Thread-topic: [TCML] Methods for VTTC soft filament startup
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In my dual 833C VTTC, I use a 12V 30A toroidal transformer which are
readily available, for the filaments. With the primary side connected
through a small variac, which doesn¹t need to be large as of course the
current draw from the 240V primary side isn¹t great to supply the 10V/20A
needed on the filaments. I have connections on my coil front panel to
connect a DMM to the secondary side of this, and I can then ramp the
filaments up slowly to 10V on the secondary side using the small variac.
I¹ve been running it this way for more than 7 years, and it works a treat.
I¹m getting better than 32² from this coil.
An old YouTube video from 2011 is here ?.
https://www.youtube.com/watch?v=Scyz1AFBlzM
Cheers,
Ian
On 28/2/18, 6:25 am, "Tesla on behalf of Steve White"
<tesla-bounces@xxxxxxxxxx on behalf of steve.white1@xxxxxxxxx> wrote:
>It would be nice to have a real 10 volt, 10 amp filament transformer.
>When I do see them on E-Bay they are rather expensive. The only 2
>advantages of using a modified MOT are that it costs nothing and it is
>fairly small.
>
>----- Original Message -----
>From: "Chris Reeland" <chrisreeland@xxxxxxxxx>
>To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
>Sent: Tuesday, February 27, 2018 10:34:47 AM
>Subject: Re: [TCML] Methods for VTTC soft filament startup
>
>I'm using a nice Stancor, for my filament transformer.
>
>
>
>Sent from my LG V20
>
>On Feb 27, 2018 10:30 AM, "Chris Reeland" <chrisreeland@xxxxxxxxx> wrote:
>
>> Hi Steve,
>>
>> I use another variac myself also with an analog AC voltmeter hooked up
>> full time.
>> I use the variac to vary the voltage for different tubes on my current
>>10V
>> 10A filament transformer within reason of course when I try different
>> various tubes. I also initially check with a good digital voltmeter and
>> "calibrate" the analog.
>> Yes, running low is not good, in addition to inrush. Running low can
>>hurt
>> output of coil also besides not being good.
>> I like to keep the tube about .2 to .4 max over rated voltage of
>>filament
>> for load fluctuations, so it will not drop too low. The load from the
>> running coil can pull AC 120v line voltage down some, which will drop
>>this
>> also. So I then adjust the filament voltage. And monitor under different
>> load conditions.
>>
>> You will have to decide the method you want to use.
>>
>> Me, I test too many different tubes to make a dedicated circuit.
>>
>> Chris
>>
>> Sent from my LG V20
>>
>> On Feb 27, 2018 8:31 AM, "David" <zipo@xxxxxxxxxxx> wrote:
>>
>>> A small variac to your filament transformer and a ac volt meter to
>>> monitor the voltage on the filament. Small 2 amp 115vac variacs aren't
>>>that
>>> expensive.
>>>
>>> On 2/26/2018 11:49 PM, Steve White wrote:
>>>
>>>> I have become recently concerned about maximizing the lifetime of my
>>>> VTTC 833A tube. I want to to apply a soft-start to the filament to
>>>>reduce
>>>> the in-rush current. I measured the in-rush current on the 833A and
>>>>it is
>>>> over 40 amps! Yikes! It then quickly settles to the required 10 amps
>>>>after
>>>> the filament heats up. I have been mulling 2 different methods. One
>>>>method
>>>> involves a thermistor and the other method relies on a fixed resistor
>>>>for
>>>> pre-heat which is then manually switched out after the pre-heat
>>>>period.
>>>>
>>>> I had a lot of hope for the thermistor method but after trying it, it
>>>> didn't work well enough. I placed a 1 ohm initial resistance
>>>>thermistor in
>>>> series with the 10 volt filament supply which would limit the in-rush
>>>> current to 10 amps. This thermistor has a final resistance of 0.06
>>>>ohms.
>>>> This doesn't sound like much, but when 10 amps is passing through it,
>>>>it
>>>> drops the filament voltage by 0.6 volts which resulted in 9.5 volts
>>>> supplying my filament which is too low for maximum filament emission.
>>>>I
>>>> could add another secondary turn to my homemade MOT-based filament
>>>> transformer to compensate. I don't like that idea though because it
>>>>would
>>>> make my filament supply highly dependent on the final resistance of
>>>>the
>>>> thermistor which has a 20% tolerance. A related idea was to place the
>>>> thermistor on the primary (120 volts) side of the filament
>>>>transformer.
>>>> Unfortunately, because MOTs are so inefficient, my homemade MOT
>>>>filament
>>>> transformer draws 6.5 amps on the primary side. I would ne
>>>>
>>> ed
>>>
>>>> about a 100 ohm initial resistance thermistor capable of handling
>>>>6.5
>>>> amps steady state. All of the thermistors that I have found with this
>>>>level
>>>> of resistance only handle 2 or 3 amps steady state. So I am giving up
>>>>on
>>>> the thermistor method.
>>>>
>>>> This brings me to the other method that I will use. I will use a fixed
>>>> 150 ohm, 100 watt resistor on the primary (120 volt) side of the
>>>>filament
>>>> transformer. This will reduce the in-rush current to the filament
>>>>below 10
>>>> amps. After the filament has pre-heated, I will then use a switch to
>>>>bypass
>>>> the resistor to obtain full heating.
>>>>
>>>> Does anyone have a better method?
>>>>
>>>> Steve
>>>> _______________________________________________
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>>>> Tesla@xxxxxxxxxxxxxxxxxx
>>>> https://www.pupman.com/mailman/listinfo/tesla
>>>>
>>>
>>>
>>> _______________________________________________
>>> Tesla mailing list
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>>> https://www.pupman.com/mailman/listinfo/tesla
>>>
>>
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