmed in other languages but so far Octave is the best when working with signals.

Hi DeepCut,

Here is the same idea using a OpAmp.

Circuit simulator for the code.
http://www.falstad.com/circuit/

Quote$ 1 5.0E-6 51.8012824668342 50 5.0 50
R 176 240 128 240 0 2 24.0 5.0 0.0 0.0 0.5
R 176 128 128 128 0 2 1.0 5.0 0.0 0.0 0.5
c 176 128 176 192 0 4.7000000000000004E-8 -5.0
c 240 128 288 128 0 1.0E-6 0.0010000001776271361
c 176 240 176 304 0 4.7000000000000004E-8 5.0
c 240 240 288 240 0 1.0E-6 0.0010000001419250282
r 176 240 240 240 0 100.0
r 176 128 240 128 0 100.0
g 176 192 176 208 0
g 176 304 176 320 0
a 320 176 432 176 1 1.0 -1.0 1000000.0
w 320 160 320 128 0
w 320 128 288 128 0
w 320 192 320 240 0
w 320 240 288 240 0
O 432 176 528 176 1
o 15 64 0 34 1.25 9.765625E-5 0 -1