Transcription
We've seen now how our primary circuit allows us to select our KVP as well as determine our exposure time. And we've seen how, in combination with the secondary circuit, through the process of step-up transformation, we've taken a low voltage alternating current and transformed it into a high voltage alternating current.
Now let's have a look at the secondary circuit and how it uses the process of rectification in order to change this alternating current into direct current. So, how does the secondary circuit fit into our whole circuit? Well, the electricity is coming from our primary circuit. We've stepped up through the process of step-up transformation and now induced a current in our secondary circuit. This secondary circuit then supplies both our anode and our cathode.
So, let's have a closer look at the secondary circuit itself. Now, I've said the primary function of the secondary circuit is to convert this alternating current into a curve that looks like this. Direct current is flowing in one direction. Now, we can see that that direct current is still fluctuating, and we need to use a process of generation in order to smooth out this current and make sure that there is a steady flow of electrons from our cathode to our anode. Those electrons aren't pulsating like it does in this current here.
Now, it's important to note that the top of these peaks is our peak voltage, and that will determine our KVP, our kilovoltage peak. So, if we want to understand the process of rectification, there are two things that we need to understand, or two rules that we need to follow. The first is a diode. Now, a diode is a semiconductor that only allows electrons to pass through it in one direction. Now, electrons can only pass through a diode in the opposite direction to an arrow. So, these diodes here will determine which way current is flowing through these wires.
The second rule we need to understand is that electrons in a current only flow from negative to positive. Electrons can't flow to the negative terminal of our coil. Now, if we have a look at the current that is coming through the secondary part of our step-up transformer, we can see that the current is alternating. Now, in this first part of the current here, this positive deflection of the current, we will get two poles on our step-up transformer: one that's negative and one that is positive. Now, electrons will flow out of the negative end here and come to a decision point.
We know that electrons can only flow through a diode in the opposite direction, so electrons or current won't flow through this diode. We have forced the current to go this way. Again, current cannot flow through the diode in this direction, so current has to flow towards our cathode. We are creating a negative charge and negative potential on this part of the cathode. Electrons are leaving our anode.
Now, get to a decision point, and we can see here that it could either flow through this diode here, or in fact, electrons could also flow through this diode here. Now, this is when our second rule comes into play: that electrons can only flow from negative to positive. If electrons were to flow through this diode, we'd be going towards our negative terminal, which is not allowed according to these two rules. So, electrons will flow through this diode and reach our positive end.
So, what has the current done then? Well, it's flowed from cathode to anode and back around to our positive end of our transformer. Now, as that current switches, as we go into the negative deflection of our alternating current, the poles of our secondary part of the transformer will change. Now we have current alternating. It's flowing this way out of our transformer. So, let's follow that current.
Now, we go through a decision point. We cannot flow through this diode this way. We have to flow through this diode. And as we come down, the same thing happens. We can't go in the same direction as a diode. Therefore, our current must come towards our cathode here. Currently leaving the anode. Now, will then travel again. Will come to this decision point where it can either go through the diode this way, or through the diode this way. Again, we know that current can only flow to our positive terminal.
So, current has to choose this diode in order to flow towards positive. It can't come this way to the negative. Now, what has happened again? Our current has flowed through the cathode, through the anode, and back around. We've kept current flowing from cathode to anode in the same direction. We've converted that alternating current into direct current, and this process here is known as full-wave rectification. We have ensured our positive deflections stay positive, and we've converted our negative deflections in our current into positive deflections here.
Now, as I've mentioned, this current is rectified, but it is fluctuating. It's got periods of peak voltage, as well as periods with no voltage, and we fluctuate between our high-end voltage and no voltage. We want to create a smooth tube potential, a constant tube potential between our cathode and our anode, and that's where the process of generation comes in.
Now, there are a couple of concepts I want to cover here. The first, we've touched upon, is our KVP, our kilovolt peak, the highest voltage in our waveform. We then have our x-axis, which represents zero voltage flow, and negative to that is our KVP in the other direction, our kilovolt peak in the opposite direction.
