Generic supply for detectors

Things at the limits.

Generic supply for detectors

Postby Doug Coulter » Sat Feb 12, 2011 9:55 pm

I am working on adding a B10 lined neutron detector tube to my lashup, and needed yet another power supply for it. For those not familiar with these, they are old as the hills (maybe older than I) but they work fairly well. Ours are GE made, around the end of WW II or so. Basically it's just a big proportional type tube but lined with B10 metal to make much more signal when hit with neutrons. In practice, these are sensitive enough for fusor use, and in fact, better than you need. But they have one caveat. The signal is tiny, and riding on the DC supply power, so you need to mind your P's and Q's when it comes to powering one (I'll document the preamp elsewhere and cross-link the posts).

This turns out to not be a hard task with a CCFL inverter, my favorite being DigiKey's 289-1025, which is 12v nominal in, and 1kv nominal out. Of all the CCFLs I've tested, this one has the best "manners", draws the least no load power, and is hardest to fry, as well as making a very stiff output voltage that is tightly related to the input voltage, so a full closed loop regulator is very rarely needed, and that only if you're going to draw real power out of one (they'll make12w if you're careful). The manufacturer is JKL and their internal part number is BXA-12529.

I've tried a ton of CCFL inverters, BTW, and this on is hands down the best -- hard to fry, most power out (it's designed to drive two lamps) and lowest quiescent current by a rather fat margin over say, the TDK brand ones I've tried. It appears designing an efficient Royer oscillator is a little past most of the manufacturers, they don't use the right core material. This part number will run reliably all the way down to .7 volt inputs, which is nice too. Something to watch for in new CCFL's is that many are now uP controlled and do dimming via PWM. Those are utterly useless for this, as they won't run on lower than spec voltages to make the output adjustable (unless you happen to have multi henry filter inductors laying around doing nothing). They also tend to be real power hogs. This is another case like getting the HV stuff meant for old CRT TV's before it's all gone -- as the world rushes to flat panel displays and LED lighting for those, all this stuff is disappearing off the market fast. So stock up now!

For this case, I needed 650v on the dot, positive, with sub millivolt AC noise on it. Since the supply operates at 50-60khz, filtering is a snap -- a .01uf output capacitor is all that's needed.
In this case, we know the tube will never (we hope) draw much current at all -- it works out to millivolts across 10 megs, so we don't need a super output power or super stiffness under load either. I always run these with at least a volt doubler, as that means they waste less power running at a lower input voltage for a given output. In this case, I simply used one of the 27pf caps on the original board as the series cap in the doubler, as that's plenty and helps current limit the thing in a fault condition, and the usual two fast diodes and that nice ceramic .01 uF output cap.

To control the input, I used an LM 317, with a 200 ohm output to adj pin resistor, and a 2k 1k pot to ground for the adjustment, which in this case puts me in the middle of the pot when it's on bogey - perfect, and plenty of range should I need this for something else. The one special trick here is that I use the same bench 13.6 v supply to run all these, and you want to avoid ground loop issues in this sometimes noisy environment. So to help with that, I put a 120 ohm resistor in series with the output ground, and a 10k ohm resistor in series with the positive 650v output.
There is another .01 uf filter in the preamp to take care of any "antenna noise" and this way the real ground is provided by whatever audio amp or counter I connect it to, not the noisy 13.6v supply that has "antennas" all over it to power other detectors, the ion extraction power supply and so on (also CCFL's btw).
PSInnards.jpg
Innards of the HV low power supply

Almost doesn't deserve a picture, since it's so simple, but here's what it looks like on the inside. I put the trimpot on the other side of the board so I can tweak it through a hole in the front panel.
PSOuters.jpg
Outside of supply


Now, these guys radiate quite a lot of 50khz stuff, so I grounded the box panel and put a bit of copper tape in there to shunt that to ground to keep the air around it quiet.

And that's it. If I do any more of these, oh please let me lay out a PCB for it -- I've made maybe 5-8 of them so far for various things; the basic design goes to 3kv fine with parts value adjustments, so a similar one is also running our 3He tube which has most of the same issues with noise, or worse. The reason I haven't is I don't have a stock of standard little boxes to put them in, you really do need a box, and so I wind up customizing each one to fit in whatever box comes out of my junk-box-box.

