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The Concise RDWorks Learning Lab with Russ Sadler, Session 5: The High Voltage Power Supply.
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Now, here’s our simplified diagram of the tube and the high voltage power supply.
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Now, in the previous session, I did indicate to you that the high voltage power supply has got two functions.
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One, it has to supply high voltage here, 25000 volts ish.
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It varies depending on the tube length and various other things to get the nitrogen in the gas mix to ionize.
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Once the gas is ionized, it changes away from being a non conductive material to being a highly conductive material, just like a piece of wire.
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And so now we have got a circuit which can carry huge amounts of current.
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But this is where the second function of the power supply comes in.
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Somewhere in here we’ve got something that is a bit like a variable resistor that can be
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programed by the controller to allow different amounts of current to flow in this circuit.
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Now, as I indicated in the previous session, if you’ve bought a machine from eBay, a supposed 50 watt machine,
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it’s an almost certainty that you have got an 800 millimeter long,
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40 watt tube. And it will be supplied with one of these pretty cheap, hmmm it’s a sort of a universal power supply,
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which can be used in the analog controller machines like the K40.
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It can also be used with a digital control system. Now, here is one of those power supplies that I took out of another machine.
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And in this instance, it says it’s 50 watt. Well, maybe it is, maybe it’s not.
Transcript for High Voltage Laser Power Supply (Cont…)
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But it’s still one of these universal power supplies which has got these connections here for like a K40 machine,
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which doesn’t have a digital controller. These connections here will allow you to use a digital controller.
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And here we’ve got the mains input connections. So these are basically mains voltage – dangerous.
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And these here are all about five volts DC. So they’re not dangerous at all.
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So if you’ve got a different set of connections to this and any replacement power supply will have,
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you will have to look up your power supply and it will give you a wiring specification, which I’ll talk about in a minute.
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Now, inside the power supply, we can see here this white thing with this red cable coming out. The red cable is
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the clue. This is the high voltage generator, it’s called a flyback transformer.
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Some high voltage power supplies have one, because for a low wattage tube, you don’t need too much voltage, it may well only require 18 or 20000 volts.
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Whereas when you get a slightly bigger tube, maybe a 60, 70, 80 or 100, watt tube or even more,
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it might have two or even three of these in it, to boost the voltage up to the correct voltage. Basically,
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this is a fairly crude mechanism an inductor type system, a transformer which allows you to put low voltage in and get high voltage out.
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But it takes time for this to happen. This process here is not an instant process.
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If we take a look through there, we can see all sorts of electronic components.
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Electronic components work at microseconds or even nanosecond times.
Transcript for High Voltage Laser Power Supply (Cont…)
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This is a thousand times slower at a millisecond.
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So we’ve got two conflicting things happening inside here.
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We’ve got rapid electronic switching and pretty slow high voltage output.
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Some devices will tell you what the maximum output current of the device is.
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This one doesn’t, so you’ll have to go onto a website and hunt for that information.
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Here we’ve got Maximum output current, 20 milliamps D.C., so 20 milliamps,
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if you look back at my chart previously, you’ll find that that will run a 50 watt tube.
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So the maximum output voltage of this is 23 kilovolts.
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What that means is you need to make sure that your tube has got a switching triggering voltage less than this because you need to trigger it at,
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say, 20kV. And this will go up to 23 K.V. kilovolts.
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So here’s something called response speed. And basically what this means is if you’ve asked for your program to deliver, say, 60 percent power,
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it will get to 90 percent of 60 percent power, 54 percent power in less than or equal to one millisecond.
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You might think that ah a thousandths of a second is quite fast. We’ll put that thousandth s of a second into some sort of perspective in a minute.
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But before we do that, let me just try and explain to you how this power supply operates.
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Now, one of the main misunderstandings that I’ve experienced with people coming to this new, is that you’ve got a 50
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watt tube and you’ve got a controller and the controller shows you anything between zero and 100 percent power.
