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Question regarding pass transistors

Discussion in 'General Electronics Discussion' started by jackorocko, Aug 29, 2011.

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  1. jackorocko

    jackorocko

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    Apr 4, 2010
  2. jackorocko

    jackorocko

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    Apr 4, 2010
    http://www.circuitdb.com/circuits/id/75

    How do they figure this? I don't get how you can divide the two resistors to find the current gain of transistor. What am I missing?
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    One difference is that one circuit will give you 5V, the other will give you 4.3.

    If you have 5V to the base of the transistor, the base is 0.7V(ish) higher than the emitter.

    edit: Of course I failed to notice that they're using a 5v regulator as a variable regulator. This is not recommended because the various fixed regulators are not optimised for minimal current through the ground lead. And it also limits your minimum voltage.
     
    Last edited: Sep 3, 2011
  4. jackorocko

    jackorocko

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    Apr 4, 2010
    And the one with NPN transistor will provide all the amps through the transistor vs only turning on to help augment the power provided by the VR. Is this a horrible design or is this a little safer?

    On that webpage they both claim to be 5 amps. But I am still not sure how they figure that. Is 5 amps the max amperage the transformer will provide?
     
  5. jackorocko

    jackorocko

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    Apr 4, 2010
    Also, what is the recommended way of attaching a to-3 to a heatsink with the output being the case. Doesn't this mean the heatsink needs to be isolated? I have a really heavy heatsink that is more then capable to handle 3 of these transistors at full dissipation I would imagine, but that makes it considerably harder to isolate the heatsink.

    Someone please advise

    My way of thinking is the pins point away from the heatsink and the circle part of the case sits inside a hole drilled in the heatsink and a isolating washer is used to isolate the case from the heatsink and a non-conducting screw is used to bind the two, but this seems silly. Poor heat transfer I would imagine.
     
    Last edited: Aug 29, 2011
  6. Resqueline

    Resqueline

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    Jul 31, 2009
    Because, as the heading says:
    Also, since the 78L05 is only capable of 100mA you'd need a gain of 50 from the 2955 to get 5A out. This can not be guaranteed under all conditions.
    That being said I'd rather use a 78M05 (500mA) or a 7805 (1A), one resistor, and a 2955 (if I had them at hand).

    The first circuit will, as Steve stated, have a lower voltage output than the second. It will also have a less than ideal load regulation and a very undefined current limit.

    You may also notice (calculate) that the 5k pot will give an adjustment range of 5-130V. (The 4.5V spec' is wrong for the second circuit btw..)
    Either lose the 220R resistor to get 5-15V, or use a 500R pot to get 5-12.5V or a 1k pot to get 5-25V.

    The current limit formula is an arithmetic simplification, and I guess it only holds true for the condition of 2A and dropping half the voltage b-e.
    The full, universal formula is Ilm + ( ( ( Ilm * 0R7 ) - Ube ) / 0R3 ).
    The limiting current of all these voltage regulators are only approximate. Studying good datasheets will reveal that they vary greatly with conditions.
    The transformer has its limits (not to be exceeded), but it is not the current limiting value calculated here.

    The heatsink needs to have 4 (small) holes drilled for each TO-3. Two for the mounting screws, and two for the pins.
    Then you need a TO-3 insulation kit, consisting of two T-shaped plastic "tubes" and one stamped-out mica/plastic film.
    You also need two suitable screws, washers, nuts, and one solder tab.
    I'm sure Google will help find the way it's done.
     
  7. jackorocko

    jackorocko

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    Apr 4, 2010
    What is llm and Ube stand for? I don't understand sry

    I have drawn a schematic of what I think would be the correct way to build a variable PSU. I want this to be a select-able PSU with say a rotary Switch, so I can have say 3.3, 5, 9, 12, 24V output. I will redraw the schematic with a rotary switch once I figure out the formula for Vout with the correct resistors. That being said, I don't understand how to get Vout when Vout = Vref(1+R2/R1) + IadjR2. I don't see how you can figure Iadj with ohms law if you don't know the Vout to begin with??? Or is Iadj always gonna be Vref/R1? So confusing...

    Also, I want to use the LM338T so I can have an easier time mounting it to a heatsink, but in the datasheet it says the TO-3 package is 50W dissipation and the TO-220 is only 25W dissipation. Does this mean I can't use the TO-220 or I will just need a bigger heatsink?

    P.S. daveelectric, I know you are into PSU design, maybe I can pick your brain in more detail in PM sometime. I want to start building useful circuits, I see PSU's as the basis of what I want so maybe we can collaborate???
     

