High voltage meter

Have planned a high voltage (hv) meter addition to my laser for almost 8 months. The initial problem was procuring the resistors that I needed. The first and second orders never materialized. I needed 600 Mohms of resistance to produce a 50ua (full scale 50ua meter) deflection at 30kv.

It’s quite simply a resistor and a meter to ground. There is a neon across the output to ground in case of a loose or open connection.

The hv resistors were not the issue, it was more of ‘my safety’ that was on my mind. Working with hv you learn respect for it, if you don’t start with it.

3/4" acrylic tubing, 1/8" walls gives me a 1/2" hole for the resistors to reside.

With a dielectric strength of between 17.7 - 60 kv/mm. 1/8" is 3.17 mm, conservatively 3mm x 17 kv/mm = 51 kv. About 50kv protection in a 30kv circuit.

I purchased 300 Mohm resistors.

Soldered them end to end…

Check fit tube

The resistors were then potted with epoxy resin which is known for dielectric strength, but I could not really find significant actual data. Watching a crazy guy on the internet who built some high voltage multipliers (at or near the 30kv area) just used regular hobby epoxy and has them in his hands. So I used what he used.

Here is the ‘hot’ end.

This is where the meter connects. The ‘top’ one in the photo is the required ground. The neon is only a safety precaution. If the load is removed (like the meter ‘opens’) you could have full hv on the meter. The reasoning is that over about 90v the neon will conduct.

I would like to mention that the identical connectors were on my 60 watt supply that I purchased from Amazon. Found out differently when I went to install. The inside male/female were of different physical sizes and would not mate. Fortunately l had purchased them as a ‘pair’ so I changed out the lps set, so I can remove the meter and reconnect the lps back to original.

I know you can’t see the difference in size, but trust me, it didn’t work… :frowning:

It’s ‘stuck’ in the machine with the 30lb 3m double sided tape. I will make a mount for it.

This is a video (albeit poor) shows it operating. It’s cutting out a meter drill hole template.

Still a working project, but there was some interest. I did have another 30ma meter I stole the face off it, but didn’t put it on this 50ua. Now that I’ve moved I can’t find it, so I have one on order. Just need to figure out how to change the lettering on the bottom from ma to kv…

Console now looks like

Meter is marked (with pencil) so I have some idea…

:smiley_cat:

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Pretty cool project but wouldn’t there be a lower voltage in the LPS you could get to and measure which would represent what the high voltage was? Front end of the flyback xformer or something like that.

Probably, but it still wouldn’t tell me what’s being applied to the tube.

Hasn’t generated much interest. Thanks for checking it out.

:smiley_cat:

What does knowing the voltage on the tube get you?

I’ve seen it stated that knowing the current provides feedback with regards to the wearing of the tube.
Example, if you once cut through 3mm ply at 20mm/s 80% power with max current set to 18mA but later you see the same settings for 3mm ply not cutting and see your current has dropped below 18mA then you are experiencing tube degradation and only so much adjusting will get you back to 18mA max and eventually a full 5V on the LPS IN line won’t get you 18mA.

What does an oil pressure gauge give you over a ‘oil light’. You can see an increase/drop before it’ a problem.

If, in your example the lps was having problems making voltage then you would also see a drop in current. Hopefully you would notice it before it became apparent and could confirm via the ma meter. The assumption the tube is bad is based on a good lps.

One of the most common questions here, tube or supply… I’m hoping this will clue me into abnormal operations.

I purchased this to learn on and it’s working quite well for that. Not to mention there seems little on hv and how to measure or apply it to the hobby world.

I’ve found when I use something I get used to the ‘indicators’ that show how the machine is running in it’s normal mode. It’s when you see changes that you can start wondering about the issue could be and not just quit.

Just another instrument that tells me how the machine is running, or at least a major part of the process.

Only time will tell how useful it really turns out to be.

:smiley_cat:

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I think that it might be useful to tell if the LPS’s output = 0 but other than that I doubt its value.

It creeps me out that there is any kind of connection from the back of a control panel meter to the 20000V.

The tube is a negative resistance device. The important operational consideration is that the supply can reach the onset potential and then provide enough current during discharge.
A tubes health is characterized by its Volt/Ampere curve. This is why I suggest that when you get a new machine run a Current vs IN(voltage) curve. Over time you can see how that changes and determine the rate of deterioration of the tube.

Of the hundreds of these K40’s I have troubleshot,
Tube-LPS failures usually are:

  1. Tube dead: no or very low current, violent arching in the laser cabinet.
  2. LPS supply dead: no current, arching in the LPS’s HVT, hissing, erratic current
  3. 1&2

Learning is a good thing :). There is plenty of HV hobby information in the Tesla coil realm.

