photomultiplier tube circuit translation from vacuum tube interface to modern - Page 1
Forum section: electronics / projects

author: CopperCone
author_rank: Super Contributor
timestamp: January 12, 2018, 10:06:12 pm
content: https://www.orau.org/ptp/Library/InstrManuals/PRI%20i111b%20and%20117.pdfI have this scillinator, but it died. I would like to replace the electronics with semiconductors.Can someone explain to me basically whats going on, so I can implement the circuit with op-amps?Specific questions:1) 6 on VT1 is biased by 1meg in parallel with 1uF (why is the capacitor across the resistor here?? I thought they wanted pure DC not some kind of HF feedthrough)2) 7 on VT1 connected to some kind of conditioning/amplifier circuit?3) 12 on VT1 connected to a negative voltage regulator?

author: james_s
author_rank: Super Contributor
timestamp: January 12, 2018, 10:15:22 pm
content: You'll probably have to redesign the whole circuit if you want to use solid state, it's rare that you can do a direct conversion. You might look around for various PMT amplifiers and see if an existing circuit will work for you. Personally I would just repair the original circuit and keep using it though, vacuum tubes are not inherently bad and no matter what you do you'll still need the PMT. No solid state sensor I'm aware of can come close in sensitivity.

author: CopperCone
author_rank: Super Contributor
timestamp: January 12, 2018, 10:21:13 pm
content: No, I'm not looking for a direct replacement. I can build the circuit out of transistors or preferably high voltage op-amps, but I am not sure whats going on.A block diagram would helpI'm pretty skilled in contemporary electronics, but I don't know whats going on near the tubes.Also, I don't really wanna repair it, I was hoping that modern devices would draw alot less current (i.e. no heaters) so I can power it off a 4.5v lithium battery and a bunch of switching regulators, add a USB port on the side, and keep it fully contained in the gun part.Also, I spy a 1600V rated cap, is the circuit on the bottom acting like a boost converter?

author: Andy Watson
author_rank: Super Contributor
timestamp: January 12, 2018, 10:59:51 pm
content: Quote from: CopperCone on January 12, 2018, 10:06:12 pmI've never seen PMT operated like that, but I guess it is the equivalent of cathode/emitter degeneration, i.e. negative feedback, to stabilise the DC operating point of the tube. The capacitor is there to retain the high-frequency response. Usually, the current through the anode is amplified, however, here they appear to have foregone the extra stage of PMT amplification and the output is being taken from dynode 10 - pin 7.QuoteYes, with an input resistance of R2 100k, VT2 amplifies the signal from the PMT and drives VT3 and VT4 - which looks like a pulse-conditioning monostable.QuoteOh yes, indeedPull up the datasheet for the PMT. It appears to require 500-1250V (negative) on its cathode. Note that it needs to be a well defined/regulated voltage. This voltage is divided through the resistor R3 through R14 and determines the gain of each stage of the PMT. I.e., the gain of the tube is exponentially proportional to the voltage across that divider chain.QuoteYes. VT9 appears to provide the feedback. Look-up the specification of a 5841 - it should give you some indication as to the operating voltage.

author: Andy Watson
author_rank: Super Contributor
timestamp: January 12, 2018, 11:07:00 pm
content: Also,Quote from: CopperCone on January 12, 2018, 10:06:12 pmAre you aware that the scintillator materials typically used in that era are sensitive to moisture ? Sensitive as in they fail if exposed to moisture normally present in air.

author: CopperCone
author_rank: Super Contributor
timestamp: January 12, 2018, 11:10:34 pm
content: How noise free do these high voltages have to be? Can I use zener ladders? Problem is the switching converter modules are like 140$ each. I would prefer them due to size however I though that this could be a 50$ project, not a 300$ project.

author: CopperCone
author_rank: Super Contributor
timestamp: January 12, 2018, 11:12:15 pm
content: I think its OK, but I am not sure. I don't know how to asertain the crystals integrity.The unit had a major malfunction (I was running it off of like 6 power supplies to meet the requirements, but the switch on the side was loose, and it twisted shorting a buncha stuff out. It's also kinda crusty inside, looks like a pain in the ass to troubleshoot.Before it fried it was showing a DC offset but no response to a gamma source.Making a new circuit seemed easier then fixing it. I don't particularly enjoy repairing things, rather design/build. I get frustrated with other peoples design choices quite often.