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So if I’ve got a 50 watt tube,
Transcript for High Voltage Laser Power Supply (Cont…)
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I would expect to get 50 watts out at 100 percent power and I would expect to get 25 watts out at 50 percent power – wrong!
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That’s not the way it works.
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There is no direct relationship between percent power from the controller or your program and the watts coming out of your tube.
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The controller turns percent power into a signal.
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Zero volts, zero percent power, 100 percent power, five volts D.C. down this line here.
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So 50 percent power is 2.5 volt, D.C., It’s a completely linear relationship between voltage and percent power.
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High voltage power supply has got a specification of maximum output current.
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And the maximum output current occurs at 5V DC signal in. So what we get out of this, in other words, the current that’s flowing through the tube.
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Zero votes in will give me zero milliamps out.
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2.5 volts in here, will give me eleven milliamps through the system.
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And five volts will give me full 100 percent, 22 milliamps, maximum output from the high voltage power unit.
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So we’ve got a direct linear relationship between current flow and percent power.
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They’re one and the the same thing, 100 percent power. 22 milliamps.
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50 percent power. 11 milliamps.
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So it’s the percentage of the maximum output of your high voltage power supply that you’re dealing with when you specify percent power.
Transcript for High Voltage Laser Power Supply (Cont…)
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It’s not percent watts is percent current.
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So here’s what reality looks like.
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You put in zero to 100 percent power and you generate anything from zero to 22 milliamps of current flow through the tube.
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If I plot the power output against the percent power that I’m putting in, I’m going to get a linear relationship for the current through the tube,
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but I’m not going to get a linear relationship for the power out of the tube.
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So, for example, here we’ve got 50 watts maximum output for the tube.
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So if I look here. That’s 25 watts, if I look at 50 percent power, which is somewhere about here.
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That doesn’t match up with 25 watts. The watts are non-linear in relation to the percent power.
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So that’s why you cannot use the percent power to control your watts.
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You have to have a calibration chart of some sort to know what watts are related to percent power.
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One thing I want to mention is, on the power supply. You will find that there are two other terminals.
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One is called G for ground and the other one will either be W or P.
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Different manufacturers use slightly different terminology.
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But basically what we’ve got here, we’ve got water which is flowing through our tube like that,
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and that water going back to a reservoir passes by some sort of, I’ll draw it like like a water wheel, with a flow switch on it.
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So this is a normally open switch, which when the water is flowing, it will be closed.
Transcript for High Voltage Laser Power Supply (Cont…)
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And that is connected between here
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and here on a K40 or some machines.
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OK, so that is to make sure that you cannot switch the tube on if there’s no water flowing.
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It’s a safety system which is most essential, because if you run this tube without water,
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you will destroy it very quickly, within a few seconds. We’ll go into this later, but on the controller, we’ve also got G and WP.
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And generally, what you’ll find is that this system here is not connected across here, it’s connected to
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here and here. And instead of G and WP looking like that, what you’ll find is that you’ve got a link wire which is connecting those two together.
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So what we’re doing, we’re monitoring the flow with the controller and not with the high voltage power supply.
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And the high voltage power supply is being fooled into thinking that, yeah, it is switched on because we link G & WP together,
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which creates the impression that the switch is closed and the real monitoring is being done by the controller.
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But we’ll come on to the controller and its functions in another session.
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You will always have a red wire, the high voltage wire coming out of the power supply.
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Not every power supply has got a wire coming out the back.
Transcript for High Voltage Laser Power Supply (Cont…)
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Sometimes it comes out the back here and it goes to the negative side of the tube like that.
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OK, so it’s a separate black or sometimes blue wire that comes out the back of the power supply and you connect it to that pin there.
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It’s also where you would put in an ammeter. You break the wire here and you put an ammeter in just there.
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Naught to 30 milliamps is normally good enough for the range, and they’re only something like about five dollars on eBay. Now on this particular controller,
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what we’ve got, we’ve got no black wire coming out the back of this unit.
