Breadboarding HV Supplies

[K4RT]

In saying "breadboarding" do you mean home-brewing ?

[W9ZSL]

Nope, I mean putting all the components on the work bench, using a variac on the primary of the power transformer, wiring everything together with clip-on test leads and then applying AC while monitoring the DC voltage on a 3 KV meter salvaged from a Gates BC-1T.  

I then substitute various filter caps, chokes and bleeder resistors in the lash-up as I bring the primary voltage up to its working voltage so I can confirm the transformer is OK. You could always apply full primary voltage immediately, but if the transformer is defective, you will blow a fuse or trip a breaker.

Using all the combinations on hand I eventually come up with a DC voltage that I can work with in order to get a rough estimate of what I need for an amp.  The bleeder draws a certain amount of current so the bread boarding does not represent what the voltage would be when loaded by an amplifier.  The voltage drops.  I guesstimate that the voltage I read with only the bleeder resistor will drop 250 to 350 volts under load.

For example.  My current project has a 4600 VCT transformer.  That's 2300 volts to the rectifier.  Using a 12 Hy choke, a 12 mfd filter cap and a 100K bleeder, I get 2250 VDC no load.

Mind you this is not rock-solid, but I'll probably end up with between 1900 to 2000 VDC for my 813 final.

NOTE: NEVER, EVER TOUCH OR CHANGE ANYTHING IF THE POWER IS ON; MAKE SURE ANY CAPACITORS ARE COMPLETELY DISCHARGED.

[W4AMV]

Good day. Timely topic for me since I am building a straight forward 811A amplifier using a pair. The amplifier is fairly compact and the transformer I have is large on size so I am looking at an external supply on a separate chassis.

Question, what is the best practice for cabling the HV from the supply to amplifier? Looking at about 1500 V and was considering either RG8 or #22 Teflon insulated wire, several in parallel since the run is about 2-3 feet. Also, proper choice of connectors. I was considering large somewhat oversize screw-lug barrier strips with a plexiglass protective plate shield. Any thoughts. Thanks!

Alan

[K1JJ]

Hi Alan,

There's a lot of good ways to run HV wire and I'm sure you will get some good suggestions here.

The Tron has always told me to run a grounded shield, like braided ground shield, over the high voltage wire in case it gets pinched. RG-213 can be a good way to simulate this too. This way the HV will arc to the shield and blow the breaker if there is a pinch problem.  

I usually use the standard 40 Kv HV red wire stuff.  But for the last few years I have used car ignition wire with the copper silver-plated stranded core. It uses a nice rubber-like insulation and rated well. It used to be found at NAPA but is now discontinued. It can be found on the web under various labels. I put that into a braided shield AND then run 3/4" black flexible plastic water tubing over it. This is a reasonably safe and inexpensive way to go for HV interconnections between rigs.

I do not use HV connectors on the back of rigs. Good quality commercial ones rated for the job are OK, but I prefer to run the HV assembly (as described) directly into the rig and then use a big ceramic standoff as a mechanical and electrical HV terminal.  

Bear in mind that my 4-1000A plate modulated rig sees up to 8Kv peak at the final input under full modulation, so I need to be careful.

The bottom line is I could turn the HV on and chop the HV lead with an axe and still be safe...

BTW, if you do use terminals for the connections, be sure they are well insulated and mounted on Plexiglas in addition to the Plexiglas covering on top.  


T

[W4AMV]

Hi Alan,


BTW, if you do use terminals for the connections, be sure they are well insulated and mounted on Plexiglas in addition to the Plexiglas covering on top.  

T

 Great inputs Tom. Thanks. The idea of forming a sandwich for the terminal strip between Plexiglas makes sense, try to minimize any flash over. I am not sure I have room to accommodate ceramic standoffs, which I do have. That is a possibility I will consider.

[W9ZSL]

Does anyone know where to get some shield braid?  Would RG-58 work for HV?  I have a lot but doubt it would hold up.  Thoughts?  I'd prefer going with HV wire and braid.  Good idea with the tubing!

[K1JJ]

Does anyone know where to get some shield braid?  Would RG-58 work for HV?  I have a lot but doubt it would hold up.  Thoughts?  I'd prefer going with HV wire and braid.  Good idea with the tubing!

