13 – Let’s Compare RF And Glass Tubes

The Tangerine Tiger Series with Russ Sadler

In this Series, Russ has purchased a new 500 x 300mm, 50W laser machine from eBay with a view to modifying and upgrading it. In fact, he rips out the glass laser tube and high voltage power supply and replaces them with an RF laser source and PSU from Cloudray. Let’s Compare RF And Glass Tubes and find out if the performance of the expensive RF Tube is worth it! Prepare to get your hands dirty!

Compare RF And Glass Tubes: Some Calculations
Compare RF And Glass Tubes: Some Calculations

If you are considering purchasing a CO2 laser machine with an RF laser source from one of the big boy suppliers, I would suggest you check out this series before making a decision!

Contents

I was very excited by the prospect of creating a very fast RF laser engraving machine to match the performance of those very expensive commercial machines. A few months into the project I now have a lot of detailed learning about how this different technology works, I discover there is no magic in it after all just lots of smoke and mirrors. This session lays the published facts about glass tube and RF systems side by side so that it is possible to make realistic judgments on the merits of each.

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Transcript for Let’s Compare RF And Glass Tubes

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0:00well it’s a nice sunny day welcome to another session with my tangerine tiger

0:07this is not a tangerine tiger no it’s in the workshop behind you and that’s where it’s going to stay

0:12today because we’re going to have a little bit of a reflection day we’re going to take what i consider to

0:18be an honest look at the difference between a glass tube laser

0:23and an rf laser now i know i haven’t got a great deal of experience with the rf laser so far but i’ve dug in

0:30pretty deeply and i’m pretty sure i’ve come to some pretty strong conclusions already even though we’re probably only

0:3720 20 of the way through our test program not only have i had the opportunity to gain

0:42a great deal of experience with the glass tube laces i’ve also had a great opportunity

0:48afforded to me to play with a fiber lacer

0:53now i’ve always sat back and i wouldn’t say envy because that’s the

0:58wrong word marvelled no that’s still not the right word

1:03wondered probably that’s a better word i’ve wondered why the rf laser has got such a great

1:09reputation why it cost so much money what’s so fantastic and marvelous about it

1:17and then claude ray with great generosity gave me a 20 watt

1:24rf laser to fit to a machine that i bought now that gave me the opportunity to try

1:30and really understand what an rf laser is all about

1:36i’m afraid any magic that i thought was attached to these devices has

1:42disappeared even though i’ve only spent a small amount of time with the rf laser

1:49i’ve done a lot of research and i can already tell you that this project as it stands at the

1:56moment is likely to be a bit of a damp squid it’s going to work don’t get me wrong

2:02we’ve got a machine that will run extremely fast but we’ve got a 20 watt laser that will not keep up with the machine

Transcript for Let’s Compare RF And Glass Tubes (Cont…)

2:10and that’s my problem now i was hoping that i might be able to make this machine comparative

2:16to something like an epilogue zine okay it would never look like an epilogue zing but it might well perform

2:23the same except when i start looking a bit more at the detailed specification the zings

2:30run anything from 40 to maybe 80 watts i think you might even be able to get 100 watts in as well that’s a big

2:37difference from what i’m trying to do with 20 watts now 20 watts is not even enough

2:43to light a bonfire to be honest um i mean a k-40 has got more power

2:48than the 20 watt tube and i suppose you’d use 20 watts for

2:54dermatology you certainly wouldn’t use it for cutting or for

2:59doing any serious work but hey that’s what i’ve got it was gifted

3:05to me and you know what they say about a gift horse i’m going to make the most of it i’m going to see just how fast it will go up

3:13so what i want to do today is to just take you through an honest set of comparisons between

3:20a glass tube laser and an rf laser to try and take away the magic aura

3:27that seems to be hanging around this very expensive technology

3:35so as i always say to you come on in and let’s get started you will still have to suffer some of my kindergarten drawings but

3:42hey i’m interested in getting the principles and the ideas over and not producing a van gogh type

3:47artwork now one of the most important things about the laser is power watts now

3:55i’ve given you this analogy before of a candle flame and how when you put

4:01your hand over a candle flame if you hold it still you know what’s going to happen but if you move across

Transcript for Let’s Compare RF And Glass Tubes (Cont…)

4:07it at a certain speed then yeah it will not hurt let’s try and

4:12turn that analog upside down now and here we’ve got a piece of material and we’ve got a miraculous machine that

