Search results

Search Forums New Posts
  1. sureshot

    Transistor hfe

    Why is it no one can advise me if a value for R7 of 2.5 ohms 5W will achieve a current limit of 0.800mA for the l7812, and a base emitter voltage of 2 volts ? With all due respect to everyone that has helped me. All I am asking is this correct or is it wrong ?
  2. sureshot

    Transistor hfe

    I'm aware 100R to 0R is resistance to infinity, why 0.870mA ? Because that seems like a reasonable work load for the l7812. As for going round in circles this might be due to conflicting advise on the value of R7 ! I'm sure that I don't really won't to go round in circles, but just acertain a...
  3. sureshot

    Transistor hfe

    Thank you for these replys above ! It might have looked like I was sounding off In my "frustrated" post above. But as a novice over the last four pages we've gone from.... You don't need to change the value of R7, to you need to decrease the value of R7 to 25ohms, to you don't need a resistor...
  4. sureshot

    Transistor hfe

    Does 25 ohms make sense.. it did two pages back. But this Mj11033 transistor has double the maximum base current. Ideally i would like the regulator to be working up to half its maximum rating, say 0.500mA before the transistors start conducting. So over 0.500mA the transistor switch on. From...
  5. sureshot

    Transistor hfe

    Looking at this screen shot, reading it back through for the original calculations, i'm not sure i've got it right, its this base chain bit of text that's got me confused. I am only going back over this for the formula used. Although i still don't know if the post above workings out is correct.
  6. sureshot

    Transistor hfe

    I think i have... but what Steve has said has thrown me a bit, no need to chang R7 value from the suggested 25 ohms 0.5W which was for the 6 X MJ11015 transistors. i've now got my sights on the MJ11033 transistors x 6. So if the maximum base voltage is 5 volts, and i want close to 0.500mA of...
  7. sureshot

    Transistor hfe

    Thanks again for your help here ! So I think I've got this now.. To limit the current through the l7812 to 0.860mA I should use a 1.2 ohm resistor, and if I've done the power calculation for this resistor its 1.63 watts, so using a 3 watt resistor should be suitable ? It is 6 x MJ11033...
  8. sureshot

    Transistor hfe

    The ceramic resistor I plan on using would be 10 watts 0.1R. I understand your description there. Further on in your last paragraph is interesting, I understand this as well, apart from what would stop the l7812 trying to pass more current than 1 Amp ? This is why I though a resistor value for...
  9. sureshot

    Transistor hfe

    Thanks for the description ! As much as know transistor are like a pair of diodes, they won't conduct unless a current is applied to the base junction. To have no resistor for R7 leaves me perplexed to be honest, as I've always known transistors to need a base current to function, as to if...
  10. sureshot

    AC motor speed controller as a regulated transformer?

    I'm only a hobbyists, and to me it screams dangerous idea, like blujet said. Better off looking for something more suitable.
  11. sureshot

    Transistor hfe

    Errmm no current limiting the input to the regulator ? Not sure that's a good idea, won't the transistors go into thermal runaway.
  12. sureshot

    Transistor hfe

    Thanks for your help Colin ! So does this mean I can use the an R7 value of 25ohms 0.5W as suggested by Analogkid back on page 2 of this thread. In other words the base current is 1 Amp for the MJ11015 and 2 Amps for the MJ11033, would the same value of 25 ohms 0.5W be suitable to bias the...
  13. sureshot

    Transistor hfe

    Back again... Before I commit to the use of the 6 x MJ11015 PNP transistors, I've happened across some old surplus stock from a supplier. This other transistor device is the MJ11033 it looks very good for the price I can get it for. The only difference I can see in the data sheets is the base...
  14. sureshot

    Transistor hfe

    Thanks Ratch ! I've done so many searches looking for people that have implemented this simple emitter follower type circuit. In my searches I have happened across the thread you posted a link for, after so many searches you forget some stuff, it only becomes aparant when your memory is jogged...
  15. sureshot

    Transistor hfe

    So at this point I'd like to say Thank you for everyone that has help me here, much appreciated ! I will have a go with this MJ11015 transistor, and have a play with resistor values, whilst measuring current and voltage with a couple of multimeters to see in real practical terms how things pan...
  16. sureshot

    Transistor hfe

    I see what you mean, but the maximum power for the TIP2955 transistor is 90 watts, and the maximum power for the MJ11015 transistor is 120 watts... So would the MJ11015 transistor have a higher upper margin on power over the TIP2955 transistor. Also as Steve pointed out to me, as its gain is far...
  17. sureshot

    Transistor hfe

    I know the power dissipated will not change ! The term ballast resistor is simply a term I used for resistors making transistors share current equally. My primary question was to try and assertain the R7 value via some help with the maths, as the MJ11015 transistor might not conduct with R7...
  18. sureshot

    Transistor hfe

    Sure I didn't say TIP2955. Had a gain of 20 - 70 anyway they work ok in this circuit, pretty good indeed to be honest. I was only interested in the MJ11015 transistor for its greater power handling capability, and where the gain is 1K that looks desirable in terms of perhaps less final voltage...
  19. sureshot

    Transistor hfe

    So the output current of the original schematic is quoted as 30 Amp and the current passing through the regulator is 0.860mA this gives a gain of 34.8 but the TIP2955 has a gain of 20@4A .. It doesn't add up ! Just want to say thanks to all who have helped me this far. At this stage using the...
  20. sureshot

    Transistor hfe

    In the pictures I posted ? The single transistor version uses 6 Amp rectifier diodes, and in the double transistor version, a 15 Amp bridge rectifier package. The transistors are the TIP2955's.
Top