OT: S-100 card power -- voltage regulators in parallel? - CP/M

This is a discussion on OT: S-100 card power -- voltage regulators in parallel? - CP/M ; On Thu, 16 Aug 2007 16:19:22 -0700, MdntTrain wrote: > > >> If it is the one I think it is, the regulator connects with its >> input on the base of the pass transistor, and its output on the ...

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Thread: OT: S-100 card power -- voltage regulators in parallel?

  1. Re: OT: S-100 card power -- voltage regulators in parallel?

    On Thu, 16 Aug 2007 16:19:22 -0700, MdntTrain wrote:

    >
    >
    >> If it is the one I think it is, the regulator connects with its
    >> input on the base of the pass transistor, and its output on the
    >> collector of the pass transistor, which is also the output of the
    >> combined regulator.

    >
    >Pretty much, yes. The emitters are connected individually through
    >0.1ohm resistors to 8V, then the bases + the regulator IN are
    >connected to 8V through a 100ohm resistor.
    >
    >The outs are combined as you say.
    >
    >
    >> The three terminal regulator passes just enough current to keep
    >> its output at the specified voltage. In this case, it pulls that
    >> current from the base of the pass transistor, which then
    >> increases the collector current of the pass transistor. The

    >
    >Ok, I follow so far.
    >
    >
    >> output current, then, is that through the regulator plus that
    >> through the pass transistor. The pass transistor current is beta
    >> times the base current, which all comes from the three terminal
    >> regulator. The total, then, is (1+beta) times the three
    >> terminal regulator current which is usually up to one amp.

    >
    >I follow that.
    >
    >But I still don't get how the 8v at the emitters is dropping to 5
    >volts at the collectors. The 0.1 resistors are way too small to be
    >dissipating the extra voltage. So is the regulator compensating
    >somehow?


    Yes, the power is dissapated in the transistors, very little in the
    resistors. They are behaving as variable resistors passing current
    around the regualtors internal pass transistor.

    Thats what they do.

    The resistors (.1 ohm) are thre for making sure one of the transistors
    do not hog the current.

    Basically the system is a simple servo, the three terminal regulator
    is trying to maintain it's output voltage. Doing so, the three
    terminal regualtor passes the increased power drain through that
    100 ohm resistor. In doing that that causes the transistor(s) to
    conduct some. If the voltage rises at the output the Three terminal
    regulator draws less current and the current through the resistor
    drops and the bias to the transistor is reduced and it conducts less.



    Allison


    >
    >~ J
    >



  2. Re: OT: S-100 card power -- voltage regulators in parallel?

    no.spam@no.uce.bellatlantic.net wrote:

    (snip on high current regulators)

    > Basically the system is a simple servo, the three terminal regulator
    > is trying to maintain it's output voltage. Doing so, the three
    > terminal regualtor passes the increased power drain through that
    > 100 ohm resistor. In doing that that causes the transistor(s) to
    > conduct some. If the voltage rises at the output the Three terminal
    > regulator draws less current and the current through the resistor
    > drops and the bias to the transistor is reduced and it conducts less.


    100 ohms sounds a little large, though 3 ohms a little small.

    It needs to be small enough to guarantee the pass transistor
    is off with the quiescent current of the regulator.

    -- glen


  3. Re: OT: S-100 card power -- voltage regulators in parallel?

    On Sat, 18 Aug 2007 00:07:51 -0800, glen herrmannsfeldt
    wrote:

    >no.spam@no.uce.bellatlantic.net wrote:
    >
    >(snip on high current regulators)
    >
    >> Basically the system is a simple servo, the three terminal regulator
    >> is trying to maintain it's output voltage. Doing so, the three
    >> terminal regualtor passes the increased power drain through that
    >> 100 ohm resistor. In doing that that causes the transistor(s) to
    >> conduct some. If the voltage rises at the output the Three terminal
    >> regulator draws less current and the current through the resistor
    >> drops and the bias to the transistor is reduced and it conducts less.

    >
    >100 ohms sounds a little large, though 3 ohms a little small.


    depns on how much current you wish to pass though the regulator before
    it starts driving the pass transistor(s). At 100ohms that would be
    around 6-7ma (assuming a transistor, it would be twice that for a
    darlington connected pair) which is low but the 100ohm number was
    picked from prior posting. The 100ohm number is high as the three
    terminal regualtor has a basic internal power drain of 1-4ma (vendor
    and type dependent). It's necessary to keep the transistor cutoff
    until it has to acutally carry load.

    Most of the regulators I've done in that style used around 10ohms to
    27ohms (mostly on whats handy). I usually include an over currrent
    transistor circuit and if the system to be powered is important also
    a overvoltage crowbar.

    Allison



    >
    >It needs to be small enough to guarantee the pass transistor
    >is off with the quiescent current of the regulator.
    >
    >-- glen



  4. Re: OT: S-100 card power -- voltage regulators in parallel?

    *glen herrmannsfeldt* wrote on Thu, 07-08-16 22:44:
    >The total, then, is (1+beta) times the three terminal regulator current


    Two things to keep in mind here:
    As beta is very much larger than one the whole setup may well start
    ringing.
    As the current through the transistor is beta times higher than that
    through the regulator so is the power lost. Any safe area protection is
    thus invalidated.

