Re: [neonixie-l] Filament transfomer current for driving six NIMOs?

[Dekatron42]

I contacted Ametherm regarding inrush current limiters as I’ve used their 
products to limit inrush current on toroidal transformers and they told me they 
had limiters used for heat lamps and similar applications and they believed 
those would work. I’ll try to order some and see how they behave.

Otherwise I’ll probably go for the relay solution shorting out series resistors.

/Martin

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Re: [neonixie-l] Filament transfomer current for driving six NIMOs?

[gregebert]

Perfect solution is a supply that is 100% efficient and gives 1.1V @ 1200mA 
(power for 6 NIMO tubes), and magically limits the current to 200mA.
Nothing will ever be this good; it's just a reference point for 
cost-comparison.

The FPGA solution I described is an added feature for the 2.5V filament 
transformer. The only advantage it could provide is reducing the peak 
current from 260mA to about 200mA. My gut feeling is that the 260mA surge 
current is probably low-enough that it wont cause any degradation of the 
NIMO tube.

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Re: [neonixie-l] Filament transfomer current for driving six NIMOs?

[Jon Jackson]

@gregebert  What is "perfect solution" with respect to the 1.1V supply?  Is
that your fancy FPGA soft-start setup ???

Jon J.

On Tue, Oct 23, 2018 at 11:30 AM gregebert  wrote:

> If you want absolute minimum energy consumption, then you will want to
> ramp the power supply, etc. I looked at a lot of tradeoffs and calculated
> the cost of energy for each.
> Circuit/component costs were not modeled.
>
> I have the option of soft-starting my 2.5V filament transformer, because
> it's controlled by an opto-triac by the onboard FPGA, which in-turn, has
> AC-line sync signals, but I did not think it was necessary given the huge
> reduction in surge-current with a series-resistor. Basically, the FPGA
> would trigger the opto-triac just before the AC-line made a zero-crossing,
> then on the next cycle a bit sooner, and so-on for about 10 seconds, by
> which time the opto-triac was triggered for the entire AC-line cycle and
> the filament is fully heated.
>
> I have yet to define how software will decide when to turn filaments on vs
> off based on PIR sensor activity, time-of-day, etc. Leaving filaments on
> for extended periods vs power-cycling them has a tradeoff point, but I dont
> know where that lies. Also, phosphor lifetime is finite, which I'm assuming
> is 25,000 hours per the MTBF in the datasheet. An older NIMO datasheet
> shows 10,000 hour MTBF (ouch, that's just over a year when running 24/7).
>
>
>   Total Filaments Current (Amps) Volts Efficiency Power Tube Life (hrs) Elec
> Cost Kwhr Filament Energy Cost
> Perfect solution 6 0.2 1.1 100% 1.3 25000 0.1 $3.30
> 7v Supply 6 0.2 7 100% 8.4 25000 0.1 $21.00
> 10V supply cur limit 6 0.2 10 100% 12.0 25000 0.1 $30.00
> 5V curr limit 6 0.2 5 90% 6.7 25000 0.1 $16.67
> 3.3V curr limit 6 0.2 3.3 86% 4.6 25000 0.1 $11.51
> 20v supply cur limit 6 0.2 20 100% 24.0 25000 0.1 $60.00
> 16v supply cur limit 6 0.2 16 100% 19.2 25000 0.1 $48.00
> 2.5VAC xfmr 6 0.2 2.5 80% 3.8 25000 0.1 $9.38
>
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Re: [neonixie-l] Filament transfomer current for driving six NIMOs?

[gregebert]

If you want absolute minimum energy consumption, then you will want to ramp 
the power supply, etc. I looked at a lot of tradeoffs and calculated the 
cost of energy for each.
Circuit/component costs were not modeled.

I have the option of soft-starting my 2.5V filament transformer, because 
it's controlled by an opto-triac by the onboard FPGA, which in-turn, has 
AC-line sync signals, but I did not think it was necessary given the huge 
reduction in surge-current with a series-resistor. Basically, the FPGA 
would trigger the opto-triac just before the AC-line made a zero-crossing, 
then on the next cycle a bit sooner, and so-on for about 10 seconds, by 
which time the opto-triac was triggered for the entire AC-line cycle and 
the filament is fully heated.

