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  1. sureshot

    Transistor hfe

    I removed the regulator l7812 and the MJ11015 transistor to check them, and there fine. So i put them back in circuit and the output was the same. So i measured from ground to the base side of the R7 100R resistor, and i got a reading of 21.50 Volts. My input is exactly 24 Volts (Two xbox...
  2. sureshot

    Transistor hfe

    Ok Steve i will give this a go, tomorrow in evening , and post the results then. Thanks !
  3. sureshot

    Transistor hfe

    Its a single transistor Steve, just to see if this MJ11015 worked. If i put the TIP2955 in there it will do they same as the other unit i built with a single TIP2955 does. As that gives the regulated 12 Volts output, less a small voltage drop at full load, well 50 watts for a single transistor...
  4. sureshot

    Transistor hfe

    Well i have some very odd results... I can't give current reading for the input, and output to the l7812. My output voltage with 24 Volts in is 17 Volts on the output, so i removed the collector from the output rail, and got the expected 12.04 Volts. So i tried putting 12 Volts on the input and...
  5. sureshot

    Any idea what this is?

    That's tear down material !
  6. sureshot

    Wanted: Very large heatsink.

    I really like both ideas, thank you very much ! No Steve not yet, it will be overkill anyway, as i'm only going to be drawing 15 Amps continuous and 20 Amps absolute maximum. As a biker many years ago the aluminium head heatsink idea sounds really good, i have a local machine engineering work...
  7. sureshot

    car power circuit thingy

    No its not offending, I'm sorry if you saw my post as such. It was just a reply no offence intended ! Lighten up (-;
  8. sureshot

    Wanted: Very large heatsink.

    Wanted very large heatsink capable of accommodating 6 x TO3 transistor packages. Drilled or undrilled is fine.
  9. sureshot

    car power circuit thingy

    Why over engineer it, what's wrong with a switch. Seems a lot of extra unessersery circuitry for an ignition turn off, or some would be time delay.
  10. sureshot

    Transistor hfe

    Ok yes I understand the reference and your anolagy, thanks. I was already to start with 200R for R7, But... I will start with 100R for R7. I can see know its not critical, but yes a design applied specifaction, depending on what you want from the regulator and transistors. Yes its been a long...
  11. sureshot

    Isolation washers.

    Thanks for the information on this guys. Yes sure an unisolated package I would use thermal compound, and the T0220 package needs isolation, so ok I'll just go with the silicon isolation washer dry. In this case its a model to test this emitter follower PSU, the last of my scrap CPU heatsinks...
  12. sureshot

    Transistor hfe

    So where you said transient response, the two senarios, high value of R7 would mean slower to start the transistors conducting and carrying the load current. And low value resistance, faster response to the transistors conducting and carrying the load current. Also high resistance, the...
  13. sureshot

    Isolation washers.

    Just a general question, with the grey silicone isolation washers, would you use them dry, or with thermal compound ? I know with mica washers you should use thermal compound. But watching Dave Jones on the eevblog on his adjustable current source he made, the power devise was mounted dry, but...
  14. sureshot

    Transistor hfe

    Thank you AK ! I have taken all that onboard. I know I'm not going to pin down an exact value for R7, that fell into place 2 pages or more ago, as there are so many variables. My grammar understanding in context of a subject, any subject far out weights my maths capacity. I use this to gauge...
  15. sureshot

    Transistor hfe

    Yes I got what you mean Steve with short circuit terminology now, my misunderstanding. And thanks for explaining what AK was meaning back in his previous posts. I think from what's been said 200 ohms 0.5W is a good starting point, well good for less losses in heat and wasted energy, if it fails...
  16. sureshot

    Transistor hfe

    No Steve absolutely not, my understanding of a short circuit is a fault situation ! I see you mean it as no R7 but rather a clear path through to the transistors base and regulator input. PS Wasn't sure if you meant arguing or agreeing, but short meaning no R7 so if agreeing, yes I am. And...
  17. sureshot

    Transistor hfe

    I'm still to determine why AK might think the turn on for the transistors "might be off".... Although I am going to start with a 200R 0.5W R7 resistor. From that measure current and voltage as load goes from 0 - 4 Amps, hopefully the transistors will start up early and carry the current. I'm...
  18. sureshot

    Transistor hfe

    Surely short circuit is no R7 in series with the l7812 input, if that constitutes a short, although I can't see how its a short, so if its voltage straight in that's a closed circuit, and open circuit is a break in continuity so no voltage flows across the R7 path. Only one can, or should say...
  19. sureshot

    Transistor hfe

    Why would the point at which the transistors conduct be off AK ? This bit I don't understand. If these values are known then why can't the transistor turn on be worked out. And thanks Steve I can see where the 12 Amps is now. And AK I have been over post #26 quit a few times now. Its hard to...
  20. sureshot

    Transistor hfe

    Thanks for coming back guys ! I see 2.5 Volts @ 100 mA, and 1 Volt @ 5mA. Which will work ! I'm not sure yet, I understand ohms law, and some basic formulas. But when I sit there trying to diggest transistor formula and physics its just not happening, not from text anyway I can't see where...
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