# Help! Whats the RMS voltage across this resistor?

#### trynabeabetterme

Joined Dec 16, 2020
5

Whats the RMS voltage across resistor r? I have tried 20 - (0.7*2) = 18.6rms but I have gotten conflicting answers :/
Someone I asked for help from said it was 13rms. This makes me scared because its worth a lot of marks!

#### dl324

Joined Mar 30, 2015
12,873
Welcome to AAC!

Neither answer is correct. It might help if you used the definition of what RMS voltage represents.

#### crutschow

Joined Mar 14, 2008
27,241
Think about what the voltage looks like across R.

#### MrChips

Joined Oct 2, 2009
23,560
The question is ambiguous.
The definition of rms voltage is the DC voltage that produces the same power.
Is it 20VDC (rms) or 20VAC (rms)? Send it back to your instructor.

#### trynabeabetterme

Joined Dec 16, 2020
5
The question is ambiguous.
The definition of rms voltage is the DC voltage that produces the same power.
Is it 20VDC (rms) or 20VAC (rms)? Send it back to your instructor.
I can't really send it back it's a question in a final year exam. Someone helped me out by saying

"It is approximately 13 VRMS.

Half rectified sinewave, Vrms= Vpk/2

Vpk = 2\^0.5 x 20V - 1.4V = \~26.9V 1.4V is to silicon diode forward voltage drops

Vrms = 26.9V/2 = 13.5V"

Should I go with that or the 18.6 I previously had.

#### Ian0

Joined Aug 7, 2020
2,276
Who said it was a sinewave?

#### trynabeabetterme

Joined Dec 16, 2020
5

#### MrChips

Joined Oct 2, 2009
23,560
The answer depends on the shape of the input waveform.
Let us assume that who wrote the question intended it to be a sinewave centered about 0V.
Then it becomes a trick question.

What level course is this?

#### crutschow

Joined Mar 14, 2008
27,241
Should I go with that or the 18.6 I previously had.
Assuming it's a sinewave (and you would need to state that in your answer) why would you go with 18.6 if you think it's half-wave rectified?

#### trynabeabetterme

Joined Dec 16, 2020
5
The answer depends on the shape of the input waveform.
Let us assume that who wrote the question intended it to be a sinewave centered about 0V.
Then it becomes a trick question.

What level course is this?
First year electronics in university.

#### trynabeabetterme

Joined Dec 16, 2020
5
Assuming it's a sinewave (and you would need to state that in your answer) why would you go with 18.6 if you think it's half-wave rectified?
Someone I had asked on another forum solved it as a half wave rectified. I do not know what wave form it is, it does not say. But since it is rms I assume sine wave? I am new to electronics this is a first year exam.

#### MrChips

Joined Oct 2, 2009
23,560
RMS value can be applied to any wave shape.
You were not told the shape of the wave hence you must state the assumed shape in your answer.
This is still a trick question.

Since this is an actual exam for credit we cannot give you the answer.

#### dl324

Joined Mar 30, 2015
12,873
I can't really send it back it's a question in a final year exam.

I don't think it's ambiguous...

#### RBR1317

Joined Nov 13, 2010
633
It seems plausible that the input is either an AC or DC waveform, but it is customary to identify a DC voltage as VDC and an AC voltage as RMS. In either case the voltage across the resistor is not a sine wave so you need to go back to the basic definition of RMS in order to find the answer.

#### Ian0

Joined Aug 7, 2020
2,276
The answer is well-defined for a waveform that is symmetric about 0V; but not for an asymmetric waveform.

#### ericgibbs

Joined Jan 29, 2010
12,958
hi ,
Welcome to AAC.
As this is homework we can only give you a hint.
Hint: those two diodes act as half wave rectifier.

As the wave form is not specified, I would assume a Sine wave, state this in your answer.

E

#### Attachments

• 93.2 KB Views: 10