Now, the second concept I want to draw your attention to is what's known as KV ripple: how much fluctuation does that current have when we're looking at our KVP and our minimum kilovoltage potential? Now, the difference between our KVP and our baseline is 100% kilovoltage ripple. We get a 100% change in our voltage potential. Now, a single-phase alternating current, because it goes into the negative deflection, or because that current changes direction, we say our KV ripple is 200% here.
Now, we've looked at a rectifying current. What we looked at was a full-wave rectification. But if we just half-wave rectify this current, our positive deflection should stay positive, and we would prevent our negative or alternating current from passing through here. We have voltage peaks that are the same as our single-phase alternating current, but we no longer have that negative deflection. Our kilovoltage ripple here is 100%. The issue here is we've got long periods of no voltage. We've got no voltage and no current flow, no electrons going from our cathode to our anode for half of our cycle.
Now, what we can do is full-wave rectification, is what we looked at in the secondary transformer when we looked at the full-wave rectifier here. We've got single-phase current with two pulses that have been rectified, or a single-phase current that has been full-wave rectified. Again, our KV ripple is still 100% because we still have periods of zero voltage and KVPs that are the same as our single-phase rectified, while single-phase alternating current.
So, now we've got a more consistent flow of current. We don't have periods of time where there's no flow for prolonged periods of times, but our ripple is still 100%. Our current is still fluctuating. What we can do is take multiple phases of current and overlap them with one another. So, instead of a single-phase alternating current, we can have a generator that applies three separate phases of current. And if we were to full-wave rectify those phases of current and time them in a way that overlaps these currents with some delay between them, we can see that we'll get voltage that is much more steady. Our peak voltage ripple between the three phases is only about 20%.
Now, we've got much more consistent flow with no periods of time with zero voltage, and this is what's known as a three-phase rectified current. Now, we can use a generator that's known as a high-frequency inverter that places multiple phases of current over one another, getting those peaks very close to one another, and we can see how these peaks being close to one another and these currents being rectified allows us to have a very small voltage ripple, normally less than 4% voltage ripple here.
Now, our potential between our cathode and our anode is much more consistent, and we're getting much more constant flow of electrons heading towards our anode and a much more constant stream of x-rays leaving our x-ray machine. Now, our KVP, our kilovoltage peak, determines the energy of those electrons going from the cathode to the anode, and it has an effect on the number of electrons going from the cathode to the anode. So, if we have a look at this graph, I want to draw your attention to a concept called tube current.
Now, tube current is the number of electrons flowing from the cathode to the anode over a period of time. Now, we can change two things to manipulate our tube current. The first is we can increase our KVP, just as we've looked at. Now, we can choose in our primary circuit a higher KVP. That step-up transformer is going to increase our KVP, and as we increase our KVP, say from 60 to 80 to 100, we get a shift of this graph to the left.
Now, at a filament current that is set, say, four and a half, you can see that our tube current for a four and a half mA filament current and a 60 KV tube potential, we've got a fairly low tube current. We've got a fairly low number of electrons flowing from cathode to anode. As we increase that KV slightly from 60 to 80, you can see we get an exponential increase in our tube current. As we increase that to 100 KV, we get an even further increase in our tube current, and this shows you that the relationship between KVP and our tube current is exponential. It's normally KVP to the power of 2 will determine the tube current.
So, increasing our KVP has a drastic impact in the number of electrons heading from our cathode to our anode and eventually has an impact on the quantity of our x-ray beam, which we will look at when we look at the x-ray spectrum. So, what I want to show you here is that increasing KVP increases our tube current and ultimately increases the number of x-rays that we produce.
Now, there's a concept called the 15% rule, that when we increase our KV by 15%, we need to halve our filament current in order to compensate for that increase in tube current, which is a concept that we will look at in a separate talk.
So, now we've seen how the secondary circuit rectifies current and changes that alternating current into direct current, and how a high-frequency generator can smooth out that current to prevent that KV ripple. We've also seen that increasing our KV increases our tube current, which is an incredibly important topic when it comes to an exam and something that comes up over and over again.
Now, if you are studying for exams, I've got a question bank that I've linked in the description below. Otherwise, we are going to move to our final component of the x-ray circuit, known as the filament circuit, where we can then manipulate this filament current and also increase the amount of electrons flowing from our cathode to our anode. So, I'll see you all in that. All goodbye, everybody.