Step up transformer for up to 300 volt pulse using 9V

metron9

I am playing with some ultrasonic transmitters from MSI and I need some help.

They have a circuit on page 5 of this pdf file that shows how to design a pulse driver.

http://www.stickerpub.com/images/40khz_transmitter.pdf

I don't have a clue as to what to use for the part T1, I think it's a step up transformer
I think I would need a winding ratio of 16:1 to step up 9volts DC to 150 Vp looking at DIGIkey I dont have a clue what type or specs to look for.


I am also unsure of the meaning of Vp, I think it's Phase Voltage and I know what single phase and three phase are and I have read some stuff on angles of phase of voltage but that's still trying to make its way to my think center of the brain.

Kevin Wood

This transformer should just be coupling the signal from the pulse generator to the transmitter. Note that transformers couple AC signals, so you aren't stepping up the 9Vdc supply voltage. The Vp of the signal would be determined by the pulse generator circuit, and the resonant frequency between the generator and transimitter is determined by the transmitter housing used (page 4 of datasheet). The tranformer you use will be determined accordingly.

Also, page 1, Vp is used as a value for Sound Pressure Output, so it probably refers to a voltage value that correlates to the pressure of the audio signal. Again, this should be determined by the pulse generator circuit.

metron9

Yikes, good think I asked, or I would have let the somke out, so page 6 Min 12V and Max 15V is the correct drive voltage

Correct if wrong here:

Peak to peak maximum would be -15 volts to +15 volts for example a sine wave at 40khz is what the driver basically does and the Transformer simply isolates the drive circuit from the load circuit.

Question, if the above is correct, why is this necessary? Why could you not drive the device directly with a 40khz sine wave?

Kevin Wood

I took a look at the manufacturer's site for better docs and I found an updated datasheet. The sheet shows on page 5 that with a typical supply voltage of 12Vdc you get an output voltage of 350 Vp-p with the transmitter as a load. The transformer could either be used for blocking DC from the transmitter, or to isolate it from direct AC voltage contact to keep it from being "hot". As for the transmitter, it looks like its just a crystal with some resonance matching components. So the AC voltage applied to the crystal causes it to vibrate at the frequency of the AC voltage applied.

The manufacturer's website also included a piezo technical manual that goes in depth to the technology involved in their products. You can also buy a dev kit with the needed board and circuitry. If you want to build it yourself, just be aware of the high voltages involved. One last suggestion. I posted a link in the Sandbox for electronic training materials. It has links to a US Navy electronics course that you can use to get some background theory.

I'll include the newer datasheet, you'll have to get the tech manual from the site. Anybody interested in piezo film sensors should also check it out.

metron9

Thank's so much for taking the time I understand it better now.

I have put this circuit in the java simulator
The scope output shows a spike signal, I used a 100 ohm resister to simulate the load But measuring the resistance of the device is like a capacitor, it's resistance increases with time to infinity. Using a 10uf capacitor to simulate the device load it makes a saw wave output. So I am ready to breadboard it and put it on a real scope.

$ 1 5.0E-6 7.0208191203301125 44 5.0 50
T 272 144 384 144 0 4.0 1.0 8.881729274089331E-8 0.011975783775907176
w 272 32 272 144 0
f 240 288 272 288 0
w 272 176 272 272 0
w 272 304 272 352 0
g 272 352 272 416 0
r 240 288 160 288 0 1500.0
R 160 288 96 288 0 5 100.0 5.0 0.0
c 384 144 448 144 0 0.0010 -0.04455787800058106
r 448 144 448 176 0 2800000.0
w 384 176 448 176 0
w 448 144 512 144 0
w 448 176 512 176 0
r 512 144 512 176 0 100.0
R 272 32 96 32 0 0 40.0 9.0 0.0
o 12 64 0 3 10.0 0.1 0

Kevin Wood

metron9,

I read through the tech manual, and double checked the datasheet. T1 on the datasheet schematic shows an adjustable transformer, and you can see it on their board layout. After looking at all of the data that I could find, I want to say that I'm certain high voltage is present in this circuit, but I'm not certain whether it is produced by the transformer. As per your original question, when you asked about the turns ratio, it is possible that the transformer is adjustable for voltage gain. It is also possible that the tranformer is adjustable simply for impedance matching the circuit based on the resonance differences of the suggested transmitter housings. It may do both. Here is what the tech manual says about high voltages:

High Voltage Techniques
The use of piezo film as a vibration exciter requires separate consideration. Since the impedance of a
capacitive transducer decreases with frequency and approaches infinity for low frequencies, very high
voltages (a few hundred volts typically) may be required to drive, for example, full audio-range
loudspeakers. Frequently, transformers are used to step up moderate voltages to supply the required
drive signal. Under these circumstances, extreme stresses may be placed upon the connections.
Consider first applying a voltage step of 30V to a capacitor of 100nF with an overall circuit resistance
of 2 ohms. The initial current pulse peaks at 15 amps (assuming the supply is capable of supplying
this). Such a current "spike" may well show up defects in connectors.
Consider next a transformer which steps 12V signals up to 240V. A DC current in the primary of 200
μA (corresponding to an applied voltage of 0.5 volts), when broken, may cause a voltage surge of 830
volts across the secondary circuit, well in the excess of the expected X 20 magnification factor. Even
with heavy capacitive loading, high voltages may be seen. Worse still, if the secondary circuit is
broken, current pulses exceeding 60A with durations of only tens of nanoseconds may arise. Such
phenomena should not trouble well-formed connections. But if a lead-attach method has been used
which has any trapped air, the effect of the reduced dielectric constant may be to promote
breakdown. Such events may be catastrophic, as the familiar crackling sound and lively blue sparks
will testify.
Solutions are:
1. Silver ink electrodes are a must - the thin sputtered electrodes cannot withstand the high voltages
2. Large area contacts to reduce stress. We paint silver ink around eyelets/rivets to provide extra
conduction paths to the film electrode.
3. (Possibly) a semi-resistive contact pad to reduce current surges—equivalent to including a series
resistance in the circuit. Practical values up to about 1 k will produce only a fractional loss in output
and will reduce the magnitude of current spikes.

I state all of this simply because I don't want you to get hurt building your circuit! Based on the docs I reviewed, I would personally ask the manufacturer to clarify the issue. So, be careful...

metron9

Yes I had hoped to get a reply as I did email them, I have posted a question on their forum now as well.

From reading the above High Voltage Techniques, I think I understand the capacitive nature of the device simply reduces the current flow to 0 on a lower frequency just like a capacitor, so high frequency bursts positive and negative are needed to drive it. Voltage must be high enough to quickly push the current or no movement will occour.

This is why I did not buy the testing driver for 100 bucks. It would make the device work, but I am interested in how it works.

to be continued....

metron9

Continued...

Here is the reply for the transformer, it indeed is a step up transformer, specs below. Now how about some help making one or finding a substatute?


Reply....
The transformer we included in the schematic is unfortunately a custom component that we produced in-house to optimize performance with these specific transducers, but we have not set up to manufacture and supply the transformers. The specs are as follows:

Primary: 200 to 400 uH, 0.5 ohms DC resistance (approx 15 turns)
Secondary: 38 to 45 mH, 108 ohms DC, approx 800 turns

Primary side is shunted with parallel 51 nF capacitor, giving resonance at 40 kHz. Q value of secondary side is 4 to 6 at resonance.

I hope this helps.
.........................

Kevin Wood

You might want to Google "custom transformers" and see what you get. You could probably contact a couple of places with the above specs, and see what they say. Beyond that, you might be able to redesign the circuit in stages with smaller steps using more commonly available transformers. Or, there is the $100 option...

Here is a link I found doing a search:

www.yesmec.com/index.html

They build custom tranformers, and their resources page has some links on transformer design.

metron9

Thanks for the link, bookmarked.

Someone else reminded me a 300 volt spike or so is necessary for a flash on a camera, I will perhaps buy a throw away camera and take it apart to see what's inside, or google flash camera circuits...

Tracy Allen

An audio output transformer in reverse might give pretty good results. Many such transformers work okay up to the ultrasonic range. Get one with say, 1.5kohm primary and 8 ohm secondary. (Mouser has some in the Xicon brand for less than $2) In order to step up the AC voltage, connect the secondary on the driver side and connect the MSI transducer on the primary, high impedance side.

The primary DC resistance will be about 80 ohms and the secondary about 1 ohm, and the turns ratio about 12:1. (Turns ratio equals the square root of the impedance ratio). With a 10 volt AC input, the output should be around 120 volts AC. The MSI docs say to limit continuous drive to less than 150 volts peak. The transformer they told you about has a turns ratio of 53:1, which I find kind of hard to believe, but they are shooting for 350 volts for high power pulse operation.

There is nothing magical about that circuit in the PDF. You could start out with a 555 timer oscillator adjustable around 40 khz and a little audio amplifier chip to drive the transformer. The circuit they have in the PDF to set the drive level is very crude indeed. The MSI device has a very low Q so the exact drive frequency is not critical and it won't display sharp resonances. With standard 40khz ultrasound transducers (which are based on a ceramic resonator and have much higher Q), I find that an inductor of around 8 millihenries in series with it can improve the output matching. But that is a refinement and you can experiment. The msi device behaves electrically like a capacitor of around 1200 picofarad.

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Tracy Allen
www.emesystems.com

Beau Schwabe

metron9 said...

Primary: 200 to 400 uH, 0.5 ohms DC resistance (approx 15 turns)
Secondary: 38 to 45 mH, 108 ohms DC, approx 800 turns

This sounds similar to a transformer that I found in a project in a magazine
once that used a single "AA" battery to flutter the wings of a mechanical
butterfly. The wings were made of a piezo electric film.

We ended up using just the transformers for a T.E.N.S. (Transcutanious
Electronic Nerve Stimulator) device at the place that I worked at the time.

The real trick with these transformers, because of the extremely low windings
on the primary coil, was to trickle charge a small capacitor (47uF) and then
rapidly "dump" the capacitor charge into the primary coil. I designed a circuit
that used an SCR (Silicon Controlled Rectifier) in a relaxation oscillator
configuration that served the purpose of pulsing the transformer in this manor.
The pulse rate was adjustable from about 30Hz to 70Hz.

BTW) I think these are commonly called "Pulse Transformers" .... If I remember
right "TOKYO Coils" has a large selection to choose from.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.