This particular one draws almost 20ma under the design load. I could about cut that in half with tweaking the lm 317 resistor values, where most of the actual power is going!
It'll run on 6v or so on up to whatever the lm 317 can take, 40v or so.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
 
Posts: 2526
Joined: Wed Jul 14, 2010 8:05 pm
Location: Floyd county, VA, USA

Re: Generic supply for detectors

Postby Doug Coulter » Sun Feb 13, 2011 12:04 pm

I should note that there are a few reasons I've gone with the approach of putting these in their own separate box. For one thing, they are noisy -- a 1kv 50khz square wave will get into other things unless you take fairly heroic measures -- like putting it in it's own box inside the box with the other stuff! Also, we have found here that things that are very low power -- CMOS, LCD displays, some uP's tend to mysteriously fail when in the same box with kilovolt DC supplies. Almost no matter what you do, some charges will build up on any insulators in there, and eventually discharge into something that can't take it. Capacitors abound, and it's the accidental ones that get you in this game -- anything with a dielectric constant and low conductivity (eg all insulators) are capacitors, like it or not. Even an exposed (unshielded) half inch of well insulated wire with 1 kv on it in the same box with an LCD display has been known to fry them regularly.

I should also mention that to do this, one needs fast diodes. 1N4007 class diodes will simply burn up as they don't turn off quick enough to handle even 1 kHz, much less 50.
Diodes meant for CRT TV use work well, damping diodes and expensive hard to find ones on ebay work well -- all are becoming hard to find as TV's and other high voltage things are moving to other technologies, so really -- I tell you three times -- anything high voltage and good is worth stocking up on, as our Just In Time world of manufacturing is dropping the making of all these parts very rapidly. They are stocking up too to be able to sell you replacements later at 10 times the current pricing long after production is completely shut down.

When using high voltages and high impedances together, you have a recipe for making electrostatic microphones -- anything that can move at all in the field will turn into a signal source. Even a ceramic cap on 1/2" leads makes quite a bong when the box it's in is tapped -- you must lock this kind of thing down mechanically. Also corona noise currents so small you wouldn't normally consider them at all are great noise sources when compared to small signals.

All these problems are easier to handle if the main HV is in its own box. Further, since most applications of this are very low current, the lines between the power supply and the detector can have deliberate series impedance added so that ground loop issues are a thing of the past -- fairly high values of resistance don't affect the output much at all, and serve to cut ground loop currents and "antenna" currents from this source to next to nothing. I tend to use about 100 ohms in the ground lead, and 10k to 100k in the hot lead of these, with a bypass cap in the driven unit. This lets me use the same "Battery replacement" supply in the big rig to run them all and no worries about induced noise in the wiring.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
 
Posts: 2526
Joined: Wed Jul 14, 2010 8:05 pm
Location: Floyd county, VA, USA

Re: Generic supply for detectors

Postby Joe Jarski » Mon Feb 14, 2011 12:27 am

Doug, Thanks for the post on your CCFL supply. After having the flu for most of last week I'm finally getting back into doing something productive and ordered a batch of the inverters to start powering up some of my various detector equipment.
Doug Coulter wrote:Almost doesn't deserve a picture, since it's so simple, but here's what it looks like on the inside.

The pictures/schematics/descriptions still help me out a lot. My electronics background is nothing compared to most of the people on this board, so as far as I'm concerned, the more information the better. It's not that I can't figure things out, but I expend 100X the time and brain power to figure simple things out compared to most of the others here that just know it by second nature.
User avatar
Joe Jarski
 
Posts: 231
Joined: Thu Sep 16, 2010 9:37 pm
Location: SouthEast Michigan

Re: Generic supply for detectors

Postby Doug Coulter » Mon Feb 14, 2011 12:59 am

Well, not quite second nature -- I've been doing this for what, ~50 years now? (yeah, my first projects didn't have semiconductors in them) I'll put up a schiz real soon for this with some notes on how to adapt it for various other things. I'm going to buy another 10 of them myself, these little toys are just so useful in this field it's not funny. But everyone should know I wasn't born with all this knowledge -- I got it the usual way and skipped quite a lot of social development and time doing it. So if I act socially retarded, in things for which others have that as second nature, that's why. No practice.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
 
Posts: 2526
Joined: Wed Jul 14, 2010 8:05 pm
Location: Floyd county, VA, USA

Re: Generic supply for detectors

Postby Doug Coulter » Mon Feb 14, 2011 6:20 pm

Hopefully this will help. Attached is the data sheet for one brand of lm-317. Digikey is a gold mine for data sheets, if you use the search and get all the way to the actual parts listings, then click on the one you want a sheet for, the page that takes you to will usually have a link for the data sheet for that part, and that manufacturer. That's very likely where this one came from, as it's a National Semi part originally, but this one is from another brand.
lm317.pdf
LM-317 data sheet
(493.04 KiB) Downloaded 309 times

