Another thing is that your 5v supply may actually have it's gnd as mains gnd, so connecting the two grounds together may lead to a bang....
On Thu, 16 Apr 2026, 07:43 Tomasz Kowalczyk, <[email protected]> wrote: > The problem with the half-wave assumption is that even with a three prong > plug, you're not *guaranteed *to connect your ground to neutral. I am not > sure about UK specifically, but here in Poland it's only a guideline to > have phase in specific hole in the outlet, and the double outlets sold in > the stores simply make it impossible to implement in practice. > > I think it's safer to assume half wave rectification doesn't make the > circuit any safer and treat it as hot at all times. The increased safety > will show up only when someone ignores the warnings. > > czwartek, 16 kwietnia 2026 o 07:55:30 UTC+2 gregebert napisał(a): > >> It's fine, as long as you know what you're doing. I've made several "hot >> chassis" clocks before, with no problems. That said, you MUST be very >> careful about what is exposed, so that nobody can get shocked, and be sure >> to put warning labels on it. Make sure you have properly-sized fuses (ie, >> smallest possible). I place small-ish series resistors in the power supply >> to limit the surge current at power-up, and size them to double as fuses if >> for some reason the actual fuses dont blow (1 one neutral, 1 on hot). It's >> best to run SPICE simulations on your power supply to balance the inrush >> current with the wasted energy (heat) of the series resistor. >> >> Half-wave vs full-wave rectification makes a **HUGE** difference. >> Half-wave is likely to have your circuit ground connected to the neutral >> wire, which is normally within a few volts of earth ground, hence minimal >> shock hazard *under normal conditions. * However, things go wrong and >> when your life or someone else's is at-risk, you cant take chances. >> >> If you used full-wave rectification, then **EVERY** point in the design >> is a serious shock hazard. Basically, the (-) lead from the bridge >> rectifier, which is likely to be your circuit ground, will vary from 0 to >> minus 310 volts (for 220V mains), or from 0 to minus 180 volts (for 120V >> mains). I have a few gizmos doing this; again you have to be very careful. >> >> Obviously, connecting anything like a scope will be disastrous unless you >> have an isolation transformer, because the ground of your scope probe will >> short-out your hot supply. >> >> But to answer your question, sensitive devices, like raspberry pi >> computers, are fine in hot-supply systems. Since I use the onboard WiFi, I >> do all of the software and logging via network, so there is never an >> external physical connection. As long as everything is at the same >> potential, there is no hazard. This is why utility workers can do their job >> on 500kV power lines that are energized. >> >> On Wednesday, April 15, 2026 at 10:19:33 PM UTC-7 Richard Scales wrote: >> >>> ... and just checking something else: >>> >>> I have this rectified mains output powering the tubes. That has +V and >>> GND. I have a small piece of electronics powered from a 5V supply which in >>> turn is also connected to the same mains supply. >>> >>> Whilst it seems obvious that the GND from the control electronics needs >>> to be connected to the GND of the rectified mains - everything about that >>> scares me a little - am I right to be scared or is that absolutely fine? >>> >>> - Richard >>> >>> >>> On Wednesday, 15 April 2026 at 16:09:01 UTC+1 Richard Scales wrote: >>> >>>> That is helpful - thankyou - I shall get some MPSA44 and give it a go! >>>> - Richard >>>> >>>> >>>> On Wednesday, 15 April 2026 at 16:02:52 UTC+1 gregebert wrote: >>>> >>>>> Your MPSA44 transistor is fine for 3 reasons >>>>> 1- You are below BVceo >>>>> 2- You are within the safe operating area (SOA) with 4meg of >>>>> resistance that limit the max current below 1.5mA at any voltage (in your >>>>> case, it's 85uA) >>>>> 3- Bipolar devices, unlike MOSFETs, can actually sustain voltages >>>>> above BVceo as long as your circuit limits the current. At higher >>>>> voltages, >>>>> there is reverse-junction breakdown resulting in current, but it's not >>>>> destructive as long as the current is limited. MOSFETs, however, will have >>>>> permanent oxide destruction at any current. >>>>> >>>>> Those IN-28 boards are cool !! >>>>> >>>>> >>>>> >>>>> On Wednesday, April 15, 2026 at 7:48:37 AM UTC-7 Richard Scales wrote: >>>>> >>>>>> [image: 7SegIN28.jpeg] >>>>>> >>>>>> On Wednesday, 15 April 2026 at 15:30:51 UTC+1 Richard Scales wrote: >>>>>> >>>>>>> Hello, >>>>>>> >>>>>>> I have a number of these '7 segment' panels which are made up of >>>>>>> discreet groups of IN-28's. >>>>>>> [image: 7SegIN28.jpeg] >>>>>>> >>>>>>> I am in the UK and am driving them with bridge rectified UK Mains >>>>>>> (238V before rectification). >>>>>>> >>>>>>> Each 'segment' has all it's grids connected to VCC via a 3M, a 1M >>>>>>> resistor is also connected so when the end of that is pulled to ground, >>>>>>> the >>>>>>> grid voltage is reduced to about 85V (assuming 340V peak). >>>>>>> >>>>>>> If I ground a segment via the 1M resistor, the tubes turn off, if I >>>>>>> let it float, the tubes turn on - all good so far. >>>>>>> >>>>>>> [image: IN28.jpg] >>>>>>> >>>>>>> My question is this, what safe and reliable mechanism might I employ >>>>>>> to make that switch? I was thinking of using MPSA44 NPN transistors >>>>>>> which >>>>>>> have a Vceo max of 400V, is it as simple as that or is there more to it? >>>>>>> >>>>>>> Right now I am using a hand made bridge of x 4 UF4007, I would use a >>>>>>> proper bridge if this project ever gets anywhere near the finish line! >>>>>>> >>>>>>> Many precautions are being taken during testing to ensure that the >>>>>>> rectified does not go anywhere near me (or anyone else for that >>>>>>> matter!). >>>>>>> >>>>>>> - Richard >>>>>>> >>>>>> -- > You received this message because you are subscribed to the Google Groups > "neonixie-l" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To view this discussion, visit > https://groups.google.com/d/msgid/neonixie-l/61eb1b0a-e9a9-45a8-9d70-e66a323e0c8cn%40googlegroups.com > <https://groups.google.com/d/msgid/neonixie-l/61eb1b0a-e9a9-45a8-9d70-e66a323e0c8cn%40googlegroups.com?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. 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