Filament Inrush Protection


Filament inrush protection is required for most tubes. Upon applying AC (or DC) power to the cold filament this inrush current is limitted by the source resistance and cold filament resistance. In case of AC operation this source resistance is mostly the secundairy transformer resistance in series with, if applicable a series resistor (serie resistor to set the filament voltage for correct filament voltage under steady state operation). Cold filament resistance will be a fraction of the "hot" filament resistance as calculated from the datasheet (filament voltage divided by filament current).

Protection mechanisme

Mostly used as inrush protection is to put some additonal resistance in series with the transformer windings (secundaire or primary). For relative small filament current ratings this can be done by putting a small valued power resistor in series and after a certain time short circuit this resistor via a relay contact. In case of big power filaments (over 100W) ratings this will led to a serie resistor with large power rating.  

Approach taken here is to implement an AC phase control circuit. Initially only a small portion of the AC cycle is fed to the transformer, this portion is slowly increased until the full cycle is being utilised. In the figures below this shown.


Fig. 1: power is applied to the filament when the peak value of the sinewave portion is almost zero (170 degrees)


Fig. 2: power is applied to the filament when the peak value of the sinewave portion a small moment earlier (160 degrees)

Phase angle is slowly decreased till 90 degrees. At that moment the maximum value of the sinewave is applied to the filament and maximum current would flow.


Fig. 3: power is applied to the filament when the peak value of the sinewave portion is maximum (90 degrees)

From that moment onwards full power is applied and AC current will flow through the full 360 degrees.

Design implementation

Design is implemented by using a small single chip microcontroller driving a triac. Phase control timing is provided by determing as accurate as possible the AC voltage zerocrossings and triggering the SCR at the correct time (just before the zero-crossing). Due to the nature of SCR's they will switch off (and kept switched off) after the current will be below the holding current of the SCR.

Phase control is done in 10 steps with 5 seconds between each step. After 50 seconds full filament voltage is applied. 


Determination of zerocrossing is not trivial, with the circuit used here (rectifier/opto-coupler) zero-crossing accuracy will be around 100us. To accomplish a higher accuracy, circuitry with accurate comparators is needed.   

In addition some precautions are programmed in software, a.o. when during normal operation some zero-crossings are missed (maybe due to a short power failure) the contraption is entering the power up cyclus. In the resource section (bottom of page) the AVR C source code is available in 2 versions, 50Hz and 60Hz operation. 

At the moment this is in use for my 160mtrs amplifier with TH289. Filament specification for that tube is 6V at 45Amp. In the video below ou can see how the filament is ramped-up. The first phase control cycles are not visable with the ambient light.



During development I did use a number of car headlights (they are much cheaper than a TH289 filament...). Short video of this:


A circuit enhancement would be to omit both of the opto-couplers and feeding the complete circuit directly from mains. 

Copyright © 2011 Carel Mobach, PC5M

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