Developing a low frequency High Voltage - Page 1
Forum section: electronics / projects

author: Qmavam
author_rank: Regular Contributor
timestamp: May 16, 2023, 11:31:21 am
content: I'd like to have a low frequency, adjustable, say 3hz to 50Hz High Voltage 9kV sinusoidal AC signal.Is it possible to de-rate a microwave transformer to work at such low frequencies?If so, then I could series several to get my voltage.But, I don't know what de-rate entails, except lower field in the core, (lower current in the primary, but that also means lower output voltage) help me there if you can.Is there a better way?Thanks, Mikek

author: CaptDon
author_rank: Super Contributor
timestamp: May 16, 2023, 12:30:37 pm
content: How much current does this thing have to source? Microwave transformers run in near saturation at their design frequency and even a 10% reduction in frequency will be a problem. I think their unloaded secondary voltage is around 3Kv when run at the design input voltage. You may need some sort of a PWM flyback circuit to generate the ramp up of the positive half of the wave with the output smoothed with a capacitor and voltage sampled to provide a voltage control loop. Not sure how I would transition to the negative half of the cycle. Maybe a second identical circuit with the rectifier diode polarity reversed. Give us a hint what the circuit will be used for, there are probably better answers. B.T.W., the high voltage secondary of a M.O.T. has one side tied to the frame of the transformer.

author: Qmavam
author_rank: Regular Contributor
timestamp: May 16, 2023, 01:43:05 pm
content: Quote from: CaptDon on May 16, 2023, 12:30:37 pmIt's low current, so no need to go near saturation of the core, "I think".I was thinking more of a sinusoidal input waveform at the low frequencies maybe a good audio amp and frequency generator.Thanks for any help given, Mikek

author: TimFox
author_rank: Super Contributor
timestamp: May 16, 2023, 01:54:26 pm
content: Saturation of the core is a function of the voltage and frequency, not current.The emf is the time derivative of the flux, which is proportional to the B field (which has a maximum saturation value) times the frequency, so high voltage and low frequency together cannot be supported.

author: Qmavam
author_rank: Regular Contributor
timestamp: May 16, 2023, 02:04:36 pm
content: Quote from: TimFox on May 16, 2023, 01:54:26 pmHow would a transformer be designed or rewound to provide a low B field and HV output.Do I want more or less turns on the primary?I'm still thinking audio amp driven by a signal generator. Only 1ma maybe 2ma HV output, probably in the under 100 uamps, but, I want to be prepared for more.Thanks, Mikek

author: TimFox
author_rank: Super Contributor
timestamp: May 16, 2023, 03:36:58 pm
content: As pointed out above, microwave oven transformers are built for cheapness, so that their normal operation is just below saturation.If you want to rewind the coils, you can start from assuming that 120 V, 60 Hz (or whatever) represents the maximum operation capability, and choose the primary and secondary windings to use that flux at your lower frequency.

author: Wolfram
author_rank: Frequent Contributor
timestamp: May 16, 2023, 06:10:00 pm
content: Flux is proportional to applied voltage and time, so inversely proportional to frequency. If you want to operate a 50 Hz rated microwave oven transformer at 3 Hz, you either have to increase the number of primary (and secondary) turns by a factor of 50/3 =~17, or derate the voltage by a similar factor. Neither is very practical, these transformers usually have ballpark 1.1 V/turn, so you'd need to rewind the secondary with 2100/1.1 * 50/3 = 32000 turns, or run each transformer at 2100 * 3/50 = 126 V out, requiring 72 transformers in series to reach 9 kV.If you only need low current (milliamps), then it could be an option to rewind a microwave oven transformer secondary with some 30+ thousand turns, keep the original primary and drive it from the output of an audio amplifier. In this case the nominal input voltage would be 120 * 3/50 = 7 V. Insulating the secondary sufficiently for 9 kV is not trivial, but doable. Immersing it in oil greatly simplifies the task here.The other option is to make a solid state or tube amplifier to provide said voltages from HV DC, which can again be provided by a small switching converter. This could be more compact, but it's not a trivial task either.

