# Value of the AC line input

Discussion in 'General Electronics Chat' started by Iodem_Asakura, Mar 27, 2005.

1. ### Iodem_Asakura Thread Starter Senior Member

Sep 14, 2004
140
0
I need a circuit that give in the output a voltage value proporcional to the AC line (120 Vrms). The circuit must provide an output from 0 to 5 Volts, proporcional for an input from the line of 0 to 150 Volts (rms). I'll use this output (0 to 5 Volts) as an input to comparator.

I was thinking of use a transformerless power supply, but maybe the acurracy of the output it's not enough reliable cause of the ripple, and it's dificult to make a good proporcionality respect to the input.

I'm not sure if i'm in the right way. May you give me some advices? By the way, for my application i can't use transformers (yeap, it's more difficult).

Thanks.

2. ### Brandon Senior Member

Dec 14, 2004
306
0
Couple of things first. 120 VAC line is not 120Vrms. Its more like 73 Vrms. You take your VAC voltages and divide by sqrt(2) to get the Vrms for sinusoidal signals.

First off, pick a fuse to use since you have the potential to create a very dangerous situation. Since your looking at pumping this into an op amp, probably would want to go with a low rating for this.

Rectify the signal with a 120VAC bridge rectifier. You'll get out about 118Vac now, but only positive. Use a voltage divider and bring it down from 118 to 0 to 5. You will have a fluctuating signal now from 0 to 5v where the peak is the peak of the original signal into the system scaled down.

A good comparator can deal with the ripple since its only 60Hz in the end. Most good comparators can do 20-30kHz without an issue. Connect the other side of your comparator to the reference level your comparing to and you'll get a 60Hz flash LED off of the output of the comparator. Since 60Hz is so fast, it will look solid.

Hope this has helped.

3. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Hi,

I agree with the rectification and voltage divider to get the proportional voltage, but be careful. The 120 VAC figure is rms. The rectifier will produce just about 175 VDC on the filter cap. AC lines are always reported as the rms voltage - at least in the US.

The other fun part is adding the "right" hysteresis to the comparator so it doesn't chatter with a slowly-changing input. There should be a technical PDF on the device that covers that issue.