#45973  12/09/04 04:16 PM
ohm's law , low voltage also?

Member

Registered: 10/17/02
Posts: 812
Loc: NC


I'm doing a low voltage project, and not use to the amperage etc. the smaller wires. When calculating the amperage, I'm assuming the math technique is the same. I have 1 alarm that is 15 watts. the voltage is 12 volts. Therefore 15 watts divided by 12 volts gives me 1.25 amps. This is the right formula no matter what the voltage etc. is, isn't it? I need a little assurance here:; Thanks ..

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#45974  12/09/04 04:27 PM
Re: ohm's law , low voltage also?

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Registered: 07/25/02
Posts: 680
Loc: Bangor Me. USA


Ohms law applies at low voltage.

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#45975  12/09/04 04:46 PM
Re: ohm's law , low voltage also?

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Registered: 10/17/02
Posts: 812
Loc: NC



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#45976  12/09/04 07:29 PM
Re: ohm's law , low voltage also?

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Registered: 07/05/04
Posts: 625
Loc: Palo Alto, CA, USA


That's not Ohm's Law, however...

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#45977  12/09/04 07:43 PM
Re: ohm's law , low voltage also?

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Registered: 10/08/03
Posts: 156
Loc: Tulsa, OK


Ohm's law applies, but when working with LV, wire and connection resistance comes into play. I assume your device consumes 15 watts over a range of voltage say from 11 to 14 volts applied at the device.

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#45978  12/10/04 03:31 AM
Re: ohm's law , low voltage also?

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Registered: 08/30/01
Posts: 325
Loc: Loudoun Cty, VA


Just as an FYI: if your project is security alarm or fire alarm there are other requirements for the load calc.
Also be a bit careful about the length & size of the wires. #18/20/22 alarm wiring is often very long runs and can add a lot more resistance than the 14/12/10 we normally use.

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#45979  12/10/04 02:07 PM
Re: ohm's law , low voltage also?

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Registered: 11/29/02
Posts: 782
Loc: Oradell NJ USA


A certian length of say 14 gauge wire will have the same voltage drop at the same amps no matter what the supply voltage is. However, that voltage drop will be a bigger percentage of the supply voltage when the supply voltage is lower. Thus, what is an acceptable drop at 120V would be bad when the supply is 12V. 12 volt light bulbs will look dimmer than 120V bulbs seeing the same voltage drop. This is a big reason why POCOs use high voltages for long distance transmission.

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#45980  12/10/04 05:15 PM
Re: ohm's law , low voltage also?

Member

Registered: 07/25/02
Posts: 680
Loc: Bangor Me. USA


That's not Ohm's Law, however... Isn't it derived from ohms law though??. Seem to remember someone proving that one time or another?

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#45981  12/10/04 05:58 PM
Re: ohm's law , low voltage also?

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Registered: 07/05/04
Posts: 625
Loc: Palo Alto, CA, USA


Ohm's Law is E = IR.
Watt's Law is P = IV.

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#45982  12/10/04 06:57 PM
Re: ohm's law , low voltage also?

Member

Registered: 10/17/02
Posts: 812
Loc: NC


My "Ugly's book" says that the three basic Ohm's law formulas are: I=E/R, R=E/I, and E= IxR I = Amperes E = Volts R = Ohms P = Watts Maybe Ugly's wrong Thanks anyway for the input... Steve

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