Weird result with an x-ray tube, power not as expected - Page 1
Forum section: electronics / projects

author: Neukyhm
author_rank: Contributor
timestamp: September 08, 2018, 12:11:15 pm
content: Hi there, I'm building an x-ray machine, I have built the main parts like the HV power supply and the box containing the voltage multiplier and the tube, covered with oil.Everything works and I can make radiographs, however the machine does not work as expected. It should have around 100W but it only draws ~40W from the PSU. At first I thought it had something to do with the grid resistor of the tube.This is the datasheet of the tube, I was adviced by the manufacturer to use 47K as grid resistor, OK. If you look at the filament current and anode current vs filament current, you see that if I apply 2.5V to the filament I have around 2A at the filament and let's say almost 3mA at anode when voltage is 65kV, so power is big.To my surprise, today I measured the current at the filament, at the moment I apply 2.5 to it, current rises to >2A as expected by the datasheet but then it decreases instantly to 1.5A, I now think that's what makes the machine work at 40W and not at 100W as I expected.Do you guys have any idea why this happens? If the datasheet doesn't lie, current at filament should be 2A and not 1.5A. You may suggest to increase the filament voltage but again I was adviced by the manufacturer to not apply more that 2.5V.

author: TurboTom
author_rank: Super Contributor
timestamp: September 08, 2018, 12:29:54 pm
content: Modern X-rax heads provide limits for filament current/voltage and control the filament current in order to stabilize the anode current in a control loop. Have you got provisions in your design to measure total tube voltage and current?For an example how a (fairly) modern X-ray had is constructed, you may want to have a look at this thread:https://www.eevblog.com/forum/projects/x-ray-generation-and-detection/msg1507648/#msg1507648

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 08, 2018, 12:37:37 pm
content: Quote from: Neukyhm on September 08, 2018, 12:11:15 pmCold filaments have lower resistance, so resistance increases, and current decreases as it warms up.

author: mikerj
author_rank: Super Contributor
timestamp: September 08, 2018, 12:55:54 pm
content: Quote from: Neukyhm on September 08, 2018, 12:11:15 pmThe emission characteristic curves on the datasheet don't start until 2.75v and the permissible voltage range is shown as 2.5-5v. I think you are under driving the filament.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 09, 2018, 11:26:14 am
content: Quote from: Neukyhm on September 08, 2018, 11:56:55 pmMaybe they are a bit old-school and actually mean a nominally-2.5v-under-max-load secondary winding on a transformer, which would put out more under the reduced loading of a hot filament.They probably ought to be specifying the max filament drive as a current rather than a voltage.

author: Wolfram
author_rank: Frequent Contributor
timestamp: September 09, 2018, 02:16:45 pm
content: Do you measure the anode voltage, and are you absolutely certain that it is 65 kV? For a given filament current, the emission current will be lower with a lower anode voltage, mostly due to space charge effects. The datasheet you linked only gives emission curves at 65 kV, here's a datasheet for a similar Toshiba tube with more comprehensive information:https://etd.canon/eng/product/read_binary.php?cid=100200100100&pid=20140730_1058&fn=PE-D-045_2017-06.pdf

author: james_s
author_rank: Super Contributor
timestamp: September 09, 2018, 04:16:05 pm
content: The anode voltage will sag heavily with load, even an old style iron transformer has a substantial impedance due to the very long thin windings of the secondary. If you're using a voltage multiplier with a high frequency drive then you'll get even more sag. You need to be able to measure the anode voltage and adjust the drive according to the filament voltage. Also you need to be careful not to energize the anode without powering the filament or the voltage across the tube can rise to dangerous levels.Here's some info on a Kodak inverter type dental xray head I reverse engineered several years ago:https://www.repairfaq.org/sam/xraysys.htm#xraytro

author: helius
author_rank: Super Contributor
timestamp: September 09, 2018, 04:25:23 pm
content: Voltage multipliers store energy, so their final output voltage varies depending on load and the input frequency. One way to increase their output voltage is to increase the frequency of the input.QuoteI believe what is meant is that at higher frequencies, more power is lost in the magnetic loop of the transformer. You are correct that a higher frequency reduces the sag in the voltage multiplier, but the transformer has inductance. It seems like you can't win!

author: james_s
author_rank: Super Contributor
timestamp: September 09, 2018, 04:34:33 pm
content: The tube manufacture is probably not going to be able to help you, it's obvious that your anode voltage is sagging, fixing that is your responsibility. Your frequency is plenty high but the supply is still going to present a substantial source impedance. You *must* measure the anode voltage, there is no way around this. If you are going to be playing with an xray tube you need to know the anode voltage, not only to get the tube operating correctly but for safety reasons. As the voltage increases, the penetration and scatter of the radiation increases. Did you look at the Kodak schematic I linked to? It's not difficult to build a voltage multiplier, you can use a load of 1M resistors in series.