I have yet to define how software will decide when to turn filaments on vs 
off based on PIR sensor activity, time-of-day, etc. Leaving filaments on 
for extended periods vs power-cycling them has a tradeoff point, but I dont 
know where that lies. Also, phosphor lifetime is finite, which I'm assuming 
is 25,000 hours per the MTBF in the datasheet. An older NIMO datasheet 
shows 10,000 hour MTBF (ouch, that's just over a year when running 24/7).


  Total Filaments Current (Amps) Volts Efficiency Power Tube Life (hrs) Elec 
Cost Kwhr Filament Energy Cost 
Perfect solution 6 0.2 1.1 100% 1.3 25000 0.1 $3.30 
7v Supply 6 0.2 7 100% 8.4 25000 0.1 $21.00 
10V supply cur limit 6 0.2 10 100% 12.0 25000 0.1 $30.00 
5V curr limit 6 0.2 5 90% 6.7 25000 0.1 $16.67 
3.3V curr limit 6 0.2 3.3 86% 4.6 25000 0.1 $11.51 
20v supply cur limit 6 0.2 20 100% 24.0 25000 0.1 $60.00 
16v supply cur limit 6 0.2 16 100% 19.2 25000 0.1 $48.00 
2.5VAC xfmr 6 0.2 2.5 80% 3.8 25000 0.1 $9.38 

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Re: [neonixie-l] Filament transfomer current for driving six NIMOs?

[Dekatron42]

A simple solution then would be to use one resistor, or two resistors one 
in each branch, of the heater chain to maintain a centered reference 
compared to ground even during startup, and a relay shorting that/those 
resistors after some time - just like slow starters for toroidal 
transformers are designed. This could then be used with AC and DC heaters. 
A more complex design would use some extra components with a voltage 
regulator ramping up the voltage, but would only work on DC designs.

Using the same circuit combined with an inrush limiter on the primary side 
of the transformer supplying the heater voltage would also limit the 
startup voltage spikes, adding a MOV would protect against over voltages.

Does this seem like a usable design?

/Martin

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Re: [neonixie-l] Filament transfomer current for driving six NIMOs?

[GastonP]

H... a transformer short circuit current, which is the maximum current 
that it can supply, is not the design or specified one, which in this case 
would be 1.2A and which is the current it can supply indefinitely while 
maintaining all of the design parameters.
The ideal transformer has, of course, infinite output current, but as the 
real transformers coils are made of copper, impure, there is its resistance 
that limits the current output (and of course heats the transformer). I'm 
dismissing losses due to the iron core as we are talking of the transition 
start up, which is likable to a short circuit.

One can calculate the short circuit current as the transformer secondary 
design voltage divided by the secondary total DC resistance, that would be 
the secondary DC resistance plus the primary resistance reflected on the 
secondary (the primary DC resistance divide by the transformer ratio).

In this case, the transformer resistance is Rsec + Rpri * Vsec/Vpri, and 
the short circuit current would be Vsec/(Rsec + Rpri *Vsec/Vpri).

The tube startup current without limiters of any kind (assuming there is 
only one being fed by the transformer) would be then Vsec/(Rsec + 
Rpri*Vsec/Vpri+Rfilament)

So, the transformer actually supplies several times the specified current 
at startup, given the very low cold resistance of the filament. This 
accounts for the "flash" one sees in some receiving/transmitting tubes.

The moral is: one needs to put some limiting element when feeding delicate 
tubes.

