RF High Voltage Shielding

[W9ZSL]

Back again!  I scored 20 feet of tinned copper braid and 20 feet of 40KV wire that fits inside like a glove.  Do I have to shield the B+ from the PS to the modulator and from the modulator to the RF deck or is the shield only necessary within the PA enclosure?

[W1ITT]

I rather prefer shielding it everywhere.  In the very unlikely event that your 40kv wire is damaged, that shield will take the HV to ground before it goes somewhere else.  There's no need to be afraid of high voltage, but it deserves respect, and a belt-and-suspenders approach.

[K1JJ]

Back again!  I scored 20 feet of tinned copper braid and 20 feet of 40KV wire that fits inside like a glove.  Do I have to shield the B+ from the PS to the modulator and from the modulator to the RF deck or is the shield only necessary within the PA enclosure?

The HV wire does not normally need to be shielded because of RF getting into the power supply, etc. The bypass HV caps and PS filter caps will take care of that.  You could run unshielded HV wire everywhere and the rig would work fine, though shielding it inside the rig is sometimes done.

But as Norm said, the safety issue is what counts.  For outside-the-cabinet runs I prefer to use a grounded shield on the cable to absorb accidental wire pinches to ground and I also put a plastic flexible tubing over the shield as a secondary protector.  

The idea is that you could cut the live cable with an axe and it would blow the breaker.

T

[KA2DZT]

Agree with JJ,  no need to shield your HV wiring anywhere.  Commonly done years ago for what reason, I don't know (probably TVI).  You can run your exposed HV wiring inside some plastic tubing for added insulation.

[W9ZSL]

OK; thanks!   

[DMOD]

Somewhere in the HV circuit there should be one or more 0.001 uF to 0.01 uF cap about 2XHV to shunt any RF to ground.

That should take care of RF floating around the system.

Phil - AC0OB

[Opcom]

I only shield the HV and modulated HV if it comes outside the equipment. Just for safety purposes.
As mentioned there are bypasses around and RF gives no trouble.
The wire is always bypassed when passing through the RF enclosure wall.

[ka1tdq]

A little reminder on HV safety...

On Friday morning I was working on my amp, but didn't give enough time for the B+ to bleed down. I don't know exactly what happened, but I stuck my test lead into the RF deck near the plate capacitor and there was a bright flash and a bang. My pinky came into contact somehow with the plate cap. Luckily, my forearm happened to be touching the chassis, and there is a HV exit burn at that point. You can see it in the picture.

My arm went very tingly for 10 seconds, but I was ok. Everything seems to be healing fine and I still have the sense of touch everywhere.

Jon

[W9ZSL]

I have a pair of 690 pf @ 20 KV for the main HV bleeders.  Schematic calls for 2 x 500.  All HV wiring will be inside a 4 foot Atlas cabinet.  That's Bella Cat. 

[W1RKW]

Had the same thing happen to me a number of years ago. Exact same situation. PS was turned off but filter caps did not bleed down fast enough. Got the back side of my and and thumb across the filter caps and chassis.  Based on bleed down time and the contact I figured about 1.8kV across the hand.  A flame shot out of my thumb where it contacted the filter cap terminal and had a burn mark below the pinky finger where contact was made with the chassis.  Had to have minor surgery to clean out the burned tissue in the thumb due to infection.  Jon is 100% correct on HV safety.

A little reminder on HV safety...

On Friday morning I was working on my amp, but didn't give enough time for the B+ to bleed down. I don't know exactly what happened, but I stuck my test lead into the RF deck near the plate capacitor and there was a bright flash and a bang. My pinky came into contact somehow with the plate cap. Luckily, my forearm happened to be touching the chassis, and there is a HV exit burn at that point. You can see it in the picture.

My arm went very tingly for 10 seconds, but I was ok. Everything seems to be healing fine and I still have the sense of touch everywhere.

Jon

[W1ITT]

A few months ago we had a thread that mentioned Grounding Sticks, sometimes called "chicken sticks".  It's an insulated stick, with a hook on the far end connected to an insulated wire that goes to ground, either through a big alligator clip or, better yet, bolted to ground.
When you think everything is off, touch everything in the rig with it, just to make sure.  Most every broadcast transmitter has one built right in.  And if someone calls you a wuss for using one, just rap them with it.  Bleeder resistors can and do open up with age and should never be trusted.  Some of you guys give me the heebee-jeebies.  They only used 2200 volts at Sing Sing, but you can do the job with a lot less.

[K1JJ]

I always like to hear these kinds of horror stories because I know you guys have successfully learned and will now go on to live long, safe lives around electricity.

The pain and resulting emotional trama that we get by eating bad food (food poison) getting mauled by a dog, hit by a car, falling down the stairs, getting burned on a stove, pulling a muscle in our back, slicing our hand with a knife, etc., etc...   is what it takes to wire the neurons in our brain to evoke a very strong negative emotional response every time we try it again.  And so it is for a bad electric shock.

Without the first-hand personal shock experience, few of us can learn by reading about it of even seeing it. We must experience it first hand to really get mentally programmed. Otherwise our safety procedures may become half-hearted.   This is a pity because some things, like 2KV ++ electrocutions don't give us a second chance. That's the hell of it.

