Thread-topic: Methods for VTTC soft filament startup
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
>>>
>>
>>
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>
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