Circuit design help - power supply - Non FIRST

[Alan Anderson]

Originally Posted by Al Skierkiewicz

Remember the Hindenburg!

The lesson to be learned from the Hindenburg has little to do with hydrogen. Mostly, it's just a bad idea to paint a big bag of fabric with the equivalent of rocket fuel, especially when it's going to be put to use in an operation that tends to involve large discharges of static electricity.

But Al is right to warn about the potential for the hydrogen to collect in high places. If you're really generating more than a liter of gas per minute, I hope you have effective ventilation.

[Mike AA]

If you were closer I might be interested in buying your "excess" hydrogen for my fuel cell I'm looking at getting.

Hydrogen really isn't all that "bad" in terms of fear or explosions, atleast when bottled up. I guess if left to accumulate in a room it would. There have been tests that show when bottled hydrogen is about the same reaction as gasoline in a car if it were to get into an accident, possibly safeR.

Hadn't heard of using different frequencies to perform electrolisis faster, I guess I'll have to try that when I get my fuel cell.

-Mike

[petek]

Allow me to add to Al's words of caution about the flammability of hydrogen. I know a fellow who blew the doors and windows out of his mother's garage when he tried generating hydrogen by electrolysis. He was very fortunate that no one was nearby to be hit by the flying debris and that somehow the garage didn't catch fire.

If you go ahead with this, please make absolutely sure you have a safe (as in explosion proof) way to remove any excess hydrogen from the building - or better yet, do it outside.

[Danny Diaz]

Originally Posted by petek

There have been tests that show when bottled hydrogen is about the same reaction as gasoline in a car if it were to get into an accident, possibly safeR.

This discussion brings back memories of the '96 Olympics in Atlanta, Georgia. The Georgia Tech Research Institute (GTRI) worked with the City of Atlanta and several companies in the Atlanta area to work on and demonstrate a hydrogen/electric hybrid bus to publicize Atlanta's efforts to help reduce air pollution (and also assess the performance of such a hybrid). I was unfortunate enough to come in just after the bus was developed and put into its short-term service, but I did get to ride the bus before it was shut down. Anyway, the designers and the City were extremely cautious about the bus and its fuel supply - so cautious the bus had hydrogen sensors running up and down the bus (outside and inside) in a fairly dense grid tweaked such that it could detect the smallest amount of hydrogen on the bus. It was tweaked so tight that the bus eventually got the moniker, "The Rooty Tooty Mobile", after the fact that passing gas on the bus would cause the sensors to go off and force the immediate evacuation of everyone on the bus. After one and a half days in service it was retired from active service as a shuttle bus and was used for research purposes from then on.

-Danny

[DonRotolo]

Originally Posted by Josh Siegel

I'm not going for "over-unity" since I don't really believe that it's possible with current technologies,

...or according to modern theories of how the world works.

Josh, it appears that you want to generate a relatively large signal (tens of amps) in the frequency range of 5 to 150 Hz, with a lower limit of say 5 volts and an upper limit of say 12 or more volts. This is not AC, but pulsating DC, there is a difference.

If I have that right, then it's easy to do for fairly cheap.

First, use a low-power variable frequency source (555 timer, signal generator, Beastie Boys tape, etc) and feed that into a few-hundred-watt single-channel audio amplifier (like used for a car). Not terribly efficient, but not too bad either. Then, just connect a (5?) volt power supply between one of the amp outputs and "ground", and you have your 5 volt bias. Fiddle with the numbers (you'll probably want a 10 to 12 volt bias, really) and drop the electrodes into the water.

Note that you cannot power the bias supply from the same source as the amplifier, or you'll have a short to ground and other issues. (=smoke).

Good luck with that, let us know how it turns out, and yes, ventilate well, like outdoors.