    A better solution but one with some extra voltage loss might be to use
    an extra diode and a resistor in every path to make the current through
    the regulator and that through each external transistor nearly the same
    or even make that through the transistors a bit less so that with
    identical heat sinks it's the regulator that gets hottest and shuts
    down.


  5. Re: OT: S-100 card power -- voltage regulators in parallel?

    Axel Berger wrote:
    > *glen herrmannsfeldt* wrote on Thu, 07-08-16 22:44:


    >>The total, then, is (1+beta) times the three terminal regulator current


    > Two things to keep in mind here:
    > As beta is very much larger than one the whole setup may well start
    > ringing.


    High power transistors tend to have lower beta, but they may
    still be improving.

    > As the current through the transistor is beta times higher than that
    > through the regulator so is the power lost. Any safe area protection is
    > thus invalidated.


    There is another suggested circuit with current limiting, which has
    a current sensing resistor, and a transistor to reduce base
    current based on the collector current.

    Figure 13 in: http://www.datasheets.org.uk/datashe...article=623405

    -- glen


  6. Re: OT: S-100 card power -- voltage regulators in parallel?

    glen herrmannsfeldt wrote:
    >
    > no.spam@no.uce.bellatlantic.net wrote:
    >
    > (snip on high current regulators)
    >
    > > Basically the system is a simple servo, the three terminal regulator
    > > is trying to maintain it's output voltage. Doing so, the three
    > > terminal regualtor passes the increased power drain through that
    > > 100 ohm resistor. In doing that that causes the transistor(s) to
    > > conduct some. If the voltage rises at the output the Three terminal
    > > regulator draws less current and the current through the resistor
    > > drops and the bias to the transistor is reduced and it conducts less.

    >
    > 100 ohms sounds a little large, though 3 ohms a little small.
    >
    > It needs to be small enough to guarantee the pass transistor
    > is off with the quiescent current of the regulator.


    One amp through a 3 ohm resistor requires 3 volts, and dissipates 3
    watts in the resistor proper. Lack of care will start fires.

    --
    Chuck F (cbfalconer at maineline dot net)
    Available for consulting/temporary embedded and systems.



    --
    Posted via a free Usenet account from http://www.teranews.com


  7. Re: OT: S-100 card power -- voltage regulators in parallel?

    On Sat, 18 Aug 2007 16:47:15 -0400, CBFalconer
    wrote:

    >glen herrmannsfeldt wrote:
    >>
    >> no.spam@no.uce.bellatlantic.net wrote:
    >>
    >> (snip on high current regulators)
    >>
    >> > Basically the system is a simple servo, the three terminal regulator
    >> > is trying to maintain it's output voltage. Doing so, the three
    >> > terminal regualtor passes the increased power drain through that
    >> > 100 ohm resistor. In doing that that causes the transistor(s) to
    >> > conduct some. If the voltage rises at the output the Three terminal
    >> > regulator draws less current and the current through the resistor
    >> > drops and the bias to the transistor is reduced and it conducts less.

    >>
    >> 100 ohms sounds a little large, though 3 ohms a little small.
    >>
    >> It needs to be small enough to guarantee the pass transistor
    >> is off with the quiescent current of the regulator.

    >
    >One amp through a 3 ohm resistor requires 3 volts, and dissipates 3
    >watts in the resistor proper. Lack of care will start fires.


    The voltage across the resistor is one BE junction ar about .7V and
    the deisred current though it will be .2333A or roughly .164W.

    Usually designers using the circuit described run the regulator
    current and hence the Base current at around 30mA so power
    dissipation is usually not an issue for the resistor for normal
    cases (operation). For abnormal cases there isn't overcurrent
    protection unless added to protect everything. Adding a fuse
    is not a terrible idea.

    For quickie tasks a three terminal regulator with an external
    pass transistor is ok. If I want a more sophisticated regulator
    with overcurrent or variable output or the option of remote
    sensing I use a venerable 723 as the parts count converges
    between the extended quickie and a full bore 723 design.

    Allison


    >
    >--
    > Chuck F (cbfalconer at maineline dot net)
    > Available for consulting/temporary embedded and systems.
    >



  8. Re: OT: S-100 card power -- voltage regulators in parallel?

    On Aug 18, 7:37 am, no.s...@no.uce.bellatlantic.net wrote:
    > I usually include an over currrent
    > transistor circuit and if the system to be powered is important also
    > a overvoltage crowbar.
    >
    > Allison


    By all these wonderful posts (thank you especially Glen and Allison,
    and also Mike, Barry, dkel..., Axel), I've decided to give the pass
    transistor method a try. I'll start with a 7805 and 2 TIP2955
    transistors for a 10amp supply -- it's less parts than my first idea
    even if that *had* worked.

    Sounds like I should have a "crowbar".. but to start, I'll have two
    fuses -- 1amp on regulator, and 6amp on main (since system only draws
    4.4).

    ~ J


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