High Voltage Electronic Load (500V-500mA) - Page 1
Forum section: electronics / projects

author: motyr
author_rank: Contributor
timestamp: March 08, 2023, 08:25:13 am
content: Hi All,I am looking for a solution to build an electronic load which will be able to work with voltages up to 500V and currents up to 500mA. At the moment I am using resistors only as a load, but this does not allow for a constant current load. Has anyone seen this kind of circuit. It could be for a slightly lower voltage so maybe it could be modified. Any info much appreciated.Thanks

author: Berni
author_rank: Super Contributor
timestamp: March 08, 2023, 08:32:09 am
content: You can use existing electronic load designs and just swap out the power transistors for ones that can handle the voltage.This will likely be BJT transistors since the hotspot thermal runaway problem of MOSFETs in the linear region tends to get even worse at higher voltages. The IGBTs commonly handle even higher voltages, but they are not suited for linear operation.

author: Martinn
author_rank: Frequent Contributor
timestamp: March 08, 2023, 12:13:53 pm
content: Check out the linear mode MOSFETs from Ixys.https://www.littelfuse.com/products/power-semiconductors/discrete-mosfets/n-channel-linear/standard_linearmode.aspxThis onehttps://www.littelfuse.com/media?resourcetype=datasheets&itemid=133ea6f6-8de1-42af-88b0-1ef750f911d0&filename=littelfuse-discrete-mosfets-n-channel-linear-ixtb30n100l-datasheetwill handle 500V/0.5A even at 90°C, although not with lots of margin.

author: motyr
author_rank: Contributor
timestamp: March 08, 2023, 01:14:24 pm
content: Quote from: Berni on March 08, 2023, 08:32:09 amThanks for your reply.That what I was thinking about but for the current I require I will need to parallel couple of them. I was just wondering if someone has already done something similar so I can learn from it.

author: Berni
author_rank: Super Contributor
timestamp: March 08, 2023, 02:00:25 pm
content: That MOSFET could indeed do it.But at these sort of powers you would definitely want to parallel a few transistors to spread out the heat. Just add balancing resistors between the transistor and ground in order to evenly distribute the current across them and then you can just treat it as one giant transistor.There is no special magic in it, just take a electronic load design that you like and stick in components that can go to the voltages you want.

author: motyr
author_rank: Contributor
timestamp: March 08, 2023, 02:52:16 pm
content: Quote from: Berni on March 08, 2023, 02:00:25 pmYes, that is understandable to have a few transistors in parallel. I was considering MOSFETS as those are more widely available in the higher voltages, but trickier to control (so I have heard).Those IXYS/Littlefuse Linear Mosfets looks really good, but quite expensive and not too many are in stock. I will definitelly look into it.Thanks

author: Wolfgang
author_rank: Super Contributor
timestamp: March 08, 2023, 03:06:28 pm
content: Quote from: Martinn on March 08, 2023, 12:13:53 pmAgree. But two things:- dont go to the limit with power dissipation. Use two or three in parallel mode, and use equalization resistors. in linear mode, this is a must.- Make sure you drive the gates from a really low impedance and that fast drain voltage excursions dont kill your gate by coupling back via the gate/drain capacitance. A Zener at the gate is a good idea, IMHO.

author: JohanH
author_rank: Frequent Contributor
timestamp: March 08, 2023, 03:27:34 pm
content: An alternative is SiC JFET. They have negative temp coefficient that prevents thermal runaway. The SiC devices have good DC SOA characteristics.https://unitedsic.com/datasheets/DS_UJ3N065080K3S.pdf

author: tszaboo
author_rank: Super Contributor
timestamp: March 08, 2023, 04:28:49 pm
content: Quote from: Martinn on March 08, 2023, 12:13:53 pmThose FETs are nice, but regular transistors are much simpler to make stable. Unless your sink needs to work with low voltages, I would just use a BJT. No capacitive load, paralleling them doesn't create thermal runaway, and no oscillations that blow up your FET.

author: Wolfgang
author_rank: Super Contributor
timestamp: March 08, 2023, 05:02:47 pm
content: Quote from: JohanH on March 08, 2023, 03:27:34 pmInteresting part. Has a DC SOAR curve, with no derating (suspicious). On the other hand, it needs a lot of negative bias to turn off.Have you seen an active load design using this part ?

author: Wolfgang
author_rank: Super Contributor
timestamp: March 08, 2023, 05:04:25 pm
content: Quote from: tszaboo on March 08, 2023, 04:28:49 pmDont think BJTs will work for HV linear operation at high dissipation. All I know have a minimal SOAR due to second breakdown restrictions.

author: JohanH
author_rank: Frequent Contributor
timestamp: March 08, 2023, 05:38:43 pm
content: Quote from: Wolfgang on March 08, 2023, 05:02:47 pmI read an article that referred to it:https://www.powersystemsdesign.com/articles/normally-on-sic-jfets-characteristics-and-applications/138/14198

author: tszaboo
author_rank: Super Contributor
timestamp: March 08, 2023, 05:48:10 pm
content: Quote from: Wolfgang on March 08, 2023, 05:04:25 pmEither they specify the FBSOA, or you can see it on the SOA if it's made from 4 lines, not 3 for DC. Forward bias is a problem when it's both high voltage and current. And regular MOSFETs have the same issue.

author: jonpaul
author_rank: Super Contributor
timestamp: March 08, 2023, 05:51:51 pm
content: actual load? eg arc lamp, plating, ion source ?load I fix o dynamiquej

author: Martinn
author_rank: Frequent Contributor
timestamp: March 08, 2023, 06:44:08 pm
content: Quote from: Berni on March 08, 2023, 02:00:25 pmI thought that instead of balancing resistors, commonly one duplicates the current control op-amp as well. At least it is done this way in the Peaktech (e.g. 2280) models.

author: Zero999
author_rank: Super Contributor
timestamp: March 08, 2023, 07:39:06 pm
content: Quote from: Berni on March 08, 2023, 08:32:09 amYou're mistaken. There are no BJTs which are capable of operating in the linear region at high voltages, with any significant current flowing. MOSFETs are a much safer bet.All high voltage linear regulator ICs use a MOSFET, rather than a BJT for a pass element.Here are a couple of example data sheets.https://www.onsemi.com/download/data-sheet/pdf/ncp785a-d.pdfhttps://www.ti.com/lit/ds/symlink/tl783.pdfIt's possible to use a linear regulator as a current sink. If there's a large budget, many small linear regulator current sources could be connected in parallel with relays to adjust the current. Another option is to add transistors as pass/cascode elements to reduce the power dissipated by the IC.EDIT:How about the IRFBC40A. The SOA isn't specified for continuous linear operation, but the graph indicates the limitation is purely thermal. In other words, it will be fine, providing the power dissipation is limited to ensure the maximum junction temperature rating isn't exceeded.https://media.digikey.com/pdf/Data%20Sheets/Vishay%20Siliconix%20PDFs/IRFBC40A_SiHFBC40A.pdfThere are plenty of other MOSFETs similar to the above. Low gate charge is more important than low on resistance, since it will make it easier to control and the voltage drop is high anyway.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 08, 2023, 08:12:20 pm
content: Si MOSFET is cheapest.A pair of FQA9N90C will handle that no problem. I have three in my 500V 4A electronic load (the remaining capacity being taken up by a switched bank of resistors).That type is obsolete now, so maybe shop around for something newer, but it should still be a good starting point. High power dissipation per dollar, no 2nd breakdown. Examples abound in all families; even the newest, highest-power-density, SuperJunction types sometimes (often?) have full SOA. (You can tell SJ from the extremely steep drop in Coss by 10-50V, then slight rebound at high voltages.)Tim

author: Wolfgang
author_rank: Super Contributor
timestamp: March 08, 2023, 10:07:44 pm
content: Quote from: Zero999 on March 08, 2023, 07:39:06 pmDont even try this. If it has no FBSOA spec it has a good reason for not having one. Another point is that with every chip shrink the FBSOA behaviour of switching MOSFETs gets worse and worse. And as all the other specs are OK, they wont even tell you about that. The reason why linear MOSFETs are more expensive is that they have a larger chip size, equalization resistors and/or a graded threshold voltage from the middle to the outside.The cheap price of a switching MOSFET in a linear application must be compared to the cost of what happens when the part (and all what is attached to it) goes short circuit in a HV environment. Its might be OK to use very old HV MOSFETS in linear mode if you go up to a small fraction of allowed power, I would never try this with new parts (beware of Chinese fakes).

