# power rating of series and parallel circuits

Discussion in 'General Electronics Discussion' started by PRIYADHARSHINI, Aug 25, 2014.

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137
8
Feb 6, 2014
hi,
1.how do we calculate the power rating for series connected resistors?
2.how do we calculate the power rating for parallel connected resistors?
3.which type of connection is mostly preferable ?

thanks regards...

2. ### Harald KappModeratorModerator

11,162
2,550
Nov 17, 2011
1 and 2: For each resistor P=I²*R or P=V²/R. This is universally valid and independent from series or parallel connection, although it may be easier to use P=i*R for series connection (as each resistor has the same current) and P=V²/R for parallel connection (as each resistor sees the same voltage).

3: This question cannot be answered universally. You have to take into account the circumstances. A series connection can withstand higher voltages, as the total voltage is distributed among several resistors. A parallel connection can withstand higher currents, as the total current is distributed among several resistors.

Gryd3 likes this.
3. ### Gryd3

4,098
875
Jun 25, 2014
Formulas:
V = IR (Voltage = Current * Resistance)
P = IV (Power = Current * Voltage)

Adding series resistors to get total resistance:
Rt = R1 + R2 + R3 + ... + Rn

Adding parallel resistors to get total resistance:
1/Rt = ( 1/R1 + 1/R2 + 1/R3 + ... + 1/Rn )

Please remember that the voltage across a single component may not be the same as the voltage supplied to the entire circuit.
These formulas assume you can determine the voltage across each resistor individually to correctly calculate power rating.
All example assume simple series or parallel circuits where there is two or more items connected in series or parallel and not any combination of both!

*Harald is speedy

137
8
Feb 6, 2014
1.k.thanks for ur explanation.here i m using 2400M ohm and 1M ohm in series.power rating of this two series connected resistors are 7.5W.
now i want to built equivalent series or parallel resistor circuit for 2400M ohm..
if i built the equivalent parallel resistor circuit for 2400M ohm.then i will calculate the power rating as per ur previous post.
it should have the same power rating 7.5W right?
2.how will be the power rating when the resistors connected in series when compared to parallel?

5. ### Harald KappModeratorModerator

11,162
2,550
Nov 17, 2011
The total power rating is the same in both cases since for a total resistance of 2.4MΩ the power is P=V²/2.4MΩ. regardless of how the 2.4MΩ is constructed.

If you construct the 2.4M from 2*1.2M in series or 2*4.8M in parallel, the power for each resistor will be 7.5W/2 since the total power is then distrubuted among two equal resistors.

All, of course, assuming the 7.5W given initially by you are corrrect - which we cannot verify lacking knowledge of either voltage or current.

137
8
Feb 6, 2014
its not 2.4M ohm 2400M ohm that is 2.4G ohm..k in this case series and parallel resistor has the same value.if it is combination of different values means how can we calculate the power rating of each resistor?

7. ### Merlin3189

250
69
Aug 4, 2011
Something odd going on here.
7.5W in a 2.4 GOhm resistor would mean 134000 V across it.
Even if at only 2.4 MOhm it is over 4000 V, which is at least conceivable, but you might still be more concerned about the Voltage rating of the resistors.
In that case maybe a mixture with both parallel resistors might mitigate against a single component failure? (eg 4 of 2.4 M ohm, with 2 in series and that in parallel with the same.)
If it really is G Ohm, you might want to worry about the resistance of the mountings or circuit board?

8. ### davennModerator

13,786
1,937
Sep 5, 2009

You REALLY sure about that ??

never seen a resistor over 22 M Ohms or so

11,162
2,550
Nov 17, 2011