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I have recently gotten into high voltage with the help of another member here, Killa-x (or something like that..) and well, I have some questions about SSTC's. I am currently gathering components for a SGTC, but my local science center has asked me to build a singing coil, and because of my lack of knowledge of a DRSSTC, for now, I will be sticking to an SSTC, particularly Steve Ward's design, seen here:
One of my primary questions was the existence of pre-made PC boards to make the process easier, and while it may cost me more, the presentation for the science museum is more professional. My friend recommended making my own, but not having any experience with it, it is unlikely that will be a possibility. I do not mind proto-boarding it, no issue at all, but it is definitely less professional.
My second question resides with the capacitors. While Mr. Ward provided the microfarad values, voltage values were not provided. I am also wondering about the antenna, where Killa-x said that most people put the antenna in the transformer, and I am not familiar with that process, if someone could point me in the right direction or explain that, it would be wonderful.
In looking at the power supply, it seems the unit only needs 12 and 5v to operate, and my first thought was a PC power supply, I have literally boxes full of them (probably 450 watt max from old computers) and I was wondering if I could use those instead as I have them in stock, they are fairly bullet proof, and it would decrease cost, it would increase size, but I am not too much concerned about that. If I am unable to use a PC power supply, what rating does the transformer need to be? The rectifier diodes are 1A so I figure it will not draw more than an amp at 20v meaning it would need to be just some BS tranny as 20 watts is nothing. I could be wrong there though..
I would ask about the other bridge rectifier, but another conversation said get a 35A so that was good. I do have an excess of 24awg magnet wire though, and to reduce costs I would prefer to use that, would anyone know a good way to calculate the difference in length needed? Also the length in general is not specified, just the length of the pipe. I would not plan on a toroid, just a piece of tungsten on top (TIG welding rod that I get from my welder, 5/32in.).
I also see the 16T trifiliar 24awg ferite thing. Is there a recommended place to get the ferites for this? Preferrably one that is cheap, or even a common place that I can find them, like say in a certain electronic component that I can salvage. I do have access to the IT departments old computers to pull parts before recycling as they get paid by computer, not by the pound, so what I take generally does not lower the cost per computer. I know like the chips and the diodes and even some of the caps it just does not pay to get them out of another board, and I do not mind that, I just cant think of a place to get that ferite from, or of a possible work around to remove it from the equation.
I also have a question about the 555 timer which I believe is just used as an interrupter? If so, is there a way for me to input either a midi signal or a pure audio signal? Basically, looking to install a 3.5mm female jack or something like that on the board to allow people to plug in an mp3 player or whatever and hear the music. I would still like to keep the 555, so I would most likely have a 3.5mm interface for it as well, seems the best way to make both work.. Killa-X did express a concern for full blown MP3 and the lack of volume it can produce. Is there a cost effective solution to this?
Anyways, I think that is all for now, I do apologize for the long winded response, as Killa-X will attest to, I am a slower learner than most, but I retain information easily once learned. I tend to ask a lot of questions and have them fairly long winded and I feel like I bother him sometime, hence why he recommended i come here. Terribly sorry again for the long post, I would just prefer to ask in too much detail rather than too little.
Thank you for your help in advance, I will post updates as I can get them.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
On the circuit you are looking at, there is a 12V and 5V regulator. You want a supply capable of providing at least 2V above that. So any 14V-15V, 1A DC plug-in power supplies will be fine. You could also use a 12VDC regulated plug-in supply and eliminate the LM7812 regulator altogether.
The bridge rectifier you can use just a KBL04 type bridge rectifier.
I have built this circuit many times, it is one of my favorites!
Cores: EVR sells cores which are great for GDTs, and that is where I reccommend you get them from, but if you are ordering a lot of parts, you can also find suitable cores on Mouser - that is where I have been getting mine, but they are about $5 each there.
Caps: As for voltage ratings for capacitors, I like to use at least 2x the voltage expected on that part of the circuit. This is harde to judge if you are not that knowledgeable of the circuit design yet. For example, all the low voltage side capacitors (everything not around the half bridge and mains rectifier) should be at least 2x the voltage of your low voltage transformer. I use 50V caps since I'm using a 24V stepdown transformer for the 12v and 5V rails.
C7 and C8 are the DC blocking caps on the halfbridge and will see rectified mains voltage and back-EMF voltages, so I reccommend 3x mains voltage. I use 600V CDE 940C6P68k-F pulse caps.
