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Cheap low-power 12V DC-DC isolator?

Discussion in 'Home Power and Microgeneration' started by Scott Willing, Jun 25, 2005.

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  1. Hiya folks,

    We just got a cordless phone with the ubiquitous wall-wart AC adaptor.
    (9V) Obviously I'm not keen to keep my main inverter running 24/7 for
    the sake of this thing. I have a 12V outlet in the perfect location
    but due to the telephone line positive ground issue, I can't just use
    a cheap linear step-down. (Creates a permanent off-hook situation.)

    I need something like an isolated DC-DC 500mA converter. 12V in, with
    an adjustable output,would be great, but 12V -> 12V would do.

    There are wonderful little modular PC-mount converters available but
    I'm not sending Digikey $100 to fix this little problem. I didn't
    spend that much on the phone.

    Would be a fun project to design/build, if I had time for fun. I need
    a built unit or a kit.

    Meanwhile I'm going 12V outlet->cheesy 125W inverter->AC adaptor.
    Works and is even pretty efficient, but... yuck!

  2. George Ghio

    George Ghio Guest

    Last time I bought a power supply as a kit to run my phone it cost
    $8.50. Looking at the catalouge right now, the price is $6.95 Aud. The
    cat No. is KA1797. for the US and CA

  3. Guest

    Just make a cheap and dirty multivibrator circuit feeding an isolation
    transformer (can be a simple "filament transformer" type unit with 2
    low voltage secondaries) and rectify and regulate the output. Set to
    600hz or higher filtering is relatively trivial.
  4. Vaughn

    Vaughn Guest

    Three thoughts:
    1) Before you do anything else, try swapping the wires on your phone line to see
    if the off-hook situation goes away. I think that there is a 10 to 20% chance
    that this will cure your problem.

    2) Assuming that you run your inverter for a few hours a day for other reasons;
    use a 12 V. Gel cell and a charger as a combined DC/DC converter and UPS for
    your cordless phone. I actually saw a commercial version of this in an office
    supply store recently.

    3) If you still want a DC/DC Converter, check out Marlin P. Jones . I don't see exactly what you want on their web site, but
    it is worth a phone call. Some of their DC/DC converters are less than $10.00.

  5. Appreciate the thought, and I hear you bro, but that's why I said
    "Would be a fun project to design/build, if I had time for fun." And
    because I know how easy it should be, I'm not keen to spend a lot.

    Truth is I probably even have all the parts I need on hand. But I also
    had the parts on hand for a little motor speed control project for
    over two years -- schematic ready to go -- and that didn't get built

    I ordered a kit to solve the speed control issue for $10. I was buying
    something else on a website, and this wasn't going to add any shipping
    cost so I went for it. Felt like a beginner hobbyist.

    But when a had a brief opportunity, I put it together in 10 minutes. I
    didn't have to think, plan, lay out. It was a little holiday for my
    weary brain, instead of the mental torture I usually subject myself to
    when I built from scratch. :)

    BTW I see Backwoods Solar offers a solution to this very problem for
    US$40. I love those guys, and I bought my first gear from them after
    moving into the electrical nightmare of my home. But the shipping cost
    sometimes kills me when importing small items.


  6. That's an intriguing prospect. It's been a few decades since I took
    analog telephony stuff in school (as related to modem circuitry) and
    the memory is not good beyond "telco = positive-grounded system." I
    dunno wether that will fly or not... but with a powered phone... hmm.

    I'll certainly look into it. It would be a riot if that worked. I'd
    owe you a case of brewskis fer shur.
    I'm ahead of you there. For some reason I love little indendent
    systems. I do have some gell cells and a few surplus 3W amorphous
    panels... you see where I'm headed...
    OK I will.

    Sheesh, three clear concise possibilities in one post. You're a credit
    to usenet sir. Thanks.

  7. Thanks George. That's a non-isolated unit, and I'd bet the price of it
    that it's another linear LM317-based circuit, with a pot to set the
    output voltage.

