This morning my study room light popped and thought of throwing some oil
into this fire by mentioning this equation:
[image: \operatorname{R}(T) = \operatorname{R}(T_0)(1 + \alpha\Delta T)] copied
straight from the all-knowing wikipedia. The full page is
here<http://en.wikipedia.org/wiki/Temperature_coefficient>
I think we have the same nixie clock Gideon :-)
Alex.
On Saturday, December 7, 2013 9:43:28 PM UTC, Adam Jacobs wrote:
>
> Hi Alex,
> I think that you are describing the same thing that I tried to describe.
> Yes, V/I is substituted in place of R into the equation as you recommend
> and your number is the same as what the math produces. It's easy to say
> 'why not just' and then proceed with the common sense understanding of an
> electronics topic, but that understanding breaks down rapidly when things
> get more complicated. Maybe I spelled it out too slowly (and I'm sure my
> broken equations in the first email didn't help). :)
> Also, normally we talk about Impedance when it is alternating current.
> Filaments are normally AC. I think filaments may have a small reactive
> component at certain frequencies, but I doubt they do at 60hz. Usually I
> don't worry about significant reactive components unless I'm building a new
> antenna for transmission.
>
> -Adam
>
> On 12/7/2013 2:04 AM, Alex wrote:
>
> Sorry Adam, I sent a reply to you and not the group...
>
> I do get your approach now (shouldn't read such things before finishing
> the morning coffee), but it still seems a slight unnecessary over
> complication? If the current is specified as 100mA and voltage is 1.5v then
> surely that gives the manufacturers specified resistance, when running hot.
> (I still argue its resistance and not impedance due to the lack of
> significant inductance and capacitance?)
>
> This would give a mfg resistance of 1.5/0.1 = 15ohm?
>
> If you treat two of them as one 30 ohm resistor with 3v over it then you
> can get your 2v drop at 100ma do you not need a 2/0.1 = 20 ohm resistor?
>
> A soft start arrangement would be kind, and I am sure I have seen it
> used on VFD filaments somewhere, but I doubt their resistance changes that
> much to as require one as they don't get too hot (compared to a
> incandescent filament...)
>
> I would still always run it though a bench PSU with a current limit set,
> then just play with some resistors (or a wire wound pot / rheostat) until
> the figures work out...
>
> - Alex
>
>
> On Friday, 6 December 2013 19:41:13 UTC, Adam Jacobs wrote:
>>
>> Hi Gideon,
>> You're doing it wrong. :)
>> We do not normally refer to filaments by their impedance, but rather by
>> their power draw. What is the equivalent resistor value of a 100watt
>> incandescent lightbulb? There isn't one, because filaments and resistors
>> behave differently. For starters, a filament has a much lower impedance
>> when it is cold than after it has warmed up. Instead, we identify a
>> filament by the current draw. I don't know offhand what the current draw of
>> an IV-11 filament is, but the datasheet will have it. I think it is roughly
>> 100ma. Same thing with the filament voltage, the datasheet will have the
>> exact number for the real math. I'm going to call it ~1.5v.
>> Going back to ohm's law, we know that I=V/R. The formula for a resistor
>> divider without a load is:
>> R_1=(V_1*R_2)/(R_1+R_2)
>>
>> Now, normally there would be load in parallel with R_2. However, in our
>> case, R_2 *is* the load.
>> So, our equation becomes:
>> R_1=(V_1*R_L)/(R_1+R_L)
>>
>> We don't know the equivalent load resistor, only the current draw, so we
>> use substitution algebra:
>> R_1=(V_1*(V_L/I_L))/(R_1+(V_L/I_L))
>>
>> [image: R_1=(V_1*(V_L/I_L))/(R_1+(V_L/I_L))]
>>
>> V_1 is the supply voltage, in my case 5v. V_L (normally V_out) is the
>> filament voltage, in my case 1.5v.
>> R_1=(5*(1.5/0.1))/(R_1+(1.5/0.1))
>>
>> Also, the reason you weren't seeing success using those batteries
>> probably is because you didn't tie them both to a common potential. There
>> is no assumed potential difference between two separate dry cells, I think.
>>
>> -Adam
>>
>> --
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