Working out resistor calculations...

[bilbo_88]

Late at night....tearing my hair out....need help with working out resistor calcs.

230V supply....R1=70...R2=90

What is the value of a resistor which when connected in parallel with 70 ohm resistor will cause a total current of 2.4 amps to flow in the circuit?


any guidence to push me in the right direction greatly appreciated

 

[telectrix]

Rt =(R1 x R2)/ (R1 + R2)

 

[bilbo_88]

thanks, the questions thrown me a bit, do I in anyway need to involve the 2.4 amps in the calculations, or is it just there as information

 

[bilbo_88]

scrap that, ive just read the question properly.....time for bed

 

[MarkieSparkie]

Let the unknown parallel resistor = Rx
The total circuit resistance (Rt) that would cause 2.4A to flow:
Rt = V / I
The equivalent resistance of the parallel resistors (R1 & Rx):
Req = Rt - R2
The unknown parallel resistor Rx:
1/Rx= 1/Req - 1/R1

Rx= 6.36Ω (to 2DP)

 

[Knobhead]

If you PM me I’ll send an Excel spread sheet. It originally set off as transformer and motor calculations, but it’s got a bit like Toppsy it just keeps growing. Parallel resistance calculations, unbalanced neutral currents and all kinds of things have found there way in there.

 

[bilbo_88]

hi mark
struggling a bit with the calculation as im getting a defferent answer to you.
my working out is as follows....
230/2.4=95.83ohms
95.83-90(R2)=5.83
(1/5.83)-(1/70)=0.17-0.014
1/0.156=6.41
Rx=6.41

anyone able to tell me where i've cocked up? cheers

 

[MarkieSparkie]

hi mark
struggling a bit with the calculation as im getting a defferent answer to you.
my working out is as follows....
230/2.4=95.83ohms
95.83-90(R2)=5.83
(1/5.83)-(1/70)=0.17-0.014 = 0.171526586 - 0.014285714
You have rounded up the result of each reciprocal above, if your final answer is to be accurate you need to use all the decimal places, as i have shown above, and once you have the accurate final answer then you can round up and simplify to give a real world answer. Where even a precision wound resistor is unlikely to be accurate to more than 2 decimal places.
1/0.156=6.41 1/0.157240072 = 6.359669628
Rx=6.41 therefore Rx = 6.36 (to 2DP) Always show all of your workings before rounding, so that the examiner can award you maximum marks for each step you get right.
anyone able to tell me where i've cocked up? cheers

See the corrections above, hope this helps, Mark.