Sizing current sensing resistors power rating
LoopyByteloose
Posts: 12,537
I want to use an op amp for high side current sensing.· I've figured that the supply voltage is 5 volts and the maximum current is 1 amp.
So Amps x Volts = Watts.· That would be 5 watts.
If I use a 0.2 ohm resistor, should it be 5 watts too?· Or could I use something much smaller?
What I've figured is 5 volts / 1 amp = 5 ohms total.· With the .2 ohms being only a small fraction of the 5 ohm load [noparse][[/noparse] .2/5 = 1/25th ], it seems like the power disapated by the current sensing would be 1/25th too. or .2 watts.·
So, I'm left with wondering if I really need big 5 watt resistors or if 1/4 watt resistors can serve this purpose. My gut feeling is that I should use the 5 watt resistors, but they are huge.
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So Amps x Volts = Watts.· That would be 5 watts.
If I use a 0.2 ohm resistor, should it be 5 watts too?· Or could I use something much smaller?
What I've figured is 5 volts / 1 amp = 5 ohms total.· With the .2 ohms being only a small fraction of the 5 ohm load [noparse][[/noparse] .2/5 = 1/25th ], it seems like the power disapated by the current sensing would be 1/25th too. or .2 watts.·
So, I'm left with wondering if I really need big 5 watt resistors or if 1/4 watt resistors can serve this purpose. My gut feeling is that I should use the 5 watt resistors, but they are huge.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
PLEASE CONSIDER the following:
Do you want a quickly operational black box solution or the knowledge included therein?······
···················· Tropically,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
Comments
It's all Ohms law and the power equation.
E = I * R = 1 A * 0.2 Ohms = 0.2 V
P = E * I = 0.2 V * 1 A = 0.2 W
So, a 1/4 Watt resistor or greater will work just fine. You figured it out a different way.
Well, I bought a few 5 watt resistors as I was at the store. I've just burned up so many 1/4 resistor of higher values that it didn't seem that it was quite right. So I guess I can have these for much higher amp motors or fool around with both higher voltage and higher amps.
I overlooked·exactly the fact that·how each component in a·series sees voltage individually as part of a voltage divider.· And then, that figure decided the power rating.
I just was in a hurry to get started with some LM324 op amps I've had for sometime. I want to amplify the max .2 by 10 [noparse][[/noparse]to a max of roughly 4 volts] and then use a ADC to read it digitally.
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PLEASE CONSIDER the following:
Do you want a quickly operational black box solution or the knowledge included therein?······
Post Edited (Kramer) : 3/21/2008 8:40:58 AM GMT