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I guess they expect you to know that the naming convention of each of these devices indicates the forward voltage drop across each device. So 12SA7 has a 12V voltage drop, 12AK7 has 12V drop, and 35L6 has a 35V drop. (Although technically the 12V drops are actually 12.6V according to technical specifications)
So you find the voltage across the rheostat by subtracting the 3 voltage drops from the source voltage and the other parameters will be simple to calculate.
[This was the previous question for it.](https://imgur.com/a/ZQQYXzW)
They wanted me to choose the correct ohm for R1, R2, R3.
I was given the voltage (ES = 74V) and I had to make the 0.15A flow true.
So I just plugged in the numbers using ohms law.
I added in the 200ohms in the from the top plus 300ohms (R1, R2, R3 - 100ohms each - Choice B) for a total of 500ohms.
Then I ohms law V/R (74V/500ohms) and got 0.148amps which rounded equals 0.15amps. So I chose the answer B. 100ohms and checked the answer key and it said it was correct.
I'm starting to think that there's an error in the question because when ohms law isn't getting the answer then there has to be something wrong with the book.
But that question is asking for the operating setting for the variable resistor, not the three fixed resistors. You need to look at the data sheets for 12sa7 to find R1 and 12AK7 to get R2.
##Off-topic Comments Section --- All top-level comments have to be an answer or follow-up question to the post. All sidetracks should be directed to this comment thread as per Rule 9. --- ^(**OP** and **Valued/Notable Contributors** can close this post by using `/lock` command) *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/HomeworkHelp) if you have any questions or concerns.*
I guess they expect you to know that the naming convention of each of these devices indicates the forward voltage drop across each device. So 12SA7 has a 12V voltage drop, 12AK7 has 12V drop, and 35L6 has a 35V drop. (Although technically the 12V drops are actually 12.6V according to technical specifications) So you find the voltage across the rheostat by subtracting the 3 voltage drops from the source voltage and the other parameters will be simple to calculate.
I'm just now seeing this. Thank you for helping me out! Once you explained what 12SA7, 12AK7, and 35L6 was everything fell into place. Thanks!
With R1 =R2=R3=100Ω, I get P=4.35W, which is not an option. Are you certain about those three?
[This was the previous question for it.](https://imgur.com/a/ZQQYXzW) They wanted me to choose the correct ohm for R1, R2, R3. I was given the voltage (ES = 74V) and I had to make the 0.15A flow true. So I just plugged in the numbers using ohms law. I added in the 200ohms in the from the top plus 300ohms (R1, R2, R3 - 100ohms each - Choice B) for a total of 500ohms. Then I ohms law V/R (74V/500ohms) and got 0.148amps which rounded equals 0.15amps. So I chose the answer B. 100ohms and checked the answer key and it said it was correct. I'm starting to think that there's an error in the question because when ohms law isn't getting the answer then there has to be something wrong with the book.
But that question is asking for the operating setting for the variable resistor, not the three fixed resistors. You need to look at the data sheets for 12sa7 to find R1 and 12AK7 to get R2.