Negative Voltage Regulators

Discussion in 'The Projects Forum' started by PCBoy, Dec 14, 2011.

  1. PCBoy

    Thread Starter New Member

    Dec 14, 2011
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    Hi.

    I'm looking for a negative voltage regulator that has an output current that's higher than 2A.

    So far, I've found LM333 but realized it's been discontinued. Any suggestions?

    Specifically looking for a fixed negative 15 regulator. But a negative variable could work as well as long as it can be set to -15.

    Thanks in advance!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    What is your input voltage (minimum and maximum), and available current?

    What is the maximum output current that you need at -15v?
     
  3. PCBoy

    Thread Starter New Member

    Dec 14, 2011
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    Thanks for the reply.

    My input voltage should be at maximum 50V. And output current at least 2.5A or 3A.
     
  4. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    A common approach is to use a std. regulator with an external pass transistor(s). It should not be hard to find a circuit ( using the usual resources, no need to post a question ).
     
  5. PCBoy

    Thread Starter New Member

    Dec 14, 2011
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    I have thought of that but our instructor specifically instructed us not to use transistors.
     
  6. k7elp60

    Senior Member

    Nov 4, 2008
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    You could possibly us a + regulator in the ground lead such as the ruff diagram posted.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    I can guarantee that this will NOT work.

    PCBoy,
    You say a maximum of 50v; do you mean +50v or -50v? Since you did not specify a polarity, I am assuming you mean +50v.

    What is the MINIMUM input voltage and polarity?

    If your input is indeed positive, you will need a switch mode power supply to generate a -15v supply.

    If your input is actually -50v, and you require 3A @ -15v out, that means you would have to dissipate (-50v - (-15v)) * 3A = 35 * 3 = 105 Watts of power in a linear regulator. You would need a very large heat sink to get rid of that much heat, and even then it probably won't be enough to keep the junction temperature within the safe operating region.

    You're going to need a switching regulator whatever way you look at it.

    Is 3A really the maximum output current that you need? You don't seem terribly certain about that point.
     
  8. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    If you need a negative voltage regulator, you can use, for instance, a LM79xx (LM7905, LM7909, LM7912) or a LM337 coupled to a 2N3055 transistor (used here as pass transistor). With an adequate heatsink on that transistor, it should give 2A. There are lots of circuit examples that use positive voltage regulators with MJ2955 pass transistors. You circuit will probably be very similar, except it will be inverted and it will use the corresponding complementary transistors).

    Here are some good links (for positive regulators):
    http://www.fairchildsemi.com/ds/LM/LM7805.pdf (contains interesting schematics on page 24)
    http://www.teachers.ash.org.au/jfuller/electronics/regulators.htm
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    See our OP's reply #5; transistors are not allowed.
     
  10. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    Sorry, didn't saw that. I've just found out that the L200 can deliver up to 2A. It is a positive voltage regulator, but the L200 designer's guide suggests that it can be used as a negative voltage regulator. Unfortunately, I can't find a better solution.

    A designer’s guide to the L200 voltage regulator:
    http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/APPLICATION_NOTE/CD00003773.pdf
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    I see that in Figure 21. I believe that is someone's wishful thinking. Ask yourself how can a negative voltage be regulated, when current is sourced from pin 5?

    There is no terminal on an L200 that sinks current, unless you wish to consider the input pin 1 as a current sink. In figure 21, they have the lower L200's pin 5 connected to ground. I don't see this as being a very happy situation.
     
  12. k7elp60

    Senior Member

    Nov 4, 2008
    478
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    If the + input to regulator is isolated from ground and the output is grounded it will work. If both inputs - and + are isolated from ground it will work.
     
  13. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    Though pin 5 is sourcing current, the voltage ends up being regulated. The difference is that you consider pin 5 ground, and thus the common (pin 3) of the regulator turns up to be negative compared to ground. After all, the regulator regulates the voltage between pin 5 and pin 3.

    I don't like this solution either, but I can't find an adequate negative voltage regulator capable of delivering 2A or more.
     
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