- #1

Berzerkfury02

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Hello I am currently designing a circuit but have run into a couple dead ends. I was wondering if anyone would be willing to take the time to help me with this problem. I am a little rusty at finding unknowns in a circuit, but

basically I am trying to find the rating of a resistor I would need to

use to prevent a higher voltage battery from damaging a lower voltage

device. I am including a schematic of my problem to better aid you in visualizing my exact problem.

I am using a 9v battery

(rated 8.4v for actual output voltage) to power three strobe lights

which are in parallel to each other. (each strobe light is normally

powered by two 1.5v AAA batteries which means the device is designed to

be powered by a total of 3 volts). The resistor directly before these

devices must therefore lose 5.4 volts of potential difference before

reaching the strobes. I have measured the resistence of each strobe and

have come up with 3.5 ohms for each. Since these strobes are parallel

to each other, I have used that inverse equation to come up with a

total resistance through these three devices as 1.167 Ohms. From here I

have calculated a little bit more but am unsure I have done them

correctly. (Since the three strobes should only have a potential of 3 Volts. I used the equation V=IR and plugged these numbers in ... (3)=I(1.167) and got a current needed to run these devices as 2.57 amps. I then went back up to the main battery (8.4 volts) and used the current I found in the equation again to find the total resistance I needed for the entire circuit. Here is where I am not sure I am doing the calculations correctly. If I'm not mistaken I can use the amperage needed to run the paralleled strobes (2.57) in this equation V=IR to plug in (8.4)=(2.57)R to find that the total resistance of the circuit must be 8.4/2.57 or 3.268 Ohms. Since I have a resistance in the paralleled strobes of 1.167 I subtracted this from the 3.268 Total resistance to get about 2.1 ohms of resistance. I then needed to find the wattage rating on the resistor where I used the P=IV equation to find the wattage for the resistor which is 2.1 ohms and must lose 5.4 volts. From P=IV I have, P=(2.57)(6) to get a wattage of 15.42 which this resistor must dissipate before the circuit reaches the three paralleled devices. In the electronics store I am visiting The resistors come accurate to the tenths decimal place. These resistors come in the following wattages.

1/4, 1, 2, 5, 10, and 25. I was wondering If you could help

me figure out which resistor I should use since I am not sure I had done this problem correctly. The resistor I purchased could not be matched exactly however I had tried to come as close as I can. It came out to be a 10 Watt resistor, rated 2.1 ohms. I think there is enough information to determine the correct resistor I need, but if there is any thing else I may need to measure, please let me know. I apologize for this extremely long explanation of my problem but any help would be greatly appreciated.

Thanks in Advance,

Daniel

basically I am trying to find the rating of a resistor I would need to

use to prevent a higher voltage battery from damaging a lower voltage

device. I am including a schematic of my problem to better aid you in visualizing my exact problem.

I am using a 9v battery

(rated 8.4v for actual output voltage) to power three strobe lights

which are in parallel to each other. (each strobe light is normally

powered by two 1.5v AAA batteries which means the device is designed to

be powered by a total of 3 volts). The resistor directly before these

devices must therefore lose 5.4 volts of potential difference before

reaching the strobes. I have measured the resistence of each strobe and

have come up with 3.5 ohms for each. Since these strobes are parallel

to each other, I have used that inverse equation to come up with a

total resistance through these three devices as 1.167 Ohms. From here I

have calculated a little bit more but am unsure I have done them

correctly. (Since the three strobes should only have a potential of 3 Volts. I used the equation V=IR and plugged these numbers in ... (3)=I(1.167) and got a current needed to run these devices as 2.57 amps. I then went back up to the main battery (8.4 volts) and used the current I found in the equation again to find the total resistance I needed for the entire circuit. Here is where I am not sure I am doing the calculations correctly. If I'm not mistaken I can use the amperage needed to run the paralleled strobes (2.57) in this equation V=IR to plug in (8.4)=(2.57)R to find that the total resistance of the circuit must be 8.4/2.57 or 3.268 Ohms. Since I have a resistance in the paralleled strobes of 1.167 I subtracted this from the 3.268 Total resistance to get about 2.1 ohms of resistance. I then needed to find the wattage rating on the resistor where I used the P=IV equation to find the wattage for the resistor which is 2.1 ohms and must lose 5.4 volts. From P=IV I have, P=(2.57)(6) to get a wattage of 15.42 which this resistor must dissipate before the circuit reaches the three paralleled devices. In the electronics store I am visiting The resistors come accurate to the tenths decimal place. These resistors come in the following wattages.

1/4, 1, 2, 5, 10, and 25. I was wondering If you could help

me figure out which resistor I should use since I am not sure I had done this problem correctly. The resistor I purchased could not be matched exactly however I had tried to come as close as I can. It came out to be a 10 Watt resistor, rated 2.1 ohms. I think there is enough information to determine the correct resistor I need, but if there is any thing else I may need to measure, please let me know. I apologize for this extremely long explanation of my problem but any help would be greatly appreciated.

Thanks in Advance,

Daniel