How do UPS trigger circuits work?

After using a pair of inverter/charger circuits as UPS units for 
some months I've become curious as to how the trigger decides
when to switch from line power to the battery and inverter.

Supposedly they trigger on wrong voltage or frequency, but they
do it in less than a half-cycle, so it seems to be more than a
simple relay. A cursory web search found nothing but I don't 
know the proper name for such an "AC comparator". 

The units in question are from Amazon:
https://www.amazon.com/Ampinvt-Inverter-Charger-Frequency-Batteries/dp/B098QL2VBZ/ref=sr_1_1?crid=1ID8F7MUJFYH8&keywords=ampinvt+800+W+inverter&qid=1683582424&sprefix=ampinvt+800w+inverte%2Caps%2C557&sr=8-1&ufe=app_do%3Aamzn1.fos.f5122f16-c3e8-4386-bf32-63e904010ad0

The docs are useless and tech support is worse. No point asking them.
Apart from those gripes, the units seem to work OK for the price.

I don't need to build anything, but I am curious as to how it's done.

Thanks for reading, any any hints.

bob prohaska

On Mon, 8 May 2023 21:55:47 -0000 (UTC), bob prohaska
<bp@www.zefox.net> wrote:

>After using a pair of inverter/charger circuits as UPS units for 
>some months I've become curious as to how the trigger decides
>when to switch from line power to the battery and inverter.
>
>Supposedly they trigger on wrong voltage or frequency, but they
>do it in less than a half-cycle, so it seems to be more than a
>simple relay. A cursory web search found nothing but I don't 
>know the proper name for such an "AC comparator". 
>
>The units in question are from Amazon:
>https://www.amazon.com/Ampinvt-Inverter-Charger-Frequency-Batteries/dp/B098QL2VBZ/ref=sr_1_1?crid=1ID8F7MUJFYH8&keywords=ampinvt+800+W+inverter&qid=1683582424&sprefix=ampinvt+800w+inverte%2Caps%2C557&sr=8-1&ufe=app_do%3Aamzn1.fos.f5122f16-c3e8-4386-bf32-63e904010ad0
>
>The docs are useless and tech support is worse. No point asking them.
>Apart from those gripes, the units seem to work OK for the price.
>
>I don't need to build anything, but I am curious as to how it's done.
>
>Thanks for reading, any any hints.
>
>bob prohaska

Partly depends on the topology of the inverter/charger or UPS.
Not sure about the one on Amazon.

An inverter/charger like the one you referenced could use either a
separate charger circuit not shared with the inverter  OR it could use
the same circuitry for charging than it does for inverting.

If it is a separate charger, then all it has to do is wait for the AC
input to go away and turn off its AC input relay but keep inverting
off the battery storage.

If combined inverter/charger, it can be more difficult to know when
the grid AC is gone so that the inverter does not try to drive the AC
input line since the AC load output would be connected directly to the
grid or shorepower input.  That is more or less how a grid tie
inverter works that can sell power back to the grid or AC input.
That is called anti-islanding and has strict regulations the company
must follow but I don't think this inverter/charger does this
function.  So that inverter/charger might take a couple of seconds to
find that the AC input went away and invert properly.

Usually, when the AC grid goes away, there will be a short time where
there is NO AC output until things stabilize. If it uses a completely
separate charger and inverter, the it can be truly UPS or
uninterruptabe power when the grid goes away.

One or two AC cycles is typical for a good/fast inverter/charger to
transfer.  Relays take a couple milliseconds to switch.

Lettuce know if you happen to take it apart.  The inverter company
support or sales people probably won't know how it works.  The manual
won't say either.

boB

On 5/8/2023 2:55 PM, bob prohaska wrote:
> After using a pair of inverter/charger circuits as UPS units for
> some months I've become curious as to how the trigger decides
> when to switch from line power to the battery and inverter.
> 
> Supposedly they trigger on wrong voltage or frequency, but they

Or "harmful transients", etc.  Depends on the UPS and the UPS's
topology.