    Attached Files:

    Last edited: Sep 2, 2011
  8. Resqueline

    Resqueline

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    Jul 31, 2009
    Ilm = current limit of the LM(317T). I = current [A].
    Ube = base-emitter voltage drop [V]

    Make sure to use a make-before-break switch or else you'll get a jump to full voltage between each step.
    I VR1 = Vref / R1 + Iadj. (Iadj is only 45uA for the LM338 and (can be ignored since) I R1 is 10.3mA in this circuit.
    R2 seems a little small being 0.14 ohms. You'd need 10A to make 1.4V across it, which is needed to make the 2955 conduct and get 0.7V across R3.
    For a 30V in-out differential you'll find in the datasheet that the LM338T will only deliver 1A. At 6V differential it can deliver over 9A.
     
  9. davelectronic

    davelectronic

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    Dec 13, 2010
    Pass transistor

    Hi there. yes i see what your working at, i only at the moment go with fixed pass transistor voltage or varible from the referece voltage of a variable package like the LM 338's there is no problem on multiple units on one heat sink as long as you use isolation bush and washers, mica or silicone washers, i do what i find on line, circuits others have made, my main interest is on simplicity, trying to go very small for a given power rating, low component count. first thing in the morning for me.
     
  10. davelectronic

    davelectronic

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    Dec 13, 2010
    Pass transistor

    I am on my phone at the minute, i will fire up the pc later, look more in detail at your project, the other guys posts are sound, and there experiance is above mine, but i will look on the pc later, this phone iam on is so limiting. But i will look more in depth on the pc at your project. until later today. Dave.
     
  11. jackorocko

    jackorocko

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    Apr 4, 2010
    So let me see if I understand this. If I was to use a 30V input and design for worse case current max, then I would need to take my input 30V and subtract my lowest output voltage 3.3V to get my biggest in/out differential. That is equal to ~26V @ 2A.

    This would mean that my 2955 wouldn't even turn on for the voltages 3.3,5,9 and for those voltage ranges I would be limited to the LM338 current max? Unless I changed the R4 resistor to a higher value, which doesn't seem like the right thing to do since it would require me to use a higher wattage resistor. Right now I am looking at a 10W resistor for R4 when my current is 5A.

    Correct me if I am wrong here, but the datasheet also states that at a 18V differential, 30Vin-12Vout, the max current I will get is only about 3A after 100ms, so even my 12V supply would still be limited to the max current of the LM338

    Put this all together and what it means to me is that my pass transistor will only provide extra current only when my output would be 24V. Do I have this correct?
     
    Last edited: Sep 2, 2011
  12. jackorocko

    jackorocko

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    Apr 4, 2010
    Now that I look at it this way, Would I be better off to use a VR for every voltage I want and then use a rotary switch to select the VR and incorporate a 1 resistor per VR to turn on 1 pass transistor?

    Edit: I will try and draw a schematic of the above in case my description is poor.
     
  13. davelectronic

    davelectronic

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    Dec 13, 2010
    Pass transistor

    Hello jackorocko.
    Ok first off ive never used a pass transistor on a variable voltage regulator, thats not to say it wont work, from what ive read the problem seems to be the voltage differential from volts in to volts out, and the heat generated by the big power being dumped on the regulator or transistor at low voltage use, IE when you use a lower voltage there is a huge differential between the volts not being used it still enters the circuit and on the VR and or the transistor.

    There is always a trade off between power in to power out and what the transistor and voltage regulator does with it, as you know it going to retain that power and try to handle it, but this is a max threshold problem in terms of heat the bottom line, you could switch in power resistors for the lower voltages to soak up this wasted power while on the lower voltages, but current is bound to be affected.

    I use the regulators and transistors with in there parameters, if i wont 12 volts 5 amps i dont use 30 volts in and hope the voltage regulator can cope, its all to do with the volts in to volts out, keeping this close IE a few volts higher than you want out gives you reliable current at the voltage you wont, if you want a differential between 3.3 volts and 24 volts any circuit will struggle with the volts your not using, its a kind of cant have your cake and eat it in terms of a very wide voltage and power range, i am a hobbyist and can handle the normal maths in a circuit, but my maths knowledge is not my strong point, so in very complex equations i struggle.

    As for isolation to the heat sink with regard to case polarity, you use steel machine screws about 3mm 1/8 of an inch, but there is a mica or silicone pad or washer you use and a plastic bush that fits either the TO220 / TO3 case, yes there is a slight thermal issue so i use thermal paste between the surfaces, non conductive, then your voltage regulator and transistor are electrically isolated from each other.

    I try not to get bogged down in the maths if i can help it as its a lot more complex, being ill my head wont do it, but i observe data sheet info and circuit values in terms of resistors semiconductors, and understand there are limits i cant go beyond, so the circuits value in basic maths for me is ok. I dont or wont get wrapped up in complex equations and scientific data of a components characteristics, it spoils it for me, thats for the maths boffins, and a good job they do, as i would be rubbish at it.