The circuit I am using is their figure 3 "basic adjustable regulator", with a 200 ohm R1 and a variable R for R2. In my case, I used a 1k pot, but that will limit maximum output voltage to
1.25vref + 5 x (vref). In other words, this chip makes 1.25v reference between the adj pin and the output pin. The adj pin is an input, draws almost no current itself, and the output pin will always be 1.25v above this if there's enough input voltage available to make that possible. So, we will draw 1.25/200 amps current through R1, or 6.25 ma. This same current will also go through R2, which in turn sets the voltage at the adj pin off ground. So, with say 500 ohms there, we would have .00625 * 500 = 3.125 volts on the adj pin, and add 1.25 volt to that for a total of 4.375 volts on the output pin. The lowest voltage you can get out of one of these with the adj pin grounded is 1.25v. The highest is set by the spec on input->output drop, and is not an absolute number, one can use these in a 300v regulated supply if you are careful.

One trick for a quieter output is to add a capacitor to ground on the adj pin, but I didn't do that here, and putting a big one there needs added protection diodes (see data sheet). I did use a cap on the input, and one on the output 47uF for both in this case -- this isn't critical, I just grabbed the first two caps that came to hand out of the junk box. The input one is required due to the expected long leads to the real power source, and the output one smooths out the peaks of current draw from the CCFL portion of the circuit to help keep noise down. Ripple is not otherwise a worry here, as there's plenty of drop from the 13v input to where we are running -- 3.xx volts in this particular case, adjusted to set the output voltage from the supply as a whole.

Some LM-317 tricks:
  1. The smaller packages can be used to get a lower current limit, and a lower power limit as they also have a thermal shutdown. This does stress the chip, but they cost about the same as a fuse anyway...and are much more effective than a mere fuse.
  2. You can use an LM-350 if you need more current. It's otherwise the same thing.
  3. There is an LM-337 for negative regulators, and all the same tricks apply to it.
  4. All the other examples on the data sheet work fine, as in constant current, programming voltage outputs and output impedances and so on.
  5. You can even use them as an NPN audio output transistor, with the bias built in and in the other direction -- I've done it and it works fine. If you use a darlington PNP and emitter resistors, you can make a bipolar current booster with one, and the bias is free.
  6. You can drive the adj pin with the output of an opamp to close feedback loops. That wasn't required this time.
  7. LM-317's have a minimum output current you have to draw, or the output will rise. They're designed so all the internal drive currents come out this pin. In our case, the set resistor loads take care of that, but in a case where you're skimping on power and/or the load will be above minimum, you can use bigger R values for R1 and R2 to cut the parasitic losses to just the point where the output starts to rise from internal currents, usually lots less than the worst case data sheet spec.

For the CCFL part of things above, the main issues were things like noise, and how much power and output "stiffness" is required. Noise is always with us, and in this case I used the fact that the little board has two ground pins and used one for the input ground and the other for the output ground. The way I wired the board, there's also a grounded wire that tends to shield the adj pin on the lm-317 from the 50khz noise the inverter generates which is considerable -- that's one big square wave with fast edges.

I determined that we just did not need a lot of output stiffness or maximum current. After all, we are drawing sub-microamps during a pulse with the tube, and that's it, I didn't use a bleeder at all anywhere (would be OK to do that, but beware the extra power draw). So rather than substituting their tiny 27 pf current limiting caps with something bigger, I took one out instead, and used that spot for the diode to ground part of the standard voltage doubler circuit. I did use a pretty good high quality ceramic .01uf output capacitor for the output filter. Since I'm doing both parts of this, I also know there's another one of those .01uf caps in the preamp to help filtering. Since I know that, I put a series resistor in the output, so we have a second stage RC low pass filter as part of the design. This resistor is also a bit of current limit, but likely it will burn out if the output is shorted, a 100k one surely would, and arc over in the bargain at high voltage CCFL drives. Here again, it's cheaper than a fuse, so no particular worries.