4:20i’m going to be using because it burns square dots

4:26okay now i i need you to suspend reality for a few moments and go along with this idea

4:32now very conveniently this happens to be one millimeter by one millimeter yeah we’ve got a

4:38pretty coarse lens in here as well okay now let’s assume through my special

4:43magic lens here i’m able to focus one watt

4:49okay and i keep that one watt on that spot there for one second

4:58okay so we’ve got a one watt for one second energy density on that

5:05square millimeter now i don’t know how you technically express that in terms of units um

5:11but let me now move on and say look if we now instantly move on to another dot

5:18which is here and again we apply one watt for one second we’re going to

5:24do the same amount of burning damage on the second dot

5:29because one watt for one second gives a certain depth or type maybe just a scorch

5:36on the material it doesn’t really matter but it’s going to produce damage of some sort and so

5:43if i carry on like this and we produce instantly jumps

5:50of one millimeter we should produce eventually a line like that all the same density of

5:57depth of cut or color [Music] now that’s because we’ve used one watt

Transcript for Let’s Compare RF And Glass Tubes (Cont…)

6:04for one second now this would work we do not have to move instantly from unit to unit because we

6:11could move smoothly along there at one millimeter per second

6:18which is effectively what we’ve got every second we’re going to move along here one millimeter so that’s effectively the

6:25uniform speed that we’ve just used so at one millimeter per second with one

6:32watt we should produce this amount of damage okay now i’m building this up gradually

6:40so what happens if i go to two millimeters a second

6:49remember the candle flame we’re now moving twice as fast how much damage will i do

6:57well the answer is obviously half as much damage because i’m spreading the same one watt over

7:04the same distance times two in the same period of time so

7:12i’m now moving two millimeters a second so i’m applying half as much energy to each one of these little

7:17blocks here so therefore i’m going to do half the amount of damage

7:23now it is a long-winded way of getting around to showing you that look to produce the

7:29same amount of damage on that line we need to move two watts

7:34of energy at two millimeters a second

7:39or four watts at four millimeters a second if i increase the speed

7:46to keep the same amount of damage into the material i’ve got to increase the watts as well

7:54and there is my problem with this 20 watt machine

8:0220 watts is 20 watts whatever sort of laser you use to

Transcript for Let’s Compare RF And Glass Tubes (Cont…)

8:08generate 20 watts that’s it so

8:14slightly different with the 20 watt fiber laser because the fiber laser was not only was

8:21it 20 watts if you like uniform power where that represents 20 watts

8:27this was able to produce spikes of energy very short spikes of energy which were up in the

8:34kilowatts region because it was storing energy between the pulses

8:41and building it up the rf laser and the glass tube laser

8:50do not do that they have got 20 watts continuous energy

8:57and with the rf laser you switch on and off with a glass tube you keep it on all the time and you

9:03switch it on and off so there are differences between these two

9:08but the principle is still the same we do not have any magic amplification

9:13with either of these and that was my problem i thought there

9:20was some sort of magic associated with an rf laser the only difference between these two

9:27laser sources let’s call them that is the manner in which the

9:33laser beam is produced now i have no intention of going inside these boxes so here we’ve got

9:41an rf laser and here we’ve got a glass tube laser

9:50now the one thing we do know is that what comes out of there is 20 watts

9:59and what comes out of here is 20 watts

Transcript for Let’s Compare RF And Glass Tubes (Cont…)

10:07but what goes in to make that 20 watts well in this instance we’re putting in

10:14rf which means radio frequency we’re feeding into this unit 48 volt dc to create a very very high

10:22frequency 100 megahertz signal within here which is vibrating

10:29the gas molecules and making them effectively become lightning just like

10:36we see in our glass tube but we can’t see what’s happening in here because this is a solid block of metal

10:41very crudely in here we’ve got two plates with this 100 megahertz zipping between them and that causes the

10:49breakdown of the gas and it ionizes within that section there okay

10:56now the ionized gas again what we’ve got is two mirrors very crudely we’ve got two mirrors and

11:01exactly the same system operates here as it does with our glass tube

11:07okay we’ve got two mirrors in our glass tube we’ve got two electrodes here in our glass tube

11:13where we feed 25 000 volts dc

11:220 volts and we’ve got gas in between here which and it’s in a tube

11:30this is not a tube this is two plates whereas this is a tube and the light reflects off these mirrors

