Driving Russian IN-28 '7 Segment' display panels

[Richard Scales]

Hello,

I have a number of these '7 segment' panels which are made up of discreet groups of IN-28's. 

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.

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

[Richard Scales]

[Nicholas Stock]

Paul may well have the answer....:)

https://www.instagram.com/p/DXCmoXuGs03/?img_index=1

What a beauty!

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[gregebert]

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 !!

[Richard Scales]

That is helpful - thankyou - I shall get some MPSA44 and give it a go!

- Richard

[Richard Scales]

... 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

[gregebert]

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.

[Richard Scales]

Very many thanks for your detailed and helpful reply - all duly noted. 

I am using full wave rectification on UK mains (238V)  - all GNDS will be connected  - things ONLY get turned on from a distance - especially as, right now - everything is hanging in free space. The shall be no probing!
I'll research more on the series resistors for 'surge protection' - I have yet to enter the world of SPICE though it seems that I really should be heading there soon.

Thank you again.

- Richard

[Tomasz Kowalczyk]

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. 

[David Pye]

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....

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[Richard Scales]

Oh wow, i had better check that for sure. 

-Richard

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[newxito]

That all sounds scary to me. I would add a small mains transformer and a 5V regulator and drive the "segments" with optocouplers... 

[David Pye]

https://know.innon.com/powering-ac-possible-ground-issues

Seems to have some useful hints.

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[David Pye]

Ensuring that the -ve DC output from the bridge rectifier is connected to mains earth and that the 5V PSU ground is also earthed should solve that, but it requires some knowledge of the operation of both power supplies in order to do safely....

On Thu, 16 Apr 2026, 07:44 David Pye, <davi...@gmail.com> wrote:

[Dekatron42]

The Russians quite often ran tubes like these on unfiltered (no smoothing capacitor, just rectifier diodes and a load resistor) half wave rectified voltage so that they would only light up during the positive half wave period and automatically being extinguished during the "negative" half wave period (or zero voltage after rectification). They would be lit up again in the next positive half wave period as long as the driving signal was still on. Quite often they used SCR's to drive these and they would also be extinguished during the "negative" half wave period, this so they could use low voltage signals to drive these SCR's.

Using Opto-Couplers is a modern way instead of SCR's.

/Martin