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#45973 - 12/09/04 08:16 PM ohm's law , low voltage also?  
sparkync  Offline
Member
Joined: Oct 2002
Posts: 817
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 08:27 PM Re: ohm's law , low voltage also?  
walrus  Offline
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Joined: Jul 2002
Posts: 680
Bangor Me. USA
Ohms law applies at low voltage.


#45975 - 12/09/04 08:46 PM Re: ohm's law , low voltage also?  
sparkync  Offline
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Joined: Oct 2002
Posts: 817
NC
Thought so; Thanks ....


#45976 - 12/09/04 11:29 PM Re: ohm's law , low voltage also?  
SolarPowered  Offline
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Joined: Jul 2004
Posts: 625
Palo Alto, CA, USA
That's not Ohm's Law, however...


#45977 - 12/09/04 11:43 PM Re: ohm's law , low voltage also?  
dereckbc  Offline
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Joined: Oct 2003
Posts: 156
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.


#45978 - 12/10/04 07:31 AM Re: ohm's law , low voltage also?  
Steve Miller  Offline
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Joined: Aug 2001
Posts: 325
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.


#45979 - 12/10/04 06:07 PM Re: ohm's law , low voltage also?  
wa2ise  Offline
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Joined: Nov 2002
Posts: 782
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.


#45980 - 12/10/04 09:15 PM Re: ohm's law , low voltage also?  
walrus  Offline
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Joined: Jul 2002
Posts: 680
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?


#45981 - 12/10/04 09:58 PM Re: ohm's law , low voltage also?  
SolarPowered  Offline
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Posts: 625
Palo Alto, CA, USA
Ohm's Law is E = IR.

Watt's Law is P = IV.


#45982 - 12/10/04 10:57 PM Re: ohm's law , low voltage also?  
sparkync  Offline
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Joined: Oct 2002
Posts: 817
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 [Linked Image] Thanks anyway for the input... Steve


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