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Resistor to limit current to circuit

Discussion in 'Project Construction Technologies' started by starscream, Apr 19, 2018.

  1. starscream

    starscream

    17
    1
    Sep 27, 2015
    Hi all. I have a circuit that draws 85mA when in use plus two LED's with a current draw of 20mA each so the total current draw should be 125mA if i am not wrong.

    Power is supplied by 8 X AA batteries in series giving 12vdc with an average of 1000-3000mah (depending on what type of AA is used). The switch i am using to supply the power is only rated for 200mA so a resistor is required so as to not damage the switch.

    Could i use a resistor to limit the supply to the circuit to 12v dc at 200mah? Or would i encounter any problems?

    Using an online calculator the value of the resistor should be 1ohms, 1/8w.

    Thank you for any help.
     
  2. Robert_fay

    Robert_fay

    124
    34
    Jun 15, 2017
    Why would you need to limit current to 200mA? If your draw is only 125mA then that is all it should pull. If you are worried about a short you should look at a fuse.
     
    davenn and Bluejets like this.
  3. davenn

    davenn Moderator

    13,246
    1,745
    Sep 5, 2009
    huh ?

    completely agree

    there is only 125mA going through the switch
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,690
    Jan 21, 2010
    And if the two LEDs are in series, the total current will be 105 milliamps.

    Presumably you have the correct series resistor for the LEDs so they only draw the required current.
     
  5. starscream

    starscream

    17
    1
    Sep 27, 2015
    Okay so the current drawn through the switch will not be determined by the capacity of the power supply but rather the draw of the circuit itself? Meaning no resistor should be required.

    And yes the LED's have their own resistors.
     
  6. davenn

    davenn Moderator

    13,246
    1,745
    Sep 5, 2009
    yes :)
     
  7. hevans1944

    hevans1944 Hop - AC8NS

    4,201
    1,988
    Jun 21, 2012
    Capacity of the power supply has nothing at all to do with the amount of current that is delivered by the power supply. This concept is so basic to the understanding of electricity that you should limit yourself to "playing" only with a handful of low-voltage cells (AA or otherwise) until you fully grasp its significance and importance.

    The capacity of a battery (or cell) to deliver power to a circuit is defined by its energy storage capability at a given (specified) terminal voltage. It is expressed in ampere-hours (AH) or milliampere-hours (mAH). Thus a 12 V DC battery consisting of eight 1.5 V DC "AA-size" cells connected in series can theoretically supply anywhere from 1000 to 3000 mAH of energy, depending on battery construction. That could be one ampere for one hour (1000 mAH) or up to three amperes for one hour (3000 mAH) or various combinations of current multiplied by time that do not exceed the mAH specification... within certain practical limits.

    You might, for example, expect an AA battery stack to be able to deliver one ampere (1000 mA) for one hour, with 12 V DC terminal voltage, with a rated capacity of 1000 mAH. That would be 12 W of power (12 V x 1 A) dissipated over a period of one hour (3600 S) and require (12 W) x (3600 S) = 43200 watt-seconds or 43.2 kJ of energy. Some "AA-size" cell chemistries may actually support this rate of power dissipation.

    But you cannot expect an AA battery stack to deliver ten times the current for one tenth the time (ten amperes for one tenth of an hour or six minutes) even though the ampere-hours are identical. Nor one hundred times the current for one hundredth the time (one hundred amperes for one hundredth of an hour or thirty-six seconds). Nor one thousand times the current for one thousandth the time (one thousand amperes for three point six seconds), The ampere-hours is the same in all these examples, but the internal construction of the AA cell just isn't capable of supplying the increased current while maintaining the 12 V DC terminal voltage. Other cell chemistries will give different results. For example, lithium-ion cells provide insanely large discharge currents in a small volume package. Lead-acid cells also provide large discharge currents for short periods of time, several hundred amperes at twelve volts for automobile starter applications, for example.

    The point is this: ampere-hour (or milliampere-hour) ratings provide a means to compare battery capacities under certain defined discharge conditions, but the actual discharge capacity depends on other factors besides current drawn from the battery and the amount of time the current is drawn. However, in all instances the current delivered by the battery depends only on the load resistance, not the battery capacity. Operating a 1000 mAH battery at 100 mA of current would theoretically allow ten hours of discharge time, but your mileage (or kilometers) may vary depending on temperature and how low the terminal voltage can go before you consider the battery to be fully discharged.
     
  8. Audioguru

    Audioguru

    2,549
    574
    Sep 24, 2016
    A car battery can supply 600A to the starter motor when it and the oil are cold in winter. The clock in the car draws 15mA, not the entire 600A.
     
    hevans1944 likes this.
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