author: Zero999
author_rank: Super Contributor
timestamp: March 08, 2023, 11:00:00 pm
content: Quote from: Wolfgang on March 08, 2023, 10:07:44 pmThe reason being no one will use it for linear, because it's not the usual application for a MOSFET?Compare the SOA graphs for the FQA9N90C (which is specified at DC) and the IRFBC40. Note they're the same shape.https://www.mouser.com/datasheet/2/149/FQA9N90C_F109-96052.pdfhttps://media.digikey.com/pdf/Data%20Sheets/Vishay%20Siliconix%20PDFs/IRFBC40A_SiHFBC40A.pdfNow look at the SOA graph of a 400V BJT. Note the different shape to the MOSFET. Even at 1ms the current rating steeply declines with increased voltage.https://www.mouser.co.uk/datasheet/2/308/1/FJP13009_D-1809266.pdfAs I said, the shape of the MOSFET SOA graphs indicate the limit is purely thermal, unlike the BJT.It's definitely worth a try. As with anything prototype first.QuoteThe size of the die depends on the on resistance, maximum drain voltage and gate charge specifications. The greater the gate charge, lower the on resistance and higher the voltage rating, the larger the MOSFET. I admit, I did look for a low gate charge device, to optimise transient response and stability, but that would mean a smaller device, thus more of them in parallel to achieve the same power dissipation, so it probably doesn't gain much.

author: Hydron
author_rank: Super Contributor
timestamp: March 08, 2023, 11:04:37 pm
content: If you have the money and also aren't looking to do this as a project in and of itself, there are a couple of options for cheapish 500V loads from the usual chinese sources. I've got a DCL6104A, and there is also a an EastTester option that may or may not be better. They are certainly not perfect (I had to mod mine to reduce it's current ripple, and it's still not perfect as the fan still affects the current sensing adding ripple, and the UI is a bit shit), but do work and are relatively affordable.

author: Wolfgang
author_rank: Super Contributor
timestamp: March 09, 2023, 12:15:46 am
content: Quote from: Zero999 on March 08, 2023, 11:00:00 pmGoogle for the Spirito effect, there you can see how the destruction mechanism in the subthreshold region works.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2023, 01:20:09 am
content: Quote from: Zero999 on March 08, 2023, 11:00:00 pmLack of DC curve is no guarantee of successful operation, nor of failure.It only means, you don't know.Notice the curves at 10us are almost always full area. Is this any indication of 2nd breakdown later? Of course not.It's unlikely, but hardly impossible, for 2nd breakdown to occur in the 10ms+ range.If youneedto know, for design-in purposes: keep shopping.If the part is too useful otherwise (used elsewhere in the design? very cheap?), perhaps qualification testing can be considered. Fine for one-offs (either it works or it doesn't!). But such processes are generally burdensome/annoying in production. YMMV.I will note one example I've tested to work at DC -- Siliconix IRF740PbF I believe, which not only handled 300V DC fine but at apparently ~1.8 times lower than RthJC(max) -- or similarly higher Tj(max), which seems unlikely. If you scrounge around for old datasheets, you may find a DC SOA for this part (ST's version, I think?). Old parts tend not to get die-shrunk (the capacitance would be all out of whack if nothing else?) so you get a lot of die area for your buck. And being lower power density, they tend to be more free of 2nd breakdown. So there is some value there -- but if they don't give DC SOA, your only option is qualification testing.Tim

author: Wolfgang
author_rank: Super Contributor
timestamp: March 09, 2023, 02:15:47 am
content: A lot of people have tried to cut corners in electronic loads and use switching mosfets instead of expensive linear types.One of them is Rigol; their DL3000 series uses just normal MOSFETs as pass transistors (some TO247 IRFxxx type as I remember).And - their preferred failure mode is 150V and maximum current. The voltage and power is way below transistor specs, only the FBSOA obviously isnt.Its a matter of safety and reliability. If your risk is small (hobby circuits) why not try. If a failure is critical and can kill expensive circuits or equipment - better play safe and get a linear.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2023, 03:21:02 am
content: A key part of why I did my design the way I did. The transistors are fed with resistors, which both reduce dissipation / increase SOA, and provide short-circuit protection. I've actually had two transistors cook off over the years, which only ever caused limited failure (the transistor and its gate protection zener). Failures which appeared thermally driven (grease was lacking or absent under the tab, not sure how that happened exactly..).The downside is the minimum "Rds(on)" the resistors set. Not desirable for a wide range load, but I also haven't needed anything between 20 and 200V for some time now. Not to say I never will, but I can make another version for that when the time comes, or just keep plodding along with load resistors.Tim

author: tszaboo
author_rank: Super Contributor
timestamp: March 09, 2023, 11:06:24 am
content: Quote from: T3sl4co1l on March 09, 2023, 01:20:09 amEven with DC SOA, you cannot be sure that the FET will not do thermal hotspotting and runaway. Because the specifications for the SOA are at room temperature, and you cannot just linearly de-rate. There is an entire "no-go" area on the SOA, where the hotspotting happens for FETs. High voltage, and high current, and die temperatures above ~70C is almost guaranteed to kill regular FETs.If this is a testing equipment, and OP has enough experience to make this work, than the easiest way to go is with a linear MOSFET with known FBSOA. Just oversize it, and pay that ~50EUR for a SOT227 Linear FET from IXYS.Thinking about it for a second, I take back, the BJT is probably not a good idea, because you need to parallel them and that can lead to more headache than it's worth. Also smaller FET's like in a TO220 package, you would need maybe 5-10 in parralel to dissipate that 250W. The power dissipation is just too much.Or if you want cost optimized setup, because you want to build a lot of these... Then get ready to blow up some FETs.

author: jonpaul
author_rank: Super Contributor
timestamp: March 09, 2023, 12:16:32 pm
content: Just seeing this long, interesting thread now. Had experience in such designs....same requirements for a series or shunt HV linear voltage or current regulator for power supply.1. Any 600V linear application of FET, IGBT, BJT power devices will encounter a few issuesa. tendency to high frequency parasitic oscillations supersonic to VHF, UHF due to stray L and C and high gain in some areas of operationuse 10..1000 ohm carbon comp in series at base or gates. Additionally some other surpression be used in the power circuit.b. despite SOAR, and other design considerations, the exposure to a fault will still destroy the pass devicec. series or parallel device's connection is more susceptible to,parasitics2. an old school ham would use a tripode or tetrode beam power TX tube (valve, ampoules) such as 813, Penta 3-500 GZ, Eimac 3-500ZTubes Need filament supply, but huge V and I overload capabilities, power can be KW pulsed or over 500W continuous ( fan) and easily replaced on a socket.Unlike the semiconductors discussed, tubes have NO SOAR, overload, voltage transients issues.Have an absolutely fantastic day!JonPS In 1960s, built an improved 0..500V 250 ma lab PSU, based on the classic RCA tube manual and Heathkit designs, used 3 X beam power tubes in parallel, with wide range V, I meters and thyratron relay protection.The massive linear lab PSU is still working!

author: Kleinstein
author_rank: Super Contributor
timestamp: March 09, 2023, 12:39:06 pm
content: One can reduce the power the the MOSFETs by having resistors in series at the drain to take over some of the power and reduce the voltage. This would at least limit the worst case current in case a FETs fails.Having an SOA curve that shows just a power limit is more like a warning sign. There are plenty of data-sheets for switching type MOSFETs that do not show an extra drop at high voltage. Especially when there is also a curve for the dynamic thermal impedance one should be carefull. There is some way to kind of calculate a kind of SOA curve based one the thermal impedance that ignores current bunching. So best condition for a unreliable worthless SOA curve, that just ignores the important point.A vccuum tube is indeed not such a bad idea.