author: CopperCone
author_rank: Super Contributor
timestamp: January 12, 2018, 11:26:20 pm
content: I think these circuits can be used to power ithttp://cds.linear.com/docs/en/application-note/AN118fb.pdf

author: CopperCone
author_rank: Super Contributor
timestamp: January 12, 2018, 11:49:13 pm
content: Ok, so 264 megaohms, from 900v, loaded down by 100k, means it will be around 0.24V.Does the top section of the tube need to be +67.5, or can it be grounded (I see them grounded on google circuit search results).

author: CopperCone
author_rank: Super Contributor
timestamp: January 13, 2018, 01:34:41 am
content: Any idea of what the time constant for the monostable pulse circuit is (dead time)? I can see a LT Timerblox being useful here.and the gain of the circuit. I have trouble imagining how big the pulse amplitude is from the electron multiplication effect

author: vindoline
author_rank: Unknown
timestamp: January 13, 2018, 01:36:35 am
content: Quote from: CopperCone on January 12, 2018, 11:12:15 pmA working scintillation detector will show significant background radiation even in the absence of a "gamma source" I'm 99% sure your unit was dead before you shorted out the electronics. As previously pointed out, the NaI scintillation crystals are extremely moisture sensitive and the old seals in these units generally fail with age.

author: CopperCone
author_rank: Super Contributor
timestamp: January 13, 2018, 02:30:54 am
content: It has a metal cup with the crystal in it, glued down to the tube, with a plastic bag over it thats wrapped tightly in tape.I know the electronics were fried because the offset adjustment pot did nothing.Anyway, I figure this:http://cds.linear.com/docs/en/application-note/AN118fb.pdfCircuits from these app notes to power the -900V and +62V railsHowever, the output stage looks like this:-dc bias of -0.24 volts (from the voltage divider).-signal pulse height of?If the signal pulse amplitude is low, then a simple inverting amplifier can turn the negative pulse into a positive pulse of significant amplitude, which can be then fed into a LTC6993 acting a monostable pulse generator, so the pulses can be averaged to a DC for the gauge on the back of it. (PWM to DC converter with a low pass filter of adjustable time constant).Does anyone know what the expected amplitude will be from the pulses? And what the expected count rate is (or how it can be calculated?)

author: Kleinstein
author_rank: Super Contributor
timestamp: January 13, 2018, 11:03:53 am
content: It depends on the application how stable the high voltage needs to be. For just counting one can accepts quite a bit more ripple / dirft than if you want to do resolve the energy as well (which is not that difficult anymore today). AFAIR it takes something like less than 0.1 V of drift and ripple to really do gammy spectroscopy.This high of the pulses very much depends on the voltage at the PMT tube. Usually one uses a voltage high enough to make them easy to measure, but not to high to give stress to the tube and HV supply. So one should have resonable quality amplification, but no need to go to extremely low noise parts. To a void the voltage at the last stage of the divider to drop to much, the pulse hight is kind of limited to a fraction of the divider current, thus less than about 0.1 mA. So at a 10 K sense resistor this would be in the 1 V range, though 0.1 V might be enough too. This is for the largest pulses - small pulses might be at 1/10 to 1/100 that size. In addition the actual pulse from the tube is very short ( < 100 ns) and usually external capacitance usually stretches the pulse and this way also reduces the amplitude.The background pulse count is expected somewhere in the 10-500 counts per second range, depending on the level for accepting a pulse. It also depends on the size of the crystal. There are usually quite a lot of very small pulses, so the count rate can go up quite a bit if a low threshold is used. That is what I observed from such an detector where i once build an electronics for. The high voltage regulator circuits from the LT link look kind of OK. I used a similar circuit, though with linear amplitude regulation instead of the switched mode regulator. For building the electronics, getting the stable HV supply is the more difficult part.A modern circuit would normally not need the +62 V rail, more like +-5-8 V for the amplifiers and a -800-900 V supply for the PMT. However it depends on the PMT tube and housing if its better run with a -800 V or a +800 - so the question of to ground which side. The old circuit seems to use a -800 V. The extra +62 V could be kind of a separate supply for the lowest stage, so that the divider can operate with a lower current.