> do it in less than a half-cycle, so it seems to be more than a
> simple relay. A cursory web search found nothing but I don't
> know the proper name for such an "AC comparator".

A UPS has limits on what it expects of the AC line.  E.g., most will gripe
if you connect it to a 67Hz 145V supply -- it's not what the device
expects.

Within the operating limits of the device, you assume a sine wave is
present at the input.  You know what a sine wave looks like for any
given frequency and peak voltage.  So, if what you are seeing *now*
deviates from what you *expected*, the AC line is not compliant.

An online topology just alarms on this condition; it's already been
powering the load from battery power even with mains voltage present.
They are (amusingly) more common for *big* loads.

An offline (standby) UPS will use this signal to activate a relay that
routes power from the inverter to the load (the inverter, hopefully,
having been kept in phase with the mains).  They are common for tiny
loads (and people with limited resources)

A line-interactive UPS is a bit of a combination; an autotransformer
works to boost/buck under/over-voltage line conditions.  The inverter
takes over when the UPS has determined that the mains are malfunctioning
beyond those limits.  This tries to bridge the load/cost for modest
sizes at modest costs.

In each case, you don't need to see the mains "gone" in order
to determine that it is "going away".  You just notice that the
input isn't what it should be -- if it was behaving according to the
operating constraints placed on the UPS by its designer.

UPSs often gripe when connected to a "synthetic" mains source (e.g., genset)
because the genset often exhibits variations in frequency and voltage
that go beyond what the UPS *expects*.  So, it engages more often than
*might* be necessary (depends on how tolerant your loads are of these
variations)

[Try powering a UPS from other UPSs of varying grades to see how
it likes *their* outputs!  :> ]

> The units in question are from Amazon:
> https://www.amazon.com/Ampinvt-Inverter-Charger-Frequency-Batteries/dp/B098QL2VBZ/ref=sr_1_1?crid=1ID8F7MUJFYH8&keywords=ampinvt+800+W+inverter&qid=1683582424&sprefix=ampinvt+800w+inverte%2Caps%2C557&sr=8-1&ufe=app_do%3Aamzn1.fos.f5122f16-c3e8-4386-bf32-63e904010ad0
> 
> The docs are useless and tech support is worse. No point asking them.
> Apart from those gripes, the units seem to work OK for the price.
> 
> I don't need to build anything, but I am curious as to how it's done.
> 
> Thanks for reading, any any hints.
> 
> bob prohaska
> 

On Monday, May 8, 2023 at 5:57:48 PM UTC-4, bob prohaska wrote:
> After using a pair of inverter/charger circuits as UPS units for 
> some months I've become curious as to how the trigger decides 
> when to switch from line power to the battery and inverter. 
> 
> Supposedly they trigger on wrong voltage or frequency, but they 
> do it in less than a half-cycle, so it seems to be more than a 
> simple relay. A cursory web search found nothing but I don't 
> know the proper name for such an "AC comparator". 
> 
> The units in question are from Amazon: 
> https://www.amazon.com/Ampinvt-Inverter-Charger-Frequency-Batteries/dp/B098QL2VBZ/ref=sr_1_1?crid=1ID8F7MUJFYH8&keywords=ampinvt+800+W+inverter&qid=1683582424&sprefix=ampinvt+800w+inverte%2Caps%2C557&sr=8-1&ufe=app_do%3Aamzn1.fos.f5122f16-c3e8-4386-bf32-63e904010ad0 
> 
> The docs are useless and tech support is worse. No point asking them. 
> Apart from those gripes, the units seem to work OK for the price. 
> 
> I don't need to build anything, but I am curious as to how it's done. 
> 
> Thanks for reading, any any hints. 
> 
> bob prohaska


That's not a UPS, it's battery charger/ generator combination. The AC output is isolated from its AC input. If you want to power AC loads with its AC output, you must do so manually or use what's usually a high priced transfer switch.
The product will blow up if you disconnect the battery while its charging. It may also blow up if you connect a battery while its AC input is live. 
It's manufactured by Luckysolar. The product design was probably done by Haveaniceday LTD or maybe Goodluckwiththis Design Group. Hard to say.

boB <boB@k7iq.com> wrote:
>..... 
> all it has to do is wait for the AC
> input to go away
> ....

