Inverters vs wallwarts

On Sat, 25 Jun 2022 02:10:59 -0000 (UTC), bob prohaska
<bp@www.zefox.net> wrote:

>I'm setting up a UPS for my computer/comms equipment using an
>inverter/charger and battery from Amazon. The equipment draw
>is only about 40 watts measured with a Kill-A-Watt, but all
>the associated wallwarts use capacitive-input switching  power 
>supplies. That means they only draw current at line peaks.
>
>My seat-of-the-pants guess is that the duty cycle is around 10%,
>meaning that the average 40 watts is really 400 watts 10% of the time.
>That's well within the continuous power rating of the inverter, which
>is 800 watts, so it's likely the setup will work as it is.
>
>The question is: Can the peak load be made closer to the average 
>load by putting an inductor in the AC line feeding the wallwarts?
>
>If anybody's been through this exercise I'd be grateful for guidance.
>
>Thanks for reading,
>
>bob prohaska
> 
>

I did some passive correction for off-the-shelf 60Hz linears in 
the 80s. Best effects achieved using a saturable choke and 
quasi-resonant capacitor, over a limited range of power levels 
for any specific installation.

The parts are generally impractical for a hobbyist to get 
ahold of, though restacking laminations from unvarnished scrap 
is possible. Requires good VP Impregnation to silence the 
final iteration.

The actual current phase angle shifted from leading to lagging 
over the useful range. Output voltage into the 60Hz capacitive 
load was flat-topped, affecting the low-line voltage performance.
The choke/cap combination supported the output difference 
during line current reversal.

It's no good guessing what the current waveshape is; you've 
got to measure it / scope it. Line current transformers are 
pretty cheap these days, often included in <$10 wattmeters 
from off-shore sources. A scope is useful, but more expensive.

A lot of modern wall-warts are actually PFC compliant, through 
the use of dedicated low power integrated controllers. These 
employ valley-fill or critical-conduction (FM) off-line switchers 
economically, at power levels as low as 5W.

Don't guess. Measure. Read specs of devices involved.

Don't go overboard. Your UPS output may be more tolerant of peak 
loads than you assume, and your loads may be less peaky, simply 
due to industry commodity trends and available parts.

RL

On Saturday, June 25, 2022 at 5:04:19 AM UTC+2, Don Y wrote:
> On 6/24/2022 7:10 PM, bob prohaska wrote: 
> > I'm setting up a UPS for my computer/comms equipment using an 
> > inverter/charger and battery from Amazon. The equipment draw 
> > is only about 40 watts measured with a Kill-A-Watt, but all 
> > the associated wallwarts use capacitive-input switching power 
> > supplies. That means they only draw current at line peaks. 
> > 
> > My seat-of-the-pants guess is that the duty cycle is around 10%, 
> > meaning that the average 40 watts is really 400 watts 10% of the time. 
> > That's well within the continuous power rating of the inverter, which 
> > is 800 watts, so it's likely the setup will work as it is. 
> > 
> > The question is: Can the peak load be made closer to the average 
> > load by putting an inductor in the AC line feeding the wallwarts? 
> > 
> > If anybody's been through this exercise I'd be grateful for guidance.

You can actually make some very strange inverters 

https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1062238

from 1983 talks about doing some interesting stuff. Buying two transformers rather than just one back then made the whole approach too expensive to be interesting. It might be worth looking again while thinking about printed windings.

-- 
Bill Sloman, Sydney

On Saturday, June 25, 2022 at 5:31:43 AM UTC-7, bill....@ieee.org wrote:
 
> > On 6/24/2022 7:10 PM, bob prohaska wrote: 

> > > The question is: Can the peak load be made closer to the average 
> > > load by putting an inductor in the AC line feeding the wallwarts? 
> > > 
> > > If anybody's been through this exercise I'd be grateful for guidance.

> You can actually make some very strange inverters 
> 
> https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1062238 
> 
> from 1983 talks about doing some interesting stuff. Buying two transformers rather than just one back then made the whole approach too expensive to be interesting. It might be worth looking again while thinking about printed windings. 