11:38and amplifies and comes out the end the pink beam that you see in here is ionized

11:43nitrogen that ionized nitrogen is used to bump into is a very crude analogy

11:51the co2 molecules and change their energy temporarily raise their energy and as the energy drops down

11:58that’s how we create our laser beam now exactly the same thing that’s happening within here now this 48 volt

12:05dc probably is using something like about 12 ounce that’s the power that we’re

12:12feeding into this device here we’re feeding 25 000 volts dc and we’ve got a current

12:19flow through here which is somewhere in the region of about say for 20 watts it’s ridiculously small

12:28at something like about 14 milliamps

12:33now the reason i’m pointing this out to you is what happens inside these black boxes is

12:40relatively unimportant what is more important is that when we look at the input

12:45we find that if i do a quick calculation there that’s 576

12:53watts of energy 48 times 12 defaults dc times

13:00current as watts that equals 576 watts that we’re putting in and what are

13:08we getting out 20.

13:14that’s interesting isn’t it what are we putting in here well to be honest although we put 25 000

13:22volts dc in once the tube has what they call struck or ignited

13:28we only need probably something like about 9 000 volts to sustain

13:35the beam so in this particular instance what we’ve got we’ve got 9000 volts

13:43times point zero four amps

13:54volts dc times amps and what does that give us

14:00126 watts so immediately you can see that

Transcript for Let’s Compare RF And Glass Tubes (Cont…)

14:08the power requirement of this device is a lot less than this device this is a

14:14very power hungry device now not only is it power hungry and relatively inefficient as you can

14:20see we’re getting a little out for a lot in we’re getting a much better return

14:25on our investment with a glass tube when you’re putting all these watts in we get a huge amount of

14:36i really ought to draw it like this don’t i waste heat

14:42now for 20 watts on the device that i’ve got in there we have about 570

14:50watts of waste heat well look that’s more or less all of what we’re putting

14:55in i mean the numbers don’t compete exactly and i’m sure if i did it very exactly and they would balance but very crudely

15:04we’ve got something here which shows you clearly that this is throwing a lot of this energy away as

15:09heat which is why the fans are on there and why they they cause so much noise now you can

15:16have it water cooled but it doesn’t change this fact you’ve still got to drag away 570 watts and you’ve also then got to

15:23have a cooler or chiller that’s capable of cooling 570 watts so this starts to get quite expensive in

15:30terms of power i don’t have a 20 watt glass tube

15:35i have a 70 watt glass tube here we’ve got a chart so across the

15:42bottom we’ve got tube wattages so i’ve been able to check for rf tubes

15:49wattages right up to about 400 watts and at 400 watts this

15:55chart down the side here tells me that i’ve got 400 times 15.

Transcript for Let’s Compare RF And Glass Tubes (Cont…)

16:02well 400 times 15 is 4 000 so that that is effectively 6 000

16:10watts of heat energy that i’m going to output so

16:17for 400 watts i’m going to create six kilowatts of heat energy

16:24that’s stupidly inefficient okay so that’s a factor of 15. but look as we

16:30start going down lower and lower for an rf tube the fact it gets even worse so let’s take a look here at

16:38something like about 10 watts at 10 watts we’ve got a factor of

16:43370 something like that so for a 10 watt output we’ve got 370

16:49watts of waste energy so my 70 watts if it was an rf tube

16:57would have something like about 1.2 kilowatts of heat now that’s that’s a small

17:03electric kettle now 10 liters of fluid in my reservoir

17:12would be up to boiling probably within about 10 minutes

17:18i was to put 1.2 kilowatts of heat into it i never get anywhere near those

17:23temperatures i mean i get maybe a one or two degree c

17:29temperature rise depends what the ambient is because we’re getting obviously lost through the side of the tank

17:34but but the point i’m really making here is we have minuscule amounts of heat generation

17:40from this system in relation to what we see here well let’s let’s just keep the mass

17:46simple shall we let’s just say we’ve got 20 watts in and therefore we’ve got 100 watts out

17:51well 100 watts is not going to cause a great deal of heating in 10 10 liters of water

17:57okay i’m going to finish up with 570 watts over half a kilowatt of heating that is

Transcript for Let’s Compare RF And Glass Tubes (Cont…)

18:02going to have a big effect on a bulk of 10 liters of water so the point is this is a very

