Circuit design help - power supply - Non FIRST
 

Hi guys (and gals),
First off, sorry that this post isn't exactly FIRST related. I posted this here, though, because people on CD know what they're talking about and are always willing to help. Plus, it may help our robot (among other things) if my theory works out.

Anyway, I need to design a power supply with dual frequency generators that can do the following, with a minimum of parts and cost:

1. Take in 12VDC at up to 30 amps (so this is a high wattage application).
2. From this, split the voltage into 6V and 6V (running in series) for use in two VARIABLE frequency sine wave generators (both use the same frequency – from about 5Hz-50Hz is what I’m looking for, though 0-150 would be great too) that are phase shifted from one another by a few degrees/radians/seconds
3. From the generators, I will rectify the outputs so that they are all positive
4. Then I will recombine the signal

What I’ve seen leads me to a few things…

First, I could use an op-amp or a wein bridge as my sine wave generator, but these do not handle high voltage or amperage well. If I could somehow get these to work without extremely expensive components (300 watt resistors and such), these would be great because the frequency is variable by a resistor, and I could share one POT between the two circuits (in parallel) so that the frequencies are the same in both circuits. The delay could simply be a variable 555 timer circuit, if my logic is correct – more time delay initially (“single fire,” if you will) means a bigger phase shift. The only thing I am unsure about is how to make sure I get full amplitude out of the circuit (6V in, the voltage oscillating from +/- 6V peak to peak rectified into +6V).

Second, I could somehow build a power inverter (DC to AC converter) and rectify the output, but I wouldn’t use a transformer and would somehow make the frequency adjustable, with a POT or similar.

Third, I could make a MAX038 or similar circuit, but my electronics skills are limited and the MAX038 has very low current/voltage limits.

The hardest part of this is making things to handle high-wattage. I hate to sound as simple as I am with electronics, but there has to be some way to use low-wattage resistors and other components in high-wattage circuits, either using the low-wattage circuit as a driver for relays or something else... Not that this is necessarily the best option; I'm just thinking out loud.

Since I'm using the output as a power supply and not to produce sound waves or anything, accuracy of the sine is not really critical as long as it is predictable. The key in this circuit would be efficiency and accuracy of the frequency, so that it makes the powered devices function as predicted.

Sorry if I'm being wordy or convoluted, I'll gladly explain as much as I can about this if it helps design the supply. I have some jpeg files of the desired output at every step, if those would help - they're on my desktop, so I can post them tomorrow.

Thanks a lot!
Josh

Re: Circuit design help - power supply - Non FIRST
 

What exactly are you trying to do? Maybe there's a simpler way to do it. You also say that you're going to "recombine" the signals. What do you mean by that? You also mention power on the input, but how much power do you need on the output?

Re: Circuit design help - power supply - Non FIRST
 

Just a few days ago I ran into a site on the net talking about how to build a microprocessor-controlled DC power supply. It's a good article that discusses the principles behind the design, and I think you might be able to apply some of the same concepts to what you're trying to do.

http://www.tuxgraphics.org/electroni...ticle379.shtml

Not really an answer to your problem but perhaps it will help a bit.

Re: Circuit design help - power supply - Non FIRST
 

Maybe a 400 watt audio amplifier would work. Generate and control your signals at low (manageable) voltage, and send them to the amp's input.

What kind of device are you going to feed this oddly specified output to?

Re: Circuit design help - power supply - Non FIRST
 

The most important question is whether you really need the output to be half wave rectified AC at specific frequencies, or are you going to use the output as DC? (filtered, ripple free)?

If you want 6V DC out then you are describing a switching power supply. There are many parts you can buy off the shelf to put one together.

If you really need a variable frequency half wave rectified sine wave then you are looking at an oscillator and what is in essence an audio amplifier.

Re: Circuit design help - power supply - Non FIRST
 

If you do need amplified & rectified AC, and have the budget, the easy way would be with an oscillator, a phase-shift network and a pair of power op amps - Apex's PA03 for example.

Re: Circuit design help - power supply - Non FIRST
 

Thanks for all the info so far, everyone!
Sciguy - As I said, I can't talk too much about specifics, but this unit will power an electrolysis chamber and resonate the electrolyte for increased output. That's where the frequency portion comes in.
The dual sine-waves with phase shift come in, because when shifted a minimal amount, the upper limit of the voltage is effectively the same as it was prior to generating the waves. The lower limit, however, is shifted slightly and therefore keeps the minimum voltage above 0 (say 6V, but that's just a guess). By not dropping to 0, production does not stop but rather becomes endothermic while the "active" production on the electrodes pauses and the product is allowed to clear.
The file at http://www.spline-designs.com/FrequencyGenerator.xls shows what I'm trying to achieve, though now I think if I could set a voltage range, I would only need one generator (ie., one sine wave from +6V to +12V at a variable frequency), which would make this much easier.

Dave - Thanks for the link, every little bit that gets me and others thinking helps :).

Alan - Good idea with the amp output, my only concern is the efficiency lost in the additional device, but I'm probably not going to be extremely efficient to begin with. The output is going to an electrolysis chamber, as described above.