Gastón

On Monday, October 22, 2018 at 2:58:54 PM UTC-3, jrehwin wrote:
>
> > I need help with how much current a regular mains transformer would need 
> for the center tapped filament winding for six NIMOs. 
> > 
> > The filament for a NIMO is specified at 1.1V AC or DC +/-0.15V at some 
> 0.2A. The cold resistance of the filament is some 2.1 Ohm. 
> > 
> > Do I need to specify some overhead for the heater transformer to be able 
> to power six NIMOs or do I just need 1.2A current capability (6*0.2A)? 
>
> 1.2A should be sufficient.  The cold filaments will attempt to draw more, 
> but the transformer will be unable to supply it, so the voltage will drop 
> initially, which is what you want:  this will give a gentler warmup than 
> smacking them with full voltage would.  It is, of course, possible to add 
> more circuitry for an even softer start, but a transformer sized for the 
> equilibrium load is pretty simple and works well. 
>
> - John 
>
>

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[neonixie-l] Filament transfomer current for driving six NIMOs?

[Dekatron42]

@jrehwin: If I order a transformer with 1.1V center tapped transformer with 
1.2A rating for driving six NIMOs, do I have to load it with 5 ”NIMOs” if I 
want to test just one NIMO?

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Re: [neonixie-l] Filament transfomer current for driving six NIMOs?

[gregebert]

>
> @gregebert - Do you have to take the fuse resistance into account in your 
> circuit to maintain a centered heater voltage (I guess so as the fuse 
> resistance can be several ohms with high speed low current fuses)?
>
> Eventually, yes. Right now I have safe-enough values to get the filament 
warm enough so that the tube should function. After I get more tubes and 
have the I-V data plotted for their filaments, and also confirm the 
brightness is correct (nominal 1850V anode supply @30uA per tube), I will 
fine-tune the series resistor value. The goal for me is to use somewhat 
less than the spec value of 200mA of filament current at the highest 
line-voltage I record at my house over a weeks' time, and verify that the 
series resistor is sufficient to view the tube during low-line-voltage 
periods. 

So far, I have only energized the filament of my one-and-only NIMO tube 
once, and that was to gather IV data.

I'm still tuning the HVDC inverter and have not actually fired-up the tube 
yet; going very, VERY slowly to make sure I dont risk any damage to the 
NIMO. Everything else is ready-to-go. Only after I have several days of 
clean operation of the HVDC supply during no-load and 150% overload will I 
attempt to fire the tube. I just found another bug in the FPGA that 
controls the inverter. It's a new design of my own, and probably way more 
complicated than it needs to be, but it's regulated and allows software 
control of the voltage, contrast, and even the tube current.

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Re: [neonixie-l] Filament transfomer current for driving six NIMOs?

[Dekatron42]

Thank you all for your input!

@jrehwin - Thanks for explaining how "a transformer sized for the 
equilibrium load" works in a circuit like this!

@gregebert - Do you have to take the fuse resistance into account in your 
circuit to maintain a centered heater voltage (I guess so as the fuse 
resistance can be several ohms with high speed low current fuses)?

@Kevin A. - I've seen those small EHT supplies and they are very nice 
except for the price, it is simply too high for me, and I'd also like to be 
able to repair the design myself so I'll go with a home built diode doubler 
design.

@Tony Adams - I have only used that kind of protection on the primary side 
of a transformer but will ask manufacturers if they have anything that 
would work on the secondary side with these low voltages/currents.

/Martin

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Re: [neonixie-l] Filament transfomer current for driving six NIMOs?

[Tony]

The old-school method used was a thermistor in series with the heater
chain, chosen with a suitably high resistance when cold which would
slowly drop as it warmed up to a low operating value.

Tony.

On Mon, 22 Oct 2018 14:10:40 -0700 (PDT), you wrote:

