9v to 7.2v regulator?

Discussion in 'General Electronics Chat' started by QMD, Jun 18, 2011.

  1. QMD

    Thread Starter Member

    Jun 30, 2010
    30
    0
    Anybody know of a good 9v to 7.2v regulator IC?

    My friend has a paintball gun that uses a 7.2v battery, but hates having to buy the special 7.2v battery, so he asked me to look into an IC that will allow him to use a 9v battery (or multiple ones in parallel if current isn't enough)

    I can design a regulator circuit with transistors and diodes, but I am sure that there are some very low cost IC's that can do the job

    Thanks
     
  2. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    Think of it this way. A 7.2V battery is probably Lithium and thus is (2) 3.7V batteries in series. When charged they are 4.2V each, adding up to 8.4V. That's just about a 1.2V less than a 9V alkaline. It may work just fine. If not, just a diode would bring it down to about 8.9V and a second diode would bring it to 8.4V. I would guess that paralleling them would be necessary.

    You could use a voltage regulator, like the LM317 Adjustable one and regulate it to 8.4V. The LM317 regulator, however, has a dropout voltage of about 1.5v and would take a brand new 9V battery down to 8.5V right off the bat. After that it keeps getting lower.

    You would be best off by buying 2 LiIon rechargeables, the kind that are used in laptop battery packs and tie them in series for the correct voltage without regulation. Then buy a charger that works with them and your set for a long time.
     
  3. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    Old Li-ion cells had a nominal cell voltage of 3.6V. Newer cells (graphite anode, IIRC) are 3.7V.

    NiMH and NiCd are 1.2V per cell so 3 can look like a Li-ion. So it could also be a 6 cell Li-ion.

    Now here's the nice thing: a fully charged NiCd or NiMH can look like 9V...

    So you could probably get away with it. If you're worried about exceeding input limits, use a few series diodes to drop the voltage, although the voltage will vary with load, it is common for batteries to do this so is unlikely to upset the device.
     
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