transistor with a lot of beta

bitrex · 2016-02-14T18:07:07+00:00

I'm working on my little graphic equalizer meter display project. The graphic EQ IC has a "split rail" reference output and since the supply is 5 volts I'm getting around 2.5, 2.6. But I don't really need this for the rest of the circuit. I'm thinking about using some VFD "bargraph" indicator tubes for the display. The ones I have in mind need a filament voltage of around 2 volts at about 30-40 mA each, times 7. I'd like to just slap a transistor fed from that reference on the base to regulate the filaments... 5 volts in on collector, ~2 volts out, just under 1 watt dissipation. Darlington NPN will have too much B-E drop to work here. Can anyone suggest a suitable jellybean tranny for this purpose?

Feb 14, 2016 47 Replies

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whit3rd 10 years ago

i.e. they are filaments and cathodes both... So ground one end of each filament, and capacitor-couple a square-wave (switched 5V) to the other end. Maybe two or more square waves, different phases, would be best for power supply ripple...

5V, you want 35 mA, at 100 kHz C is about .027 uF to drive a single filament

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George Herold 10 years ago

n put

Huh, Is the current listed anywhere on there? I was testing these little CUI buck regulators, fixed output voltage and ~0.5 Amp in a little through hole pac like an LM78XX.

George H.

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George Herold 10 years ago

Huh, nice ripple... but according to DK it's discontinued.

George H.

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Syd Rumpo 10 years ago

Sziklai pair?

Cheers

Syd

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Lasse Langwadt Christensen 10 years ago

Den mandag den 15. februar 2016 kl. 15.12.04 UTC+1 skrev George Herold:

can put

seller says 1.8A, 3A peak if the pictures are to be believed the IC is an MP2307 rated for 3A cont.

4A peak

-Lasse

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Rich S 10 years ago

OK, there's been other better solutions posted here. But just to finish my poor little thread... my go-to choices for medium-power (TO-126) have been BD139 (NPN) and BD140. Rated 80V, 1.5A. Fairly cheap, 0.11USD and 0.14USD respectively, from Tayda Electronics.

Cheers, RS

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tabbypurr 10 years ago

All VFDs are direct heated afaik.

I'm not seeing the problem. If all you want is low Vbe with high gain, a sziklai pair gets you that. But why not just run the filaments from a suitable winding to begin with. Or if you must, use a dropper resistor or diode.

If for some odd reason you wanted to put heaters in series, you could if you used relatively high anode V & drove the grids from this via resistors. VFDs should do 90 or 150v happily enough.

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