Something perhaps counterintuitive that I did here was insert a resistor in the output ground as well, in this case about 120 ohms. This is because the bulk 13v input supply is grounded at one power strip, and the counters and audio amp grounded at another spot via a power strip. The result is about a 6 foot loop antenna, and a fusor can make some real EMI, which means ground over here is not the same as ground over there when there's noise present. There can be enough current (it's all fat wire) across this shorted turn to drive a signal effectively into the counters or audio amp. So we break that loop with a resistor, and have a low resistance cable to the audio amp, which grounds the preamp directly to that rather than some random spot on the "one turn inductor" that is our system ground(s). This prevents things like arcs from causing false counts and clicks. In this case, it was better to put these on the output side, because there the current is very low, so there's no appreciable drop across these resistors, and basically no other currents flowing there. Had we done this on the input ground, we'd have let in a bunch of 50khz current noise created by the CCFL input load, not good. One could improve on this by adding some turns through ferrite beads on both the input and output wire pairs, but in tests here (fairly brutal ones) they weren't needed.

Here's the data sheet for the CCFL I use, not too useful perhaps, but for completeness and the mechanical specs.
BXA-12529CCFL.pdf
CCFL datasheet
(147.95 KiB) Downloaded 345 times


Note that I would do a couple of things differently depending on the expected load. For a phototube, which has some real current drain due to the divider R chain, I'd use a larger, maybe even .01uf series cap in the doubler instead of their 27 pf ones, for starters. I'd have to lose the series resistor in the output most likely as well. These changes in turn require more care to not blow out the fuse on the CCFL -- like using the smaller lm-317 with lower peak current limit. Phototubes have a gain curve that is very sensitive to supply voltage, so if I wanted to make a gamma spectrometer, I'd go full boat and use an opamp, a divider off the HV output, and a reference diode to close the loop and have truly well regulated output voltage. That's another post, though. I did make one up like that, and use it as a bench supply for detector testing and it's very handy. Since there's no size issue with that one, I went whole hog and used .47 uf output filter caps, and it will run a geiger tube for minutes with the input power switched off just from the charge those retain (or give you a darn painful shock/burn, there's no free!).

While virtually all counter type tubes want a positive output, which this is wired for, sometimes you need a negative one too, as with some very nice phototubes I have that have the output ground referred -- very handy. In that case, you simply turn all the diodes around. If you need both polarities (like in my bench supply version of this) you simply add the other two each diodes and capacitors and you'll have a nice split supply where both outputs track -- this is what I did for my ion source extraction supply in fact. What's super nice about the negative polarity powered phototubes is that the resistor and dynode currents don't pass through the signal output like they usually do when the other polarity lashup is used. You have to have one more wire, but then not needing any way to remove the HV DC off the signal (transformer or capacitor) is a pretty large benefit, and less noise from resistor current is always nice. Most of the phototube/socket combos out there don't have this feature, but can be rewired in most cases if you want to. If they are sealed, like the little NaI heads we stripped off a PET scanner, then you're out of luck and have to deal with that. You can't just put the load in the ground wire, because in that case, it's also the case/shield ground and will collect noise like a magnet.

Note, I've also floated the transformer secondary off ground as a test, with not very interesting results. I have no idea if that side will stand off the full rated output voltage....but it will float some. You can do this by cutting a track on the PCB on the secondary side.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
 
Posts: 2526
Joined: Wed Jul 14, 2010 8:05 pm
Location: Floyd county, VA, USA

Re: Generic supply for detectors

Postby Joe Jarski » Tue Feb 15, 2011 12:24 am

Thanks Doug! I was on track with a lot of the basic stuff. Once I get some of CCFL inverters in hand I can try some of the other tricks. The mods for PMTs will help too.

You're about ~49 years ahead of me on this stuff. I tend to get stuck on silly things like - I need a 10uF 100V cap, now which of the 3,000 different types should I get? Does it really matter? I know sometimes it does, sometimes it doesn't, but I don't always know when that is. Or things like a voltage doubler with two diodes and one cap - probably simple stuff, but not the way I'm used to seeing a doubler in the books that I've read. I'd spend hours trying to find the caps spec'd on the LM-317 datasheet, where you just grab a couple and go because they'll work. I know it's all experience and I'm way ahead of where I was a year ago, but I have have a long way to go. That's why I try to understand the what's and why's of this stuff instead of just blindly copying it. Spending more time just fiddling with stuff will do me some good instead of trying to design something on paper and have it work perfectly on the first try.
User avatar
Joe Jarski
 
Posts: 231
Joined: Thu Sep 16, 2010 9:37 pm
Location: SouthEast Michigan

Re: Generic supply for detectors

Postby Doug Coulter » Tue Feb 15, 2011 11:04 am

I get what you're saying -- and it helps if I know that so I put in the right stuff, without flooding in too many words so no one can wade through it all -- there's a balance to try and hit.
In this case, the caps don't matter much. .1uf might be on the small side, 1000uf would stick you with a pretty nasty startup current surge. The fact that say, electrolytics aren't perfect capacitors doesn't matter in this particular case -- they only have to be good up to frequencies that the LM-317 can "understand" or oscillate at. I didn't go into that because in this case it really isn't critical that you use some special super good thing (I usually will when the other case is true).

