Switching HV with triodes - Page 1
Forum section: electronics / beginners

author: thesmug
author_rank: Contributor
timestamp: October 02, 2020, 07:44:40 am
content: Hi everyone! I’m asking this in the beginner section because I feel like it’s a simple question but I’m confusing myself trying to think through this problem. I have a transformer outputting 15kV half bridge rectified to -7.5k pulsed DC (no smoothing, capacitors scare me at this voltage). I need to be able to switch the HV side of the transformer on and off quickly for a safety system and so far my research has led me to conclude that the best way to do this is with a triode. The device (a neutron generator) runs at around 10mA so thyratrons seem like overkill. I think that I should set this up with the HV applied to the anode, and the cathode going to the device and then hold the grid at cutoff voltage (-12V for the one I’m using). I’m mostly confused about the whole negative voltage thing; should I put it the other way around (HV into the cathode and connect the anode to my device), and should I reverse the grid current (+12V)? I’ve never worked with vacuum tubes before and I’m having a really hard time finding good information online since all of it is audiophile stuff and it’s not helping.Thanks in advance everyone!P.S. How do you do PCB trace clearance at these voltages? The pins on the triode are about 5mm apart but calculations show you need more than 4 inches of clearance for the 25kV the device is rated for

author: MosherIV
author_rank: Super Contributor
timestamp: October 02, 2020, 07:52:50 am
content: HiWelcome to the forum.QuoteWhat makes you think half bridge means half voltage.The transformer must be centre tapped dual secondary for that to be true.Please provide a circuit diagram?QuoteTrack to track clearances or spacing.Best to keep the HV stuff seperated from the low voltages and use a physical cut out in the pcb.Isolate the HV side from the low voltage side if there any interfaces eg if measuring the HV use opto isolated or galvanic isolated device to bridge from HV to LV

author: thesmug
author_rank: Contributor
timestamp: October 02, 2020, 08:03:36 am
content: QuoteExactly right! I can provide a diagram in the morning (3AM right now here), but I’m just about to go to bed, sorry about that.

author: thesmug
author_rank: Contributor
timestamp: October 02, 2020, 08:12:58 am
content: Ignore the component values and numbers, but it’s this circuit. My apologies for the hasty drawing. The “switch” would be the triode.

author: Ian.M
author_rank: Super Contributor
timestamp: October 02, 2020, 08:27:42 am
content: Triodes (and other thermionic valves) don't work that way. The grid bias is applied with respect to the cathode, so if you are using one as a switch, its load must be in series with its anode, not its cathode, to avoid the need to drive the grid with a greater voltage swing than the load will 'see'. Also, as they are unidirectional (i.e. no anode current flows if the anode is negative of the cathode), to switch a negative voltage, you'd need to apply it to the cathode and apply the grid bias with respect to that negative voltage, which when you are dealing with voltages many KV negative of ground potential is rather awkward.Also, there is the problem of finding a reliable supplier for triodes rated for operation at 7.5KV, 10mA. You are about 50 years too late for that . . .

author: thesmug
author_rank: Contributor
timestamp: October 02, 2020, 08:45:43 am
content: I’m using new old stock tubes that were meant for other research equipment, they’ve just been waiting 50 years for this moment! I do see how biasing the voltage below that of the cathode would be an obvious problem. I can’t think of a workaround off the top of my head right now. Are there other devices that can act like a switch in the way I need? SCRs with this voltage rating are exceptionally expensive

author: Ian.M
author_rank: Super Contributor
timestamp: October 02, 2020, 09:05:23 am
content: Its not *IMPOSSIBLE* to use a triode as you could use a battery supply*for the grid bias with the battery positive terminal at -7.5KV and control it via a fibre optic cable or a >10KV isolation rated optocoupler to get enough isolation, but its certainly not an easy option.Also, the need to apply negative grid bias to cut the triode off is a major disadvantage, as it means most control circuit faults would result in it failing on.If I were you, if the application doesn't require fast switching, I'd be looking at a good deal on a pair of high voltage relays that can handle the voltage and current, one SPST-NO in series with the supply, and one SPST-NC in parallel with the load as a shorting relay for safety, with sequencing so that in normal operation, the shorting relay opens before the series relay closes, and on switch-off the shorting relay coil power is maintained long enough for whatever bleed resistor arrangement you have to do its job before its coil is switched off and the short reapplied. If control circuit power fails, that combo of relays would fail safe, though the surge from closing the shorting relay without any delay might damage the shorting relay contacts, unless you grossly overrate it.* The grid requires very little drive current so a battery supply is feasible. If you have to buy or build a PSU with >10KV isolation to provide the grid bias supply it gets even harder and far more expensive compared to a stack of PP3 batteries in a thick-walled Perspex box!

