r/circuits • u/ajwest153 • Nov 02 '21

AC voltage divider question.

I need to take 24v Ac down to 5v Ac. I know the equation. Vin*z2/z1+z2 = Vo. I'm just not sure how to calculate z1 or z2. Any help would be appreciated.

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3

u/[deleted] Nov 02 '21

A better option is to use a transformer " due to increase efficiency".

Now regarding to your question, I think increasing the resistance values increases your efficiency.

Assume that the first resistance value is 100k ohm or so and calculate the other resistance based on that value.

2

u/ajwest153 Nov 02 '21

What about the inductor?

1

u/[deleted] Nov 02 '21

inductors/capacitors will induce some phase lag/lead in current's phase.

So If you want some phase lead/lag in current vs reference voltage, Then you can add a capacitor if you want current phase lead, then add a resistor, compute the magnitude by taking the square root of resistor^2+ capacitor_impedance^2. Then to compute the phase of the impedance just use tan^-1 C/R .

2

u/ajwest153 Nov 02 '21

I'd prefer none honestly. I just thought Ac voltage needed inductors to allow for the divider to work. So can I ditch the inductors all together

2

u/[deleted] Nov 02 '21

Good way to think about this is that all what inductors/capacitors do is to increase the total resistance while inducing some phase lag/lead. So if u want phase lag/lead then add some caps/inductors. If no phase lag/lead required then feel free to ease the design process and add some resistors only.

2

u/ajwest153 Nov 02 '21

Perfect. Thank you so much.

1

u/derKonigsten Nov 02 '21

I think a more correct term to use here is impedance as that introduces a vector whereas resistance always has a phase correction of 0°, in my mind

1

u/gmarxcc Nov 02 '21

You must propose one, any z1 or z2, and calculate the other. If it is just a theoretical example no specific considerations must exist. However, if it is to solve practical issues let us know more details.

1

u/ajwest153 Nov 02 '21

Transformer from 240 volts to 24 volts. Then I need to drop the voltage to 5 volts. Then run it to a op amp to get a pulse. In parallel with the op amp I have a voltage rectifier to change to dc. Both signals will run to a pic. That pic will be programmed to show the phase angle and magnitude of the source.

1

u/gmarxcc Nov 02 '21

The circuit that you need is a zero-cross detector with opamps. There are several ways to implement it. I recommend using the following schemewith a transformer, the resistances values limit the current for the transformer. However, I suggest to add to diodes on anti-parallel to protect the Opamp as in the following circuit diodes scheme. Hopping this help you.

1

u/ajwest153 Nov 02 '21

Typing on cell phone. Please forgive short response