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controlling inrush current to toroid transformer ( 12AX7 Audio Pre-Amp)

Cris Harrison
Intellectual 285 points

Hi, all I think I need some help. I am building a Stereo Pre-Amp Consisting of 4x 12AX7 (dual triode) It glows. The problem that I am having is those normal Iron transformers in my requirements and price are basically not available or are VERY pricey. So faced with a brave new world I am using 3 toroidal transformers,

1. Heaters are powered by  one torrid. With dual 12vac output (nice for isolation)

2. HV about 220vac  by two toroids, one for each channel. 

Problem how can I 'soft start' them one at a time as inrush currents are brutal. I know how to work with IOT including MSP-430 LaunchPad. but not sure what I need to do? 

maybe have 430 or Arduino the look at not zero crossing but peak crossing and kick in a relay or is there an easier way? 

Thanks guys. 

Cris. 

  • 0
    TI__Guru 71526 points

    Hello Cris,

      Since the 12AX7's are vacuum tubes and you are, as I understand, wanting to control the inrush to the AC filament, I'm thinking one of our AC Motor Control Reference Designs might be suitable.  You will have to explore these designs, see what would be close to your design requirements. You can start here: http://www.ti.com/applications/industrial/motor-drives/overview.html  .  

    And there are other Reference Designs that might be suitable, perhaps in the Lighting Controls: http://www.ti.com/applications/industrial/lighting/overview.html .  Look under the Bulbs, tubes section: http://www.ti.com/solution/lighting_bulb_replacement to see if there might be a fit. 

    Let us know what you discover that could be the basis for this design, and we can provide more details from that point. 

    Regards,

    ~Leonard  

     

  • 0 in reply to LeonardEllis
    Prodigy 5 points
    Since this is about TUBES, and I love tubes, I wanted to offer some thoughts.

    1. Consider a Zero Crossing Triac, opto coupled, Like the IL410 IL4108 from Vishay.
    These opto couplers only turn on at the zero crossing, viewed by the triac.
    So a simple timer could stagger the output of the transformer.
    And each would apply the AC at the zero crossing, minimizing current spikes.
    Being optically isolated, the control side would be SELV, low voltage grounded.

    2. NTC Thermistors have been used for decades for AC inrush. Almost every PC offline supply has one on the AC input.
    They are not an "active " device, rather passive and will dissipate heat.
    NTC starts out as high impedance, say 5-10 ohms, then as current flows their resistance drops to 100-200mOhms, Negative temperature coefficient. NTC
    Since tube designs are not made for efficiency, rather made to work right with elegance, some NTC's on the torroid windings might do the trick.
    Ametherm is a well known supplier of NTC's.

    TI in general does not make devices intended to switch AC circuits.