Dual Regulator Dual PS, Will this work?

Discussion in 'General Electronics Chat' started by Wendy, Jul 17, 2008.

  1. Wendy

    Thread Starter Moderator

    Mar 24, 2008
    20,764
    2,534
    We've had several questions about making dual power supplies lately, and it got me to thinking. Will this idea work? I used a battery just for an example, basically to show a floating power supply.

    [​IMG]

    If not, why?
     
    Last edited: Jul 22, 2008
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    Put a load on only one of the outputs, and follow the current.
     
  3. Wendy

    Thread Starter Moderator

    Mar 24, 2008
    20,764
    2,534
    OK, understood. So lets try a variation, when I can get back.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    Bill, sorry if I seemed somewhat cryptic. I think my answer would have been more appropriate to homework, but I guess you figured it out.:)
     
  5. Norfindel

    Active Member

    Mar 6, 2008
    235
    9
    You need to have the center connection of a transformer to be able to do that, or anything were you can take a center reference.

    If no central point is available, i think you could make it with a 7824 and a 7812, but better to use it to supply little current, because the dissipation of the 7812 is going to be much higher than that of the 7824.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, here's a reasonably simple dual-supply single regulator circuit I was fiddling with.

    The nice thing about it is that the power opamp will keep the porportions between the + and - side the same, even if the regulator stumbles; sort of like a cheapie tracking supply.

    With the pot setting the input level to the power opamp, you can set just about whatever combination of two voltages that you wish, within the limits of the LM317 and the op amp of course. Setting the voltage out of the LM317 below around 15v will result in a rapidly increasing amount of heat needing to be dissipated.
     
  7. Wendy

    Thread Starter Moderator

    Mar 24, 2008
    20,764
    2,534
    I've used the op amp follower myself, it works well. I'm wanting to explore other ideas, but at the moment I'm using a balky old 98SE laptop that doesn't like .png files on MSPaint. My drawing abilities are limited until I figure several things out, and I'll be home by then.

    I'm planning on posting several ideas here before I'm through. The brain fart that was the first schematic cleared up some things, for a psudeo ground to work it has to be a power supply that can soak as well as source current.
     
  8. Wendy

    Thread Starter Moderator

    Mar 24, 2008
    20,764
    2,534
    OK, I'm writing an interactive article here, so I'm going to reserve the next two posts to make sure I have room. I've also drawn some concepts that will need review. I'll incorporate it into my blog later after peer review.

    ***********************************

    Many times it is necessary to have a dual power supply when all you actually have is one. It is possible to synthesize a pseudo ground using techniques similar to normal power supplies, but with important differences. I'm going to assume a 36V ungrounded power supply for most of my models when it is necessary to have a number.

    Here is an old technique that works well within the limits of a zener, the current is limited by the resistor, exceed the current and the voltage folds back, yet the other polarity continues to function.

    [​IMG]

    You design this as you would any zener regulator, design information can be found in the AAC eBook in Vol. III, Chapter 3, Section 11.

    The fact this works so well might encourage you to make a leap to solid state regulators, to use something similar to these configurations.

    [​IMG]

    [​IMG]

    This won't work. The reason is simple, if the plus side is loaded and the minus side is floating (no load) where does the excess current go? With the zeners the answer is into the second zener, but solid state regulators are not designed to absorb current, only to be a source. This point can be illustrated with a simple one transistor one zener regulator.

    [​IMG]

    Note that if the voltage on the emitter exceeds the base voltage the base emitter junction is back biased, and the transistor is turned off. What is needed is more than the series pass transistor, you need a transistor or device that can swing both ways, absorbing as well as sourcing current.
     
    Last edited: Jul 22, 2008
  9. Wendy

    Thread Starter Moderator

    Mar 24, 2008
    20,764
    2,534
    Op Amps can do this, some are quite high current devices. Take the following configuration.

    [​IMG]

    I have seen recommendations for LM675 for high power applications, and KA334 or L272 op amps for lesser applications. The data sheets for all of these devices include methods for making a pseudo ground. The reason this works so well is the op amps active gain actually reduces the apparent output impedance to millohms, even though the current drive may be limited to the op amps specs. Even a weak op amp or one with really low current output specs will appear to have low output impedance, until it burns out. It is possible that you don't have access to these devices, or don't want to use op amps at all for some reason, so we'll explorer other ideas.

    A BJT in voltage follower configuration is one of the most basic voltage regulator configurations. You can read more about this in Vol. III, Chapter 4, Section 6 in the AAC eBook. Basically the output impedance of this regulator is the base resistance divided by the gain of the transistor, this is directly equivalent to the internal resistance of a battery. The following illustration shows the configurations and math behind an emitter follower, and will be important for future concepts.

    [​IMG]

    We can use this concept to create the following circuit, which is two emitter followers back to back.

    [​IMG]

    You start with R2 at its maximum value, then slowly adjust down until you see a current surge, then back it off. It will work, but has several problems. It will have a small dead space where neither transistor is conducting, and if the power supply changes (specifically drops in voltage) the voltage drop across R2 also decreases and may allow the two transistors to conduct in an uncontrolled fashion, burning them out. Single transistors will have a fairly high output impedance, depending on the values selected, but this can be improved using Darlington transistors. It will amplify the problems as well as reducing the output impedance, but it will work.

    You can reduce the sensitivity to the source voltage by using devices whose junctions react similar to the transistor base emitter junction, a power diode. If you want to really match the curves put the diodes next to the transistors so they are the same temperature. Here is the schematic of this arrangement.

    [​IMG]
     
    Last edited: Jul 22, 2008
  10. Wendy

    Thread Starter Moderator

    Mar 24, 2008
    20,764
    2,534
    While these concept will work they suffer from a the big disadvantage that their output impedance is higher than it has to be. As mentioned earlier, op amps with their active gain can bring the apparent impedance down to almost zero. Given that they are also the main stay of voltage regulators it isn't too surprising their used in voltage regulators. The virtual ground is also a voltage regulator of a sort, from the outside it appears to be a tracking dual power supply.

    To merge all of the concepts that have been put forward looks something like this.

    [​IMG]

    This will be an op amp with really high current output. You can use Darlington transistors to reduce the load on the op amp, but it will probably not be necessary.

    Sgt. Wookie came up with an interesting variation in the following schematic. This will create a regulated vertual ground, but with minor modifications can be used to create a straight dual power supply, such as +12VDC and +5VDC.

    [​IMG]
     
    Last edited: Aug 14, 2008
  11. Norfindel

    Active Member

    Mar 6, 2008
    235
    9
    True, i hadn't taken into account that most regulators cannot sink current. Seems like the opamp version is the way to go.
     
  12. Wendy

    Thread Starter Moderator

    Mar 24, 2008
    20,764
    2,534
    Hey Wookie, mind if I use you're schematic and drawings? And do I attibute it to Sgt. Wookie, or your real name assuming its OK?
     
  13. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Sure, go ahead - but everyone here knows me by SgtWookie - so just go with that ;)
     
Loading...