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

Registered: 10/17/02
Posts: 811
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?
walrus Offline
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Registered: 07/25/02
Posts: 671
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?
sparkync Offline
Member

Registered: 10/17/02
Posts: 811
Loc: NC
Thought so; Thanks ....

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#45976 - 12/09/04 07:29 PM Re: ohm's law , low voltage also?
SolarPowered Offline
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Registered: 07/05/04
Posts: 615
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?
dereckbc Offline
Member

Registered: 10/08/03
Posts: 158
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?
Steve Miller Offline
Member

Registered: 08/30/01
Posts: 322
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?
wa2ise Offline
Member

Registered: 11/29/02
Posts: 771
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?
walrus Offline
Member

Registered: 07/25/02
Posts: 671
Loc: Bangor Me. USA
 Quote:
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?
SolarPowered Offline
Member

Registered: 07/05/04
Posts: 615
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?
sparkync Offline
Member

Registered: 10/17/02
Posts: 811
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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