>Congratulations on acquiring 6 NIMOs !!
>
>I've gone thru all the NIMO filament analysis and came up with the 
>following, which I have implemented in my design
>
>1. Transformer is 2.5V CT, 3A. Triad F301X.. It has dual primaries so it 
>can operate on 115 or 230 VAC mains.
>2. Series fuse 250mA (BEL C1F250). One for each NIMO
>3. Series resistor; empirically I'm choosing 6.8ohms.
>
>The filament is the most-critical reliability-impacting element of your 
>NIMO tubes, so be extra careful to make sure you design with this in-mind.
>Filaments usually fail due to power-on surge-current, just like 
>conventional incandescent bulbs. The surge-current is the result of the 
>lower-resistance of a cold filament being the only limiter of current. For 
>example, measured NIMO-tube filament resistance varies from about 2.8 ohms 
>cold, to 7.0 ohms when stabilized at 200mA. If you use a constant voltage 
>to power the filaments, you will have significant surge current. But if you 
>add some series resistance and drive from a higher supply voltage, the 
>surge current will be less. With the components I listed above, the surge 
>current is about 260mA.
>
>The fast-blow fuse will protect the NIMO from line-surges, or other 
>uncontrollables.
>
>I'm almost done with my NIMO clock, and I will collect waveforms of the 
>power-on transient for each filament once I get my other tubes (I have 1, 
>need 5 more)

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[neonixie-l] Filament transfomer current for driving six NIMOs?

[Kevin A.]

I like the discussion here. I'm working on a 2 Nimo clock w/VFD for myself, and 
a 6 tube clock for a friend. I'd like to add that I found these power supplies 
which are really small and quite trick for a low profile nimo design, needing 
between 1500-2000 vdc. They aren't cheap, but it's the latest and greatest and 
would really make for a slick and fully controllable design: 
https://www.digikey.com/product-detail/en/xp-power/AG20P-5/1470-3211-ND/5873521

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Re: [neonixie-l] Filament transfomer current for driving six NIMOs?

[gregebert]

Congratulations on acquiring 6 NIMOs !!

I've gone thru all the NIMO filament analysis and came up with the 
following, which I have implemented in my design

1. Transformer is 2.5V CT, 3A. Triad F301X.. It has dual primaries so it 
can operate on 115 or 230 VAC mains.
2. Series fuse 250mA (BEL C1F250). One for each NIMO
3. Series resistor; empirically I'm choosing 6.8ohms.

The filament is the most-critical reliability-impacting element of your 
NIMO tubes, so be extra careful to make sure you design with this in-mind.
Filaments usually fail due to power-on surge-current, just like 
conventional incandescent bulbs. The surge-current is the result of the 
lower-resistance of a cold filament being the only limiter of current. For 
example, measured NIMO-tube filament resistance varies from about 2.8 ohms 
cold, to 7.0 ohms when stabilized at 200mA. If you use a constant voltage 
to power the filaments, you will have significant surge current. But if you 
add some series resistance and drive from a higher supply voltage, the 
surge current will be less. With the components I listed above, the surge 
current is about 260mA.

The fast-blow fuse will protect the NIMO from line-surges, or other 
uncontrollables.

I'm almost done with my NIMO clock, and I will collect waveforms of the 
power-on transient for each filament once I get my other tubes (I have 1, 
need 5 more)


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Re: [neonixie-l] Filament transfomer current for driving six NIMOs?

['John Rehwinkel' via neonixie-l]

> I need help with how much current a regular mains transformer would need for 
> the center tapped filament winding for six NIMOs.
> 
> The filament for a NIMO is specified at 1.1V AC or DC +/-0.15V at some 0.2A. 
> The cold resistance of the filament is some 2.1 Ohm.
> 
> Do I need to specify some overhead for the heater transformer to be able to 
> power six NIMOs or do I just need 1.2A current capability (6*0.2A)?

1.2A should be sufficient.  The cold filaments will attempt to draw more, but 
the transformer will be unable to supply it, so the voltage will drop 
initially, which is what you want:  this will give a gentler warmup than 
smacking them with full voltage would.  It is, of course, possible to add more 
circuitry for an even softer start, but a transformer sized for the equilibrium 
load is pretty simple and works well.

- John

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[neonixie-l] Filament transfomer current for driving six NIMOs?

[Dekatron42]

Hi,

I need help with how much current a regular mains transformer would need 
for the center tapped filament winding for six NIMOs.

The filament for a NIMO is specified at 1.1V AC or DC +/-0.15V at some 
0.2A. The cold resistance of the filament is some 2.1 Ohm.

Do I need to specify some overhead for the heater transformer to be able to 
power six NIMOs or do I just need 1.2A current capability (6*0.2A)?

/Martin

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