author: Wolfgang
author_rank: Super Contributor
timestamp: March 09, 2023, 03:22:45 pm
content: I love equipment that is built to outlast its inventor.I and hate "compromised" engineering at high voltagesWhich tubes were in there ? PL519 ?

author: Martinn
author_rank: Frequent Contributor
timestamp: March 09, 2023, 05:51:09 pm
content: I got one of the Ixys monsters as a free sample (try it - there's a button for sample requests), but still tubes sound interesting!Are there any design resources - example schematics, calculations... for building a tube HV electronic load?At least this datasheethttps://patric-sokoll.de/R%C3%B6hrenmuseum/Datenbank/PDF/Datenblatt%20PL519%20Telefunken.pdfhas quite a lot of information.I am currently working on a CRT drive (scope clock style) and there's essentially zero documentation on how to properly control these things. Collectively forgotten.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2023, 07:26:01 pm
content: Quote from: tszaboo on March 09, 2023, 11:06:24 amApparently my MOSFETs are dead...Weird that they still work. *shrug*The DC SOA curve is for Tj=150C (or whatever the limit is). I'm not sure what you're getting at. Did you mean Tc? But elevated Tc means less power, less into the instability curve, for the same Tj. It only gets better.Which is the same reason you (almost?) never see 2nd breakdown on very small devices (sometimes TO-220FP, often SOTs). There's just no power handling in the first place, nowhere to go up the instability curve.Quote from: tszaboo on March 09, 2023, 11:06:24 amI mean... good luck finding a BJT with useful power dissipation at 500V. That's rather the more important fact.You need to parallel FETs anyway: it's just as trivial, use generous source/emitter resistors. Assuming you don't mind the added drop (which you surely don't for BJTs, even less is needed than for FETs).TO-220 handles a bit more than that. Common drain/collector, a greased joint can be used, no insulator. (Preferably still with insulation so they can be individually resistor-ballasted, but paralleling is a viable sacrifice.) 50W certainly, and I wouldn't feel bad pushing 100W with one, as long as it's got low enough RthJC to suit. They're available over 300W (Tc=25C) nowadays.Tim

author: mawyatt
author_rank: Super Contributor
timestamp: March 09, 2023, 08:09:37 pm
content: Has anyone placed an incandescent bulb in series with the power MOSFET? Might help make paralleling devices easier and keep within the SOA as with higher currents the bulb resistance increases which reduces the Drain voltage and reduces device dissipation. Not sure if in the Source would be any benefit, as with regular ballast resistors.Anyway, just a thought!Best

author: Wolfgang
author_rank: Super Contributor
timestamp: March 09, 2023, 09:52:25 pm
content: Quote from: Kleinstein on March 09, 2023, 12:39:06 pmAgree. Just to play the devils advocate: Even tubes can short out due to flashovers, especially at higher voltages (>1kV) and when they have accumulated gas due to long periods of inactivity. Otherwise they are a lot more robust than semiconductors, thats for sure.

author: jonpaul
author_rank: Super Contributor
timestamp: March 09, 2023, 10:36:56 pm
content: Seeing some interest in the tubes/valves, Rebonjour a tous.....A few notes....Unsure the OP needed full control down to zero, or a fixed specific value. my design was for 250 ma at 0...400V.1/ The PS was built in NYC/USA we designed with 6080 low u triode series reg tubes but later found some horizontal output/sweep TV tubes with plate caps, beam power 6BG6, used three.Rated at 20 W Pdiss but we added a rotron fan...2/ Any tube will do as triode op is all thats required.Choose by plate diss, socket, mounting/cooling and max cathode I, plate V.But within those limits the max param can be exceeded with littl change of burnout...2/ We used three tubes in parallel, but a singe TX triode or tétrode like Varian/Eimac 3-500 etc are familiar to HAMs and broadcast folk.3/ The original designs were repeated in many magazines, kits etc.Popular Electronics 1959, Electronics World 1962, QST,Professional lab PSU made by the large PS mfg like Kepco, Lambda.All based on the original 1940s RCA app note.4/ The series valve regulator is easily adapted to a constant or variable voltage lab supply, or constant current.5/ Adding a series R to the pass element indeed reduces pass diss but limits the I and affect the min/max output V, I. Not good fore a wide range spec.Pix is various views of this now ~ 60 yrs old dinosaur, still capable of service. Note the deteriorated caps and insulation!6/ Can post a hand drawn schematic or likes to the varions refs and articles if of interest.PM me for deeper details or if OP wants contact.Bon Soiree,Jon

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2023, 10:37:02 pm
content: Quote from: Wolfgang on March 09, 2023, 09:52:25 pm"But other than flashover, gas leakage, cathode evaporation,what have the [vacuum tubes] ever done to us?!""...Consume heater power?""OH SHUT UP!"(To the tune of: )"Robustness" might be a better word? Specifically with respect to overload. That's the only respect I can think of they're any more reliable than semiconductors; the heat capacity is pretty high. But that's not saying much, as SOA is easily managed with proper design or controls (i.e. just turn it off within some ~100µs after an overload).But a tube is extraordinarily hard to recommend when MOSFETs are easy to use, cheap and readily available, compact, and handle considerable voltages. Remember the largest receiving tubes are only 50W or so, while transistors are regularly many times that. And such tubes aren't very cheap these days: they're in fair demand for audio, some transmitters, and restoration purposes. To get more power, you need many in parallel, or transmitter types that are even more bulky and expensive -- and few of which have agreeable dropout voltages, costing you even more dissipation.Tim

author: tszaboo
author_rank: Super Contributor
timestamp: March 10, 2023, 01:30:41 am
content: Quote from: T3sl4co1l on March 09, 2023, 07:26:01 pmIt just means you got lucky. I qualified dozens of types of FETs that were supposed to be OK just to see them die. And the designs were built into machines that are operated almost 24/7, and we made thousands of these. It's not simple, and the graph alone is no guarantee. You can push a TO220 to 75W, if it's about 0.5K/W rated, and you have a huge heatsink. Above, you are rolling a dice with it's reliability. TO 247 is somewhat better, because its not a "trendy" package, and the FETs are made on larger processes, but still, selecting them is not easy (or you pick a Microsemi or IXYS linear one).But you can figure this out. Look at the transfer characteristic of a NFET. There will be a few lines with different temperatures on it, they typically cross at the same point, at around 3-4 volts. So if you are below this (you are, the current is less) then at the same gate voltage, a warmer FET will conduct more. Now, imagine the package of a FET as a bunch of tiny FETs in parallels, with the same gate voltage. The one which is hotter will conduct more, which will make it hotter...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 10, 2023, 01:58:41 am
content: By your logic I don't seem to be lucky at all, doomed rather?(SOA is DC constant-power from 30mA to 9A so that's basically the whole range here.)Aha, your image is linked from here,https://www.rohm.com/electronics-basics/transistors/understanding-mosfet-characteristicswhich on the related page discusses SOA, and I don't see anything about these effects mentioned. It's simply...that's the point of the SOA curve. It's plotted taking all that into account already, or it's simply wrong. There's nothing else to do from a design standpoint. The remedy is legal not technical, at that point (in theverylimited sense of, being misled by false advertising and buying whatever few parts one did; obviously there's no contract terms or liability beyond thatat best).It sounds more likely that you had thermal cycling failures or something like that. Also missing from this is manufacturer weigh-in, surely you reported the failures and brought in an FAE to investigate them? Surely they would be highly interested in failures within their published SOAs.Which is probably what I've seen on my unit, seems like migration of grease, maybe the grease is old, too much applied (or not enough?!), uneven clamping, no idea. If it keeps happening, I may switch to spring clamps (instead of a single screw), change the grease (maybe one of those super-thick types popular on CPUs?), or use a thermal pad (hm, phase-change would probably be excellent for my application).Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: March 10, 2023, 08:31:53 am
content: For the MOSFET SOA curves a problem is that there are quite a few false SOA curves around, especially those calculated from transient thermal impedance. These can be OK for the short time scales like < 1 ms but not for the longer pulses or DC.Another problem with the SOA curves is that most of them are typical values. Only special parts are actually tested at high power so that they can guarantee the SOA curve. A few audio grade BJTs and likely the linear rated MOSFETs are such candidates. There is a chance for production faults (e.g. an air bubble in the solder to attach the die or contaminations).At the very least one should to a test of the ready made electronic load before actually using it, so it won't fail with a more sensitive circuit.Using vacuum tubes would need tubes rated for high power - not the normal receiver tubes, but more like those for power amplifiers and these tend to get expensive by now.Compared to this even the linear rated MOSFETs are not that bad in the price.To reduce the problem with a fail short, I would consider multiple (e.g. >3) paths in parallel and series resistors with the FETs. If than 1 fet fails short the current to that channel would still be limited to a somewhat moderate value and the other FETs would more or less turn off. Even if the resistor only takes off 30% of the power this would be 30% of the voltage and this can help quite a bit with the SOA limit. A typical use for such a high voltage load would be testing a power supply for a tube circuit and thus a relatively well known voltage / power range. So one could choose the right series resistors.