author: james_s
author_rank: Super Contributor
timestamp: May 16, 2023, 06:14:07 pm
content: A transformer that can operate down to 3Hz is going to need an enormous core, the bigger the core, the fewer turns you need.I think a high voltage amplifier is probably going to be the easier approach here, it's not trivial, but it shouldn't be immensely difficult either.I'm curious though, what is the use case for 9kV at 3Hz?

author: Benta
author_rank: Super Contributor
timestamp: May 16, 2023, 09:24:11 pm
content: Might be worthwhile to play around with an old-fashioned automotive ignition coil. You'll probably get one for a couple of dollars at a wrecking yard.

author: peter-h
author_rank: Super Contributor
timestamp: May 16, 2023, 09:26:52 pm
content: I used to work in high voltage (precision ppm-level stuff, up to 100kV or so).This is quite a difficult thing to do. At 3Hz, one would use a standard Cockroft Walton designhttps://en.wikipedia.org/wiki/Cockcroft%E2%80%93Walton_generatorand modulating the supply voltage to the circuit will give you 0 to 9kV easily. If you want the 9kV to swing either side of GND then you can use a capacitor in seriesBut achieving 50Hz with the same circuit will be harder, due to the C-W output capacitance. It should be doable if you run the multiplier at a high frequency (say 100kHz plus, which one tends to do anyway to avoid audible magnetostriction effects).A straight DC amplifier, converting 9kV DC (which is trivial to generate) into 9kV P-P AC, is another way, but you will need a lot of power transistors/mosfets in series, and ensure even voltage distribution across them.If the output current was very small (microamps) then the above amplifier could be implemented with two large photoresistors as a potential divider. I did that in the 1970s. I painted some cadmium sulphide onto ceramic substrates and illuminated them with LEDs. But CdS is quite slow so 50Hz may not work.

author: coppercone2
author_rank: Super Contributor
timestamp: May 16, 2023, 09:33:25 pm
content: I wonder if you can find a old transformer to work on as a base, say from a 16.5Hz grid.For instancehttp://www.bueker.net/trainspotting/voltage_sixteentwothirds.phpthat might be a real long shot, but I suspect there might have been quite a few 25Hz transformers built. They would work alot better for 5Hz then a 60Hz would.looks bighttps://www.ebay.com/itm/354428025030

author: Benta
author_rank: Super Contributor
timestamp: May 16, 2023, 09:57:59 pm
content: Erm, the OP mentions 1, perhaps 2 mA, so 10...20 VA.I'm not sure railway transformers in the kW/MW range really fit here...

author: Marco
author_rank: Super Contributor
timestamp: May 16, 2023, 10:01:35 pm
content: Quote from: peter-h on May 16, 2023, 09:26:52 pmI'm pretty sure I once saw an application note which used high voltage optocoupler like the OPC10M or OPTO-150 (these are not normal optocouplers, they have large stacks of photodiodes which can directly block high voltage) as an amplifier ... but I can't find it any more.PS. foundit, it was from VMI. Uses a normal opamp using two high voltage optocouplers in push pull.

author: Benta
author_rank: Super Contributor
timestamp: May 16, 2023, 10:13:50 pm
content: So how does this generate 9 kV, 2 mA?

author: Marco
author_rank: Super Contributor
timestamp: May 16, 2023, 10:16:56 pm
content: Just use 10 parallel optocouplers :p (Bring a lot of money.)