Don

[Al Skierkiewicz]

Originally Posted by Don Rotolo

...Then, just connect a (5?) volt power supply between one of the amp outputs and "ground", and you have your 5 volt bias. Fiddle with the numbers (you'll probably want a 10 to 12 volt bias, really) and drop the electrodes into the water.

Don,
This is not generally a good idea with modern audio amp design. The DC on the output that does not appear as part of the input signal will cause the amp to go unstable as the error correction circuitry tries to fight what it sees as an illegal output. This could have disasterous results for the power amp. The DC may also upset the damping/feedback control causing the amp to oscillate, sometimes enough to cause localized interference.

As to the notion that hydrogen is relatively safe, I think this can be attributed to the hydrogen cell research that has been publicized for the last few years. In this technology, the gas it locked up in a matrix that releases the gas in a very slow, low pressure stream useful for engines but slow enough to be non combustable under normal circumstances. I saw a demo where the container was shot with an incendary bullet and did light but did not explode. Hydrogen without precautions is explosive in a confined space.

[Andrew Blair]

Al, you're completely right. I would hazard a guess that several people in chemistry have done the experiment where you combine Mg with HCl, creating a few ml of H2. When you flip the containing vessel and light the H2 contained, it makes a popping noise. Remember: A few ml, in open air makes a loud pop! By performing electrolysis, both the O2 and the H2 are released, and if unseparated upon creation, is a reaction waiting to happen.

So, with a little electrolysis with enough current/voltage, in a confined space with both the O2 and the H2 mixing from uncontrolled hydrolysis, you have yourself a nice way to be seriously hurt. I will repeat. BAD!!! Unless of course full, absolute safety precautions are taken. I'm not being a jerk, but its important to be thourough. Sounds neat though, never knew pulsing DC would accelerate the process.

By the way, is hydrogen in a fuel cell matrix semi-liquid, or is it still far below that kind of pressure?

[sciguy125]

I was doing a little more thinking about this today.

You're essentially trying to work with a rectified sine wave, but keep it from dropping to 0V. Why not take the straight rectified signal, then add a DC portion to it? If your amplifier can do DC, then you can just add it before the amp. If not, then you'd have to add it to the output. Maybe put some other DC supply in series with the output of the amp?

[Gdeaver]

Hydrogen goes "BOOM" very easily and nicely. I would suggest extreme caution in this pursuit. You should have extreme fear of this research. You probably have not had the unfortunate experience to to see what a fuel air explosion does to a human being. I have. First, the smell of burnt flesh and hair stays in your nose for days. The sight of a human face blasted with shrapnel is a little unpleasant. The worst was the way he was flailing those stubs around that used to be hands. You get the picture?
If you still want to play, may I suggest the concept of micro batching. Your in the experimental stage. Why not scale down the experiment. A little "pop" is better than a room leveling "bang".

[Josh Siegel]

I'm well aware of the potential dangers, and never work with hydrogen indoors or around any sources of heat. I also ground myself before I work on the generator to prevent static discharge.

That said, I've been doing a lot of research on this topic over the past few days.

I know there are function generator kits out there (kit 6023 KT at MPJA, for example or the 3023 at Quasar) which use a "4 core" LM348 op amp to generate sine, square, and triangle waveforms. These use an op amp with an identical pin-out to the MXL1179 from Maxim IC, which can handle very high current and voltage - now, at first I thought that I could simply replace the IC, but then I thought about it and there's a) no way the traces would hold up to 30 amps, b) I'd still need high wattage resistors, and c) I don't know how to set up offset voltages.
That led me to think about the way PWM works, at least from my primitive knowledge of it. If there were some way for me to set a bias voltage on that function generator kit, I could change the potentiometer for the range I want and use a MOSFET to control voltage, couldn't I?

So what I'm thinking I'd like to do is this:
Function Generator (powered by 9V or similar) - pseudo sine wave out to a rectifier - to a MOSFET.

