Ah battery to momentary switch

Discussion in 'General Electronics Chat' started by BinaryAssassin, Mar 17, 2016.

  1. BinaryAssassin

    Thread Starter New Member

    Mar 17, 2016
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    Hey all,

    I have a circuit that starts with two 12v batteries 7.2Ah wired in series which connect to a momentary push button switch and then to a 1.2K ohm resistor and 5mm LED (3.2 forward voltage, 20mAh). My question is if the momentary switch is rated at 24v/3Ah will running the batteries with 7.2Ah damage the momentary switch?

    This small circuit is a module part of a larger one I'm trying to build.
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
    3,244
    622
    Welcome to AAC!

    The current will only be about 20mA and the switch will be fine. Did you mean to say that the switch contacts were rated at 3A? Amp hours is a measure of battery capacity.
     
  3. BinaryAssassin

    Thread Starter New Member

    Mar 17, 2016
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    Yes that was my typo! Well the other part of the circuit will bypass the resistor and led and send full battery power 24v/7.2Ah to another device. There will be another momentary push button for that part of the circuit. I should have clarified!
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    4,796
    Ah is a unit of charge, not of current.

    Switches are not rated in Ah (ampere-hours). They are rated in V (volts) and A (amps). And amp and an amp-hour are two completely and fundamentally different concepts.

    Similarly, your LED has a forward voltage of 3.2 V when a current of 20 mA (not mAh) is flowing in it.

    Your switch rating is probably 24 V / 3 A.

    The two 12 V batteries in series give you 24 V, which is right at the voltage rating of the switch. This is not good. First, you should never run things at their specified limits. Instead, always give yourself a safety factor. Depending on the device, running it to somewhere between 50% and 85% of the design limit is a common rule of thumb. So you would want to use a switch that is rated for at least 28 V and preferably 48 V. Second, depending on the batteries you are using, the fully-charged voltage might be well above the 12 V nominal voltage. Lead acid batteries are generally considered to have a full-charge voltage of 13.8 V so that would put you at 27.6 V, noticeably above your 24 V rating.

    When you close the switch, the resistor will have about (24 V - 3.2 V) = 20.8 V across it and, therefore, a current of (20.8 V)/(1.2 kΩ) = 17.33 mA through it (and the LED and the battery). This is about 15% below the nominal current of the LED, so it will be happy. It is also large enough in comparison to that nominal current that it will be clearly seen as being lit. It is WAY below the current limit of the switch, so that isn't even a factor.

    Notice that the Ah rating of the batteries has not come into play in any of this so far. What IT tells is, very roughly, how long the batteries will last. At 17.33 mA, the batteries will last (again, roughly) (7.2 Ah)/(17.33 mA) = 415 hours.
     
  5. WBahn

    Moderator

    Mar 31, 2012
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    Keep in mind that having a battery that has a capacity rating of 7.2 Ah tells you very little about the maximum current that the battery can deliver. That might be important if that other device pulls significant current.
     
  6. BinaryAssassin

    Thread Starter New Member

    Mar 17, 2016
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    0
    Sounds good, I did have some typos I see.
     
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