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Instead, what we’ve got is a spare connection in here.
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So we’ve got two mains cables and an earth and then we’ve got a spare terminal and that spare terminal
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there, is basically where this wire goes back into the front,
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there. So if you don’t have a black wire, that’s where it goes. If your tube doesn’t work for some reason or other, you get no power out of it.
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You need to check two things, first of all, have you got water flow?When you tried to turn the power on
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you will get a water error shown on your display pad. So that’s one thing, that will quickly tell you that you’ve got no water flowing and
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there’s either there’s a problem of some sort with this switch or with your pump. If that’s all okay,
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and you still don’t get any output power from here. That could be for and two different reasons.
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One of them could be that you’ve actually run out of carbon dioxide, as I described to you in the previous session.
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So if you’ve got a pink beam in there, you know that this thing is working properly.
Transcript for High Voltage Laser Power Supply (Cont…)
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If you don’t have a pink beam in there, there’s a pretty fair chance that there’s something wrong with this device.
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And the first thing you should do is check whether or not the fan is working.
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If the fan is working, it means you’ve got mains input working to drive the fan and something seriously has going wrong inside here.
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And the most likely serious thing is that this flyback transformer has failed.
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I have seen burnt burnt out circuit components as well. In general terms, if you’re not electronically competent, it’s a new one of these.
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Now, I promised you earlier that we’d come back and look at this thing called response time.
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It’s a very important part of the function of our laser machine with a glass tube machine.
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The controller works with all electronics.
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And the power is controlled between the H.V. power supply and the controller, with a signal which works in microsecond time scales.
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So when we say we’d like a 50 percent power, the controller says, OK,
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now draw this line with 50 percent power and switch on now, because it sees the beginning of the line.
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So the percent power and the on off signal gets sent at exactly the same time.
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The percent power, is instantly switched on, ready to limit the control of current in this circuit here.
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But we haven’t yet switched the tube on, because instantly we tell the HV power supply to switch the high voltage on.
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But before the current can flow through the circuit, remember,
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we got to have an ionized beam and to create the ionized beam, we have to have kilovolts.
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And you remember me pointing out that white thing in the back of the power supply,
Transcript for High Voltage Laser Power Supply (Cont…)
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and I said that it was capable of only responding in millisecond timescales. One millisecond later than the on/off signal.
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We’ve now got sufficient voltage in here to allow 90 percent of the power to be flowing in this circuit.
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So once we’ve got the laser beam switched on, it can respond very quickly.
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The thing that doesn’t happen quickly is switching the laser tube on.
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And what do we doing all the time we’re running?
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Every time we generate a new signal, we switch the laser beam off. So that we can move to the next part and cut the next part and then we switch it on.
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Well, that’s not too bad when you’re running a cutting circuit because things are moving basically quite slowly.
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The thing that really starts worrying us is when we start moving to scanning or engraving. Here, we’ve got our head with the lens in it and we want to scan
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a pattern at 400 millimeters a second. OK, so this is the line that the controller thinks we’re going to try and produce.
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So it moves the head to this position here and then it says to the controller, OK, switch on now because I want to draw a line.
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We’re traveling at 400 millimeters a second and one second is a thousand milliseconds.
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So if we take 400 millimeters and divide it by a thousand, in one millisecond, I’m going to travel naught point four of a millimeter.
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So that means if I’m trying to draw blue lines, scan lines like this, the first scan line is going to be point four of a millimeter slow.
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It’s going to finish correctly,
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and then it’s going to jump down to the next scan line and come back and it’s going to start point four millimeters slow.
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And so it’s going to happen every time we scan backwards and forwards. So we’re not going to finish up with a nice blue scanline pattern like this.
Transcript for High Voltage Laser Power Supply (Cont…)
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We’re going to finish up with a raggedy pink scanline pattern like this.
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Your image is going to be very fuzzy around the edges because of this effect.
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If you slow down to 200 millimeters a second, then you’re only going to have a 0.2mm error.