The best thing that can happen to a ham or radio tinkerer is to get a nice healthy jolt early on in his career and live - whatever it takes to wake us up out of our complacency. This is almost a guarantee that we will do everything possible to make sure it doesn't happen again.  

Blessed are those who can learn from others' blunders without experiencing it themselves.

T    (Member of the Belted Bad and Lived Club)

[ka1tdq]

You know, all this for a PW 1500 watt linear.  What's up with that stupid power limit rule anyway?  I talk on 40 meters AM, but I've yet to have a QSO with someone in China.  We need more power.

Jon

[W4AMV]

Here is my HV Supply for a pair of 811A's. I tried to over engineer this thing a bit in preparation for something larger. This unit is a V doubler with the goal at 1200 V 500 mA. The VA on this xmfr is unknown, weighs in at just shy of 20 pounds. I suspect it is at least 500VA. It is a 120-480V control transformer and with the TR and peak V available from the doubler the transfer from 120 to HV is 10.66:1. I am seeing exactly 10:1. Several of the ideas for handling the HV layout came from folks on the forum. Thanks! Alan

[K9PNP]

And don't trust the ceramic on bleeder resistors to contain the HV.  Was testing the 1.5 KV PS I built and got the back of my hand close to the resistors while it was operating.  Fortunately, as above, another part of the hand was grounded and the HV did not get up the arm to mess with the heart's electrical system.  A plexiglas shield for the bleeders is being worked on.

[W4AMV]

Hi Mitch, thanks. Yes! I was a little terse in mentioning what I did in putting this together. I do not have any interlocks, yet... However, it is not clear in photo 1, but there is a plexiglass shield all on the top of the card. As well on the bottom of the card out of view from this photo, there is plexiglass bottom that the card is mounted upon with ceramic spacers. So the HV card is sandwich between some plexiglass plates. Furthermore, the HV wire exists the card and passes to a ceramic standoff, but the wire and the standoff are in cased in a plexiglass sample vial. It's basically a pill container with the top cover secured so there is no HV line exposed. The HV line runs off to a 3/8 auto hose line and is all contained in the hose line which eventually mates with the amp. 

[Opcom]

A little reminder on HV safety...

On Friday morning I was working on my amp, but didn't give enough time for the B+ to bleed down. I don't know exactly what happened, but I stuck my test lead into the RF deck near the plate capacitor and there was a bright flash and a bang. My pinky came into contact somehow with the plate cap. Luckily, my forearm happened to be touching the chassis, and there is a HV exit burn at that point. You can see it in the picture.

My arm went very tingly for 10 seconds, but I was ok. Everything seems to be healing fine and I still have the sense of touch everywhere.

Jon

You know, all this for a PW 1500 watt linear.

Jon

I am glad you are OK.  That white spot. I'd let my doctor look at it. Think of it this way, what if it had been a 3CX3000 or a 4-1000 GG amp? It might have had 5000V waiting there and could have been far worse a burn, like emergency room horror time. I agree that for AM the limit should be higher. I always think about +6dB over the present rule as being reasonable for my taste, but I am too cheap to spend money on anything more than that. 8-)

I have a pair of 690 pf @ 20 KV for the main HV bleeders.  Schematic calls for 2 x 500.  All HV wiring will be inside a 4 foot Atlas cabinet.  That's Bella Cat.  

Those C should be fine for RF bypassing. What are you using for HV bleed?

People criticize me for wasting 90mA in bleeders but I want the plate voltage gone quick when the key is up. I use the permanently attached chicken stick no matter what. If I open the cabinet doors on my TX then the unit is un-keys including the HV supplies via their primary relays. I can bypass that but have to do it intentionally and only done so very rarely.

That is a nice looking cat. Calico are said to be good luck and every one I have ever met was nice tempered and gentle. Never been bitten by a calico.

Based on bleed down time and the contact I figured about 1.8kV across the hand.

If it had bleed down time and still had 1800V, did you install a higher current bleeder?

Here is my HV Supply for a pair of 811A's. This unit is a V doubler with the goal at 1200 V 500 mA.

Those are decent size gray resistors next to those caps. Do they make a bunch of heat that will over time ruin things? Its a nice job and layout but I always nitpick my stuff about heat from things, esp. large resistors which I don't like to place on PCBs.

[W4AMV]

Hi Patrick. I agree. Actually, the bleeder R's are not on the PCB. They along with the meter shunt and series R's are on a terminal card which is raised above the PCB as it sits on standoffs. That one bleeder R is a little close to the jacket of one of the Caps and I was a bit bothered how that worked out. You might note that the top glass top is raised a bit with some black spacers to keep the glass above the resistors and let some air circulate. However, the bleeder system is 100k and at 1200V it's under 15 W.

[WBear2GCR]

My suggestion regarding bleeder resistors for HV is to use at least TWO resistors in parallel.
Over rate for wattage and voltage. Substantially if you can. 

The reason is that in some instances - usually after long years of service - the heat can cause failure of a resistor. In this case having 2 resistors reduces the possibility of having NO bleeder to pull the voltage down.