    My only advise is a variable supply is better with linearity, and selective switching while easily achievable, can be a head ache for big differentials in voltage, so can none switching and continuous voltage travel. If you want multiple voltages at high current, either use power resistors to soak up and dissipate the excess heat, or use different regulators, a single variable voltage regulator supply is fine with or with out a pass transistor the latter offers more power, but the higher you go in voltage the lower you go in current or you end up with excess heat. its best to keep input volts no more than a few volts higher than the final output voltage, and when you use low voltage there is a problem with what the circuit does with the excess, forced air cooling offers some benefit as to just passive cooling.

    I cant match Steve's and Resqueline's experience, some times you have to try it and see, beyond the theory, i hope you find the solution your looking for. Dave. :) PS ive used a fair few regulators, the 78SXX series at 2 amps are more versatile for fixed, and the LM338K offers better performance in terms of heat it can withstand, but ive used the LM338T as well, i know the LM338K is more expensive but its a better option than the LM338T.
     
    Last edited: Sep 2, 2011
  14. jackorocko

    jackorocko

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    Apr 4, 2010
    ok, so after looking at the specs I realize a high power PSU is out of the question with a select-able output voltage. Given that I think this should cope with all that I want.

    How does this look??? Only thing I don't get it what the value of R8 should be, if I understand the equation then a lower value resistance compared to R7 will give me a higher current?
     

    Attached Files:

  15. davelectronic

    davelectronic

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    Dec 13, 2010
    pass transistor

    Hi jackorocko.
    The transistor base resistor, i would use 10 ohms 0.5 watts carbon or metal film the latter gives a closer tolerance, the power resistor, the PNP emitter can be 1 ohm 3 watts for 3 amps and 7 watts for 5 amps. Dave. :) PS i cant see a problem i will leave two links showing diagram schematics i like the look of.

    http://www.zen22142.zen.co.uk/index.html

    http://www.zen22142.zen.co.uk/Circuits/Power/boosti.htm

    http://www.zen22142.zen.co.uk/Circuits/Power/1230psu.htm

    These circuits might help i like the site.
     
    Last edited: Sep 2, 2011
  16. jackorocko

    jackorocko

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    Apr 4, 2010
    Ok, so let me know what won't work here. I have calculated the values

    1.5A llm
    4A Ice

    I have also calculated my transistor power dissipation to be 95W when the output voltage is 3.3V. Is this correct and can the 2955 even handle this? I guess now the question is what size heat-sink I will need to keep the transistor from roasting itself.
     

    Attached Files:

  17. davelectronic

    davelectronic

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    Dec 13, 2010
    pass transistor

    I dont know about the transistors base resistor, is it 1R or 10R i cant tell, anyway the heat sink does not need to be huge i use thermal axial fan cooling with simple thermal switches, one for one speed or a couple for different fan speeds and temperatures, they are small cope with thousands of make break cycles, 50,000 + its up to you huge heat sink passive cooling, or a small heat sink for forced air cooling, which is very efficient. Dave. :)

    TIP 2955.

    http://www.futurlec.com/Transistors/TIP2955.shtml
    Max rating 90 watts, temperature max 150 degrees C

    The complementary is shown in there as well, NPN.
     
    Last edited: Sep 3, 2011
  18. Resqueline

    Resqueline

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    Jul 31, 2009
    I'll be busy (on the road) for a few days onwards, but I'll take the time to point to some basics you should be aware of.
    What are your design goals; max currents at various voltages. What is your raw voltage? Can you resort to fan cooling?
    Don't forget that the max 1A @ 30V diff. needs to supply the 2955's base, so you can't spend it all on R7.
    In practice a 3055/2955 can't handle more than 50W of passive dissipation. I found I had to use no less than 4 * 3055's for a 0-30V 12A supply.
    Use series connections for the voltage setting resistors. This will avoid a dip in voltage as you switch between settings.
    Some supplies use a separate switch/relay at the transformer to avoid the big voltage differential problems.
     
  19. jackorocko

    jackorocko

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    Apr 4, 2010
    1 Ohm

    To build a supply that I can use on the bench. Doesn't have any specific purpose, But I would like to have a usable supply with the typical voltages up to 12 or 24 depending on what would be easier with at least 5A of current, since I think this should cover most things I plan on trying. My raw voltage out of the transformer would have been 20V as to get a rectified voltage of 28V so I can hopefully get up to 24V regulated out of it.

    Also, my latest design uses the LM317 since I won't have to deal with such wild swings in current over the voltage differential.
     
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Another option may be to look at a SMPS.

    Even if you want the output to be vaa a linear regulator, you might consider a SMPS pre-regulator to drop the input voltage tot he linear regulator to (say) 5 volts above the output voltage.

    This also gives more protection in short circuit conditions.

    Of course, you may wish to avoid a SMPS for noise reasons, in which case switching the secondary tap at the same time as changing output voltage is almost as good (and much simpler)
     
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