C12 (which may be a few caps in parallel, for better voltage smoothing) should be rectified mains voltage + headroom for spikes. I've been using 2x 200V 460uF electrolytics in parallel with good results at low power levels (about 550VA and lower) but it isn't enough capacitance for higher powers, as the increased current draw at >70Vac causes large voltage ripple which comes out as 120Hz modulation on the TC's output. I think I calculated that I would need about 40,000uF (ridiculous) to achieve 1V ripple at full 120V input, which would yield a silent full power arc. Anyway, I've been using only 200V because my bridge rectifier is only rated for 200V 15A, so I can't exceed 120Vac (~170V DC) input without blowing my recitifer, so no need to spend extra on expensive caps. If I were doing a presentation coil I would use a 600V rectifier rated for 20A or more and about 2000uF of capacitance rated for at least 350V.
Interuption/Singing Coil The 555timer interupter shows feeding pin 3 of the gate drive chips is the only thing I do not like at all about steve's design and I have been leaving it out in all my recent builds. The PRF frequency range and pulse width range is very limited. You are stuck to very loud annoying audio range tones and you cannot use that type of interuption with a singing coil.
I wish I could tell you a scheme which would work perfectly for audio modulation with this design, but the truth is I'm still designing it myself. I'm going with a Class A power amplifier topology which is VERY inefficient but should yield the best fidelity. I have fets and IGBTs rated for 2500W dissipation for the task but need an adequate cooling system before I begin real tests. The theory is simple: bias a fet/igbt into the linear region and then inject the audio signal into the gate. Keep the audio modulation voltages in check so that there is no clipping (don't let Vgs get above 9V or below 5V) via a potential divider and volume adjustment on the audio source. Then it is just a matter of keeping the beastly device cool since it will always be in its linear region.
Most folks would say make use of the Enable pins of the gate drive and do the modulation there, but I've decided to modulate the DC buss instead as this way the half-bridge and primary are only running on much reduced average power and can run the long durations needed for music. The linear device is going to take all the thermal load off the rest of the system, basically, and keeping it cool will be a challenge. If I knew how to make a voltage controlled pulse width PWM circuit with a high enough PRF to be out of the audio range (>30kHz) and allow easy low pass filtering without needing a ton of capacitance, I would just do that, but then I would have to worry about run times for the half bridge and primary.
On the circuit you are looking at, there is a 12V and 5V regulator. You want a supply capable of providing at least 2V above that. So any 14V-15V, 1A DC plug-in power supplies will be fine. You could also use a 12VDC regulated plug-in supply and eliminate the LM7812 regulator altogether.
The bridge rectifier you can use just a KBL04 type bridge rectifier.
What about a computer power supply that is regulated to 12, 5, and 3.3 volts DC, not that I would use the 3.3v though. It seems like it may work, and provide reliable, fairly heat free operation. Also really easy to replace if it does blow in, say, a power surge.
Thoughts?
If that will not work I will just do the LM 7812 and 7805 idea because those things are fairly cheap anyways. And with the 42inch TV and an onkyo 600 series amplifier both broken (brother broke the amp by blowing a channel and I cannot get the dc offset right to bypass protection, and the tv was given to me with a note saying if I can fix it I can have it, so I took it apart) I should have plenty of prats to scrounge from.
Registered Member #2405
Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140
For the low voltage digital circuits that should be no problem using a computer power supply I have done that with a table top unit. The bus voltage however you will need a little more that the computer supply will deliver.
Keep in mind that if you use a PC power supply you need to load the low voltage rails to a certain point or the powersupply will drop out of regulation. I can't remember if it was the 5v or the 3.3V rail that was critical, but one needs to see a constant load of a certain value otherwise the 12V rail will not be regulated.
Registered Member #2405
Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140
Yes that is true if I remember correctly I loaded mine with a resistor so it had a constant load on it. Many of the walwart transformers are the same way they also need to be loaded to some degree. It dont need much but it needs something on it.
I have built this circuit many times, it is one of my favorites!
Cores: EVR sells cores which are great for GDTs, and that is where I reccommend you get them from, but if you are ordering a lot of parts, you can also find suitable cores on Mouser - that is where I have been getting mine, but they are about $5 each there.
Caps: As for voltage ratings for capacitors, I like to use at least 2x the voltage expected on that part of the circuit. This is harde to judge if you are not that knowledgeable of the circuit design yet. For example, all the low voltage side capacitors (everything not around the half bridge and mains rectifier) should be at least 2x the voltage of your low voltage transformer. I use 50V caps since I'm using a 24V stepdown transformer for the 12v and 5V rails.
C7 and C8 are the DC blocking caps on the halfbridge and will see rectified mains voltage and back-EMF voltages, so I reccommend 3x mains voltage. I use 600V CDE 940C6P68k-F pulse caps.