author: james_s
author_rank: Super Contributor
timestamp: September 09, 2018, 04:37:01 pm
content: Quote from: helius on September 09, 2018, 04:25:23 pmWhat I meant is that using a voltage multiplier (which is typically only used with a high frequency transformer) is going to result in greater sag. Voltage multipliers are very saggy, that's just the way they are, at least when you have multiple stages with reasonably sized capacitors.

author: floobydust
author_rank: Super Contributor
timestamp: September 09, 2018, 05:16:41 pm
content: Your SMPS might have poor regulation as the tube anode current comes up and then filament voltage sags. Sub in a known resistor (2V/1.75A graph) say 1-2 ohms and see if you have enough power from the PSU. Add a 1/2 turn to the secondary. Poor (filament) connections can also do it, they can heat up and a thermal IR camera pic can show these.This is one ETD59 with two secondary (HV, filament) windings? If so, 110kHz is high so your measurements are beyond multimeters and hard to be accurate at HF.The x-ray tube is bit strange, it's has a "grid" that looks like an acceleration (screen) grid.The grid resistor graph shows lines at 35k and 40k with "Rg-3k" and "Rg+3k" labels added.So 47k ohms seems a bit high. It affects the focus and the conductance of the tube.But the low filament voltage I think is the problem.

author: james_s
author_rank: Super Contributor
timestamp: September 10, 2018, 01:12:18 am
content: The image looks pretty muddy but that might be due to the camera photographing the dim phosphor screen. It's also pretty clear that your anode voltage is low given the low penetration, you can't even see through the IC package. Get the power up to the proper level and you should get a much better image.

author: Wolfram
author_rank: Frequent Contributor
timestamp: September 10, 2018, 09:43:26 am
content: What's the capacitor value that you are using in your multiplier? Can you confirm that your voltage multiplier is 12 stages (i.e. 12 stages of two capacitors and two diodes each)? A twelve stage multiplier will be very saggy under load, and I can guarantee that the output voltage is much lower than the theoretical. The transformer output voltage can also not be calculated simply by the ratio, due to leakage inductance and parasitic secondary capacitance. Assumptions are as good as the understanding of the system, so if you want to estimate the voltage, you have to account for your drive circuit behavior, transformer parasitics, voltage multiplier sag and the operating characteristics of your x-ray tube. Just measuring the voltage is usually significantly easier.Results look good so far.

author: Zucca
author_rank: Unknown
timestamp: September 10, 2018, 10:07:30 am
content: Interested to see pics of the whole setup. TIA.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 10, 2018, 01:31:52 pm
content: I don't see much lead shielding around that box....

author: TurboTom
author_rank: Super Contributor
timestamp: September 10, 2018, 02:31:26 pm
content: May we assume that you operate the apparatus from a remote room and you've got a geiger counter to make sure your own exposure is minimized? And also that noone else is in the device's vicinity when you operate it (any pets?). Really looks like shielding is NIL.

author: grumpydoc
author_rank: Super Contributor
timestamp: September 10, 2018, 03:12:54 pm
content: Quote from: TurboTom on September 10, 2018, 02:31:26 pmLets hope so.His set-up probably isn't producing very energetic x-rays but, even so, I would not want to hang around it without shielding and with no dosimetry.I'm almost as amazed that he has a CW multiplier with a nominal 65kV output built on perfboard.

author: TurboTom
author_rank: Super Contributor
timestamp: September 10, 2018, 10:14:49 pm
content: The less energetic (or "soft") X-rays are even more dangerous since they are less penetrating and are absorbed by the soft tissue of the body more or less completely. Hence they do more damage to the body than "hard" or higher energy X-rays. That's also the reason that most of the X-ray heads for general medical use include a filter of 0.5...2mm of aluminium to block the low-energy portion of the X-ray spectrum. Things are different in a mammography machine and that's also the reason that these kind of examinations are considered more risky.When expeimenting with X-rays, you've simply got to know what you're doing. But it seems that @Neukyhm has taken appropriate precautions.

author: InductorbackEMF
author_rank: Regular Contributor
timestamp: September 24, 2018, 05:16:55 pm
content: Neat "setup" plan to make one as well,i`m lucky as i have alot of lead plates lying about.

author: floobydust
author_rank: Super Contributor
timestamp: October 06, 2018, 06:53:03 pm
content: I encourage you to measure the HV, as I've build multipliers with those same caps/diodes and found they don't perform well.The capacitors are out-of-the-box -15% at around 8.5nF and even less with DC bias on them. The diodes I have not tested but they are designed to be encapsulated (8, 10mm vs 15, 20mm) and I had them arcing across the body when I pushed it. I measured their forward voltage as much less than the datasheets, so the stack seems small.My point is, I would make measurements to confirm the HV stays up under load, instead of upping the filament voltage to get good anode current.Less X-rays generated if the HV sags.

author: james_s
author_rank: Super Contributor
timestamp: October 07, 2018, 05:56:11 pm
content: Why don't you just measure the anode voltage? You don't need anything particularly exotic to do so.