author: floobydust
author_rank: Super Contributor
timestamp: January 13, 2018, 11:41:51 pm
content: I agree the hardest part is making a 1,000VDC 0.1-0.5mA regulated, low-ripple power supply. Disposable camera strobes (transformer) work well, with added CW voltage-multiplier.Of the two6199 photomultiplierconfigurations, a -HV supply and pulse-amplifier off the anode, like Model 117.Model 111B instead it picks off the pulses from Dynode 10 (pin 7) instead of the anode. I'm not sure why.Overall it is similar to Geiger-Müller Tube designs, many circuits on the web for the Russian SBM-20 tube, but some are pretty poor designs which use a noisy, unregulated boost converter, and pulse amplifier with too low input impedance to match the tube's output.The pulse amp, pulse-stretcher (one-shot) and integrator for the panel meter can be simple circuits.

author: CopperCone
author_rank: Super Contributor
timestamp: January 14, 2018, 12:46:18 am
content: If you look at the app note I linked, there is a low noise switching converter you can use.

author: floobydust
author_rank: Super Contributor
timestamp: January 14, 2018, 05:48:05 am
content: That Jim Williams app note 118 always humbles me. Packed with wisdom and he was such a master.The cartoon says he wrote it on vacation at Columbus Isle? Not sure it it's his sense of humour, Club Med....I could not find thatCoilcraft CCFL transformer,it is discontinued but a smaller 3.5W part still exists.I wasn't sure what you were doing with the battery nightmare. The original look like quite a hassle. A 6V pack would be enough to run the converter and panel meter. I thought it was a 12V Royer circuit, that's why I suggested a flyback, but Jim's circuit is best and 5V powered.

author: CopperCone
author_rank: Super Contributor
timestamp: January 14, 2018, 05:11:50 pm
content: yea to power it initially I needed 2 dual supplies and 2 single supplies lolI found all the parts for the -1000V one through google.http://www.newark.com/coiltronics/ctx210609-r/low-voltage-transformer-1-100/dp/76T4688https://www.mouser.com/ProductDetail/Coiltronics-Eaton/CTX150-4-R/?qs=sGAEpiMZZMu3JGptLTgkAjP3QY95dIuQvcJpbQpHzpg%3DIt looks a bit pricey, but I tend to look at how much the equipment cost back in the day, so it seems like a good deal.You think it will be fine powered from 6V? I never actually made a boost converter before.Actually, what does the -R specification mean in the name? Their both different then the datasheet.What makes coupled inductors different from a transformer? Any idea how he uses it? It just shows a inductor with no name. I assume its L1 but the coupled part confuses me.It only makes sense to me that he would use it in parallel, but why? I can't exactly test this without wasting a PCB and possibly destroying a expensive IC.

author: floobydust
author_rank: Super Contributor
timestamp: January 15, 2018, 04:41:34 am
content: For the AN118 magnetics, the parts are obsolete. I'm not sure about substitutions and keeping things resonant and constant current.The CTX250-4 250uH 650mA inductor is puzzling. It's a dual winding part, you can see that in Jim William's pics. I imagine they are parallel connected. The CTX250 is now a Cooper Eaton part and no longer in theCTX datasheets.-R is RoHS compliant. It looks like 150, 200, 300uH are available.CCFL Royer transformer Coilcraft/Eaton CTX210609-R is recently discontinued. Digikey513-1729-NDSpecs 1:100, 6W, Lp=20uH Rpri=0.132R, Rsec=176R, 11mA max., no data on FB winding.I think substitute is:Coilcraft FL2015-9LSpecs 1:100, 6W, Lp=22uH Rpri=0.132R, Rsec=176R, 11mA max. but don't need so much current.I consider FL2015-3L,Specs 1:100, 6W, Lp=19uH Rpri=0.190R, Rsec=285R, 5mA max.I don't know why the difference in the two transformers (FL2015-3L and -9L), unless a 2.5W part.

author: KE5FX
author_rank: Super Contributor
timestamp: January 15, 2018, 04:59:26 am
content: Great lesson in "How not to draw a schematic."The PMT is a current source, basically something you can treat like a photodiode and use to drive a TIA. Obviously you want to arrange the supplies to keep the last dynode at the ground end.Hamamatsu is one of the larger remaining manufacturers of these beasts, and theirPMT Handbookis a great reference. Another good one is Phil Hobbs' book on electro-optical systems.For power supplies, the path of least resistance is to get a Bertan PMT-50 module on eBay. These can often be found for not much more than the shipping cost, and they're tailor-made for the purpose. The 'PMT' model prefix is probably not a coincidence.