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I’ve watched them on the scope, there is no way you going to get anything other than the rms value, No different than the ma meter. It gives you rms. The tube is only lasing when the pwm is high, otherwise it is off. The illusion of power control comes from pwm, the only control you really have is speed.

It might turn out to be a bust, but I’ve wondered about it since before I got the machine.

See a couple of the videos of the K40 arching from the anode to the case. I’m sure it’s not limited to them.

I ‘repaired’ a transmitter at the police station, back in the 70’s, when I was in school. Radio tech at the commercial radio shop. Mostly police types and the local CB’ers.

I watched the lighting strike the tower and have never seen one last so long.

The power supply box was full of tiny burnt metal parts and the cabinet coated with soot and metallic coat. This was a tube transmitter so it had a hefty power supply. There were not any plates of the transformer left or any other parts in the supply.

The radio worked fine when the power supply was replaced, about 70 pounds of parts. You just never know where it will go. Maybe it will just blast the neon and provide no protection at all, but I’ve seen this in production equipment, back a few decades… It’s a pretty simple electrical circuit, even with a neon failure it will have a ‘spark gap’ to discharge (I hope…)

I’ve tried to take precautions, but only time will tell.

I’ve hung my scope on the meter, but it’s such a low voltage and noisy, it’s difficult to read it on the scope. Working on that when time allows.

Any clue as to what kind of voltage they are producing when the tube fails and it arcs? Takes substantial voltage to make the distance from the anode to the case. Seems like 30kv wouldn’t make the distance.

In your number 2 scenarios where is the ‘hissing’ originating? From the lps?

I appreciate the feedback.

:smiley_cat:

IKN, but because the supply is unloaded its arc is pretty violent.

It does, in fact when they post a picture of an arc its a pretty reliable way of knowing a tube is bad.

Yes, my research revealed that the HV diodes go bad and/or the windings short and arc. Of course once this starts it all over as the wire in the HVT is very fine.

There is a lot more information about this LPS on my blog.

After a couple of years working with the LPS it became clear to me that:

  • Component failures are usually catastrophic killing most of the onboard power devices
  • Most of the circuitry sits at +600V. Dangerous to your test equipment and yourself to troubleshoot.
  • A new supply is cheaper than repair
  • The supply is intended to be a consumable. Its marginal output design and the nature of HV makes it unreliable.
  • “don’t touch this”

The K40 has two dangerous subsystems; the LPS and the laser. This danger is exacerbated by the silent nature of their operation. When operating normally they quietly do their thing not garnering caution from the unsuspecting user that gets their hands or face into the wrong place.

An interesting place to put a scope is across the ma meter as it shows the tube current. I never spent much time trying to make sense of this waveform as it needs a useful sync from the driving program.

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Trying to hit the sack, but I keep forgetting to ask you

Have you’ve ever seen one with the cathode disconnected?

That would happen if the ma meter opened up.

Wouldn’t that ‘emulate’ a bad tube also? Back to the unloaded lps?

Understand about the fine wire.

Built a Tesla coil in the 5th grade with a ‘used’ commercial AM transmitter tube for the oscillator. Had to build the 800 volt plate power supply for it.

My dad helped me wind the secondary coil by making a jig for it to rotate. It was almost 3’ long, lots of winding.

I have a plugged connection wire to the PWM output, just for the scope trigger… :slight_smile:

Thanks for your time… Take care

:smiley_cat:

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I do not recall one with missing ground per se but this is one thing I have them check when I see no current in the meter.
Yes I would expect it to act as an unloaded or dead tube but moving the ground wire to the cathode side of the meter is an easy way to verify that its grounded through the meter.


I always wanted to design a repeating test pattern of vertical bars and sync on a known point in that pattern to watch the tube current across a single horizontal scan. I theorized (and the math suggested) that a CO2 tube cannot switch at the speeds we are expecting. Seeing the current clearly would prove/disprove that.


Be careful when reminiscing about building a tesla coil as a kid. It’s like comparing a garter snake to a black mamba, after all they are both snakes. This supply has a silent energy like no other I have seen in a hobby environment. Its current capability is in excess of 30 ma. I know this because I measured it. Its current while sustaining a 2 inch arc easily pegged my meter.


I built a HV lab to test these things that had all the safety precautions and after multiple surprise arcs (no shock) I decided it was just to dangerous to mess around with. I knew that sooner or later I would get lazy and then I would hurt myself.

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