18:10inefficient system as far as power is concerned and energy conversion okay so that’s

18:17a big negative as far as it’s concerned but nobody ever worries about that because you just buy one of these machines you plug it in

18:24and you use it just to keep things nice and simple we’ll keep rf on the left hand side of

18:29the page and we’ll keep glass on the right hand side

18:36i’ve got a whole set of data sheets here from a company called syntec optronics

18:42they’re in singapore and they make they make huge numbers of all sorts of

18:48laser systems from uv laces right the way up to ir lasers like

18:54this and everything in between fiber optics you name it they make it and the great thing about

19:01them is they’re very honest with their data sheets so this is a great source

19:07to do comparisons when i look at other famous products like roofing and maybe coherent and other things

19:14you will find exactly the same sorts of performance specifications

19:20it’s a feature of the physics not of the manufacturer okay

19:26now the one thing that’s common throughout all of these rf laces is this factor here

19:34which is the mode quality now what is mode quant the energy of the light intensity within

19:41our beam is something called a gaussian distribution and here we’ve got a gaussian

19:48distribution and here we’ve got another gaussian distribution and another one and another one these are all gaussian distributions

19:56the only difference is if we look carefully we shall see that if you squash the base up you get a tall

Transcript for Let’s Compare RF And Glass Tubes (Cont…)

20:04thin distribution and if you start stretching the base out like this you get a short fat like me oh no sorry

20:11i’m tall and fat um distribution this thing here called mode quality

20:20is how close to a gaussian distribution the

20:28light intensity is within the beam this is the perfect distribution and what they’re saying is

20:34well you know our distribution is pretty good look it’s it’s less than 1.2 now m

20:42squared equals one means it’s a perfect match for a gaussian distribution

20:49so at 1.2 it’s not a perfect match for a gaussian distribution

20:55whereas if we get a tall gaussian distribution like this and it has the top knocked off like that that is not a

21:01gaussian distribution so that’s you know it may well be that the problem is there

21:06or it may well be that the problem is down here somewhere like this you know either of those factors would

21:12make this a non-conforming gaussian distribution

21:17so the quality of the b is defined by this m squared number and the m squared

21:24number for all these rf laces is 1.2 now wherever you look

21:32on these spec sheets 1.2

21:371.2 1.2 add that to our list on the left hand side here m squared

21:44equals 1.2 our glass m squared equals

21:55beam quality is less than or equal to 1.1 but typically if you look through

Transcript for Let’s Compare RF And Glass Tubes (Cont…)

22:04any specification for a glass tube you’ll find that the m squared value

22:09equals 1.1 so glass tubes if you want to use the

22:15word quality have got a much higher quality of beam than an rf

22:22laser now that’s slightly contrary to the words that i’ve heard mentioned exhibitions by the sales guys for these

22:31machines who dramatically berate the chinese glass tube machines

22:40and greatly enhance the performance of their own rf tubes

22:47the facts don’t bear that claim out now the second fact that needs to be

22:55talked about something called beam divergence angle

23:01now beam divergence angle is exactly what it says but it’s expressed in a funny unit called milli radiance now

23:09one milli radian mrad basically what that means is the beam

23:18is going to expand from a starting point here over one meter a thousand millimeters

23:29it’s going to grow by one millimeter for a glass tube

23:36somewhere in the region of approximately three milli red beam divergence

23:44okay so what do we see for a rf tube 30 watts

23:52here we’ve got it seven seven

23:58[Music] 40 watts seven seven

Transcript for Let’s Compare RF And Glass Tubes (Cont…)

24:09sixty watts and eighty watts seven seven seven so the rf b is not as

24:16compact it grows rapidly at least twice as fast as a glass tube laser i start off with a

24:24beam diameter of two millimeters diameter and i finish up with a beam diameter of

24:29nine millimeters diameter we don’t modify the beam we just fire our beam at mirror three

24:37with this you cannot fire the beam at mirror three you have to control the beam and in

24:43doing that what i’ve had to do i’ve had to increase the beam from two

24:49up to six millimeter diameter i actually put a

24:55times two beam expander in there which technically should have taken me from two to four

25:00millimeters didn’t work that way quite but hey i’m happy with the six millimeter beam

25:06that’s parallel so it hits mirror three at six millimeters diameter whereas here when i hit mirror three