That's the core of my question. How is the decision made? Is
there a dedicated power management IC for the purpose? 


away and turn off its AC input relay but keep inverting
> off the battery storage.
> 
> 
> Lettuce know if you happen to take it apart.  The inverter company
> support or sales people probably won't know how it works.  The manual
> won't say either.


I did take a few photos of the insides, but to me they weren't 
very informative:
http://nemesis.zefox.com/~bob/ampinvt/2nd_inverter/board_photos/

Rumor has it that most inexpensive inverters are clones or outright
copies of commonplace designs. If anybody recognizes this one it
would be helpful. 

Thanks for reading,

bob prohaska

Don Y <blockedofcourse@foo.invalid> wrote:
> 
> 
> An offline (standby) UPS will use this signal to activate a relay that
> routes power from the inverter to the load (the inverter, hopefully,
> having been kept in phase with the mains).  They are common for tiny
> loads (and people with limited resources)

That's what I'm dealing with. 
 
> In each case, you don't need to see the mains "gone" in order
> to determine that it is "going away".  You just notice that the
> input isn't what it should be -- if it was behaving according to the
> operating constraints placed on the UPS by its designer.
>

But how is the expetation generated and tested, in a timely fashion?

Thanks for writing, 

bob prohaska

On 5/9/2023 2:12 PM, bob prohaska wrote:
> Don Y <blockedofcourse@foo.invalid> wrote:
>>
>> An offline (standby) UPS will use this signal to activate a relay that
>> routes power from the inverter to the load (the inverter, hopefully,
>> having been kept in phase with the mains).  They are common for tiny
>> loads (and people with limited resources)
> 
> That's what I'm dealing with.

They tend not to switch to "backup" as quickly.

>> In each case, you don't need to see the mains "gone" in order
>> to determine that it is "going away".  You just notice that the
>> input isn't what it should be -- if it was behaving according to the
>> operating constraints placed on the UPS by its designer.
> 
> But how is the expetation generated and tested, in a timely fashion?

Likely purely in the time domain.  An ADC watching the mains (through
a divider of some sort) and its own internal timebase.  It can sample
the line voltage thousands of times per second (so scores per AC cycle?).

[It has to do this in order to keep its own sine-wave generator *tracking*
the mains so that the inverter will have the proper frequency and phasing
if/when activated]

Whether that sine-wave reference/generator is a physical device
or a virtual device (e.g., keeping track of what the mains should be
so that it can drive the inverter to that instantaneous output
voltage), it can compare the reading from the mains to the expected
value from the sine-wave generator.  When some difference is detected,
conclude that the mains has failed.

If, for example, the mains just had a positive going zero-crossing
~4ms ago, you would expect the (scaled) mains voltage available at
your ADC to be roughly the peak AC mains voltage (within the tolerance
of the UPS).  If, a millisecond later, it's *0* volts, then obviously
something is wrong -- because a sine wave doesn't decay to 0 volts
for another 4 ms (at 60Hz).  I.e., at 4ms it was "nominal" but at
5ms it's clearly not as expected so switch to the inverter.  If the
mains voltage reappears, then you can switch BACK -- after imposing
some hysteresis -- to the mains.

Keep in mind that the UPS has to accommodate variations in frequency
as well as voltage (and disturbances) BUT ONLY TO THE EXTENT ADVERTISED
FOR THE UPS.

