Power transistor substitution - BD140 to TIP32C

Discussion in 'General Electronics Chat' started by DXMachina, Jul 21, 2012.

  1. DXMachina

    Thread Starter New Member

    Jul 21, 2012
    4
    1
    Hi all

    Been using solar panels to charge SLA batteries for ages, running domestic mains lamps converted to use 12v multi-LED modules in my home. Now I'm ready to go a step further, having a decent amount of stored electricity I want to invert up to 220v AC for laptop power and other light uses.

    I havent done any electronics for a long time but used to be capable as a teenager (I'm 33) of building radios and the like from loose components and some of the knowledge remains

    This circuit looks promising - seems plausible and not too hard for me to make

    12v to 220 transistor inverter using darlington pairs and 2n3055s

    Most of the parts are silly-cheap on eBay but I dont like the look of the BD140 PNP power transistor used to drive the base of the 2N3055 output transistors: It says it must be cooled but its a TO-216 / SOT32 format which is difficult to heatsink. (too small, and the sinks are rare)

    If I use a TO-220 package TIP32C transistor in its place would the circuit still work as intended? I dont properly understand the gain calculations as they would be applied to a darlington pair driver circuit, the maths is a bit beyond me but it appears the TIP32C has roughly equivalent specs but at a higher power

    Would it then need less cooling and last longer / run cooler ? Or would putting a higher power rated part there unbalance the circuit, needing different resistances to get the right power on the final drive to the 2N3055 ? (which will be both on big heatsinks)

    Massive thanks in advance from this amateur.. I'm here to learn
     
    PackratKing likes this.
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    T6 and T5 have no pull up resistors on their bases to turn them off and that is never good.
     
  3. DXMachina

    Thread Starter New Member

    Jul 21, 2012
    4
    1
    OK I understand that.

    T5 / T6 are PNP, so if I put 10K ohms from Base to +ve for both, would that do? or should I use a lower resistance value?
     
  4. paulktreg

    Distinguished Member

    Jun 2, 2008
    611
    120
    Be aware that the output will be more square than sine and as the title states, it's simple, so little or no in built safety.

    I'm not trying to discourage you from building this device, that's up to you, but you can buy decent inverters for very little money these days.

    If the specification of the transistors is similar then you should be fine. Perhaps the inclusion of a fuse in the battery lead and some sort of over temperature alarm would be a good idea?
     
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    should be OK.
     
  6. DXMachina

    Thread Starter New Member

    Jul 21, 2012
    4
    1
    Thanks !

    I know I could buy an invertor for less than it would cost to make this, I just want to do something different...
     
  7. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    +1 It's a time bomb, I wouldn't spend the kind of time and effort it would take to build it on an unprotected device.
     
  8. paulktreg

    Distinguished Member

    Jun 2, 2008
    611
    120
    I'm not trying to put you off building an inverter, I just don't like this particular design! ;-)
     
  9. DXMachina

    Thread Starter New Member

    Jul 21, 2012
    4
    1
    A fuse makes sense.

    Could I add some sort of protection with some arrangement where the bases of the output transistors are shorted to -ve if the input current exceeds a certain point derived from ohms law applied to the voltage difference across a very high powered low-impedance resistor in the supply line?
     
Loading...