Here's some info on voltage multipliers (that I should probably transplant here). I used the Villard multiplier, just one stage of it in this particular build. In essence, you have a series cap from the AC supply that gets charged on (in this case) the negative half cycle through the diode to ground. In the positive half cycle, that charge is in series with the transformer output, and passes to the output cap through the second diode, the process repeating every cycle. This is essentially the same circuit used in a microwave oven, which doesn't use the second diode or cap because the tube itself is a diode, and they don't want to have it run except alternate half cycles anyway so the tube can recover during the 50% off time. This circuit has also been called the Cockroft/Walton circuit, since they are the guys who made it famous. As you can see on the linked page, one can add however many stages to this for more voltage. There are limits, of course. As you get higher voltages, you have less current available at the top of the "stack" -- just like a transformer. So with a doubler, you have twice the volts, but half the amps possible, and so on (it's a constant power). The lower stages also see higher currents, proportionally. This is why we don't just start at some really low voltage from the transformer and use tons of stages to get to really high voltage -- at some point, there's just too much current for the bottom stages to handle with same sized caps and diodes throughout the stack.

A feature of this configuration, also shared by microwave ovens is that by selecting the size of the first capacitor, you can set a current limit -- a smaller value will have more AC voltage drop across it as the current goes up. This feature is used here, and also in microwave ovens. For the uWave ion source supply, I used this to limit the current to the tube for CW mode, by using a .1uf series cap (instead of the .7 uf normally used), and added back the full circuit with the second diode and output filter cap to provide continuous DC to the magnetron. This series cap is analogous to a simple series resistor for current limiting, with one nice feature -- since it's ac and a good cap, it simply reduces max current, but doesn't actually waste any power like a resistor would -- it doesn't get hot. At high currents, it simply doesn't have enough charge to dump into the DC filter cap to fully charge it, and is discharged during that time. So the size of that cap controls how many joules per cycle the stack can be supplied with. It's all reactive power, so no actual power gets lost (except in any parasitic series R in the cap), kind of slick.

People who first examine the multiplier often think it would be good to have the capacitors at the low end of the stack be larger values to reduce the voltage sag and ripple under load.
But on a second look, notice that if (when!) there is an arc, all those caps are in series discharging at very high currents during the arc. If they're all equal, nothing really bad happens. If some are larger than others, some of the discharge current has to go through the diodes, and more often than not, it's the end of the diodes in question, as peak currents in an arc can get to thousands of amps pretty easily. The top diode on the "AC side" of the stack has to carry the full current of the caps on that side as well, and in some cases, I'll use a higher current diode there to handle that case better, and also use smaller caps on that side, and accept the sag under load because that's better than replacing diodes a lot.

Going backwards, in an arc, that AC side set of caps tries to push back through the transformer secondary in an arc, which can cook things on the primary side of the transformer as well.
Say I have a 12v primary there, and a 1kv secondary. Now I multiply that secondary with 2 stages, for a 4kv output (more or less). When it arcs, I will be trying to push 4kv back into the transformer secondary, which will be 48v on the primary, and it's a pretty irresistible force -- will probably fry the drivers. Two things can help prevent this. One is that if the transformer was already near saturation, the full 4x can't easily get back through it, and second, if you did this full wave, which means in this case you have a center tapped secondary and two of these multiplier chains (which can share the DC filter caps), then the same thing happens on either side of the center tap in an arc, which more or less cancels out. This is one of the reasons why all the big, pro, power supplies do that. The other reason is that one side or the other is always supplying current to the DC output, instead of only every half cycle as in the simpler circuit we're using here.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
 
Posts: 2526
Joined: Wed Jul 14, 2010 8:05 pm
Location: Floyd county, VA, USA

Re: Generic supply for detectors, unit deux

Postby Doug Coulter » Fri Mar 04, 2011 4:36 pm

So, I'm rebuilding the preamp for the 3He tube, and decided I may as well re-do it's power supply too. I had been running it at 3kv, which is right at the limit of the original voltage doubled CCFL running on 14v...A little iffy, and I'm at the point where none of that is permissible anymore, I need this stuff totally rock-solid.