    But this is in the right direction. I've never run across these folks
    before, so I'll check them out regardless.

  8. Hank McCall

    Hank McCall Guest

    Actually the fellow who said to use an astable multivibrator and a
    filament transformer had a good cheap isolating device. The other way is
    to make a simple flyback mode switched mode power supply. I think
    National Semiconductor or T.I> have complete schematics on their web sites.
  9. wmbjk

    wmbjk Guest

    At the risk of being burned at the stake... ;-) I can see the
    attraction of not having the extra drain on system capacity, but is it
    really worth much trouble to save running the main inverter 24-7? At
    say, 10 Watts idle and 100 hours extra run time, the total is 1 kWhr
    per week, perhaps 50Watts PV. Which is a still sort of a lot if the
    phone is the only appliance wanting full time power. But I expect
    there are others, or will be. Perhaps it's time to bite the bullet and
    run the inverter 24-7?

    Wayne (the heretic)
  10. Guest

    Some phones have no problem with telco's positive ground. I use a
    cheap GE (the one without caller ID and display). Last cheap Uniden
    worked fine for a couple years, died. I take a guess at Walmart, try
    it, exchange till I find one that works. Don't much want noise from
    converter being radiated around my place.

    Tom Willmon
    near Mountainair, (mid) New Mexico, USA

    Net-Tamer V 1.12.0 - Registered
  11. Follow-up:

    Sonava gun, it turns out the wires *were* reversed and have clearly
    been since before we moved in. However correcting the reversal didn't
    fix the line-holding issue; I still need isolation. Too bad, you'd
    have been twice the hero if that little trick worked. :)
    While out of town I picked up another 12V gel cell. I already have
    some surplus 3W panels, so I'm going to put together a little
    independent system to run the phone and a 12V radio that is often our
    only other load when the office isn't running. The latter doesn't draw
    enough juice to keep the inverter out of search mode even at its
    highest sensitivity, so it normally runs on rechargeable NiCds.

    I also picked up a very simple phone line tester when I was at it.
    Just a bicolour LED molded into an RJ-11 plug with a little handle on
    it. At the price, not even worth the trouble to make myself.

    I just thought I'd toss it into the kit for troubleshooting elsewhere,
    but (though I'm embarrassed to admit it) the dollar tester is what put
    me onto the line reversal at our place. Could've found out with a
    meter - and confirmed it with one - but since the wiring colours match
    up correctly at the distribution point inside the house, I've always
    assumed it was right.

    I should know better.

  12. Vaughn

    Vaughn Guest

    I really appreciate "follow up" posts like this.
    Oh well, I still think it was worth the try.
    I think that is a good choice. Don't forget to cobble up some type of
    regulator. I have ruined more than one battery with an unregulated trickle

  13. No worries. I bought a little multifunction device that I ran across
    after buying my 13W suitcase solar panels. It's such a nice
    combination of features in a compact (cigarette package-sized) metal
    chassis that I couldn't resist. Indeed I think I'll get a couple more.

    You can see it here:

    I managed to find it for CDN$38, and then from another source for
    CDN$28, so at nearly US$40 it's pretty seriously overpriced at this

    It combines a shunt PV regulator with a LVD, and has a couple of
    outputs on 3.5mm phone jacks that aren't even mentioned in the
    descriptions - 6V (7806 regulated) and a 3V output that looks to be
    zener-regulated off the 7806 output. (I didn't bother to desolder the
    component inside the spagetti to be sure.)

    The main control circuit is clever, if cheap. The quality of
    construction was a little iffy too - I had to resolder part of the
    terminal block and reposition one of the LED's. Still, to wire such a
    thing point to point and work up a nice little chassis for it would
    take a lot more than $28 worth of my time.

    I'll probably fudge the 7806 up to 9V with a couple of resistors and a
    cap to provide my phone power supply, and use the 12V output to run
    the radio. Badda bing.

  14. Sure is.

    Does look/sound like a clever gadget. Google 'sol4ucn2' turned up: (back
    order until about 7/15), US$28 , $30

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