Circuit suitable for battery or not

Discussion in 'The Projects Forum' started by lanfear, May 17, 2012.

  1. lanfear

    Thread Starter New Member

    Nov 7, 2009
    Basically I have a circuit which works off the mains through a 12v dc adapter. The circuit has a 9v regulator for a pair of op-amps. I was wondering if there is a method of determining if a battery is suitable for it and also if a normal 9v battery would be enough.
  2. bertus


    Apr 5, 2008

    Depending on the current drawn by the circuit, you can take a 9 Volts block battery or a block with 6 AA batteries.
    With the 6 AA cells the circuit will last longer.

  3. lanfear

    Thread Starter New Member

    Nov 7, 2009
    i just tested it and it draws around 1.57mA. however what i am wondering is if there are any rules of thumb or anything when determining if you can use one or not, or if one just has to look at how much it draws and gets a large enough battery.
    Last edited: May 17, 2012
  4. KJ6EAD

    Senior Member

    Apr 30, 2011
    There are a couple of issues to consider.

    First is capacity and current. Small batteries have a capacity rated in mAh (milliamp hours). If I have a device that draws 20mA and I connect it to a battery with a capacity of 2000mAh, I can run the device for 100 hours from the battery (2000mAh / 20mA = 100h), so the rule of thumb you're looking for is: runtime = capacity / rate. Battery capacity information is available from the respective manufacturers. To keep this simple, I've not included some other factors that will affect performance.

    The second important consideration is whether the circuit in question can operate correctly from a decreasing voltage source that varies over a wide range as it drains. An alkaline 9v battery for example can start at 9.6V and fall to 6.6V as it's used. Some circuits will work well with this changing supply voltage and others not.

    In the specific situation you cite, if the draw remained constant at the low rate you stated, a typical 9V battery with a capacity around 700mAh would run it for quite a long time but there are a couple of caveats. The rate may not be constant. What happens to the rate if you turn up the volume or an LED flashes or a vibration motor turns on for example. Op amp circuits running from a single supply may have offset and operating point problems if the supply voltage changes depending on their design. You're quite unlikely to damage the circuit by testing it either with a battery or an adjustable DC supply so long as you stay below 12V.
    Last edited: May 17, 2012
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