C12 (which may be a few caps in parallel, for better voltage smoothing) should be rectified mains voltage + headroom for spikes. I've been using 2x 200V 460uF electrolytics in parallel with good results at low power levels (about 550VA and lower) but it isn't enough capacitance for higher powers, as the increased current draw at >70Vac causes large voltage ripple which comes out as 120Hz modulation on the TC's output. I think I calculated that I would need about 40,000uF (ridiculous) to achieve 1V ripple at full 120V input, which would yield a silent full power arc. Anyway, I've been using only 200V because my bridge rectifier is only rated for 200V 15A, so I can't exceed 120Vac (~170V DC) input without blowing my recitifer, so no need to spend extra on expensive caps. If I were doing a presentation coil I would use a 600V rectifier rated for 20A or more and about 2000uF of capacitance rated for at least 350V.
Interuption/Singing Coil The 555timer interupter shows feeding pin 3 of the gate drive chips is the only thing I do not like at all about steve's design and I have been leaving it out in all my recent builds. The PRF frequency range and pulse width range is very limited. You are stuck to very loud annoying audio range tones and you cannot use that type of interuption with a singing coil.
I wish I could tell you a scheme which would work perfectly for audio modulation with this design, but the truth is I'm still designing it myself. I'm going with a Class A power amplifier topology which is VERY inefficient but should yield the best fidelity. I have fets and IGBTs rated for 2500W dissipation for the task but need an adequate cooling system before I begin real tests. The theory is simple: bias a fet/igbt into the linear region and then inject the audio signal into the gate. Keep the audio modulation voltages in check so that there is no clipping (don't let Vgs get above 9V or below 5V) via a potential divider and volume adjustment on the audio source. Then it is just a matter of keeping the beastly device cool since it will always be in its linear region.
Most folks would say make use of the Enable pins of the gate drive and do the modulation there, but I've decided to modulate the DC buss instead as this way the half-bridge and primary are only running on much reduced average power and can run the long durations needed for music. The linear device is going to take all the thermal load off the rest of the system, basically, and keeping it cool will be a challenge. If I knew how to make a voltage controlled pulse width PWM circuit with a high enough PRF to be out of the audio range (>30kHz) and allow easy low pass filtering without needing a ton of capacitance, I would just do that, but then I would have to worry about run times for the half bridge and primary.
By the way, welcome!
Thanks for the detailed info!
Are there any specifics for the GTD ferite? Like size and whatnot?
How do you feel about recycling capacitors? If so, I can save myself a ton of money ha ha!
Kind of bummed C7 and C8 have to be such nice (and pricey) caps, I was hoping to do this for pennies and donate it, looks like I will have to give them a bill? Who knows, I may be able to talk CDE in to sending me some samples :D
For C12, I have two 1100uf 200v lytic caps laying around, would those be sufficient? They were going into a vintage power supply, but eh, I am just going to sell the supply, wont do me any good anyways, not much better than an LM317 with lots of filtering caps. I would like to run it hard if I can, but I am not sure what running a SSTC hard really is.. So if you could elaborate more on that I would be appreciative. You seem to reference adding more power, but isnt the power draw limited to the voltage regulators?
As for the audio, please let me know if you come up with any ideas because I would really like some sort of a singing coil, but one with a knob to play with should be sufficient for the museum people. Being a young audiophile I do have some good amps at my disposal including a Marantz 170DC and some tube amplifiers I made.
I know companies like EVR make interrupters, but at this time I really cannot afford that unfortunately. Being a recent college grad, money is a bit tight, and I am just trying to inspire young minds on a budget ha ha!
I feel like there should be a way to control it like one would a light organ, using some sort of a mosfet or transistor to control the lights when triggered by the sound that it interrupts the coil. It would have to be stupid sensitive though. It is just an idea as I have 0 experience designing major circuits like that, but the idea seems viable to get the voltage you need.
For the low voltage digital circuits that should be no problem using a computer power supply I have done that with a table top unit. The bus voltage however you will need a little more that the computer supply will deliver.
Any idea how many watts it will require? Since the regulators are only rates for an amp, even a POS power supply should be able to handle that kind of load, right?
Keep in mind that if you use a PC power supply you need to load the low voltage rails to a certain point or the powersupply will drop out of regulation. I can't remember if it was the 5v or the 3.3V rail that was critical, but one needs to see a constant load of a certain value otherwise the 12V rail will not be regulated.
I actually had no idea, never measured one when it was off, I will just be like everyone else ha ha! The 7812 and 7805 should be fine. Is there any recommendation for paralleling them to get more amperage through? Or maybe even running a different voltage to give you greater spark output?
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SSTC questions from a newbie
