How to get +5V from a -12V supply and use a common ground reference?

Discussion in 'The Projects Forum' started by negeungrevrhaf, May 12, 2009.

  1. negeungrevrhaf

    Thread Starter New Member

    May 12, 2009
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    I'm working on a small device to control a video projector (turn it on and off, etc.) via the RS232 serial port.
    It uses <1mA at +5V. I want to try to power it off the receive data (Rx) line, which supplies -12V DC [the projector doesn't send any data, so the line is always in a marking state].

    The problem is that I need to drive the transmit data (Tx) line to a positive voltage. I can't just use a linear voltage regulator, because that would make the Tx ground reference -5V with respect to the RS232 ground pin.

    I've thought about using an inverting buck or buck-boost converter, but I can't find any commercially-available regulator IC's that have a negative input voltage and positive output. Any ideas for getting +5V from a -12V supply?
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Use a 555 as a signal source, then use a diode capacitor for a power supply. Think about making AC from the -12V, then using it to feed a DC power supply. Finish with a simple zener diode for a regulator and you're done.

    Having said all that, the question I should have asked first is, how into electronics are you?
     
  3. eblc1388

    Senior Member

    Nov 28, 2008
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    Bill's suggestion is good. You can also try to use the ICL7660S which requires less component.

    In the image, just visualize V+ to be RS232 0V and the ground symbol to be from Rx(-12V).

    Because the circuit acts as a voltage doubler, it will double this voltage giving you +12V output less the diode drops w.r.t. RS232 0V. However, the current on the RS232 line is limited so it would be less.

    [​IMG]

    Edited: TL7660 is rated for 10V only. ICL7660S good up to 13V. You can use a zener diode to limit the input voltage to 12V or less.
     
    Last edited: May 12, 2009
  4. negeungrevrhaf

    Thread Starter New Member

    May 12, 2009
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    Well, that would certainly be easier than what I was trying. Modeled it in LTSpice with an LT1206 in place of the 7660, and everything seems to come out right. Now to order the parts and breadboard it...

    To answer Bill's question, I'd consider myself a beginner. It's a hobby I'm just getting into, and I'm more into the PIC programming side of things. I've got a pretty good grounding in DC and digital basics (Ohm's and Kirchoff's laws, BJT's vs. MOSFET's, logic gates, etc.) but I'm still trying to wrap my head around things like oscillators, inductors and switching power supplies.
     
  5. negeungrevrhaf

    Thread Starter New Member

    May 12, 2009
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    Blast you, inaccurate simulations! LTSpice gave me a reading of 20uA supply current at 9.8V with no load. Should have read the LT1026 datasheet - it actually drew 5 mA with no load and failed to produce the necessary output voltage. On to redesign for the TC1044S from Microchip...
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
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    Do I understand correctly that the RS232 port (on your PC I presume) is going to be used to supply power to your device but you need +5V? In other words the port is not being used as a signal source or recipient, just a power supply?
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Hehe, you got me thinking about the subject, might even use it for an experiment. Here is the graphic work I've done to date, maybe it might help. The outputs are unregulated, I'd use a resistor/zener on them.

    [​IMG]

    You can improve their performance by using Shottky diodes. These circuits are so low power any diode will work.
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Check the DTR and RTS pins on the projector's 9pin connector. It is quite likely that one of these pins is at the +ve potential which you can use with a diode and cap like the old serial mouse circuit to generate +5v for your circuit.

    It's fine to draw a few mA from that pin to power your device, I've done it a couple of times to power a low-power device direct from the serial port.

    You could google for "serial mouse schematic". :)
     
  9. aerospaced

    New Member

    May 28, 2009
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    To make it simple, just use a 7805. Connect the input to the Grnd and the Grnd to the -12. You are simply using the potential between the grnd and -12 in reverse. It works, we did this on several satellites. It is sometimes better to use (-) in logic intensive PCBs because the noise is then ignored.
    Don't forget the decoupling caps.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Bill,
    Neither of the "a" circuits will provide a DC output; they will output a roughly triangular wave somewhat offset from ground. You need the series diode to keep the cap charged.

    [​IMG]
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    Really? I'm trying to pump up the second capacitor to battery voltage.

    Hmmm, maybe move the diode from ground to be in series with the first cap?

    I gotta remember to add designators, sure makes taking about circuits easier.

    I'll redraw and repost in a bit.
     
  12. CDRIVE

    Senior Member

    Jul 1, 2008
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    In an earlier post I asked you if all you wanted from the serial port was a low current supply. I was going to tell you what RB mentioned in his post, as the DTR line can be made positive and will easily drive a LED or two.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Yep!
    I know that! You'll never get there without a pair of diodes, though.

    Well, you need a source for electrons (or holes) and a destination for them. You need two diodes to ensure they all stay in line. It's sorta like an air compressor; you have an inlet valve and an exhaust (pressure) valve. If either the intake valve or exhaust valve sticks open, you won't get your compressed air tank filled up; but you'll sure get a heck of a lot of noise.

    Absolutely! Otherwise, we're talking about the pointy-looking-thingie on the right of the drawing that're hooked up to pin umpty-ump of the flapdoodle IC, which sure takes a lot more typin' than "missing D2"! ;)
     
  14. Wendy

    Moderator

    Mar 24, 2008
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    The two diodes are a doubler. I don't think you need two diodes for simple rectification, which is what I'm aiming for. A diode clamper was what I was originally thinking of, though I didn't hang the name onto until later. Looking at the chapter following, I can rotate the diode 90° and get the result I was after, which is the power supply reversed (minus diode drop).

    Looking at it this will make an excellent 555 experiment.
     
    Last edited: May 31, 2009
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