Oh, the time for that HAS come, this is one of the results:

<https://www.ti.com/lit/ds/symlink/ucc12050.pdf>

Good for a half watt, and kinda... tiny.   The actual info on the circuit design is sketchy, because 
there's not a lot of good symbols available to describe an integrated magnetcs circuit.

On Friday, June 24, 2022 at 9:31:00 PM UTC-7, Jasen Betts wrote:
> On 2022-06-25, bob prohaska <b...@www.zefox.net> wrote: 

> > The question is: Can the peak load be made closer to the average 
> > load by putting an inductor in the AC line feeding the wallwarts?

> Yes but only a little, the current phase angle is typically only 
> about 20 degrees leading so a line reactor will not help much. Most 
> of the power factor comes from crest factor rather than cos(phi). 
> 
> It will work better if you put a bridge rectifier before the inductor. 
> (because now you can use a larger inductor), but now you'll have to 
> figure out which wall warts actually need AC, and only connect the 
> DC-capable ones. 

The inductor coming after the rectifier, though, means it has to handle the DC
current without saturating; that can be a problem, and it does add weight
and cost.   Doing such a design is a tricky task, not easy to simulate or
calculate, so is usually a cut-and-try exercise.

Jan Panteltje <pNaonStpealmtje@yahoo.com> wrote:
> On a sunny day (Sat, 25 Jun 2022 02:10:59 -0000 (UTC)) it happened bob
> prohaska <bp@www.zefox.net> wrote in <t95qrj$r99$1@dont-email.me>:
> 
>>I'm setting up a UPS for my computer/comms equipment using an
>>inverter/charger and battery from Amazon. The equipment draw
>>is only about 40 watts measured with a Kill-A-Watt, but all
>>the associated wallwarts use capacitive-input switching  power 
>>supplies. That means they only draw current at line peaks.
>>
[snip]
>>The question is: Can the peak load be made closer to the average 
>>load by putting an inductor in the AC line feeding the wallwarts?
>>
> 
> Interesting question, my cheap UPS seems to put out a square wave
> I wondered if the flat tops are actually not better for the wall warts
> as the charging part is longer than with a sine wave top...

AIUI, current into a capacitor is dv/dt x C, so the square wave should
be much worse in terms of peak current than my sine-wave case. Charging
is completed before the flat top of the cycle can commence. Seems likely
the leakage inductance of the inverter's transformer is dominant.

> Been working now fine for a year or so with this thing,
> comes in almost every day these days with mains company fiddling,..
> flashing light bulbs sometimes here too.
> To backup for longer times I have a pure sine wave 2 kW converter and a 250 Ah lifepo4 battery..
> So I can keep watching sat TV or even cook food.
> More than 10 wallwarts on that UPS now, some Raspberry Pi, some USB hubs, some cameras., also
> security recorder, monitors... .. audio amp... 4 TB harddisks...
> I would personally not bother with a a series inductor...
> 

Your use case sounds very close to mine. If you aren't having trouble it
seems doubtful I will. 

Thanks for writing,

bob prohaska

On 2022-06-25, whit3rd <whit3rd@gmail.com> wrote:
> On Friday, June 24, 2022 at 9:31:00 PM UTC-7, Jasen Betts wrote:
>> On 2022-06-25, bob prohaska <b...@www.zefox.net> wrote: 
>
>> > The question is: Can the peak load be made closer to the average 
>> > load by putting an inductor in the AC line feeding the wallwarts?
>
>> Yes but only a little, the current phase angle is typically only 
>> about 20 degrees leading so a line reactor will not help much. Most 
>> of the power factor comes from crest factor rather than cos(phi). 
>> 
>> It will work better if you put a bridge rectifier before the inductor. 
>> (because now you can use a larger inductor), but now you'll have to 
>> figure out which wall warts actually need AC, and only connect the 
>> DC-capable ones. 
>
> The inductor coming after the rectifier, though, means it has to handle the DC
> current without saturating; that can be a problem, and it does add weight and cost. 

The current magnitude is the same as the ac current magnitude,
so not much of a problem. What bad thing happens if it saturates?

> Doing such a design is a tricky task, not easy to simulate or
> calculate, so is usually a cut-and-try exercise.