author: Wolfgang
author_rank: Super Contributor
timestamp: March 10, 2023, 09:08:52 am
content: Quote from: Kleinstein on March 10, 2023, 08:31:53 amAnother trick than adding series resistors that reduce available range is to use MOSFETS not only in parallel, but also in series. The idea was that if one of them fails the other one in the series chain can still handle the voltage and power and the instrument turns of gracefully. I have seen this in an older HP power supply.

author: tszaboo
author_rank: Super Contributor
timestamp: March 10, 2023, 10:07:11 am
content: Quote from: T3sl4co1l on March 10, 2023, 01:58:41 amThere are just so many factors, that can make a FET ok or fail in these edge cases. A tiny bit of different thermal conductivity, the clamping method. From the same series, one FET can be OK and one can fail. Die sizes can be different between two very similar FETs. And most manufacturer is not going to test this, they just put theoretical values based on a few measurements into the SOA. And extrapolate. Or DC often times will mean 1s, which is not enough time to break the FET.https://www.ixys.com/Documents/Articles/Article_Linear_Power_MOSFETs.pdf

author: jonpaul
author_rank: Super Contributor
timestamp: March 10, 2023, 12:27:07 pm
content: " there is no 'safe'"...John Gotti, 1970s, the NYC " Téflon (mafia) DonSOAR, safe area was conceived at RCA semiconductor in NJ 1960s for early BJT switching power transistors.Unfortunately, despite the SOAR ratings, the blowup of power switch devices was still frequent.Transient thermal characteristics must be consideredhttps://toshiba.semicon-storage.com/info/application_note_en_20210331_AKX00049.pdf?did=63510Later research exposed many other SMPS switching fault mechanisms besides thermal.The adaption of SOAR to FETS,and other modern switch devices does not Change the situationUse of switching transistors and FETS in linear mode may not be covered or specified in the device spec,sheet.Jon

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 10, 2023, 03:56:02 pm
content: Many moons ago, I stumbled upon a German-language website which had implemented a 500v E-load using one of those monstrous Russian valves. Of course, to be able to have a 250W plate dissipation would still require to parallel several valves.The combination of filament and plate dissipation, would certainly keep you warm in winter!! With Russian valves, the ultimate irony.Unfortunately I can’t remember the website’s name.

author: max_torque
author_rank: Super Contributor
timestamp: March 10, 2023, 04:20:56 pm
content: Quote from: motyr on March 08, 2023, 08:25:13 amWithout wishing to derail an interesting thread, but the OP need to consider a couple of facts:1) There project is presumably NOT about making a HV DC load, therefore, if they actually want to complete their initial project they should just buy a commercial DC load that is suitable rated and get on with there actual project! Chances are they can sell that unit after they finish there project for a decent amount of the original cost and recoup most of their outlay2) Do they actually need a full bandwidth DC load? A relay or two and a couple of resistors makes a simple switchable load. Choose your resistors carefully and you can form say a 3 bit load, giving you 9 levels of current consumption. If you want to just test your project at some different loads and to see how it reacts to a change in load, then this approach is a LOT less money and effort.(personnally, i'd just buy a commercial DC load and get on with my actual project.........)

author: Martinn
author_rank: Frequent Contributor
timestamp: March 10, 2023, 07:33:29 pm
content: Quote from: max_torque on March 10, 2023, 04:20:56 pmbut then I'd never came across the idea to use a PL519 as pass element...Lots of grids btw. Pentode probably means three grids - who needs three grids? OK, dual gate MOSFETs exists, but three?! And how to connect all those?

author: pqass
author_rank: Super Contributor
timestamp: March 10, 2023, 08:09:13 pm
content: Quote from: Wolfgang on March 10, 2023, 09:08:52 amOr use switchable Vbe multipliers (a couple or three) in series with the MOSFET.As shown in the attachment, using 300V darlingtons, each can drop 145V (@0.5A = 75W), leaving the MOSFET to pickup the slack.Default them to on and as the voltage across the load goes lower, turn each off (for minimal voltage drop of 4V).http://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWcMBMcUHYMGZIA4UA2ATmIxAUgoqoQFMBaMMAKACURtC9xCquezFFBEAWWnBFUYCFtgz9uvRTxQpR4KSwBmnNCFHZhAg0b3hoSVCwBOeqoeEoEhU8Kpg4LAOaclCUQ0TF2koFgBDcAxXRxBnGLNiOKomZHhJGEhKLIwwIkgUPGIwMitYJE90lgB3KJjRQWiDPGkauuaVDrDak0CeXpawuwHOgI1paFYABwNJUTAYuYbNVtrxBwW-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-Ww8AHl-Q02L4OIcPwwjiIjfRm27KiW3jOJGzQVRG2bFEJIZJoFlaOxsVcHjVP42svFqTTlC4vjvkMnjuNLFFeKMwgTLCXUmHYzsGFsmTQggBhRFILkR3kDBrWiZ0kAYTJ0hKShsDwLguAFQokkqEBVx9bV-RcjQ3jSgN+GILwABkQEcpxGx3NzSTVTVmToag9l8QqA2I-p9FaeQqFc38mC00sAJeAqhFKurCy0WoBv0TKY3BDq4lLSbBu+GbRtc8a2km9q2ss-YzAuR4tvBVy1JANa2KGg70q6iznlqRaoWshEoJmNqMGEK7XG64bNlugrntuSbYky2IJveqEfrMV7evY37XP+nqBqDOrYm6lqTs4IHAZerRar6368m2MxWnymHhBK+kyvCCqqqsVLFqUP7cc4HK9gcvribGoMPAK6ldyPBA5RQUgrV58gQsY+BLSirhHoCe1sUgMg0kSp8dTaazMCe9LHtuQyPpulMDKaFXjLidW9g0potb1o2wyUtCkzMo2ULQht+CTY3gyTTsQykrxOO1l7uPV4RcIIoi8ntqNjCpVnoM1qF3cRbBELJZDIpEAAxWEEuyXqQFygB7cIlRZABLPCpnVbUWQANxznVwm8Og5FUCA09JTJgogABJSry+ZKua7rlggA