25:12i start off at about four millimeters diameter

25:18and i grow over the same distance one millimeter and i’m gonna hit my beam i’m gonna hit

25:24mirror three with a slightly expanding five millimeter

25:29maybe even six millimeter b so i’ve got roughly

25:36the same amount of beam diameter hitting my mirrors the only difference is

25:43that this is a an element in the path which is going to consume something like

25:48about eight percent of my energy this does bring us on to another point as well

25:55and that is this look we started off with a two millimetre beam

Transcript for Let’s Compare RF And Glass Tubes (Cont…)

26:03and okay so look it wasn’t perfect because it was an m 1.2

26:09maybe looking like this with a blunt a blunt point but because we’ve had to expand it we’ve

26:16had to take it from two millimeters here out to maybe six millimeters

26:23here now in taking it out to six millimeters

26:30look what we’ve done we’ve not changed the shape of the beam the beam has remained gaussian to the

26:37extent of 1.2 not gaussian but we’ve changed it from something that

26:44looks like this nice and sharp and pointy to something that looks like this

26:49which is a bit like a blunt pencil before i even get to mirror three

26:55i’ve degraded the beam that i’m going to be using for working through my lens so i’ve got

27:02a much better quality beam here from a glass tube by the time i get to my lens

27:08that i have here now the chances are i’m not going to be able to do as much damage because i’ve got a much softer

27:16beam now there’s an argument here which which goes something like this if you use a beam

27:22expander then you get a better focused point that’s what lens theory says and i agree entirely

27:28with what len’s theory says how do we get a focus point let’s start the other side here we’ve

27:35got this beam here it passes through this material and it gets changed its direction through refraction

27:42and it comes out the other side here and does this and this one here does the same

27:49it comes out here and does that and look focus

27:57and that’s the whole point of a lens it can it re-guides the beams from parallel down

Transcript for Let’s Compare RF And Glass Tubes (Cont…)

28:03to a small point but hey that isn’t the end of the story but what about these beams here they

28:09come in like this and very crudely i’m going to just draw this

28:14slightly wrong and exaggerated but they do not come down

28:21to the same focus point we’ve got another focus point here

28:26and those rays so all the rays passing through this lens

28:32focus at different points now this ray that’s passing down the

28:39center the magic ray as i call it it’s a ray on its own

28:45how can it focus if it’s on its own you need another ray coming in from another

28:51direction to focus so that ray carries on

28:57to infinity theoretically and never focuses okay let’s just

29:04imagine what happens to these rays that are right right close to this one

29:11now you know what happens to two parallel lines they run parallel and if you change them very very

29:16slightly so they’re not parallel hang on that means that that’s going to focus some way

29:23out distant point what i’m really getting to here is there is a not a blind spot but almost a

29:31hole where the rays are out of control they’re not really being focused

29:38properly onto what you would expect to be the focus point it’s a we’ve got i like to call these rogue rays

29:44i’m not i’m not an optical engineer i’m just looking at this purely as a logical set of conditions we’ve seen this effect

29:52with some of the cutting experiments that i’ve done you know and i can show you that there is power

29:57way beyond the focal point let’s come back to this picture

Transcript for Let’s Compare RF And Glass Tubes (Cont…)

30:03if we stretch our beam from two millimeters to eight or ten millimeters diameter sd

30:10you damage with light intensity look we’ve lost all the light intensity at the

30:15center and we’ve much more turn this into a uniform

30:21light pattern where the effect of these rogue rays has basically been

30:27minimized and what we’ve done we’ve actually corralled these rogue rays that are near to the center and we’ve now

30:33made them come back much closer to the correct focal point so yes

30:39increasing the diameter of the beam with a beam expander will allow you to focus the rays

30:47into a specific point much better that’s fine for engraving

30:54but it doesn’t help your cutting you don’t want the rays to be focused onto a specific point

30:59you need these rogue rays to get good cut basically concludes a bit of a comparison between the two

31:07systems the two laser source systems so on balance you would never buy

31:15an rf laser it’s inefficient it doesn’t do much for the beam

31:23and the other thing is if i want to buy 100 watt

31:30rf what’s it going to cost me

31:36well you’ll find it very difficult to find out but it’s in the region of

31:43probably 15 000 to 20 000

31:55if i want to buy a 100 watt

32:00glass tube what’s it gonna cost me let’s exaggerate and say something like

Transcript for Let’s Compare RF And Glass Tubes (Cont…)