Last time I looked at it in detail, it seemed pretty easy to hit the
ballpark.

The aim is not a resonant filter, it's just making a current reservoir
so that (more) continuous current is drawn from the supply instead of spikes
on the peaks of the sine wave (or the edges of a modified square wave
inverter output)

-- 
  Jasen.

 bob prohaska wrote:
======================
>
> AIUI, current into a capacitor is dv/dt x C, so the square wave should 
> be much worse in terms of peak current than my sine-wave case.
>
** Nope  -  a rectified square wave is pure DC. 
    SMPSs used in pro-audio power amps are nearly all square wave inverters. 

   Battery to AC inverters are different. 


.....   Phil 
 

On Saturday, June 25, 2022 at 3:10:38 PM UTC-7, palli...@gmail.com wrote:
> bob prohaska wrote: 

> > AIUI, current into a capacitor is dv/dt x C, so the square wave should 
> > be much worse in terms of peak current than my sine-wave case. 

> ** Nope - a rectified square wave is pure DC. 
> SMPSs used in pro-audio power amps are nearly all square wave inverters. 

If the load on an AC source were a capacitor, you'd get
current= dv/dt x C, and if the load were a rectifier into a (capacitor in parallel with a resistor)
you'd get something more benign.   In a power brick, the load is a rectifier and (resistor-loaded?) 
regulator, like a capacitor in parallel with a current sink.   

The ramp up when the rectifier diodes turn on will be abrupt, lots of peak current,
which was what the inductor was intended to moderate.

If the hypothetical inductor saturates, it'll make a buzzing sound; that's annoying.

WITLESS  whit3rd wrote:
=================
>
> > > AIUI, current into a capacitor is dv/dt x C, so the square wave should 
> > > be much worse in terms of peak current than my sine-wave case. 
> 
> > ** Nope - a rectified square wave is pure DC. 
> > SMPSs used in pro-audio power amps are nearly all square wave inverters.
> If the load on an AC source were a capacitor, you'd get 
> current= dv/dt x C, and if the load were a rectifier into a (capacitor in parallel with a resistor) 
> you'd get something more benign. In a power brick, the load is a rectifier and (resistor-loaded?) 
> regulator, like a capacitor in parallel with a current sink. 
> 
> The ramp up when the rectifier diodes turn on will be abrupt, lots of peak current, 
> which was what the inductor was intended to moderate. 
> 
> If the hypothetical inductor saturates, it'll make a buzzing sound; that's annoying.
>

** Context and relevance are not to be found in this fool's dictionary. 


....    Phil 

On a sunny day (Sat, 25 Jun 2022 20:22:46 -0700 (PDT)) it happened whit3rd
<whit3rd@gmail.com> wrote in
<682a4161-f73e-4b7f-ad78-9237c4291e62n@googlegroups.com>:

>On Saturday, June 25, 2022 at 3:10:38 PM UTC-7, palli...@gmail.com wrote:
>> bob prohaska wrote: 
>
>> > AIUI, current into a capacitor is dv/dt x C, so the square wave should 
>> > be much worse in terms of peak current than my sine-wave case. 
>
>> ** Nope - a rectified square wave is pure DC. 
>> SMPSs used in pro-audio power amps are nearly all square wave inverters. 
>
>If the load on an AC source were a capacitor, you'd get
>current= dv/dt x C, and if the load were a rectifier into a (capacitor in parallel with a resistor)
>you'd get something more benign.   In a power brick, the load is a rectifier and (resistor-loaded?) 
>regulator, like a capacitor in parallel with a current sink.   
>
>The ramp up when the rectifier diodes turn on will be abrupt, lots of peak current,
>which was what the inductor was intended to moderate.
>
>If the hypothetical inductor saturates, it'll make a buzzing sound; that's annoying.

In the UPS case you do not charge into an _empty_ capacitor, if the UPS functions right
then it is just the discharge difference from the last mains period (depends on wallwart load),
the peak current will be limited also by the resistance of the series diodes,
and by the Zi from the UPS etc..
Square wave charges for a longer time, so next period less drop to correct than with a sine...