# Frequency Question -- 24 VAC, 60 Hz versus 24 VAC, 20 Hz

## Main Question or Discussion Point

Suppose that I have two sources: 1. 24 VAC, 60 Hz and 2. 24 VAC, 20 Hz
Then I will use it to power into certain load (maybe we can assume a purely resistive load to simplify the question). What would be the effect of those sources to the load? How about the current flowing? I think (1) and (2) would produce same magnitude but does its frequency follows the the frequency of inputs? or it will differ?

## Answers and Replies

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anorlunda
Mentor
The power dissipated in the resistor is I*I*R. Where I is the RMS (root mean square) of the sinusoidal current.

The RMS of a sinusoid is 0.707 of the peak value and independent of frequency.

Suppose that I have two sources: 1. 24 VAC, 60 Hz and 2. 24 VAC, 20 Hz
Then I will use it to power into certain load (maybe we can assume a purely resistive load to simplify the question). What would be the effect of those sources to the load? How about the current flowing? I think (1) and (2) would produce same magnitude but does its frequency follows the the frequency of inputs? or it will differ?
Series connection of such voltage sources and the load (in the most simple case) gives voltage drop waveform like this one
If the load is resistor current is of the same shape i(t)=v(t)/R

meBigGuy
Gold Member
Its hard to understand exactly what you are asking. You want to connect one supply or the other to the load? Or, you are connecting them in series to the load? And I have no idea what you mean by "the effect of those sources to the load"

But the current at any time is the result of the instantaneous voltage across the load.

The power dissipation due to the sum of 2 AC waveforms depends on the correlation between the waveforms. If they are phase coherent (exact frequency multiples) the end result is different than if they are not exact frequency multiples. (read about summing RMS voltages)

NascentOxygen
Staff Emeritus
Science Advisor
Then I will use it to power into certain load (maybe we can assume a purely resistive load to simplify the question).
I understand you to be using one source at a time. As others have explained, for a resistive load the frequency does not alter the power delivered. The waveform still spends half of its time positive, and the other half negative, regardless of frequency of that sinewave. So with equal amplitudes of sinewave, the power will be the same for either source.

berkeman
Mentor
Suppose that I have two sources: 1. 24 VAC, 60 Hz and 2. 24 VAC, 20 Hz
Then I will use it to power into certain load (maybe we can assume a purely resistive load to simplify the question). What would be the effect of those sources to the load? How about the current flowing? I think (1) and (2) would produce same magnitude but does its frequency follows the the frequency of inputs? or it will differ?
If your load is not purely resistive (like if it includes a transformer), then the frequency makes a *big* difference.

davenn
Science Advisor
Gold Member
2019 Award
If your load is not purely resistive (like if it includes a transformer), then the frequency makes a *big* difference.
yes indeed, and people that bring back to their own country devices with electric motors in them, they get a transformer to convert the AC voltage 110-220V or 220 - 110 VAC for example that then wonder why their unit plays music at the wrong speed and other problems the 50 / 60 Hz makes quite a difference

Dave

Suppose that I have two sources: 1. 24 VAC, 60 Hz and 2. 24 VAC, 20 Hz
Then I will use it to power into certain load (maybe we can assume a purely resistive load to simplify the question). What would be the effect of those sources to the load? How about the current flowing? I think (1) and (2) would produce same magnitude but does its frequency follows the the frequency of inputs? or it will differ?
To Clarify My Question, I made an image to show it:

What will be the effect of each sources to the 10K resistor? is there any siginificant difference when the frequency was change?

meBigGuy
Gold Member
Assuming 24VAC RMS sinewave. Also, assuming no parasitic capacitance or inductance (which is impossible in the real world)
In both cases the circuit will conduct 2.4ma AC and the resistors will dissipate 57.6mW.
You could theoretically have a resistive element with a fast enough thermal time constant that could be detected by a temperature sensor of some sort. Other than that, there is no difference.

You could draw in parasitic elements such as capacitance and inductance, radiation resistance, etc. Those elements would be affected by frequency.

Assuming 24VAC RMS sinewave. Also, assuming no parasitic capacitance or inductance (which is impossible in the real world)
In both cases the circuit will conduct 2.4ma AC and the resistors will dissipate 57.6mW.
You could theoretically have a resistive element with a fast enough thermal time constant that could be detected by a temperature sensor of some sort. Other than that, there is no difference.

You could draw in parasitic elements such as capacitance and inductance, radiation resistance, etc. Those elements would be affected by frequency.
So in this example, you are favoring for source with 60 Hz rather than the source with 20 Hz?

meBigGuy
Gold Member
I have no idea what you mean. I'm not "favoring for" anything. In what way do you conclude I am "favoring". I don't think I said or implied anything judgmental about either source. I'm not even sure what your overall point is in this thread. Am I missing something?

The only real difference I see is that the 60Hz circuit is on the left.

When I said "In both cases the circuit will conduct 2.4ma AC and the resistors will dissipate 57.6mW." I probably should have added that the 60Hz source produces 60 Hz current and vice versa, but I thought that was too obvious to have to even mention.

What are you getting at?

I am afraid that I misunderstood your comment. But regardless of that, I think this quote answered the difference of the above circuits (which have different frequencies) :
You could draw in parasitic elements such as capacitance and inductance, radiation resistance, etc. Those elements would be affected by frequency.
Thank You for Your Answer and Concern

meBigGuy
Gold Member
I'm glad that quote works for you, because I also felt it pretty much summed up the differences. Glad to help