Can a bjt astable multivibrator run NPN transistors?

Discussion in 'The Projects Forum' started by imbaine13, Jan 1, 2014.

  1. imbaine13

    Thread Starter Member

    Oct 6, 2013
    Hello everyone,

    I am in the process of building and inverter but I am having two serious problems. ONE; As I started out with electronics, I avoided the pnp transistor wherever I could (because I wasn't sure how to use it). I now have a much better understanding of how it works, but I'm still more comfortable with the npn. This is the circuit I'm going to use ( There are so many similar circuits, and whats disturbing me the most is why use a pnp darlington pair. Wouldn't an npn darlington pair work?
    TWO; The other issue is with the transformer. I followed some formulas and chose a flux density of 1.2 Tesla. Unfortunately, the number of turns are 2400 for the 230v output, and 125-0-125 turns for the 12 volt input. This seems huge for the transformer I plan to rewind (400 watts transformer) considering the smallest wire gauge I have is 28 swg. Would using, for example, 480 turns for the 230v output and 35-0-35 for the 12 volt output work?

    Thanks for you time.
    Last edited: Jan 1, 2014
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    Note that the high-current switches are NPN (2n3055s). To turn on the 3055, there is a switch that sources current from the positive rail that causes current to flow into the 3055 base. In this configuration, a PNP is a much better high-side switch than a NPN would be. A PNP Darlington is even better.
    imbaine13 likes this.
  3. #12


    Nov 30, 2010
    You really must get comfortable with the idea that a PNP is just an NPN with the polarities backwards. It works just the same, current per current, and when it is appropriate to use a PNP, it greatly simplifies the circuit design.

    I can't help much with the transformer. Assuming grain oriented silicon steel laminations as a 420 watt core, my old notebook (from 1976) shows 124 turns of #15 wire for a 125 volt 60 Hz primary. That, about 1 turn per volt, makes your 2400 turn idea seem very wrong.
  4. crutschow


    Mar 14, 2008
    An NPN substitution would mean the Darlington would be operating as an emitter-follower. Then, to saturate it, the base voltage must be higher than the collector voltage, which would require an additional supply voltage higher than the 12V rail. Otherwise the Darlington collector emitter-voltage can never be less than two base-emitter voltage drops of over a volt).

    Alternately the PNPs are operated in the common-emitter mode so the Darlington saturation voltage is only one base-emitter drop or about 0.6V-0.7V).

    The minimum number of primary transformer turns you need are determined by the saturation flux density rating of the transformer core. You want to operate just below that level. Do you know what that is for the core you want to use?
    imbaine13 likes this.
  5. k7elp60

    Distinguished Member

    Nov 4, 2008
    To add to what others have said. The collector circuits of the oscillator do not have enough drive for the low of the 2N3055 transistors. The PNP darlington transistors are able to do that.
  6. imbaine13

    Thread Starter Member

    Oct 6, 2013
    Thanks 24, I agree with you on the issue concerning PNP's. About the transformer though, the core is grain oriented steel (I'm not sure about the silicon part). I got all my transformer calculations from here,

    and discovered that fewer turns lead to over fluxing of the transformer core which leads to over heating and affects the life of the transformer. Over fluxing results from using over 1.7 tesla on the core I'm using (the figure varies with core material), and 1.3 tesla gave those number of turns (2400 turns for the 230 volt winding). I unwound a different transformer last night, and forund 3935 turns on the 230volt winding, and just 142 turns for and output of 8.3 volts!
  7. imbaine13

    Thread Starter Member

    Oct 6, 2013
    Thanks for your explanation, unfortunately, I didn't get any of what you said about the darlington. But yeah, I do know the saturation flux density of the transformer core (1.6T.)