author: coppercone2
author_rank: Super Contributor
timestamp: May 16, 2023, 10:25:19 pm
content: Quote from: Benta on May 16, 2023, 09:57:59 pmthey might have had auxiliary circuits powered by them for smaller purpose designed transformers that are still substantial.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 17, 2023, 12:39:33 am
content: Maybe old time NST 60Hz, 12000V sec, 115V pri, 360 VA. At 3Hz, maybe 15 of them would do the trick. Nice thing is they are designed for this voltage level. You would need to make sure to run it closed loop. I would imagine they would be pretty easy to damage otherwise.Maybe something like this would be better suited. 50lbs

author: james_s
author_rank: Super Contributor
timestamp: May 17, 2023, 12:40:35 am
content: You can't run a NST in series though, they have a grounded center tap, at least every one that I've seen does.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 17, 2023, 03:38:31 am
content: Quote from: Qmavam on May 16, 2023, 11:31:21 amGoogle finds thishttps://electronics.stackexchange.com/questions/237714/low-frequency-1-10hz-transformerit seems you just proportionally de-rate (ωL is constant) so a 50Hz mains transformer accepting 240V, will accept 14.4V at 3Hz'several' may be a higher number than you imagined ?

author: Marco
author_rank: Super Contributor
timestamp: May 17, 2023, 03:59:52 am
content: According toAcalbfi'spage for Trek, they use stacked MOSFET amplifier stages with optical coupling.Finding a second hand Trek 609B-3 or 10/10B-HS is probably the safest solution.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 17, 2023, 12:46:16 pm
content: Quote from: james_s on May 17, 2023, 12:40:35 amYes, case is connected to center tap of sec. I am missing why this would prevent running them in series. Pri and Sec are isolated. Primary does not require case to be grounded. While you have potential between cases I don't see a problem unless there is a risk damaging the transformer. I've never tried to measure the pri/case breakdown.

author: Wolfram
author_rank: Frequent Contributor
timestamp: May 17, 2023, 01:01:02 pm
content: Pri-case breakdown is surely not rated for half the secondary voltage, let alone (n+1)/2 required for n stacked transformers. In this case the voltage per transformer would be lower than the design value, but I would still not put more than some 1000 V across primary-case.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 17, 2023, 03:33:36 pm
content: I suspect this is what james_s was suggesting. I did try to see if Google could locate any test data but no luck.

author: TimFox
author_rank: Super Contributor
timestamp: May 17, 2023, 06:43:14 pm
content: Have you considered building an amplifier around the 6BK4Chttps://frank.pocnet.net/sheets/107/6/6BK4C.pdfor similar high-voltage vacuum tube?The 6BK4C triode was used as a shunt regulator for color-TV anode supplies, and is rated for 27 kV continuous plate voltage.With a 40 W plate dissipation rating, it is also limited to 1.6 mA (abs max) plate current.It has a very high mu = 2000, so it requires a reasonable grid drive to get 9 kV pk-pk, but a single tube will have trouble generating 2 mA in class-A triode operation.However, the heater power is reasonable (6.3 V at 0.2 A), so if you use your transformer to generate the DC plate voltage (maybe through a voltage-doubler or tripler), you could (carefully) parallel a few tubes.For a class-A amplifier, the quiescent point would need to be maybe 15 kV at 0.75 mA (per device), swinging from 10 to 20 kV, capacitively-coupled to your load.(With high voltage from anode to cathode, this tube can generate softish x rays, so a reasonable shield may be needed.)

author: TimFox
author_rank: Super Contributor
timestamp: May 17, 2023, 09:13:22 pm
content: If you have a real budget, you could go to these people:https://www.advancedenergy.com/products/high-voltage-products/high-voltage-amplifiers/standard-high-speed-amplifiers/trek-609e6/

author: Marco
author_rank: Super Contributor
timestamp: May 17, 2023, 11:46:47 pm
content: Yah, I mentioned the Trek amplifiers a little further up. If you're really determined, you could probably build a push pull amplifier with a cascode, especially since only 50 Hz is required.Maybe something like this with optocouplers so you don't need PMOS and keep the number of MOSFETs somewhat reasonable (open loop here, but trivial to close). It's not going to have much bandwidth because the resistor divider and the gate capacitances, but you can try todecouple thoseif more speed is required. For optocouplers you'd need something like OPI1270 to withstand the voltage and for MOSFETs something like UnitedSIC so they can handle the linear mode. Even with 1700V MOSFETs, this requires a stack of 22 for 18 kV peak to peak.The optocoupler approach would work with tubes too to make a push pull amplifier, would need isolated power supplies for the heaters though.PS. the patent I linked is actually from the founder of Trek even if not assigned to the company, wonder if they use it. I had a similar idea with a NPN/PNP buffer, but was looking for a clearly explained example.