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If you slow down to about 10 millimeters a second, it’s going to be negligible.
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But you can’t scan at 10 millimeters a second.
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There are parts of the system in the programing system called reverse scanning offset, to try and compensate for this,
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but I’m not going to go into that at the moment. All I want to show you is how important it is that the response time of the power supply can actually
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mess around with the relationship between the start point, actual and the start point program.
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Now, when we move on to bitmaps, things begin to change quite dramatically.
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A bitmap could be composed of something like this.
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It could be a pixel, which is naught point one millimeters wide, where did I get that from?
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Well, if I’ve got an image which is set to 254 pixels per inch.
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It means every pixel could be naught point one millimeters wide.
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Which is 254 dots per inch, so we need to switch the laser on and off at this rate, 254 dots per inch.
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It’s it’s a slight exaggeration because it’s actually one dot then off, one dot off.
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So it’s actually only half that amount, about a 127. Let’s just say we’re trying to run this at 400 millimeters a second.
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This scan of on off, on, off, on off, on off the computer is going to say right now switch on now.
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So it’ll switch on, but nothing happens until point four of a millisecond later or point four of a millimeter later.
Transcript for High Voltage Laser Power Supply (Cont…)
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One, two, three, four. And of course, it switches off OK.
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And then it’s going to take one, two, three, four to switch on the next one.
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So hang on. Look how many signals I’ve got. Look how many pixels I’ve missed, so I’m not going to do a very good job of replicating that pattern.
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Let’s just stay at 400 millimeters a second for a moment, because we’ve got another problem.
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We’ve told the power supply to switch on at this point here.
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And we know that it’s going to take point four of a millisecond. Or point four millimeters to switch on.
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So it starts to switch on and then it gets told to switch off.
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So if we run too fast, we’re not actually going to produce any pixels at all.
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So here’s our demand. Whatever that demand current is, a certain number of milliamps, and here’s zero, the signal isswitched off.
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So if we run at a100 millimeters a second, what it means is it starts off at zero.
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It might possibly get up to 90 percent of what we’re after within one millisecond.
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Like that, and then it would drop off. And then there’ll be no signal.
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And then we shall switch on again, and it possibly might take
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the same amount of time and we won’t have a full millisecond of burn.
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We’ll only have a very short period here, where we might be able to experience enough burn to produce a dot of some sort.
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Now, the dots will be the right size apart, but they won’t be very strong.
Transcript for High Voltage Laser Power Supply (Cont…)
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If we want stronger dots, we shall have to drop this to something like 50 millimeters a second and at 50 millimeters a second,
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then it means that we should be able to get this to rise up much quicker like this.
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And maybe we shall then have enough power to produce a much bigger dot every time.
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This gets quite complicated, the way in which the power supply controls what you can do with this machine.
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So you can’t just go and think that pushing buttons on this machine is going to make it work, how you imagine it will work?
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There are bits of this machine which will slow you down and cause you to scratch your head if you don’t understand what’s going on.
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That’s why I’m taking the time to explain to you that you can’t just run at 600 millimeters a second with a thousand DPI.
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The power supply just won’t support that rate of putting dots down.
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It can’t switch on and off quick enough, we’ve got a little bit technical there,
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but you have to understand the power supply is a limiting factor on a glass tube machine. Now in reality,
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most power supplies are actually at least 50 percent better than the spec. My power supplies, on
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both my machines are capable of running four times faster than the specification states.
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You can’t say how good a power supply is going to be other than it must meet the specifications.
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Now, if you bought an eBay machine and it’s fitted with a power supply,
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there’s a pretty fair chance that that power supply is a B-Grade power supply.
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And it’s B-Grade because it failed the one thing that you most need, which is its response time specification.
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It’ll deliver the power, it’ll deliver the voltage, but what it won’t do a switch on and off quick enough.
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And on that bombshell, we will end our discussion about power supplies.
Transcript for High Voltage Laser Power Supply
