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  1. sureshot

    Transistor hfe

    I know this thread seems a bit dragged on... But I will do the practical stuff in the next few days. I've looked at the quiescent current for the lm7812 and the 78S12cv the 2 Amp regulator, and there quiescent current is 5mA@12 V and [email protected] V so the current through a 200R resistor is unlikely...
  2. sureshot

    Transistor hfe

    Well there's got to be a starting point somewhere, and a do remember 5mA on the input of the l7812 with the TIP2955 transistors. I think I get confused on the input current and output current of the l7812 I'm sure that's where I'm going wrong. Anyway I see at 200 ohms 0.5W nothings going to go...
  3. sureshot

    Transistor hfe

    As a starting point I will go with 200 ohms 0.5W and a 1% tolerance resistor. As the maximum base voltage is 5 Volts for the MJ11015 transistor, 1 Volt to turn it on seems very low. And 5mA current for the l7812 also seems very low. As I don't understand transistor physics very well.... At one...
  4. sureshot

    Transistor hfe

    Ok I see what's going on there.... I think... So I know the maths, is this correct... R7 100 ohms with a 2.5 Volts across it would be 25mA ? If I've got that value worked out right I think I've got it.
  5. sureshot

    Transistor hfe

    So doing the maths.. A value of R7 200 ohms will give me 4 Volts dropped across R7 and a current limit of 0.020 mA for the l7812, have I worked that out right ? Help appreciated.
  6. sureshot

    Transistor hfe

    Ok as I said I do really value your opinion ! So 200 ohm for a single transistor is perfectly suitable to turn on and forward conduct that transistor ? I think from what you're saying that it is. The bit I'm struggling with is.. If AK's 25 ohm 0.5W resistor limits the regulator current to...
  7. sureshot

    Transistor hfe

    I don't have a problem using a 200R 0.5W resistor for R7, just confused by the 10 x difference I've described in the post above. And I was going on the maths for a single transistor AK suggested on page 2. It is very confusing, I know a range of base resistors for R7 vary the base current and...
  8. sureshot

    Need to draw 5 amps

    The converted ATX is the favorite here, so long has you can convert one safley .I use the +5 and +12 volt rails output for many applications, and there a winner.
  9. sureshot

    Transistor hfe

    Oh, I thought it was, only reason I thought this is because the single schematic using 1 x TIP2955 transistor has a base resistor of 10 ohms 0.5W and a ballast resistor of 1 ohm 7 watts. Whilst the 6 x TIP2955 transistor schematic has 100 ohms 0.5W base resistor, and has 6 x 0.1 ohm 5 watt...
  10. sureshot

    Transistor hfe

    Well I've got the 6 x MJ11015 transistors, rather than just put all six transistors I'm going to build up a single transistor model. Using the formula that AJ suggested back on page two of this thread, once built I'll power it up, load it with 50 watts or a bit more, open the circuit and...
  11. sureshot

    Simple Conversion from Battery to Plug-In

    Hi welcome to the forum. The easiest cheapest way is to mount a low voltage DC chassis socket, say 5.5 mm x 2.5 mm diameters into the housing of this devise. Then use a universal wall adaptor that can meet the voltage and current needs of this device. Other might have different ideas.
  12. sureshot

    Substitute transistor.

    Yes it certainly powerful enough, it achieves the Darlington configuration, and the HFE is about there almost. But as I can get the BD131 for £0.80 its not to bad. Thank you for the tip ! Appreciated.
  13. sureshot

    Substitute transistor.

    Thanks for your help here, a bit more searching and I've managed to pin down a supplier with the BD 131 transistor, thanks again all.
  14. sureshot

    Substitute transistor.

    Thank you Edd. I will have a look at your surggestion, of the 2N3054, and thanks for the link ! Always a pain in the back side when you find a circuit, only to be challenged by obsolete of hard to find parts lol.
  15. sureshot

    Substitute transistor.

    Thanks Davenn. I have built an lm338K and an lm338T version linear regulator supplies. I was moving towards the more challenging circuits, not just for there PCB layout but also unit constructional layout. I've never used the 723 Regulator, but though I'd give it a go. Retail psu's I've looked...
  16. sureshot

    Solder issue.

    Yes, for sure. I reinforce heavy current tracks with solder runs, and links with a heavy gauge copper wire, using two or even three rails for very heavy current loads.
  17. sureshot

    Substitute transistor.

    Having a look for another decent psu circuit i found this in the image below. Problem is i am having problems finding a BD131 NPN transistor. I was wandering if anyone can help with a substitute, thanks for reading, any help appreciated. Having a few ideas like the BD437 but not completely sure.
  18. sureshot

    Solder issue.

    Something there i've learned.. Tin whiskers, never heard of that, my soldering needs some brushing up on, especially steadiness. Long runs on veroboard is tricky, avoiding solder bridges, i have tried 8 Amps through veroboard tracks alone O-: heated pcb lol.
  19. sureshot

    Solder issue.

    Yes I get on fine with it, but it is 4% silver content. When the lead free solders became available I did make the mistake of getting the poorer quality solder with no silver in it, I can't remember is exact content % of tin and copper off hand, but its horrible stuff. If you try the 4% silver...
  20. sureshot

    Solder issue.

    Yes I think your right, I only changed to the high silver content solder as the old 60 / 40 lead tin solder from local suppliers is usless. I was finding it hard to get a good quality lead tin solder so tried this high silver content solder, and got on well with it. I was only using up a small...
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