In the case of the Henry 2K-3 amps, the "bleeder" resistors actually act as a type of load or ballast resistor, and pull down the B+ quite a few volts all the time. They get HOT! If they open, the voltage floats up and exceeds the arc over of  quite a few components! These are >100w brown ceramic wirewounds too, not puny sandcast stuff...

Always put a stick or at least a HV meter on everything before putting hands on. I do.

                          _-_-

[W9ZSL]

I scored seven 100K at 50 watt aluminum 3% resistors and will use 4 on the RF supply bleeder; two in parallel in series with two more in parallel for 100K. I have a 3KV meter that requires 8 ma for full scale reading which works out to having about 375K for the dropping resistor.  Three of the 100K plus an 80K with slider for meter adjustment will be paralleled with the bleeder  The whole system will give me roughly 85K total bleeder resistance.

[K1JJ]

More High Voltage safety suggestions:

I have a large meter (6" X 8") that is dedicated to measuring the main HV in the shack. I use this same HV power supply for all big rigs.   This HV meter is in my face and I watch it like a hawk after I turn the supply off and watch the voltage decay. It is also a great meter to watch during modulation and key up to be sure HV regulation is good.  I have four power supply settings using the primary for various voltage levels. This meter is a double check that I have everything set right.

Another important "idiot light" is a red 15 watt bulb that sits near the HV meter. It is connected to the HV transformer 240V primary to show me when AC is applied across the transformer. The point is, what if the primary was on, and the rectifiers were open due to a problem?   There could be  4KV AC on the transformer secondary with no DC voltage showing on the meter.  The red light lets me know.  Even a Jesus stick across the filter caps would not show this mistake. Of course, we should unplug the AC mains of any power supply before working on it.

In addition to shorting sticks, physical barriers and other precautions mentioned, we want the HV area to be secure enough to allow someone to walk into the room with the HV on, be drunk as a skunk, grope around anywhere he can put his hands and be safe. Same thing for a cat or dog. The room's safety must be well thought out.

T

This is a picture of my 6" X 8"  HV meter. The other meter for tube grid current has a fail safe circuit that shuts down the drive whenever a certain threshold is hit to any amplifier in the shack. Save the grids in my 8877 linear, 4-1000A AM rig, etc. - when I make a drive mistake.

[w1vtp]

<snip>

In addition to shorting sticks, physical barriers and other precautions mentioned,  we want the HV area to be secure enough to allow someone to walk into the room with the HV on, be drunk as a skunk, grope around anywhere he can put his hands and be safe.Same thing for a cat or dog. The room's safety must be well-though out.

T

This is a picture of my mater HV meter. The grid meter measures current and has a fail safe circuit that shuts down the drive whenever a certain threshold is hit to any amplifier in the shack. Save the grids in my 8877 linear, 4-1000A AM rig, etc. - when I make a drive mistake.

Emphasis mine: Ross Hull, the well known experimenter, was killed when he did not observe Tom's important advise.  He was experimenting with a TV receiver when he groped for the CRT HV switch under the bench and connected with the HV instead.

Good advise Tom,  Al

[w1vtp]

I have a pair of 690 pf @ 20 KV for the main HV bleeders.  Schematic calls for 2 x 500.  All HV wiring will be inside a 4 foot Atlas cabinet.  That's Bella Cat. 

What a pretty kitty!

http://www.qrz.com/db/w1vtp    Scroll down to the first row of pics and meet Rosie.

Al

[ka1tdq]

I keep the 220 locked out to the amp when not in use, and I keep the filament/HV locked out as well.

I will be adding a HV meter so that I can be sure the HV bleeds off even when locked out.

Jon

[K1JJ]

I keep the 220 locked out to the amp when not in use, and I keep the filament/HV locked out as well.

I will be adding a HV meter so that I can be sure the HV bleeds off even when locked out.

Jon

Very good on the locks, Jon.  As the kids grow up, the locks will become even more important. How many dumb and dangerous things have we both done even when warned by our parents?  :-)

Another thing to consider is what would happen if you were on the air and you were suddenly called into the other room for an emergency?  A loud bang, an intruder, a person at the door who was injured,  an accident in the kitchen - whatever. It causes you to run out of the radio room with the HV still on. Could someone walk in there and accidently stick their hand in the soup?   How about a freaked out cat that runs into the radio room for cover?

Another possibility:  What if you got across the juice and were slumped unconscious over a HV power supply? Would the XYL know enough not to touch you - and instead turn the breaker off? Does she know that there could still be HV on you or in the area until it decayed down?  How about cardiopulmonary resuscitation once she is sure the HV is bled off and it's safe to touch you? How about paramedics that rush in? Could they unwillingly become part of the electrocution gang-bang or would they just freeze not wanting to touch you?

The bottom line is that we all make mistakes every day and we need to make rules and procedures to eliminate the major ones. Big mistakes are called bad accidents.   After awhile we finally think out the more common errors until we get to the outlier. An outlier error is the weird stuff we call perfect storms that will happen once in a great while. We cannot predict when they will happen, but we can prepare for them.

If we are successful, our obit will eventually say something about dying of "natural causes."  

T