Ken - I don't need DC in the strictest sense, otherwise I'd go straight in from my power supply (a DC car battery). I effectively need "pulsed" DC with a minimum "low" voltage, a max "high" voltage and some form of curve (not square wave) in between. Triangle wave would work, though I'd much prefer sine waves if possible, because sine waves have more of the type of derivative I'm looking for (sharp increase, quick level, sharp decrease).
I did think about tearing apart both an audio amplifier and a sine-wave inverter for this project, but the inverter isn't variable frequency and has a transformer, while I wouldn't know how to tune the amp.

Pete - Thanks for the link, I'll take a look at the data sheet and see what I can figure out about it.

Thank you all for your time in reading this, and I really appreciate the help.

Re: Circuit design help - power supply - Non FIRST
 

Oh! an electrolysis chamber!

Ive heard about that.

you sit in the chamber and it takes all the hair off your face, and it never grows back. Very interesting application!


:^)

Re: Circuit design help - power supply - Non FIRST
 

petek's power op amp suggestion seems perfect for this application. String together a simple oscillator, a full-wave rectifier, and an op amp with positive offset and appropriate gain, and voila: you've got what you're asking for.

Re: Circuit design help - power supply - Non FIRST
 

Well, I would keep the power op-amp, but use a microcontroller to build a waveform table in memory. Once that's done, step through the table and send the output to a DAC, which creates your waveforms. Depending on your tolerance for distortion, you may need to filter the output to remove the Fourier step components. Here's a link to Google Direct Digital Synthesis.

-Kevin

Re: Circuit design help - power supply - Non FIRST
 

From your description it sounds like you are experimenting with generating hydrogen from water.

Something to keep in mind: the overall efficiency of the system is the amount of energy you get out (the gas) divided by the energy you put in.

If you have to use amplifiers or other switching electronics to make the system work remember the power at the wall plug is the energy input, not whats across the electrodes.

The reason I bring this up: there are hundreds of websites created by people who think they have defied the laws of physics and come up with perpetual motion machines or motors that put out more power than they are driven with

and a common mistake they tend to make is pulsing the motors with low duty cycle DC pulses and measuring the output from the system as DC or AC. If you used pulsed power you have to get a true-RMS volt / amp / watt meter. Most meters (digital and analog) measure average voltage or current, not the actual RMS voltage. For a perfect sine wave, the average and the RMS values are the same. But when you start pulsing power a regular (average reading) meter will read less than the actual RMS going into the system

As a result, people measure 10W into their system, and 20W coming out, and think they have created an over-unity machine.

True RMS meters (like a Fluke 87) tend to be more expensive, so usually only people who really know what they are doing buy them.

Re: Circuit design help - power supply - Non FIRST
 

I appreciate the warning, Ken -
You're right in that this is a hydrogen/oxygen electrolysis chamber, but I have taken into account the actual vs. apparent input. I'm not going for "over-unity" since I don't really believe that it's possible with current technologies, but resonance is a contributor to gas production (it is used by the US government in military electrolysis applications, not to help split water but to allow the electrodes to clear from gas bubbles and therefore maintain a larger surface area) and with voltage dropped below the threshold voltage, electrolysis becomes minimally endothermic. This circuit is just designed to help get closer to 100% efficiency and let me test out a few theories about stretching and relaxing hydrogen bonds, along with seeing if heat energy can be used to supplement the system such that our output may come close to our input and hydrogen may prove a viable storage option for energy transport on a small scale.

Now I'll take another look at the ideas presented, and see if I can set up a schematic in my mind before I try to build the supply.

Re: Circuit design help - power supply - Non FIRST
 

Josh,
What you need is a DC coupled power amp capable of offsetting the zero crossing of the sine wave to some voltage. This is possible with audio power amps but not at the power levels you have stated in your intitial specs. Your requirements will only give you a possible 4-5 volt peak to peak signal at 30 amps or somewhere close to 200 watts. A couple of function generators and a big stereo power amp would do the trick with a variable DC suppply to make the offset but would only make power up to about 50kHz.
What you are asking to do is dangerous in most situations particularly for someone without previous experience in high power amplifiers capable of the frequencies you have stated. I hate to restate the obvious, but you do realize the flamability of hydrogen? At the quantity that this process would appear to generate, you are going to realize a rather large volume of hydrogen gas.

Re: Circuit design help - power supply - Non FIRST
 

Al, thanks for the words of warning regarding the amp's and their usage - I'm going to try to find some safe way of doing this, and certainly wont "play" with anything I feel unsafe with or that I haven't learned about. The relatively low frequencies should make this easier (0-150Hz, not kHz), but I understand the risks inherent in working with any high-wattage circuits.
Secondly, I do realize the flammability of hydrogen and I have flame arrestors/bubbler's and other precautions to ensure the safety around the generator. Given that it is not being stored and that hydrogen dissipates so quickly, the 1-1.5LPM that I've been getting isn't really a problem, unless presented with a flashback that the bubbler and arrestor will stop.

Re: Circuit design help - power supply - Non FIRST
 

1.5 Liters is still a lot of hydrogen if I read your post correctly. I don't want Bloomfield Hills to disappear off the map, I have friends out there. Remember, the hydrogen may not be dissipating but it may be gathering in high concentrations above your head. Remember the Hindenburg!
Most audio amps will go very low in frequency (some down to 8HZ) but only DC coupled amps will go to 0 Hz and you will only be able to offset the input waveform with a DC amplifier. You will need to be able to control the voltage input as the power amps will respond rather quickly to changes on their input giving you significantly more than 6 volts peak output.