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 10, 2023, 08:53:51 pm
content: Quote from: Martinn on March 10, 2023, 07:33:29 pmUsually beam tetrode, but true power pentodes exist too. Just tie g3 to k, or sometimes a modest voltage (+30V or so, a large value resistor divider will do).g2 acts like a virtual plate, meaning Ia is mainly defined by it (as a triode's plate current depends on plate voltage), but it doesn't intercept all the current; the rest is allowed to pass through to the plate proper. This gives the sharp knee of the plate curve, much better efficiency than a triode. Though you might not be able to take advantage so easily, as for example a cathode follower in a PSU, you need an isolated supply to do that from the high side. The alternative is tying g2-a which is called triode mode.g2 draws modest current in the process, and variable (depends on Va, Ia), so it really needs a fixed supply, not just a resistor divider.Basically any electrode above cathode voltage can draw current, though how much depends on its cross section. Grid current (g1) is pretty significant and also it's right by the cathode so can't dissipate much power before overheating. Screen is designed to pass current, but how much depends on type (~1/4 of plate current for many signal pentodes, to 1/20th or less for aligned-grid power tubes), and also it goes way up when there's no plate voltage around to sink what current would otherwise pass the screen.So, it's like a dual-gate MOSFET, except the one gate draws current so it's more like cascode with a low hFE (4-20+) BJT on top. Except Va can saturate somewhat below Vg2, not the sharp Vce(sat) the above would imply. And the FETs are all in triode mode all the time. (Tubes are more like a SIT (static induction transistor), but no one makes those anymore, or, they're very obscure anyways, so that's not a useful reference.)Tim

author: motyr
author_rank: Contributor
timestamp: March 10, 2023, 09:20:30 pm
content: Hi,Thanks for all of your replies. It looks like it has started really nice discussion here.Just to clarify few things and maybe answer some questions people have asked about what and why.- the DC load will be used to test various power supplies in the range between 300-500V and currents from 10mA to 250mA (at the moment but there is a talk about going up to 500mA).- those power supplies are already designed or being designed along with some other staff.- we are using resistors at the moment but this is a little problematic as we have to use different values for different voltage outputs to maintain desired current.- was thinking of the commercial DC load but have only seen those which are rated at 5-8A at this voltage levels (unless I have missed some with lower ratings) and the prices are quite high (I will get an approval for it but not too easily). Unfortunately the company I work for is very reluctant from buying anything from China (don't ask me why, as I don't know) so that option is not available.- as for the control it does not need to be from zero volts, as low as 100V will be suficient. I am more interested in controling the load current.I really like the valve idea but at this point my knowledge of this subject is very limited and probably sourcing parts may be quite problematic so will probably leave it for now.I think I will try to get some linear MOSFETS (will need to check what is available through Mouser/Digikey etc). I have some of them from my previous job but for lover voltages, when I was working with audio power amps.I will also get some standard MOSFETS with good ratings and will build some prototype boards t ocheck how they behave. Once I have it built I will report here to share my findings.Thanks for all of your input.

author: jonpaul
author_rank: Super Contributor
timestamp: March 11, 2023, 01:52:16 am
content: Teslacoil....Thanks to clarify the spec.We used many power resistors and rheostats over the decades to make such loads.The vacuum tubes are easily avail in any desired V, I rating for hams and broadcast, see ARRL, QST, Ham forums and ham fleas.Any triode or beam power will do.We have used 813, 811, 3-500, 813.Trick is the sockets and filament traf.For BJT/FET, besides the other issues mentioned, a series connection of pass devices in linear needs a voltage divider for DC and AC, and floating control drives, thus more and more complex as # devices grows.In case of a fault eg one device shorts, all other will be overvoltage en fail in sequence.Jon

author: Martinn
author_rank: Frequent Contributor
timestamp: March 11, 2023, 04:00:36 pm
content: Quote from: T3sl4co1l on March 10, 2023, 08:53:51 pmThanks a lot for the explanation!I'm just wondering if for an electronic load application the pentode or triode mode would be better. The PL519 datasheethttps://patric-sokoll.de/R%C3%B6hrenmuseum/Datenbank/PDF/Datenblatt%20PL519%20Telefunken.pdfhas characteristics for both. It specifies g2 voltages from 175 to 205 V (80 mA max), which would require a separate power supply.Pentode mode seems mostly constant transconductance (like a MOSFET in saturation region), while triode mode is more... resistive? Not sure what's preferable from a stability point of view.Apart from that, it seems you could replace the MOSFET in a classical electronic load circuit with a triode (on top of the shunt resistor). You'd have to level shift the op-amp output from 0...10 to -80...0 V (assuming around -80 V cutoff, don't see the spec in the data sheet).Are there any specialties for anode voltage going down to 0 V? As I understand it, the g2 voltage accelerates the electrons independent on anode voltage.If I wanted to try this (I have a 400V 1A chinese lab supply for testing), what tube would you recommend buying? I honestly have no clue, PL519 was mentioned, 6L6GC seems popular for audio amps (and a less exotic 6.3 V heater voltage) and is widely available.

author: djerickson
author_rank: Regular Contributor
timestamp: March 11, 2023, 05:38:03 pm
content: What's wrong with a beefy IGBT? My DIY-E-load uses a surplus IGBT Half-Bridge module and I regularly dissipate > 100W. IGBT modules are designed for abuse: big power dissipation. Stay well inside the SOA curve and provide lots of cooling.This one should do the job. You need 500V at 0.5A, so 250W. This one is rated 1000V at 185A, and 500W dissipation. $50. You'd be well inside its SOA curve.https://www.digikey.com/en/products/detail/vishay-general-semiconductor-diodes-division/VS-GT180DA120U/8854201I like the compact SOT-227 packages. There are beefier packages available.If you don't trust one part for 250W dissipation, use 2 in parallel. That's a mere 125W each.Dave

author: jonpaul
author_rank: Super Contributor
timestamp: March 11, 2023, 05:48:00 pm
content: IGBT is designed for on/off sw, not linear opj

author: jonpaul
author_rank: Super Contributor
timestamp: March 11, 2023, 05:52:14 pm
content: no need for screen supply or constant currant tube!Just any triode or tetrode/beam power triode connected will work as the const V/current you seek is regulated by the feedback loop.Thus the device characteristics are not critical except for the Pdiss, Vp, IkUnfamiliar with EU/Russian tubes but most are knockoffs or equivalents to USA mfg.We used 3 x 6BQ6 Hor Out tubes in parallel, for a 400V 250MA linear lab PS.See ebay and ham radio fleas for large TX triodes and sockets.Bon ChanceJon