author: CopperCone
author_rank: Super Contributor
timestamp: January 15, 2018, 08:38:40 pm
content: Quote from: floobydust on January 15, 2018, 04:41:34 amNo, the parts are not completely, to others that may want to use the circuit:Here is the dual inductor:https://www.mouser.com/ProductDetail/Coiltronics-Eaton/CTX150-4-R/?qs=sGAEpiMZZMu3JGptLTgkAjP3QY95dIuQvcJpbQpHzpg%3DThis guy is no longer manufactured, but 304 are in stock, so its not that bad. I don't think alot of people will make these.http://www.newark.com/coiltronics/ctx210609-r/low-voltage-transformer-1-100/dp/76T4688Do you know the consequences that might happen if other parts are used? What specification is most important.What do you mean by "don't need so much current"?I need to read about the royer topology and those types of transformers. I find the whole mess rather confusing.Also, any idea on what the input voltage range will be? I read royer dc/dc converters have a narrow one.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 15, 2018, 09:20:14 pm
content: The Royer converters are usually relative well behaved and not very picky about the parameters. However the voltage ratio is kind of fixed, so one has to control / regulate the voltage or current seen by the Royer part. In the Royer converter the turns ratio sets the voltage ratio, similar to a normal transformer, but with an additional factor due to the resonant waveform. The series inductor value is not that critical, it kind of set current ripple at the input side. The transformer inductance sets the operating frequency, with some room for adjustment by the parallel cap.The AN118 part used a switched mode regulator chip for this, but at low power this can also be done with a linear regulator. In the circuit I had build, I used a simple linear regulator (TL431 and 1 or 2 transistors) to control the input voltage to the Royer converter. If the turn ratio is adjusted to a good value, the power lost due to linear regulation is not that bad.The original tube circuit used a very low current for the main string and a separate 62 V supply for the last stage. This is kind of clever as it reduces the power requirement and the 62 V seemed to be good choice for the tube amplifier anyway. If low power is required one could use that same trick as well - the voltage for the lowest stage should be less critical anyway. So an accurate regulation is only needed for the main string (excluding the last steep). The lowest step could be just another transformer tap. However this might get a little tricky with a ready made transformer. Adding a few extra turns to a ready made transformer could be an option.With the very high impedance string with 22 M resistors the current would be only in the 5 µA range, thus about 5 mW For an accurate regulation one might go for a slightly higher current and more modern resistors. The low current (e.g. < 50 µA) is kind of nice as a relatively small filter cap can be used and thus less danger from the HV supply. Using the PMT tube resistor string for voltage feedback is also kind of elegant, though is includes a small error due to the electron current inside the PMT.The lowest stage might need a little more current and thus the extra source for around 40-70 V. With just a long string one would likely need smaller resistors, like 1 M each.

author: mark03
author_rank: Frequent Contributor
timestamp: January 16, 2018, 04:29:08 pm
content: On my indefinite-future "to do" list is building a HV supply for some surplus PET-scanner PMTs acquired cheap from Sphere electronics. I haven't found the time yet, but I did bookmark this design, which looked interesting:https://www.google.com/patents/US8437149It is unusually well written and helpful, for a patent.

author: james_s
author_rank: Super Contributor
timestamp: January 16, 2018, 07:40:37 pm
content: Quote from: Kleinstein on January 15, 2018, 09:20:14 pmI once tacked one together literally out of parts I found on the floor near my bench and it actually worked on the first go, I was impressed. It passed enough current to easily light a 12V automotive bulb.

author: floobydust
author_rank: Super Contributor
timestamp: January 16, 2018, 10:06:48 pm
content: Hamamatsu PhotonicsC4900 PSU module-1,250VDC 0.6mA, ±0.01% line/load regulation and 38mV ripple.patent US 5548502.It seems to be modulating the Royer base-drive bias, which I am not sure gives total control...For good PMT sensitivity, the HV power supply best be low ripple and well regulated.I ordered some Coilcraft CCFL transformers to try out, see how they do making 500-1000VDC at a few mA.