32:08about 500. if you want to waste your money on a recce then yeah you’ll pay a thousand dollars or more

32:16all right but discount different people claim different numbers

32:21for their systems five to ten times the lifetime of a glass two firms they will assume that

32:28this takes probably about two years and its life is gone whereas in fact from all practical experience of not

32:34only myself but other many people around the world that have got tubes at five six seven years old

32:40we know that glass tubes do not expire in two years i do know people that say that these

32:46only last about five years but let’s just give it the benefit of that and say 10 years

32:51if i have to replace the tube twice in the lifetime of the machine 20 years

32:58what’s it cost me it’s cost me 40k

33:04because it’s cheaper to replace than it is the regas and i hate the idea of regassing because you don’t get a chance to

33:10clean the cathodes which is where all the oxidation takes place okay so this is four four of these

33:17four years four times five years so

33:23so it’s going to cost us 2 000

33:29to replace the tube four times in 20 years

33:34now it doesn’t require a genius to say why would you spend your

33:41money on an rf laser tube that costs the earth does not perform as well as a glass tube

33:51i mean i’m not an advocate for either of these two systems i’m just putting the facts in front of you i

33:57really wanted this to succeed and be a fast really good machine

Transcript for Let’s Compare RF And Glass Tubes (Cont…)

34:03that would run rings around the glass tube machine i didn’t think it would ever be in terms of cost

34:09how do you control the power that you get out of these machines well with the rf they always use something

34:16called pwmc it’s a square wave like that which is normally at the uh transistor

34:23level five volt d and then along here we’ve

34:29got hertz frequency this frequency here is a roundabout maximum of 20 sometimes 25 kilohertz

34:40okay now that’s got nothing to do with the 100 megahertz that’s used

34:49to generate the arc that’s within the tube these are two completely separate

34:54frequencies that do not get involved with each other this one has a control of the output

35:00signal and this one is for generating the laser itself

35:05as we’ve already discussed in earlier sessions when i’ve talked about this control system the five volt dc is only

35:12switching on fully switching on the power that’s coming out of the laser so if you’ve got

35:18a 60 watt laser you will be switching on 60 watts

35:26it can’t do anything else it either turns the laser on or off the idea of this pwm signal

35:33is pretty simple if you allow the signal to be zero

35:40five volts like that in other words what you do you close down these gaps here so that

35:47the signal is on all the time then you have something called cw continuous wave you don’t have a chopped

35:54up 60 watt power you have 60 watts of continuous output power and that’s what you need for cutting you

36:00don’t want to compromise your power when you’re cutting you need it all now 100 power means we’re going to switch it on

36:05and it’s going to stay on all the time like that and if you want 50 power this is the

36:11frequency from here to here this is one cycle what you do you put a certain proportion

36:18of the cycle 50 percent of the cycle on and 50 of the cycle off

36:25so it’s delivering 60 watts for that amount of time and delivering zero watts for that

36:31amount of time the argument goes that on average what you’ve done you’ve decreased the power

36:37to 30 watts we’re not talking about doing average damage we do

36:42real damage over a period of time so there’s a very complex relationship

36:47between frequency power and what they call duty cycle

36:54in other words the amount of time you spend on and the amount of time you spend off

36:59now i’ve already demonstrated this to you the way in which i’ve been able to

37:05manipulate and play with these numbers to get let’s call them dots but they’re

37:10not really dots because this machine cannot produce photographic dots it can only produce

37:16lines this is a very complex way of controlling the power for

37:24engraving if you look at the reader handbook for the controller they recommend that

37:30for glass tubes you use something called analog output which basically means something very simple like zero

37:36and five volts there’s a scale which a signal which comes out which is

37:42anything between these two voltages here and whatever that number is is fed into

37:48the hv power supply and the hv power supply then supplies

37:56a percentage of its maximum output so say this 8 volt power supply could supply 30 milliamps

38:05zero volts equals zero milliamps

Transcript for Let’s Compare RF And Glass Tubes (Cont…)

38:12and five volts equals thirty milliamps and everything in

38:19between everything in between is proportional so if you want 50 the output of the power

38:26supply you call up for 50 okay of that so two and a half volts and that’s the

38:33way that this works very simply when you want to change power you change power by just calling up your

38:39percent power and you’re not fiddling with this you’re purely adjusting the current that