author: Qmavam
author_rank: Regular Contributor
timestamp: May 22, 2023, 03:10:06 pm
content: Quote from: PCB.Wiz on May 17, 2023, 03:38:31 amThis is a little bit tempting, I have a transformer out of a welder, the core is 10" wide x 6" tall and 5.5" deep, it's 60 cycle. The next thing is to find an amp that would drive a couple dozen watts at that low frequency.I suspect this more of a project then I want to take on. It's something my son is doing at work, if they want it bad enough they'll need to shell out the money for a commercial unit.Thanks, for the input, Qmavam

author: Qmavam
author_rank: Regular Contributor
timestamp: May 22, 2023, 03:15:11 pm
content: Quote from: TimFox on May 17, 2023, 06:43:14 pmFunny you mention that, I'm actually awaiting parts to build a HV pulser using a 6BK4C.Do you have any info about radiation output when I'm running 8kV?I bout the C, because it is supposed to have leaded glass to help, but I wonder if I needa tube cage to be safe?Mikek

author: TimFox
author_rank: Super Contributor
timestamp: May 22, 2023, 04:11:09 pm
content: When operated at 25 kV, 1 mA the Sylvania datasheet estimates 1.5 mrad/hr (but forgets to mention at what distance from the tube, perhaps 1 meter?).The RCA datasheet for the 6BK4C estimates 0.5 mrad/hr, and refers back to some JEDEC specifications for the x-ray problem.At 8 kV, an x-ray tube rule of thumb (radiation proportional toV2) estimates 10x less radiation output, and of softer x rays (implying somewhere around 0.1 mrad/hr at that reference distance).You should have a metal shield anyway for HV safety.X-ray absorption for Bremsstrahlung sources is usually specified by "HVL" (half-value layer), but there are few measurements available below 10 kVp.Being real conservative, a published number for 31 kVp is about 0.5 mm of aluminum: 3 mm of Al would give you a factor of 1/64 at that energy.Safety specs keep getting lower (and harder to measure), but the natural background radiation on Earth is roughly 250 mrad per year, or 0.03 mrad/hr, depending on location (with additional man-made dose from medical, etc.).Staying well below that level is a conservative safety measure.

author: coppercone2
author_rank: Super Contributor
timestamp: May 22, 2023, 09:33:34 pm
content: steel is alot easier to solder then aluminum, if you even want do that. I would use spot welded steel boxes for this with edge overlap and a few spot welds to hold it together. Easy construction like a carit does not need to be made like a EMI shield. No reason not to use JB weld to glue it together either... or just glue lead. It's only had when you want to combine radiation and EMI shield, and you can totally coat it in a thick layer of epoxy to be safe with lead.

author: TimFox
author_rank: Super Contributor
timestamp: May 22, 2023, 09:37:40 pm
content: Yes, for metal shielding of an x-ray source, you just need to avoid gaps that line up with the direction from the source (which here is the internal anode of the tube).No need for lead, steel or even aluminum at this energy should suffice.

author: coppercone2
author_rank: Super Contributor
timestamp: May 22, 2023, 09:50:41 pm
content: well, to combine the HV shield and weak rad shield, I recommend nickel silver as a material that is very easy to solder, and quite easy to bend into a shape.It comes together as easily as copper but it should give more rad shielding then copper. And its robust, which is nice for a safety shield, you don't want it crinkled or rippedI think its better mechanically then copper anyway. but solders easier then steel, and it will be a better low frequency EMI shield too (better near DC)I would recommend if you can do it for low price, have it background if your fingers are on the chassis and low when its open for repair. That IMO is solid construction that you can be proud of