author: thesmug
author_rank: Contributor
timestamp: October 02, 2020, 09:16:00 am
content: That actually gives me an idea for a new design: instead of switching the HV to the device, we could have a triode shorting the HV to ground and then separately switch the power going into the transformer (120VAC, very easy by comparison)! That would fill the same safety purpose and maybe it solves the issues with the triode? It definitely is safer since it would then fail safely. I’m having trouble thinking now as it’s so late, but I’ll get back tomorrow to think more about it.Thanks for all your help!

author: Ian.M
author_rank: Super Contributor
timestamp: October 02, 2020, 09:29:49 am
content: That could work reasonably nicely for a safety/discharge circuit for a positive EHT supply, provided you interlocked it so that the 120V AC supply couldn't be applied if the triode filament was open circuit or short circuit, with the interlock also providing a filament warmup time delay, and if you included a circuit to momentarily pulse the grid positive before the interlock was released, drawing enough grid current to check that the tube still has sufficient emission. However, if its going to be cut-off for long periods of time you are likely to run into trouble with cathode poisoning.Also, as I understand it you need a negative EHT supply, so wouldn't be able to use a Triode to short it easily, as the control grid voltage would need to be with respect to the negative EHT voltage, and also you'd need a high isolation voltage heater supply as the max. heater to cathode voltage is almost certainly far far less than its rated anode voltage.

author: thesmug
author_rank: Contributor
timestamp: October 02, 2020, 09:33:04 am
content: Yes that’s the plan, but it’s still negative. In all honesty a few batteries in a well insulated case with an optoisolator and a reed switch to ensure the power can’t be turned on with the battery case open would work fine.

author: Ian.M
author_rank: Super Contributor
timestamp: October 02, 2020, 10:04:46 am
content: Quote from: thesmug on October 02, 2020, 09:33:04 amYou've still got to power the filament. Due to the relatively high filament power requirement, batteries aren't a good option. If you loose filament supply, the shorting triode fails dangerous.

author: thesmug
author_rank: Contributor
timestamp: October 02, 2020, 03:33:26 pm
content: QuoteThat was my original idea but I mostly gave up because I couldn't find a suitable thyratron anywhere.QuoteShould I give up on trying to use the tubes I have? I really would like to use them but it feels like there's really no good way of doing this. I do need fast switching which was why I originally started looking into the tubes to begin with, and not relays. I really wish there was some magical way to turn the voltage positive for the switch then turn it back afterwards...

author: TimFox
author_rank: Super Contributor
timestamp: October 02, 2020, 03:42:27 pm
content: Unfortunately, the high-voltage triodes intended for shunt regulation on TV supplies (e.g., 6BK4) that can withstand 10s of kV are limited to 1 mA or so in plate current.

author: Ian.M
author_rank: Super Contributor
timestamp: October 02, 2020, 03:56:58 pm
content: Quote from: thesmug on October 02, 2020, 03:33:26 pmHow fast is "fast switching"? Give us numbers and units of time!What tubes do you have available?Re: your earlier clearance question, HV tubes with conventional plug-in base pins almost invariably had a top cap for the anode connection to provide enough clearance and creepage distance for the voltage involved. Even so, higher voltage ones that are physically small may need to be run immersed in high purity transformer oil to prevent external flashover!Worst case: as I commented earlier, you may need to design or otherwise obtain a power supply that has 10KV isolation to provide the filament and grid bias supply to use them.

