Switch on AC supply at maximum or minumum voltage - Page 1
Forum section: electronics / projects

author: Cicada
author_rank: Regular Contributor
timestamp: May 31, 2016, 12:56:51 pm
content: HiI wonder can someone point me to a source of information on how to design a circuit that can switch power on at the max or min of an AC waveform.The circuit must be:1. As simple as possible.2. Safe3. Reliable4. And be able to handle a decent amount of power. Maybe your typical 250VAC @ 16AThere are 3 methods that I immediately think of:1. Look at the zero crossing instant and switch power on 1/4 period later. Switch on at both min and max of waveform.2. Look at rate of change of waveform and switch on at zero rate of change3. Measure the waveform with ADC and use that to switch when you want to.All these are maybe a bit to involved.Maybe there is a simple way with just a silicon device (Triac?...) and a couple of passive components.Would like to hear the brilliant ideas of the EEVBLOG users.

author: ajb
author_rank: Super Contributor
timestamp: May 31, 2016, 03:07:32 pm
content: This is usually the opposite of what you want when switching AC--switching at the peak means maximum inrush current and maximum EMI. Unless you're doing adjustable phase angle switching for power control, such as in a dimmer.If your frequency is fixed and/or you don't need high precision you could just use a triac with an RC delay, and trim the delay to suit your waveform. The circuitry will be identical to a triac lighting dimmer, in fact you could use one as-is, provided you find one with a suitable current rating.A more accurate solution under ideal conditions would be an op amp differentiator, which will give you the derivative (slope) of the signal. Assuming a sine wave, the only time the slope is at zero is at the positive and negative peaks, so feeding the output of the differentiator to a window comparator will give you a pulse at the peak. However, this will be susceptible to noise and false peaks that may be introduced from other equipment on the line.A more reliable solution is probably to set up some sort of timer synchronized to the AC frequency. An analog implementation could be:- a ramp generator that gets reset by the AC zero crossing- a peak detector that holds the maximum value of the ramp generator (which will vary with frequency)- a divider from the peak detector to provide a reference level at half the maximum ramp voltage- a comparator that compares the half-of-peak level with the ramp signal. When the ramp signal passes the halfway point, it turns on the switch.This solution will auto-adjust to different frequencies, and should pretty reliably trigger at the half-cycle point, but if the waveform is sufficiently distorted this may not coincide with the peak voltage.If peak voltage is most important, then probably the best solution is a peak detector that holds the peak (scaled) voltage from the last couple of cycles, and a comparator that will trigger the switch when the instantaneous voltage gets within some threshold of that.

author: digsys
author_rank: Unknown
timestamp: May 31, 2016, 03:46:31 pm
content: I often use an AC opto-coupler on the mains side. Series cap + resistor, zener+resistor, some filtering and 10Kish load.This gives a 1mS (or any width) pulse at a pretty precise point near zero, and that can be used to reset a timer. Mains frequency is pretty accurate, so youdon't need a max check. The big questions are - what about current lead / lag ? How often are you switching? Switching Input or Output ?How accurate does it HAVE to be? 5%? 1% 0.1%

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 01, 2016, 03:38:36 am
content: 1/4 wave switching is optimal for inductive loads: transformers, solenoids, induction motors.Zero cross switching can potentially destroy the TRIAC! For example, a toroidal transformer load.But so can peak switching, on a capacitive load (PFC capacitors, cap input rectifiers?).1/4 wave switching, into a transformer load, with a subsequent cap-input rectifier, could be problematic. For the case with a toroidal transformer, you might be better off with a compromise, like 1/8 wave. You're screwed in either direction, but perhaps you'll be least screwed there.Tim

author: Zeranin
author_rank: Regular Contributor
timestamp: June 01, 2016, 04:08:50 am
content: If switching a resistive load, then zero-voltage-switching is best, and in this case the simplest, cheapest, safest way to drive the triac is with a zero-voltage-switching optically isolated triac driver such as the MOC3041. The MOC3021 does the same thing, but will fire the triac exactly when you tell it to, rather than waiting fro the next zero voltage crossing.If you really want to trigger the triac at the peak voltage, then an easy and reliable method of generating the trigger pulses is to integrate the AC waveform, which gives another sine wave but 90 degrees lagging in phase, and then trigger the triac on every zero crossing of the integrated waveform. This is much better than using a differentiator, that inherently has issues with noise, and provides the same result. In the simplest implementation you don't even need an op-amp to get the integrated waveform, just use a RC low pass filter with time constant longer than the period of the mains.

author: Cicada
author_rank: Regular Contributor
timestamp: June 01, 2016, 11:21:38 am
content: HiI want to thank all of the poster for all of the ideas. I will ponder all the proposed solutions in the next weeks so that I am prepared when I actually start designing and building the circuit.

author: Robaroni
author_rank: Frequent Contributor
timestamp: June 01, 2016, 12:53:08 pm
content: Quote from: Zeranin on June 01, 2016, 04:08:50 amI've used this successfully too. Simple and reliable.Rob