38:46flows through the tube directly here you’ve got simply power and speed now that brings us round

38:55full circle to where we started from power for a given period of time with this system

39:01it’s intuitive so pwm is outside many people’s sphere of

39:08understanding or knowledge the only problem with the glass tube is the linear relationship is between the

39:14percent power and the current flowing through the tube and there isn’t a relationship between

39:20the current flowing through the tube and the watts that come out of the tube so at the end of the day there is not a

39:26linear relationship between here and the watts

39:33so speed is a much more logical way to control the power with the glass tube i understand how both these systems work

39:38now and i can see clearly that it would be possible to run a glass tube with an rf

39:47control system now bear in mind that rf is the mechanism by which the laser beam

39:52is being produced not the mechanism by which it’s being controlled so there is no logical reason why i

39:59couldn’t use this complex system for running a glass tube now the great advantage

Transcript for Let’s Compare RF And Glass Tubes (Cont…)

40:08of this and its only great advantage is its speed of switching

40:15the speed of switching is all to do with the way in which any gas discharge tube works before you

40:22get power out of a gas discharge system you have a zone let’s call that power

40:31watts if you like there is a zone in here where

40:40things are very uncertain and it takes time for the beam to establish a constant

40:46output power okay now there’s the constant output power let’s call it 60 watts

40:56okay now this picture here applies to both glass and rf laser tubes now you can see i put

41:04this strange block here and that’s because this is something called pre

41:12ionization

41:19it’s a zone that exists for all gas discharge systems regardless of whether it’s rf or

41:27constant current glass tube and during that period of time

41:35there is huge uncertainty about exactly what’s going on inside that tube and i don’t mean

41:41uncertainty from the outside because we know that there’s things going on but they’re not under control

41:47you get incredibly high spikes of very very high frequency energy that

41:53can be taking place during that period of time okay before it settles down to a nice

41:59steady controllable output so it’s a region of uncertainty

Transcript for Let’s Compare RF And Glass Tubes (Cont…)

42:06and on the glass tubes you can exploit that because it is very high frequency very

42:12high pulses it’s superb for doing glass engraving for cutting card without scorching it’s got all

42:19sorts of great uses but the problem is

42:26if you switch your signals i want to switch on the laser tube now

42:32what has to happen is this is the this is the if you like the switch on signal from the controller

42:39that signal then has to go through the power supply and the hv power supply has to generate

42:44some high voltage to make the tube switch on and the switch on

42:49time is such that it could be

42:54it does something like this before it reaches 61. a normal glass 2 power supply claims that you can

43:01get to 90 of your demand value in less than

43:08one millisecond now one millisecond is like a year in electronics time so it takes a finite

43:16amount of time for the power to appear when you’re trying to switch on an hv power supply

43:25with this system here with an rf system what you do you have this

43:32switch on period nullified because you hold the beam with a special thing called a tickle

43:38current you hold it here so it’s just hovering there all the time not it’s not doing it’s not got enough power to do anything

43:45but it’s sitting there so at the moment you switch on the moment you say i want to switch the power on it

43:52instantly switches on and you’ve got 60 watts there so that’s the big advantage of an rf

44:00tube you lose the pre-ionization zone because the pre-ionisation zone is

44:06nullified by what they call a pre-ignition signal is sitting there waiting to switch on

44:11the instant that you ask for the power to be available but from what everything that i’ve gathered

44:17that’s really the only positive advantage for an rf system it’s switching speed

44:26but now i’m going to surprise you and say because i understand this

44:31i could make a glass tube system do exactly the same thing

44:40that’s for another day i think we’ll stop at that point and say yes we’ve confirmed several

44:48things first of all i haven’t got enough power to do much damage with this machine at 20 watts

44:54and secondly i’m very disappointed that there is no magic in the rf system after all i’ve shown you both sides of

45:01the argument you can work out for yourself whether either side of the argument is right or wrong

45:06and you can wire up the pluses and the minus for yourself i’m afraid the whole thing on this side

45:12appears to be minus except for this final point here which is response speed well if i can do it for

45:20here i can probably do it for here as well so the only message i can really finish up with is maybe you’re not going

45:28to rush out after all and buy an rf machine so i’ll leave that with you thank you very much and i’ll catch

45:33up with you in the next session

Transcript for Let’s Compare RF And Glass Tubes

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