author: thesmug
author_rank: Contributor
timestamp: October 02, 2020, 04:40:25 pm
content: Quote from: TimFox on October 02, 2020, 03:42:27 pmNow that I look at the datasheet for my tubes they're only rated 1.5mA.Now I feel dumb!Finally I can part with those tubes and look in to a more reasonable solution.Thank you!In this case, I think I like the idea of using several relays. We were looking for ~1ms switching time but I think relays are probably good enough for what we're doing anyway. Thanks everyone!

author: bdunham7
author_rank: Super Contributor
timestamp: October 02, 2020, 04:55:47 pm
content: I'm just curious as to why a safety system can't work in this instance by disconnecting the primary side of the transformer. I've built supplies like this and never tried or needed to switch the HV part because it generally creates more problems than it solves.

author: mag_therm
author_rank: Frequent Contributor
timestamp: October 02, 2020, 04:58:53 pm
content: Search of "high voltage ceramic vacuum relay"brought up some Jennings RF72-26N1107Used for low price.8kV50 amp2.5 msec26VdcBut why not do it on primary?

author: thesmug
author_rank: Contributor
timestamp: October 02, 2020, 05:09:32 pm
content: QuoteWe're switching the primary too, but wanted isolation on both sides. Either way, we definitely need a switch to short the HV when it's turned off, which seems like a good job for a NC relay. Otherwise just a whole bunch of MOSFETs in series works too.

author: bdunham7
author_rank: Super Contributor
timestamp: October 02, 2020, 05:26:21 pm
content: Quote from: thesmug on October 02, 2020, 05:09:32 pmOK, but I'm trying to understand why. This is a relatively simple (electrically) system of a HV supply driving a gas tube of sorts (yes, mini-accelerator, but still quite similar to Crookes tube), correct? The typical, simple way would be a simple line-frequency transformer/rectifier with a bleed resistor of a value that bleeds off any system capacitance fairly quickly. You are right to avoid too much capacitance so there won't be all that much energy to dissipate. Another way would be to use a higher-frequency, lower-voltage transformer--say 400Hz or 1kHz--driven by an H-bridge and followed by a voltage multiplier. Higher frequency allows you to use smaller capacitors in the multiplier. Switching (isolating) the HV side seems pointless to me because you want to dissipate any energy, not isolate it somewhere it can remain and zap the unwary. I imagine your neutron tube could hold a bit of charge. I would think the goal would be to bleed off most of the energy in the HV section within a few milliseconds or so of power disconnect.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 02, 2020, 05:26:55 pm
content: Why not crowbar the HV side then?Tim

author: thesmug
author_rank: Contributor
timestamp: October 02, 2020, 05:54:00 pm
content: QuoteI thought that's what I was trying to doQuoteThe more and more I think about it the more I wonder that too... I'll see if we can change the design. I still would like to have a safety shunt across the HV for when it's supposed to be unpowered.

author: mag_therm
author_rank: Frequent Contributor
timestamp: October 02, 2020, 06:19:36 pm
content: If you need a safety measure due to regulations about accessing circuit at that voltage in the workplace, called "shorts and earth" in my day.Then the device can't be anything, it will probably have to comply with standards at your location.In my experience, semiconductors would not comply, not even an electromechanical relay.Usually involved manually bolted device along with keys, live line detector etc and a standard operating procedure.

author: thesmug
author_rank: Contributor
timestamp: October 02, 2020, 06:24:52 pm
content: There are no regulations to worry about, just safety rules we initially came up with. I think I'm just going to abandon the whole HV side of things and do all the switching on the safe 120V side. Thanks for the help everyone.

author: bdunham7
author_rank: Super Contributor
timestamp: October 02, 2020, 07:38:54 pm
content: Quote from: thesmug on October 02, 2020, 05:54:00 pmWell, for FWIW, with HV like this I like to see the whole HV section (including the transformer) in a separate closed box and using all passive components. No switchs, relays, etc. Diodes, of course as needed. The HV bleed shunts should bleed off a few percent of the operating current and there should be at least two for redundancy. Before you open the box the transformer primary should be physically disconnected (which makes the secondary another shunt) and a shorting bar can be mechanically installed either before or immediately upon opening the box.