Am I thinking along the right lines? Is there any way to use the kit I linked with an offset voltage easily? Also, I'm not sure of the peak-to-peak voltage on the kit, and I assume my MOSFET selection should be based upon that and the offset (ie., 6V/30A MOSFET if it's 3V peak to peak and a 3V offset voltage).

The only problem I forsee with this circuit is the voltage in the secondary circuit (through the MOSFET) being susceptible to changes in the voltage of the control circuit. It also seems pretty inefficient.

[KenWittlief]

the efficiency problem you are facing is using any type of amplifier.

Amps work by taking a DC power supply output and holding back the part of the voltage that does not match the input. Subtracting the AC output from the DC power input leaves about half the input power dissipated by the amp (the Mosfet driver mostly) - so you are looking at 50% efficiency or less with this approach. The only way you can get around this is to use a squarewave output. This would keep the mosfet driver out of its linear range, so its either full on or full off. In either state its power dissipation is negligible. (this is how a Victor speed controller work).

If you want to approach 90-95% efficiency with a sine wave output you are going to need something like a motor generator set: a motor that drives an AC generator. The speed of the driving motor will determine the frequency of the AC output. The output can then be full-wave rectified.

Then all you need for your application is a DC offset (DC power supply in parallel with the AC rectifier). The diodes in the rectifier will keep the DC bias supply from back-feeding the AC generator.

The biggest risk you face is having something wrong with the DC bias. The O2 and H2 form at the opposite polarity electrodes. If your bias goes negative for any reason you will have a small amount of O2 in your H2 collector. Hydrogen and oxygen mixed together is subject to spontaneous combustion.

The only way you can be sure your electrodes are properly biased is with an oscilloscope.

[Gdeaver]

This is outside of my expertise but, wouldn't putting a large inductor after a FET give the desired results? Monitor the voltage after the inductor. When the voltage gets close to 12 volts turn the FET off. When it drops to 6 volts turn it on. The feedback could be done with op amps or micro controller. The output would not be a true sine wave but good enough. The frequency would be variable to the load. Kind of a switching power supply with sloppy voltage control. As far as safety, one should have the mindset that it is going to blow and design the apparatus to handle it. Murphy's laws dictates in will blow at the most inopportune time.

[KenWittlief]

Gdeaver has the right idea - you could make an resistor- capacitor or resistor inductor filter circuit that would turn a square wave into something approaching a sinewave.

For the current levels you want the inductor or capacitor would have to be very large. If you use an inductor you have to make sure it cant spike negative when the pulse is shut off. This is normally accomplished with clamping diodes.

[Dave.Norton]

Originally Posted by KenWittlief

If you want to approach 90-95% efficiency with a sine wave output you are going to need something like a motor generator set: a motor that drives an AC generator. The speed of the driving motor will determine the frequency of the AC output. The output can then be full-wave rectified.

If you want to see how to get a high efficiency power amp check out the sub-woofers from Sunfire. Carver has used a switch mode power supply on both rails, as the input waveform opens up the power supply on that rail tracks it. He then has a high current amplifier that only has 12 volts across it nominally, but can drive up to 300 volts peak to peak when required. It is fun to watch operating, but if you are ever looking at one be careful with it, 2 to 3 kW is not to be trifled with.

[Al Skierkiewicz]

Josh,
You can tell a lot about the function generator you linked to by looking at the power, the circuit design (quad opamp) and the picture. A single ended power supply on this opamp will render the output DC shifted to half the voltage of the power supply, in this case a 9 volt battery. Without a big capacitor on the output to block the DC (I can't see a big cap in the picture) the output will generate waveforms that center on half the power supply more or less so expect the output to be around +4 volts most of the time. In this application you are not really concerned about efficiency in the amplifier should you use one. What you are asking can be accomplished with a switching circuit in series with a variable DC power supply or a power amp where you can adjust the offset voltage of the output to be at some DC potential. With a power amp, the efficiency goes way down in this arrangement as the amplifier stage is drawing current all the time whether there is signal or not.