In some of my VTTC's I used the resistor method with a switch to bypass it.
Or a timer can be used if you want to be fancy. It worked well. I like the method.
I used a 150 ohm resistor.
John
-----Original Message-----
From: Steve White <steve.white1@xxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Tue, Feb 27, 2018 11:23 am
Subject: Re: [TCML] Methods for VTTC soft filament startup
I forgot to mention the variac method. I actually do have several of those and had considered it. The problem for my VTTC is that I am out of front panel space to mount it and the associated voltmeter. Too bad because that would probably be the ideal method.
----- Original Message -----
From: "David" <zipo@xxxxxxxxxxx>
To: tesla@xxxxxxxxxx
Sent: Tuesday, February 27, 2018 8:31:15 AM
Subject: Re: [TCML] Methods for VTTC soft filament startup
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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