[The same measurements also let a line-interactive UPS know when to boost
or trim the mains WHILE they are available]

If you want to use a UPS as an inverter, all you have to do is lie to it
about the availability of mains power!  :>

> Thanks for writing,
> 
> bob prohaska
> 

bob prohaska wrote:
> boB wrote:
>>.....
>> all it has to do is wait for the AC
>> input to go away
>> ....
>
> That's the core of my question. How is the decision made? Is
> there a dedicated power management IC for the purpose?

The AC-OK sensor circuit of an old UPS design is found on Sheet 1, in 
the upper right corner (D8 - C5):

<https://electronics-components.ru/files/2015/732pschem_765.pdf>

AC's apparently applied to a transformer, rectified, and compared
against a zener diode reference. It's relatively simple circuit.

Danke,

-- 
Don, KB7RPU, https://www.qsl.net/kb7rpu
There was a young lady named Bright Whose speed was far faster than light;
She set out one day In a relative way And returned on the previous night.

Correction: Sheet 1, upper LEFT corner

bob prohaska wrote:
> boB wrote:
>>.....
>> all it has to do is wait for the AC
>> input to go away
>> ....
>
> That's the core of my question. How is the decision made? Is
> there a dedicated power management IC for the purpose?

The AC-OK sensor circuit of an old UPS design is found on Sheet 1, in 
the upper LEFT corner (D8 - C5):

<https://electronics-components.ru/files/2015/732pschem_765.pdf>

AC's apparently applied to a transformer, rectified, and compared
against a zener diode reference. It's relatively simple circuit.

Danke,

-- 
Don, KB7RPU, https://www.qsl.net/kb7rpu
There was a young lady named Bright Whose speed was far faster than light;
She set out one day In a relative way And returned on the previous night.

On Tue, 9 May 2023 11:09:03 -0700 (PDT), Fred Bloggs
<bloggs.fredbloggs.fred@gmail.com> wrote:

>On Monday, May 8, 2023 at 5:57:48?PM UTC-4, bob prohaska wrote:
>> After using a pair of inverter/charger circuits as UPS units for 
>> some months I've become curious as to how the trigger decides 
>> when to switch from line power to the battery and inverter. 
>> 
>> Supposedly they trigger on wrong voltage or frequency, but they 
>> do it in less than a half-cycle, so it seems to be more than a 
>> simple relay. A cursory web search found nothing but I don't 
>> know the proper name for such an "AC comparator". 
>> 
>> The units in question are from Amazon: 
>> https://www.amazon.com/Ampinvt-Inverter-Charger-Frequency-Batteries/dp/B098QL2VBZ/ref=sr_1_1?crid=1ID8F7MUJFYH8&keywords=ampinvt+800+W+inverter&qid=1683582424&sprefix=ampinvt+800w+inverte%2Caps%2C557&sr=8-1&ufe=app_do%3Aamzn1.fos.f5122f16-c3e8-4386-bf32-63e904010ad0 
>> 
>> The docs are useless and tech support is worse. No point asking them. 
>> Apart from those gripes, the units seem to work OK for the price. 
>> 
>> I don't need to build anything, but I am curious as to how it's done. 
>> 
>> Thanks for reading, any any hints. 
>> 
>> bob prohaska
>
>
>That's not a UPS, it's battery charger/ generator combination. The AC output is isolated from its AC input. If you want to power AC loads with its AC output, you must do so manually or use what's usually a high priced transfer switch.
>The product will blow up if you disconnect the battery while its charging. It may also blow up if you connect a battery while its AC input is live. 
>It's manufactured by Luckysolar. The product design was probably done by Haveaniceday LTD or maybe Goodluckwiththis Design Group. Hard to say.


An inverter/charger will work instead of a "UPS"...  But might or
might not have a lot of transfer time. 

The only question is, does this have a built in charger as well as an
inverter ?

Batteries not included of course as a real UPS would have.

boB