author: Martinn
author_rank: Frequent Contributor
timestamp: March 11, 2023, 06:38:20 pm
content: Quote from: jonpaul on March 11, 2023, 05:52:14 pmHam fleamarkets are rare these days...Can't search ebay for "large TX triode" - found a list:https://www.angelfire.com/electronic/funwithtubes/power.htmlLooking for max. plate power gives... 6CA7/EL3, 6L6GC...?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 12, 2023, 12:51:57 am
content: Quote from: Martinn on March 11, 2023, 04:00:36 pmWell, let's see. For the present case, 500V 500mA, you need maximum 250W. Na is anode dissipation, 45W. (Note also combined a+g2 max 46W, and g2 rating alone, 9W. Design center ratings are somewhat archaic; they were used to account for nominal tolerances of components, voltages, etc. We're controlling everything, so can use maximum values from the notes.) So you need six of them to start with. A bit less with resistor ballast; not quite half I think, maybe three or four.Triode mode would be fine, I think (I'm not clear actually on when the 400 or 700V rating applies?), but notice you have 40V 300mA of heater power per tube, preferably in series so three will take 120V and four will take 160V. 160VDC could be rectified (and isolated!) 120VAC, and also used for screen, so that's not a problem.Notice the screen needs a current limiting supply, in case anode saturation occurs. This will mean anode dissipation is small, so we can use the upper screen limit, 9W. 9W * 4 tubes = 36W, 36W / 160V = 225mA max, which could be a foldback current limiter (a suitable PNP with heatsink could do this; obviously, select it for adequate SOA!), or really just a dumb resistor will get the job done. Normal screen current is in the 10mA range: see page 5, at 160V and 125mA Ia (500mA total across 4 tubes), Vg1 ~ -32V and Ig2 ~ nil. Well, it ain't gonna be zero, but it's less than 10mA anyway, so it's very efficient in terms of Ia/Ig2 ratio and certainly less than 40mA total is required; a resistor dropping say 20% voltage at this would be fine (i.e. 32V / 40mA, call it a little more or 1k just for convenience), and then short-circuit is still well under the 225mA limit, which is actually more now because Vg2 drops as Ig2 rises, and... And, we might use 3.3k per tube instead, so we have even less to worry about any given tube's limit. (In other words, I used single-tube ratings going into this section, and ended up designing across all four in parallel -- that's just how low Ig2 is in this condition.)If you want a semiconductor control, it can be cascode mode with a smaller MOSFET/BJT driving the cathode, then g1 is tied GND, g3 tied cathode, and g2 resistor to +160V. Heaters in series across +160V (DC heating, how luxurious!). Then anode to resistor to load.If these were low-voltage heaters (EL519, etc.), I wouldn't worry about the screen supply, and use them triode mode instead (tie g2 to a, everything else the same).And the cascodes can be four transistors, one per tube, with a generous degeneration resistor, to further enforce current sharing. Just one in common could be used (cathodes tied), the tubes will match well enough; some derating should be used to allow for part-to-part variation (i.e. use design-center ratings instead). Voltage handling is at least 30V, since as much  Vg1 is needed at maximum Ia. Cutoff isn't listed(!) here but around 100V maximum. Note that, as Vk rises, Vg2k falls -- effectively we're dropping screen as well as grid voltages jointly, in the cascode configuration. This helps out, and certainly more than 160V rating will not be needed.Anode resistor should be... let's see. It makes a parabola, where power starts at zero at Ia = 0, goes up towards a peak (maximum power point), then drops again as Va drops faster than Ia rises. We want this positioned so that Pa(max) is not exceeded, and may have further limitations from Vload(min) (say if we want 500mA at 50V, we'll need less than 400 ohms per anode). Note the cascode drops say 30-100V as well: effectively Va(max) is reduced by as much. (The cascode can still saturate quite low, as can the tubes, because Vg2 and Va track Vk as everything pulls down towards zero. It's a whole telescoping pile of saturation. The 30-100V figure is only true out of saturation.) If we say 200V drop at 125mA, that's 1600 ohms per anode (and a 39W maximum power point per tube).I'd have to draw it out / simulate a range of scenarios to see the overall maximum power, accounting for cascode and ballast resistance, and the full voltage and current range. Something in this ballpark seems reasonable, at least.Note that screen current introduces error in the cascode's ground return; we could use an NPN just as well, since its base current contributes in a similar way. (I.e. Ie = Ic + Ib, and Ic = Ik = Ig2 + Ia. These are small errors, a few mA probably, but definitely error we can avoid.) We can use an op-amp to set total current as usual, but it needs to be from a shunt resistor in series with the source's ground return. Which is fine as long as the source and load don't need to share common ground. (If you're planning on digital control, say, consider an isolated serial comm port or something like that. A damn good idea anyway, if you plan on testing non-isolated sources from time to time.)Lifetime -- expect ballpark 5khr near ratings; that's better guaranteed for industrial tube types, and commercial grade you'll just get whatever. Less if they've gone gassy in storage of course, and probably even less if used. They may last 10khr+ at reduced ratings. Gas evolution and declining cathode emission are probably the most common failure modes. I would recommend buying at least double stock (so, 6+ tubes) and keeping them on hand for replacement as needed. On the upside, you have a metric fuckton (technical term) of cathode capacity here, so it will take a truly exhausted tube to fail to function here. Some status/monitoring circuitry may be desirable, like tracking the Vg2 or Vk of each tube just as a low priority measurement, or maybe with a warning/error/fault indicator if they become too mismatched. You can read the health directly off the voltages and replace whichever one is weakest.And, short circuits are -- maybe unlikely in this service?, but something to beware of. Anticipate a cascade failure, and design accordingly. Probably the cascode (bottom) transistors should be considered expendable, but series resistance and a clamping TVS can protect the control (op-amp?) from further backflow. Treating the tubes individually helps limit fault current so that's nice.Overall cost: expect to spend a good couple $400 if buying everything off the shelf. Everything should fit in a, oh, 15 x 20 x 40 cm enclosure, maybe? Fan recommended, so, ventilation holes, and metal is nice for fault protection (earth it!). Steel or aluminium is fine. (The circuit should be isolated from chassis, so it can be floating, allowing for non-isolated supplies.)In comparison, the solid state version spends $30 on transistors even if you splurge for "linear rated" types, heatsink can be salvaged or maybe $50-100 if you need to buy fancy and new, and everything fits in a smaller enclosure. Reliability is most likely limited by thermal performance; possibly spring clamps and thermal pads are better than screws and grease. Mind that the transistors either need to be isolated from the heatsink; or the heatsink isolated from ground, which some plastic/fiberglass/ceramic standoffs can achieve no problem. Assuming no thermal related failures, lifetime is practically unlimited, or at least I would guess in the 100khr ballpark.Remember you're also burning 48W of heater power for all those PL519s (plus a bit more for screen, if not using triode mode and when at low Vload); efficiency of a load dump doesn't seem like something that would normally come into play, but, well... Using slimmer types like 6L6GC will get you better "efficiency", somewhat at the expense of saturation voltage, and the question of screen voltage becomes more important (you get excellent anode saturation voltage with 6L6, but quite a bit of screen voltage is required; this is because the much smaller cathode has less perveance i.e. less Ik for given applied electrode voltages). 6L6 are in current production, but most suck -- you need the -GC version for high power dissipation (30W vs. 22 or so for -GB and etc.), but many are sold under that designation which start glowing red at a fraction of the rating! Overpriced with cheap construction. And since they're very much in demand for audio purposes, you'll be paying quite a premium for genuine old stock parts.Basically your choices are audio power output tubes, radio transmitter tubes, or horizontal deflection (sweep) tubes. A few vertical deflection and supply regulator types are out there with modest ratings as well, but not as high, and you need modest voltage ratings and raw dissipation. And sweep tubes are well known these days for their power handling ability so their prices tend to track audio output tubes.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 12, 2023, 01:09:29 am
content: Quote from: djerickson on March 11, 2023, 05:38:03 pmWhat SOA curve? You link a part that ends at 6ms... It's also more expensive than anything else mentioned here (except tubes), and in particular about four times more powerful than required (assuming DC SOA is available), and ten times more expensive than the cheapest alternative.Mind, not to say they'reimpossible. Shockingly, I have seen IGBTs with straight DC SOA curves. They're not common, and I have no idea how they pull that off, and I would definitely test parts before designing them in -- but it seems at least a few manufacturers are willing to put that on there.Tim

author: jonpaul
author_rank: Super Contributor
timestamp: March 12, 2023, 02:13:11 am
content: Tube valve,,searchesTransmitting tubes...see RCA.transmitting tube manual813 811a 803 3-500 3-250T V and receiving tubes like power triode, ,/ beam power 20..50W PdSee RCA receiving tube manual6L6, 6080, 6BQ6,Ham Radio FleasUSA... see,ARRL, QST. biggest in Dayton OH in MayUK see,BVWS " boot sales"EU see your country's ham radio groupsLargest is mid June in Friedrichshafen Germanywe also see local person to person ad sites with old ham valves and equipmentFinally many colleges and firms just discard old valve,equipment, still working.Bon courageJon

author: Zero999
author_rank: Super Contributor
timestamp: March 12, 2023, 02:40:17 pm
content: Is it okay to have discrete steps?You could build many lower constant current loads in parallel and enable/disable them to vary the current. For example, 8mA, 16mA, 32mA, 64mA, 128mA & 256mA, would give 0 to 504mA in 8mA steps. It would be very easy to control with an MCU and an implement isolation for safety's sake. If greater resolution is required, add more steps i.e. for 2mA resolution add 4mA and 2mA.