Hi Mike,

I have seen braided shield on eBay and the web for reasonable prices. If you buy it new from a supplier, it can be outrageous.  I have also stripped old RG-8 and used the shield.

From personal experience, I have had RG-58/59, etc  break down at around 2 Kv. I know that some guys have had better experiences with it, but mine failed. I would not use it. Choose RG-213 or maybe RG-8U.  The ratings are available on the web, but I think RG-213 with the nice mil-spec insulation is perfect for  2.5 Kv and less applications. For outside-the-cabinet higher voltage runs, (> 2.5 Kv) I would stick with the standard 40 Kv HV wire and covered with a grounded shield and finally covered with plastic tubing as I described in a previous post.

T

[W4AMV]

Yes, RG58 is an issue. Better off with RG8, and not RG8X, the foam is an issue.

I think I am going to try Teflon #22 AWG, several in parallel and same with the ground return. Slip both in separate spiral flex tubes. Use the barrier strip, sandwich with Plexiglas and buried INSIDE the chassis. As well, Plexiglas sandwich on the rectifier doubler card with the cap bank.

Alan

[K1JJ]

Yes, RG58 is an issue. Better off with RG8, and not RG8X, the foam is an issue.

I think I am going to try Teflon #22 AWG, several in parallel and same with the ground return. Slip both in separate spiral flex tubes. Use the barrier strip, sandwich with Plexiglas and buried INSIDE the chassis. As well, Plexiglas sandwich on the rectifier doubler card with the cap bank.

Alan

*  I checked and see Belden rates  RG-213 at 5 Kv breakdown. There is controversy saying that is too conservative, but I would hang my hat on 5KV and 2.5Kv being the working voltage to be safe.

BTW, in the early days, I thought of a ground between the power supply and rig as a "necessary" wire, something like grounding a rig to Earth.  But just imagine if there was no ground between the HV power supply and rig and YOU were the ground connection from arm to arm.  I have since run a fat shield for ground from all rigs and accessories to supplies AND ALSO have a separate wire that connects the power supply grounds to each rig. Redundancy to be doubly sure.

T

[Opcom]

Wire insulation is important for safety if it is close to other things or in a bundle. A cheap and useful wire is 15KV 'luminous tube wire'.

If bought right, it is cheap enough to be used for temporary hackery, and I have some in the transmitter for 8 years with no decay. It's outdoor rated so maybe it is better than indoor stuff. It tends to not be as wiggly and flexible as the pricey silicone stuff, but is only about as stiff as RG-58 or 8x.

I use this kind, $75 for 250FT, 15KV #14 (30 cents a ft.):
http://kcelectronics.com/gto-15ignitionwirerated15000voltssuppliedin250footpullbox.aspx

There's also this for 70 cents a foot.
http://www.skycraftsurplus.com/wirehighvoltagegto.aspx

"GTO wire" on google  will show a few more sources.

[W9ZSL]

Saw some #18 40KV from a reliable seller on eBag for just under $13 for 10 feet.

[W1RKW]

Ah yes, HV.  When I was building an 813 linear which eventually became GORT I was testing a 2.5Kv PS. I had just de-energized it and wanted to measure something in the PS (I don't remember what it was) but I stuck my hand in the chassis with the meter probe. The probe slipped and my thumb hit a hot terminal.  I realized I needed a better bleeder resistor.  My thumb had hit a hot terminal, a flame shot out of it like the proverbial pickle with AC applied to both ends. I figure about 2Kv got me. I burned the back side of my hand. Ended up on the floor with palpitations and  twitches and just feeling freaked out.  Several days later had an infection from the hole in the thumb which required minor surgery to clean the decay deep inside the thumb. I was fortunate to have the back side of my hand touching the chassis. If it wasn't I don't think I'd be posting this. The current path was from thumb to back side of hand.  Some other current made it through me and enough to give the jitters.  Had sore arm muscles too. After that experience, no probing while hot or even off without discharging caps. With the 813 rig it has a built in "jesus" stick. And if any measurement is needed while live, it's either a proven HV probe or direct connection to the discharged circuit then apply power.  No short cuts, pardon the pun.