Therefore, I rebuilt the supply, fixing a couple of things along the way. For one, I went to a two stage CW multiplier to get more output volts for less input volts. Along the way I got rid of the .047uF caps I'd been using -- far too much stored energy in those, and replaced them with smaller .01uf caps -- still too much stored energy, but at least not as ridiculous. I'd used 1/8w isolation resistors in the previous version, which were easily exploded by the energy in a couple .047 uf caps, so this time I went with good old 1/2 watt carbon composition resistors for that, should be more reliable in a fault condition.

A bonus of going to the lower input voltage is that I also lost half the quiescent current -- down to 20ma from 40, and since I'm now running the input at around 6v, that means 1/4 the power. In this case, it doesn't matter -- all these run off one of those bench supplies used to fix CB radios and car audio back in the day, but less waste is generally a nice thing.

Since I'd put the previous model into a fairly large box to accommodate the big original caps, I had plenty room to do this up clean and sweet. Here's a pic of the thing in the box bottom, which will be again screwed to the side of the 3he moderator, away from the preamp. I've not hooked up the input and output wires yet.
PS2T.jpg
Top view

I used a spare small cap for the 2nd AC side -- this may as well be a "no load" situation, so the 27 pf is plenty (and the price was right). I didn't really need the spark gap caps here, but they were short enough to fit in the box well after being bent over. Lots of room meant I could spread things out nicely for low corona issues. I will probably want to put some copper tape on the outside of the box and ground it to keep from radiating so much of the 50khz supply noise, but again -- with the supply at half voltage, the first 6 dB are free too.

Here's the back.
P2B.jpg
Bottom view


The other change I made is to the pot value. This time I used 2k to get a wider range. A bit harder to adjust precisely, but less bother than calculating a pot + series R, and this makes the thing more generic should it be put to some other use later on. It is adjusted correctly in the top view picture -- right in the middle, cool.

BTW, in tests yesterday I found I had to take the PS voltage down from 2.95 kv to about 2350 V or so to be in the right gain range for the preamp as built. That will get a little mod and update itself, as it has an issue with the big digital signal it puts out getting coupled back into the input -- a little shield in there should fix that one and get rid of any double pulsing.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
 
Posts: 2526
Joined: Wed Jul 14, 2010 8:05 pm
Location: Floyd county, VA, USA

Re: Generic supply for detectors

Postby chrismb » Mon May 30, 2011 5:16 pm

Doug Coulter wrote:I should also mention that to do this, one needs fast diodes. 1N4007 class diodes will simply burn up as they don't turn off quick enough to handle even 1 kHz, much less 50.


Doug, a little while ago I bought some cheapie CCFL inverters off ebay and stuck in 1N4005's without even thinking about it. I ran them as a full wave bridge with a 5V input [off of a regulator, to stabilise the input voltage] to the inverter, giving about 500VDC out. All seemed well, no excess current draw. Seems to work fine still. I run it continuously into my GM pancake [through a resistor network to tune exactly the right voltage I want]. All is fine, and I seem to get <1% (well, less than measurable) variation across the tube.

I came to rig another of these little chaps up this morning as a voltage doubler. The long-and-short of it all is that after trialling SM2000 and 1N400X diodes, yup, it blew itself up (with a little bit of help - I was showing my boy some pretty pink-purple sparks it would make!! ;) )

Anyhows, that is just a little text confirming experimentally that you need something quicker, but that in full wave bride, for reasons I am not clear on, you might get away with it.

Then the final 'question' of my post - so, what diodes do you use and/or recommend for rectifying 40/50kHz CCFL inverters? Is the trr min to look for simply 1/50kHz, or less, or..?
chrismb
 
Posts: 620
Joined: Thu Aug 05, 2010 7:32 pm

Re: Generic supply for detectors

Postby Joe Jarski » Mon May 30, 2011 6:34 pm

Chris, for what it's worth, I've been using UF4007 diodes with good results. They are lower voltage than the special ones that you see Doug using, but I haven't needed the higher voltage yet and they're readily available from the supply houses.
User avatar
Joe Jarski
 
Posts: 231
Joined: Thu Sep 16, 2010 9:37 pm
Location: SouthEast Michigan

Next

Return to High Voltage, High Power

Who is online

Users browsing this forum: No registered users and 1 guest