author: Martinn
author_rank: Frequent Contributor
timestamp: March 12, 2023, 06:28:07 pm
content: Quote from: T3sl4co1l on March 12, 2023, 12:51:57 amThanks a lot for the detailed explanations!Being a tube newbie, I'd go for a single tube setup first. Anyway I am fiddling with a scope clock PSU and power is in the order of 250 V, 10 mA so a 30 W tube should be fine.I found an LTSpice model for a 6L6GC so I played around with that one (see attachments). First was a straight replacement of the MOSFET with a 6L6GC in triode mode. Works, but needs a -80 V (in the simulation) bias to turn it off.Nice idea using a cascode - eliminates the negative HV bias, at the cost of having to find a 150 V MOSFET with DC SOA spec (why does mouser have no selection box for that!). I used an IRF530 (close enough) in a simulation.What about the g2 power limit - is ist also relevant in triode mode?Triode vs. pentode/tetrode mode: In the simulation, the saturation voltage with g2 driven high is much lower than in triode mode (which makes sense, as the electrons get an extra acceleration by the g2 field). However, I'd need an extra 250 V or so bias (with more than 10 mA), which would be an extra complication in a +-12 or 15 V (or +15 only for a single supply OP like OP777) setup.Also the g2 current would be missing in the low side shunt (best solution is probably as you suggest a low side source measurement).So I guess if I do a test setup it would be triode only.For a first test setup I'd probably go for a 6L6GC (40 EUR!), +-15 supply and heater from a lab supply. Blowing this up to a full blown digitally controlled unit is probably not worth the effort. I thought about a wide range (1 uA-10 A) HV load, but I guess I rather design this around the IXYS monster they once sent me as free sample. Still I have never designed a tube circuit so that would be a nice learning experience!

author: jonpaul
author_rank: Super Contributor
timestamp: March 12, 2023, 08:09:46 pm
content: 6L6 is costly as popular in vintage guitar amps and hifi.Many other beam power, triode work easily some with easier bias....We have seen 6080 dual low u triode, old TV horizontal output tubes, etc.I would look further before spending €40 on a 6L6.Finally reading your applications, I would keep it simple....get a few fixed power resistors, or rehostats easily available and cheap.We tested and designed dozens of power supplies at 100...2500..12kV with such resistance loads.Jon

author: Martinn
author_rank: Frequent Contributor
timestamp: March 12, 2023, 08:57:00 pm
content: Quote from: jonpaul on March 12, 2023, 08:09:46 pmDo you have any specific suggestions for 20-30W triode that I can order in a tube shop? At least 6L6GC has simulation models.Rheostats: For me this is about learning tubes. If I'd need an electronic load, I'd simply buy one (or use resistors as load).https://www.aliexpress.com/item/4001037646977.htmlFr. 130+50 DHL shipping - would be worth a try.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 12, 2023, 10:28:52 pm
content: Triode mode doesn't care much about screen power, no.If you have a floating supply for screen power, returned to cathode, that solves the current error problem -- and probably isn't much more bother, as far as adding another supply goes. But yeah, triode is fine with cascode.Tim

author: motyr
author_rank: Contributor
timestamp: March 13, 2023, 12:03:36 pm
content: HiThanks for all of your replies.I don't think building multiple fixed loads and switching them if needed is a good option for us as it will overcomplicate the whole thing. We are doing it now with different values of resistors.One idea was to have binary load resistor (100, 200, 400 800, 1600 ...) so you can select any value in 100 Ohm steps, but that will require lot's of relays and resistors (still an option).I will not go for the tube design as I am not sure if anything fails other engineers will be able to handle it (eg if I am away or no longer with the company).Linear MOSFETS seems to be the best option for now, and I will try to order some with the correct ratings.Thanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 14, 2023, 01:37:13 am
content: A power DAC of equal value resistors is feasible. Drive them from switches (small MOSFETs will do) and an LM3914.Which is a bargraph display driver, yes. It is less well known as, but better described as, a unary "flash" type ADC. The perfect complement for a "power DAC"!The downside is the recovery time between input voltage change and resistors switching out, and you still need a linear sink to fill in the gaps.Dirtier still is an SMPS (buck or boost, take your pick) dumping into a load resistor, which doesn't really need to be "dirty" as such, but your compromise is between switching noise and filtering bandwidth -- the heavier filtered it is, the slower it can respond.Tim

author: flowib
author_rank: Contributor
timestamp: March 14, 2023, 03:55:47 am
content: Lets see.I have a friend that made exactly what you are asking for with pair of EL34 pentodes and a TL783. No reason this solution cannot be scaled to 250WYou dont need triodes if the PSU's under test dont dip under 200V output or so. The most common tube in current production is an EL34. Which maxes out at about 28 watts plate and G2 dissipation combined.You can use a TL783 as a current regulator in the cathode lead of say 10 paralleled EL34 pentodes wired as triode. Basic schematic is literally half a dozen parts.Alternatively you can use say eight TO-220 Mosfets. LT1010 driver and a Opamp. You'd need some serious heatsink right around .2-.3K/W though.Older mosfets part numbers i have some experience with are : BUZ80. And IRF840.The mosfet approach requires that you either use a opamp that wont go unstable with gate capacitance. Or use the afformentioned LT1010.

author: flowib
author_rank: Contributor
timestamp: March 14, 2023, 04:33:15 am
content: Heres a quick sketch i did for a variable load with two EL34, Posting it here so it may help someone else.Be mindfull of the min current requirement of the LM317HV or TL783 for regulation. 3.5 and 15mA respectively.I chose to run a doubler off the 6.3V for the filaments to furnish a +12V for the control grid. This will increase the voltage the IC sees by just as much, but it improves efficiency.

author: max_torque
author_rank: Super Contributor
timestamp: March 14, 2023, 11:42:01 am
content: Quote from: motyr on March 13, 2023, 12:03:36 pmInteresting use of the word "overcomplicate"..... ;-)Congratulations you now have a new project!chances are, this HV DC load project will be twice the cost and effort of your actual project which in most cases will simply fall by the wayside, swallowed by the new one

author: JohanH
author_rank: Frequent Contributor
timestamp: March 14, 2023, 02:19:06 pm
content: Quote from: max_torque on March 14, 2023, 11:42:01 amIt's the journey that matters, not the destination...

author: motyr
author_rank: Contributor
timestamp: March 15, 2023, 12:04:32 am
content: Quote from: max_torque on March 14, 2023, 11:42:01 amTo have values of resistors for the different currents at different voltages is not a good option for us, so in that sense it will be overcomplicated.Buying electronic load which is already on the market is quite expensive and the lowest one I could find was 8A @800V, so massive overkill.Building own load will give us flexibility.I will learn and gain experience, not to mention all of the fun and smell of burned transistorsThanks

author: Kleinstein
author_rank: Super Contributor
timestamp: March 15, 2023, 07:58:25 am
content: Just switching resistors can get complicated too, with quite a lot of values to get fine steps. However it can be a good idea to use some resistors for part of the load, especially to reduce the worst case (with high current) voltage the MOSFETs would see. The other point is that with a sizable series resistor, a failure of a MOSFET (usually short) is not that drastic. The smell would be more burned resistors instead.With the rating of electronic loads, the maximum voltage and current are often not valid at the same time. In the extremes a unit rated for 800 V and 8 A may not be able to withstand 500 V and 0.5 A at the same time. Like with the MOSFETs there tend to be a kind of SOA curve, that may not be a simple power limit.

author: max_torque
author_rank: Super Contributor
timestamp: March 15, 2023, 11:07:52 am
content: Quote from: motyr on March 15, 2023, 12:04:32 amHow many "steps" do you actually need. 4 resistors and 4 small relays, 4 small toggle switches and 9v battery would give you 16 steps.I am in no way saying that building your own HV DC load won't be fun, or interesting or usefull, but what it absoluetly won't do is get the original task done. If this is a commercial task then this sort of diversion is usually the death knell for the original project ime. You say a COTS DC load is expensive, but i bet when you add up the hours you will spend making a working DIY version you've almost certainly paid more even at the minium wage. And if you sell the DC load you bought at the end of the project you can recover at least 50% of the direct costs too.In commerical electronic development projects we buy the tools we need and get on with it........

author: djerickson
author_rank: Regular Contributor
timestamp: March 16, 2023, 12:27:16 pm
content: My mistake, you are so right. I incorrectly assumed that the lowest SOA curve was DC as it usually is.For my E-load I use an older half-H bridge IGBT: Toshiba MG100J2YS50. It has a DC SOA curve and can dissipate up to 250W DC.Here is a photo of the load before it got all digital.Dave

