Circuit help: switching a load only at peak of mains

[JB]

I have a series of measurements to make of the inrush current of a range of
LED drivers and electronic ballasts. Is there a simple way of ensuring that
the load is always switched on at the peak of the mains cycle? I'll probably
use a substantial relay or thyristor to do the actual switching. This is for
a single phase 230V 50Hz supply.
I have done zero crossing triggers for switching loads but never peak! My
Google mojo seems to have deserted me on this one. Any help much
appreciated.
JB

 

[Tauno Voipio]

I have a series of measurements to make of the inrush current of a range of
LED drivers and electronic ballasts. Is there a simple way of ensuring that
the load is always switched on at the peak of the mains cycle? I'll probably
use a substantial relay or thyristor to do the actual switching. This is for
a single phase 230V 50Hz supply.
I have done zero crossing triggers for switching loads but never peak! My
Google mojo seems to have deserted me on this one. Any help much
appreciated.
JB

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.

 

[[email protected]]

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

If that's not acceptable I would look into the old analog TV sync separatortype circuit with slight modifications, that way it would be tolerant of frequency deviations, like if you were to try to use the thing in the US andnot 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 case" scenario not using zero crossing ? Or is it pure curiousity or somethinglike that ?

 

[[email protected]]

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

 

[Tauno Voipio]

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

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

 

[[email protected]]

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

Shit, 12.5.......

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

 

[Phil Allison]

Out of curiousity, why are you doing this ?
Perhaps looing for a "worst case" scenario not using
zero crossing

** 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

 

[JB]

How accurate do you need it ? Seems the simplest way is to just use a
conventional 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
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
case" scenario not using zero crossing ? Or is it pure curiousity or

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

 

[Klaus Kragelund]

How accurate do you need it ? Seems the simplest way is to just use a

conventional zero crossing circuit and delay the output by 20 mS.










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.

You could route the zero crossing trigger into a function generator and letthat 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 state switch. After 1000 seconds you will have hit the period at some time, provied your mains frequency is not african and running crooked

Let the scope capture the inrush even.

Cheers

Klaus

 

[Phil Allison]

"JB"

This is really only to test the absolute worst case for inrush current for
the given load.

** But you do not have control over the loads or even know what they are.

I like to have some 'headroom' when I'm specifying the relays in our
products which switch these loads in the field,

** Errr - what loads are they ?

so I need worst case scenarios.

** You obviously have no fucking idea.

All your relays have to do is withstand inrush surges a tad larger in
magnitude than the circuit breakers fitted to the same circuits are able
o - before tripping.

So you need to do is look at circuit breaker ratings, as might be used in
lighting, for the peak current that causes tripping

FYI:

For a 16A breaker - it is about 180 amps for 1 to 2mS.

For an 10A breaker - it is about 120 amps for 1 to 2mS.

Practically any mains rated relay with 16A or 10A contacts will exceed these
numbers.

It only has to work on 50Hz supplies and is only for in house testing
purposes here in the lab.

** Yawnnnn .....

Thanks for the pointers.

** That comes across as very, very fake.



..... Phil

 

[Phil Allison]

"Fred Bartoli"

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.

** A circuit that triggered just before the peak AC voltage would do as
well.

Say 320V - or a stepped down ratio of that voltage.

The OP is a bullshitting dope anyhow.





.... Phil

 

[George Herold]

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.

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

George H.

 

[piglet]

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 lower than the stored capacitor voltage). Adjust RC time constant to suit. The peak signal is then a big slewing signal and very easy to detect.

 

[mike]

I have a series of measurements to make of the inrush current of a range of
LED drivers and electronic ballasts. Is there a simple way of ensuring that
the load is always switched on at the peak of the mains cycle? I'll probably
use a substantial relay or thyristor to do the actual switching. This is for
a single phase 230V 50Hz supply.
I have done zero crossing triggers for switching loads but never peak! My
Google mojo seems to have deserted me on this one. Any help much
appreciated.
JB

Isn't this how a light dimmer works?

 

[mike]

Isn't this how a light dimmer works?

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.

 

[Spehro Pefhany]

Isn't this how a light dimmer works?

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.

 

[miso]

I have a series of measurements to make of the inrush current of a range of
LED drivers and electronic ballasts. Is there a simple way of ensuring that
the load is always switched on at the peak of the mains cycle? I'll probably
use a substantial relay or thyristor to do the actual switching. This is for
a single phase 230V 50Hz supply.
I have done zero crossing triggers for switching loads but never peak! My
Google mojo seems to have deserted me on this one. Any help much
appreciated.
JB

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.

 

[Phil Allison]

"mike"

Or, how about just using DC for your test?

** 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.

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

** 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.

To get any meaningful results, there needs to be a standard
source impedance spec.

** 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

 

[P E Schoen]

"JB" wrote in message

I have a series of measurements to make of the inrush current of
a range of LED drivers and electronic ballasts. Is there a simple
way of ensuring that the load is always switched on at the peak
of the mains cycle? I'll probably use a substantial relay or thyristor
to do the actual switching. This is for a single phase 230V 50Hz supply.
I have done zero crossing triggers for switching loads but never peak!
My Google mojo seems to have deserted me on this one. Any help
much appreciated.

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:

http://www.electronicspoint.com/sol...iring-scrs-inductive-loads-t261190-print.html

There are links in that discussion to suppliers of peak-firing SSRs, but
here they are again:

http://www.wolfautomation.com/products/30434/peak-switching-ssr-single-phasebrcarlo-gavazzi-rm1c
http://www.galco.com/buy/Crydom/PSD2450

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

 

[Phil Allison]

"P E Schoen"
"JB"

I have a series of measurements to make of the inrush current of
a range of LED drivers and electronic ballasts. Is there a simple
way of ensuring that the load is always switched on at the peak
of the mains cycle? I'll probably use a substantial relay or thyristor
to do the actual switching. This is for a single phase 230V 50Hz supply.
I have done zero crossing triggers for switching loads but never peak!
My Google mojo seems to have deserted me on this one. Any help
much appreciated.

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:

http://www.electronicspoint.com/sol...iring-scrs-inductive-loads-t261190-print.html

There are links in that discussion to suppliers of peak-firing SSRs, but
here they are again:

http://www.wolfautomation.com/products/30434/peak-switching-ssr-single-phasebrcarlo-gavazzi-rm1c
http://www.galco.com/buy/Crydom/PSD2450

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.


** If you find any 230VAC LED drivers or electronic ballasts that
use IRON TRANSFORMERS - let us know...



..... Phil