Re: [neonixie-l] Input voltage protection

[Mac Doktor]

Excellent explanation, Greg.


> On Sep 13, 2019, at 2:31 PM, gregebert  wrote:
> 
> The transformer doesn't play a significant role in limiting inrush current.
> 
> A filament behaves like a thermistor; at cold temperatures it has a lower 
> resistance, but as it heats-up, it's resistance increases. So, when you 
> connect a voltage source to it there is an initial surge current (I= 
> Vsupply/Cold_resistance), and as the filament heats up the resistance 
> increases. At some point, equilibrium is established and the radiated thermal 
> energy prevents the filament from heating any further, hence the resistance 
> stabilizes.

A PTC thermistor. Just like the one in a color CRT that's in series with the 
degaussing coil. Ever notice how when you turned a cold computer monitor on 
there was a loud hum that faded out after a second or two? And if you turned it 
off and back on you heard nothing? That's because the thermistor was still warm.


> After enough thermal cycling, the filament weakens. Not just hot/cold 
> expansion & contraction, but also electromigration from what I've researched. 
> Over time, the filament will develop narrowed regions and the wearout will 
> accelerate to the point that local current-density is so high that it 
> burns-out like a fuse. Long story short, how many times have you been 
> surprised when the incandescent bulb dies when you turn it on ? Almost 
> always, it's during turn-on.

I collect antique Xmas lights and preserving light bulbs 70~90+ years old is a 
big issue, especially with things like bubble lights that are not designed to 
be re-lamped. Miniature base lights were wired in series and typically rated 
15V RMS. Mixing different types of bulbs is asking for trouble because the 
filaments have different characteristics. The one that heats up the quickest 
will be the first to blow. I use a Variac to slowly ramp the voltage up. 
Problem almost solved.

The final step is to use parallel-wired sockets and large 12V RMS transformers 
designed for low voltage outdoor lighting systems. Under load the output 
voltage is around 12.25V RMS.


> As an example, I have a NIMO tube that varies from 2.7 ohms (cold) to 7 ohms 
> (hot). If I follow the datasheet and connect a 1.1V supply to it, the surge 
> current will be 400mA, which is twice the rated current. But if I drive it 
> from 2.5V and add a series resistor of 5.5 ohms, the surge is reduced to 
> 300mA. It may not sound like much, but considering these are extremely rare 
> devices I'm not taking any chances. In the end, I chose an even higher series 
> resistance because there was no visible difference in display brightness at 
> lower filament current.

Exactly. I run my Xmas lights at ~15% below the rated voltage. When they were 
new the mains voltage in the US was 110V RMS, not 120V, so the voltage stamped 
on the bulb can't be trusted. A further complication is that the early NOMA 
bubble lights (which were made in huge quantities and still around) are stamped 
14V and came in strings of 9 instead of 8. They can only be mixed with other 
types of bulbs safely by going to all this trouble.


> This approach is fine for single filaments; many devices, including NIMO 
> tubes, have parallel-connected filaments. When 1 goes out, the tube is 
> half-usable. But some large VFD tubes, like the ILC1-1/8 have multiple 
> parallel filaments, and I think they could still work with 1 burned-out, so 
> that means you will need a more-complex control circuit to limit the current 
> as filaments burn-out.

Just like putting LEDs in parallel with a common resistor which seems to be 
standard practice these disposable days. This is why amplifiers that use 
multiple transistors or tubes in parallel requires careful matching for maximum 
life.


Terry Bowman, KA4HJH
"The Mac Doctor"

Q: Should car stereo speakers be pointed to the rear for more thrust or up for 
more traction?

A. On long trips, the 20- to 30% improvement in gas mileage you might get with 
speakers pointing to the rear is certainly worthwhile. On the other hand, if 
you drive on snow or ice, the extra traction of speakers pointing upward gives 
you added control.

Don Lancaster

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Re: [neonixie-l] Input voltage protection

[ZY]

Do you have a suggestion on an A/D IC? Or are you using one from the uC 
directly? I'm looking to sprinkle some A/D everywhere for my nixie and non 
nixie projects just for fun.
On Tuesday, 10 September 2019 15:51:18 UTC-4, gregebert wrote:
>
> My current clock went a bit overboard with self-checking; I have A/D 
> converters on all supplies and software periodically monitors them for 
> out-of-tolerance, and will shut-down the HV if that happens
>
 


Also on a similar but separate note, anyone have a suggestion on how to 
protect the filament (~1V) for VFD tubes? I'm always scared my buck 
converter will fail and pass Vin through the filament of my harder to fine 
VFDs.

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Re: [neonixie-l] Input voltage protection

[gregebert]

My current clock went a bit overboard with self-checking; I have A/D 
converters on all supplies and software periodically monitors them for 
out-of-tolerance, and will shut-down the HV if that happens. There is also 
hardware-based detection of a gross power-supply failure that will 
instantly shut down the HVDC, just in case the RasPi crashed and cant 
monitor the supplies.

I was tempted to add a crowbar circuit that killed all of the supplies if 
an over-voltage condition occurred, but decided against that mainly because 
I did not want false triggers. The other reason is that if an overvoltage 
condition did occur, it's probably too late to save the clock because CMOS 
devices can get destroyed in nanoseconds if the zap is big enough. My main 
concern is the tubes; they are irreplaceable while the rest of the clock 
can be replaced. So, shutting-down the HV is sufficient.

There's plenty of ac-line protection, and I'm fairly certain that's where 
most of the trouble would originate from that could cause the power supply 
to run amok.

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Re: [neonixie-l] Input voltage protection

[Kevin A.]

I see your point. For a clock that's running off of a decent quality wall
wart, I dont see the added complexity of undervoltage compensation being
necessary. If it were an automotive or battery powered application, sure.

On Tue, Sep 10, 2019, 12:20 PM gregebert  wrote:

> Many nixie designs I've seen here use boost-converters to generate the HV.
> When the supply voltage drops, the converter has to work much harder:
>
>- Higher duty-cycle of the MOSFET
>- Higher inductor peak-current, which is a risk for saturation
>- And therefore, higher heat-producing RMS current in the MOSFET and
>inductor.
>
> Those could lead to overheating and eventual burnout of the DCDC converter.
>
> So, having under-voltage protection will provide better protection.
>
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Re: [neonixie-l] Input voltage protection

[gregebert]

Many nixie designs I've seen here use boost-converters to generate the HV. 
When the supply voltage drops, the converter has to work much harder:

   - Higher duty-cycle of the MOSFET
   - Higher inductor peak-current, which is a risk for saturation
   - And therefore, higher heat-producing RMS current in the MOSFET and 
   inductor.

Those could lead to overheating and eventual burnout of the DCDC converter.

So, having under-voltage protection will provide better protection.

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Re: [neonixie-l] Input voltage protection

[Kevin A.]

Why not just use a TVS diode for input protection?

On Tue, Sep 10, 2019, 10:17 AM newxito  wrote:

> I’m designing a new clock board and I would like to improve the input
> voltage protection. Until now I just have a fuse and a mosfet for reverse
> voltage protection.
>
> My favorite solution for the new board is the LTC4365 because it protects
> from under, over and reverse voltage. Does anyone use this chip?
>
> I also found the TPS2400 which has under and over but no reverse
> protection.
>
> Any other suggestions? Is it worth to spend 4$ (chip & mosfet) for the
> enhanced protection?
>
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