[John K5PRO]

Just reading this line. I've been busy today troubleshooting inside a 30 kV, 35 Amp DC supply, with 240 uF. Its actually just the capacitors, bleeder and crowbar resistances and shunt device in this enclosure, about the size of a 2 car garage. The power supply is a GE oil filled pig outdoors with rectifiers. Primary is 4160 VAC. This is the plate supply for my final amplifier. We have some very strongly written and trained safety procedures and follow them to the letter. After 23 years here, I don't want to end my career in a grave. Have never gotten bit from more than 120 Volts that I can remember, but maybe my memory got zapped from it! The other power supplies I sometimes get involved in are 88 kV klystron beam supplies, and I don't like them since they use series/parallel capacitors in the vault. Our highest voltage at work is 690 kV DC from a voltage multiplier, a Cockroft-Walton circuit which is a DC accelerator for hydrogen ions and protons. That thing sounds like a thunder storm when it breaks down.

Years ago, I saw a digital guy reach into the back of my protoype 5 kW FM transmitter, and touch an exposed 5200 VDC terminal. It happened so fast that I couldn't yell no. I pulled him off the floor and tried to get him to the hospital, but instead he got up and cursed at our company president for the working conditions. Then and there I realized that it is NOT a good thing to operate radio transmitters with the sides and back off, as you never know who might walk up and reach out. Prototyping HV components on the bench?.... no way, Jose.

When I was 15, I built a Tesla coil with a luminous tube transformer and homemade plate glass capacitor and spark gap. It
was nasty, and exposed wiring. A few tingles from the secondary winding HF side but never on the primary or transformer secondary. Since I did it all alone, my brother and sisters were "growed" up, I had to be very careful, hand in pocket, etc. Lucky times.

[W4AMV]

Boy John, I highly respect your ability to stay safe. I liken your type to the fellow(s) that walk the Grand Canyon on a tight rope.

Stay safe... And please provide us low V types some with some well learned HV knowledge.

Alan

 

[KD6VXI]

Not to hijack,  but John,  do you remember the capacity Jim finally ended up with on his huge cap bank?

Up in Canada.

--Shane
KD6VXI

[John K5PRO]

I thought he had about 200 uF for his amplifier. It was an insanely large amount for ham rig, but I think he wanted lowest IMD and NO ripple. The only reason we need 240 uF at work, is for i= C dv/dt, i being the pulsed plate current of 240 Amps, dv being the allowable droop of 1kV (out of 23 kV plate voltage) and dt being 0.001 seconds, the pulse width of each RF pulse.
But to handle such stored energy needs a very fast crowbar. We have it set up so that it will not melt a 30 gauge wire when put across the output terminals with a big HV relay. That is protection that will not burn a screen grid bar in a tetrode during a tube arc over. Series R is needed along with a current shunting device, in this case, a mercury ignitron switch.

Back to the cautions:
Most or all of the heavy metal broadcast transmitters, like the ol' Gates BC1 series of models, are quite lethal as its easy to reach over and inside to troubleshoot, and of course be completely grounded at the time. The Continental/Collins Power Rocks are really nasty as they ran twice as high a voltage due to PDM, and about 20 uF of filter cap. The final tube cathode was floating at the - HV potential from the modulator switch tube. Temptation is to probe the waveform from the modulator under the tube socket. At least one broadcast engineer died from those.

I believe the new solid state stuff that QIX and others here are building are much safer, and yet still powerful for RF production. RF burns are nasty as others have said. I did get one or two of those in the broadcast amplifier cavity development, they smelly and hurt!


 

[Opcom]

Just reading this line. I've been busy today troubleshooting inside a 30 kV, 35 Amp DC supply, with 240 uF.

At those voltages it could be used as a coin shrinker I saw at a teslathon. It had maybe 40uF but was charged to 40KV with an X ray power supply adjusted by a variac. It was discharged through a 1.5" diameter coil of a few turns #12 or #10 inside which a quarter was held in place between two halves of the wooden dowel upon which the coil had been wound. The coil was blown to pieces each time. Containment was a 55 gallon drum with a steel plate for a lid.

Because of high voltages and large size equipment I never allow the cats into the shack where their little paws may reach towards harm. I'm very careful never to get my appendages near exposed voltages. I am comfortable around it but all too aware of the danger. I saw a co-worker get a small chunk of his thumb blown off when he was working on a microwave oven. That's small stuff compared to ham equipment.