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 16, 2023, 08:39:30 pm
content: Quote from: JohanH on March 14, 2023, 02:19:06 pmThat is exactly the line I say to my wife when we are on vacation. Sure, traveling by plane is very fast, specially for distances above 1000 km, but traveling by car or even by train one will really “experience” the place. Even with the heat and mosquitoes, it is worthwhile.Not that I fully convince her, though.Having said this, if one is on a time or budget constraint, one SHOULD choose to travel the fastest way.

author: Martinn
author_rank: Frequent Contributor
timestamp: April 19, 2023, 03:36:08 pm
content: Quote from: T3sl4co1l on March 12, 2023, 12:51:57 amSome feedback. I gave this idea a try and ordered a board from JLC. 6L6GC in triode mode, cascoded above a power MOSFET.Current ranges jumper selectable 2.5, 25 and 250 mA. Set current from 10 turn potentiometer (2.5 V range). Enable with soft start (not connected in picture).As I write, I have it running at 400 V, 75 mA (30W max. for the 6L6GC) - that's the highest voltage my lab supply (HSPY 400-01) can do. What would the voltage limit of a 6L6GC be? Datasheet says 450 V, but at low current (few mA), could it deal with 1 kV? After all, it's just a piece of vacuum with wires and sheet metal. PL508 has a transient anode voltage rating of 2.2 kV.Measured with an IR thermometer, the tube anode heats to around 200 °C, the MOSFET heatsink (unfortunately it needs one, which I could have seen from the tube datasheet) to about 70°C.P1 and P2 are injection points for a bode plot measurement.There are monitor amplifiers (all OP07) for voltage and current - for connecting a panel meter. See this threadhttps://www.eevblog.com/forum/projects/decent-quality-panel-meter/msg4820225/#msg4820225First I intended to use a TLV07 single supply, but when I read about its peculiarities (varying complex output impedance), I spent a -12V supply and stayed with an OP07 (it's cheap and I did not want to be limited by offset voltage problems. For the HV monitor it was a bad choice due to its input bias current). On the board are a 12 to -12V inverter and also (alternatively) a 6 VAC to +-12 V supply (single winding heater transformer powering the entire board, not tested yet).So far it just works.I am wondering whether I should build it up properly: Main effort would be mechanics, aluminium sheet metal case laser cut, perforated bend lines for DIY bending.Could I mount the tube horizontally? Vertically gives an awkward form factor. Should I add a fan? Seems a good idea. More mechanical design.Then it would need proper isolation. This could be a heater mains transformer and digital isolators on the panel meter (SPI) and for the enable lines and and range switching (plus some MOSFETs replacing the current range jumpers).Not sure if it's worth it. Maybe I should build another one using the monster IXYS linear MOSFET I mentioned earlier, which would give far higher power and current levels.

author: Wolfgang
author_rank: Super Contributor
timestamp: April 19, 2023, 03:47:40 pm
content: Quote from: Martinn on April 19, 2023, 03:36:08 pmI would say you can get away with 1kV at low currents. As long as power is within limits (and the vacuum of the tube is good) this normally works.At just 1kV arcovers are not so much of a problem, at higher voltages the can cause immediate destruction. Another good candidtae for your application are sweep tubes(PL519). The go to higher voltages and can take a lot of overload when properly cooled.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 19, 2023, 04:26:11 pm
content: 450V design center, with a transformer load -- double is acceptable (absolute / peak / maximum). See also vertical output service (wait, was 6L6 specified for vertical? I suppose it is a bit overkill for that..), though that's pulsed peak, not continuous.Horizontal orientation, check the datasheet. Permissible orientation is specified.Tim

author: Martinn
author_rank: Frequent Contributor
timestamp: April 19, 2023, 04:37:05 pm
content: Quote from: T3sl4co1l on April 19, 2023, 04:26:11 pmAt least this onehttps://www.jj-electronic.com/images/stories/product/power_tubes/pdf/6l6gc.pdfdoes not mention orientation.Same with this PL519 datasheet:https://frank.pocnet.net/sheets/010/p/PL519.pdfI don't know if it is safe to assume that when the datasheet doesn't specify orientation, it does not matter?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 19, 2023, 05:00:12 pm
content: I mean, with new production, who knows... they don't last long in any orientation.(This is mostly sarcasm; I don't know the actual stats for any particular brand.)Hm, I want to say the usual suspects (GE, RCA?) always specified. I would assume vertical is default at least? Surprised Philips didn't (doesn't?), maybe I never noticed that.Tim

author: Wolfgang
author_rank: Super Contributor
timestamp: April 19, 2023, 05:37:28 pm
content: Quote from: Martinn on April 19, 2023, 04:37:05 pmNormally it does not matter when a specific orientation is not in the datasheet. In the old times they feared that gravity would make the filament sag towards the grid and change characteristics. I only know this from transmitter tubes.

author: mawyatt
author_rank: Super Contributor
timestamp: April 19, 2023, 06:02:33 pm
content: Recall as a kid repairing Guitar amps and such, Fender and others mounted the 6L6 output tubes upside down. They had a spring toothed clamp that held the tube base in place. Also remember that all the same tubes types from RCA, GE & Sylvania looked exactly alike, was told that each were manufactured by same OEM and they exchanged types between them to keep cost down. So a GE & Sylvania 6L6 might be a RCA OEM tube, and a RCA 12AX7 might be a Sylvania OEM, and so on.We often replaced the 6L6 with 6550s which were more reliable under the heavy stress of a Bass Guitar Amp.Best,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 19, 2023, 06:09:20 pm
content: Quote from: Wolfgang on April 19, 2023, 05:37:28 pmFilament cathodes usually too, e.g. 5Y3, 5U4, etc. The latter used a ribbon which was okay to mount edge-on, else it might sag into the plate and arc over.Tim

author: Martinn
author_rank: Frequent Contributor
timestamp: April 26, 2023, 06:29:17 pm
content: Another update.As I don't see me putting this in a proper enclosure anytime soon, I have designed a 3D printed base holding all components together. With a few hundred V of input range and mains supply, a flying wire mess just seemed to dangerous. Essentially this is just a quick test setup wiring the tube load and panelmeterhttps://www.eevblog.com/forum/projects/decent-quality-panel-meter/msg4820225/#msg4820225test print together. In the pictures you can also see what I have built this for: Testing a 250 V scope clock boost converter. A cascade (which should give the -1.25 kV cathode voltage) is yet unconnected on the board.It works!Of course there's a lot of cleanup to do. But for now, it does what I built it for.Thanks again for the tube cascode idea!

author: motyr
author_rank: Contributor
timestamp: May 23, 2023, 02:03:05 pm
content: Hi All,Just a little update on this project.I have designed and build first prototype and it works like a charm. The parameters have slightly changed and it is now up to 1000V 300mA or 500V 600mA rating switchableYet to fully test it at those levels but first trials are promising.I am using 6x IXYS IXTH2N150L transistors mounted on a large heatsink with forced air cooling. There is provision for both manual control or connecting it to the MCU. There are placeholders for ADC and DAC on board as well as temp monitoring, overvoltage, overcurrent, reverse voltage, overtemperature detection with programable thresholds (DAC), soft-start of load, isolated communication to the MCU and some other features.It took me about 3 weeks to design it (schematics, calculations, PCB layout) but it all has been accounted for in the project budget, and we are still way below the price of the COTS units (we will need at least 3 of them). Plus I have learned a lot and this knowledge will actually be useful for some other projects we are doing.Once I have a green light from our senior management I will share some schematics, pictures etc (unfortunately we do have a strict policy of not sharing anything without prior agreement).Thank you all for your help and support.

author: Wolfgang
author_rank: Super Contributor
timestamp: May 23, 2023, 02:18:35 pm
content: Quote from: motyr on May 23, 2023, 02:03:05 pmCongrats ! Will some info about your design be published ?

author: motyr
author_rank: Contributor
timestamp: May 23, 2023, 02:29:15 pm
content: If my senior management presses the green button then yes. If not I will try to find a way