Circuit help: switching a load only at peak of mains

This is a quite uncommon requirement.

How about delaying a zero-crossing trigger for a quarter cycle?

A relay / contactor is not quick enough, but a thyristor should do.

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Tauno Voipio

How accurate do you need it ? Seems the simplest way is to just use a conve ntional zero crossing circuit and delay the output by 20 mS.

If that's not acceptable I would look into the old analog TV sync separator type circuit with slight modifications, that way it would be tolerant of f requency deviations, like if you were to try to use the thing in the US and not want to change the delay to 16.66 mS. Something non-linear like a VDR might help detection in such a case.

Out of curiousity, why are you doing this ? Perhaps looing for a "worst cas e" scenario not using zero crossing ? Or is it pure curiousity or something like that ?

Oops, I think I might have meant 25 mS.......

20 ms won't do - a quarter cycle is 5 ms (on 50 Hz) or 4.17 ms (on 60 Hz).

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Tauno Voipio

Shit, 12.5.......

I'm changing my handle to mushbrain, hey I just woke up.

** Peak switching gives the max current surge with most non- transformer PSUs.

Important to have a number for this with LED lighting fixtures as they are commonly used in large numbers in commercial applications.

It sets the upper limit on how many can be on the same switch and breaker.

In bad examples with massive surges, that number can be only two or three despite the actual power level being only a few hundred watts.

... Phil

Le Tue, 17 Sep 2013 09:53:32 +0100, JB a écrit:

Just integrate the mains voltage and do a zero crossing detection. For obvious offset reasons you'd rather low pass filter the voltage than integrate.

That's as simple as:

comparator 1M ___ |\| -|___|---+--------|-\ | | >- | .-|+/ --- | |/| 100n --- | | | | | === === GND GND

The 1.6Hz cutoff frequency ensures that you're off by less than 2° at

50Hz and even better at 60Hz. You'll also have to mind the angle introduced by the comparator offset and hysteresis, but this should not have significant impact on your amplitude switching point.

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Thanks, 
Fred.

This is really only to test the absolute worst case for inrush current for the given load. I like to have some 'headroom' when I'm specifying the relays in our products which switch these loads in the field, so I need worst case scenarios. It only has to work on 50Hz supplies and is only for in house testing purposes here in the lab.

Thanks for the pointers.

JB

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You could route the zero crossing trigger into a function generator and let that one make the needed delay

Another russian method (guess why it's called russian :-)) is to just take a function generator, set it for 0.11Hz. Let the output control a solid sta te switch. After 1000 seconds you will have hit the period at some time, pr ovied your mains frequency is not african and running crooked :-)

Let the scope capture the inrush even.

Cheers

Klaus

e

Hmm and I was going to suggest differentiate and zero cross detect.

George H.

Use a classic opamp half-wave rectifier peak follower circuit and look for the op-amp output hitting the negative rail once the input peaks (becomes l ower than the stored capacitor voltage). Adjust RC time constant to suit. T he peak signal is then a big slewing signal and very easy to detect.

Isn't this how a light dimmer works?

Yep. Variable angle firing point.

In one of my "crime scene investigations", aka product liability analyses, I had to determine a failure mechanism where a certain name-brand furnace controller could turn on the gas, and leave it on, in spite of no flame.

I built up a lab rig where I could introduce line spikes and vary the angle at which they occurred.

Over a certain range of angles and amplitudes I could make their flame detector circuit fail every time.

They settled out-of-court >:-} ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

Or, how about just using DC for your test?

Inrush current into a cap is essentially infinite. So the limitation is entirely in the internal parasitics and circuit elements.

To get any meaningful results, there needs to be a standard source impedance spec. So, your device needs to be consistent with that.

Sort of.

There's a low pass filter, but it's reset every half-cycle when the diac triggers. Before it's turned up far enough, the diac doesn't trigger so you get the "snap-on" hysteresis effect.

Differentiation picks up all kinds of line noise.

This blank line insertion is becoming profoundly annoying. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

Meditating on this, I don't see why you wouldn't do a test with a DC source, current probe, and storage scope or DSO. Worst case will be with the lowest impedance power source, and you can't be taking line inductance into account with your method.

I'd be inclined to use a large value low ESR cap as the source.

"mike"

** High current (like 60A) 330V DC supplies are a hard to find.

You could charge a big electro to 330V and then switch it onto the load, but this is not gonna result in the same inrush current pulse as occurs when switching on the AC supply at the worst moment.

** The existence of series resistors, NTCs and the cap's ESR all affect the game - and that is why a measurement has to be done. Subsequent on cycles may well result in larger surges than the first too. ** AC supply impedance is a variable, but how do you arrange for it to be unusually low at a given test location ?

Stick a 1000uF, 600V film cap across it ?

FYI:

Some time ago, I experimented with an 8 amp rated domestic lighting breaker, a bridge rectifier and a 47uF, 400V electro.

When switched on at or near the AC voltage peak, the breaker tripped 3 out of 5 times.

... Phil

supply.

Peak firing SCR circuits are often used for highly inductive loads to reduce the DC offset phenomenon which can cause an initial current surge up to twice the normal current, because of the phase angle of current to voltage. Here is a discussion from June:

There are links in that discussion to suppliers of peak-firing SSRs, but

here they are again:

asebrcarlo-gavazzi-rm1c

I have some old SCR firing boards that used an adjustable phase angle to

fire a pair of large back-to-back (antiparallel) SCRs rated at about

800A and 1600V. We redesigned the boards using a microcontroller which also checks for gate continuity to avoid the disastrous consequences of firing only one of the pair, which puts several thousand amperes DC into the transformer. If you want one of the old boards let me know - they are basically scrap. I also have a pile of used/surplus 90A 600V SCR modules you can have for cheap.

Another phenomenon to consider for your testing is remanent magnetism in a transformer when the current has been suddenly cut off, and the core remains magnetized. Depending on the polarity of the next switching event, the voltage may be applied such that the core saturates and a very high current transient occurs, which will often trip a circuit breaker or fuse within

1/2 cycle.

Paul