Inverters vs wallwarts

You could scope the input current. It may not peak as high as you have estimated.

bob prohaska wrote: ================

** A well chosen choke will do that, but not by much. ** PFC corrected SMPSs were invented for this job. Doubt if you will find such in wall warts.

...... Phil

** Yeah, it more like 20 to 25% .

With 50Hz power, 100Hz current pulses are about 2mS duration.

..... Phil

Big supplies are now required to be PFC, namely not pull big current spikes. Small warts may be exempt.

Even with a simple rectifier front-end, the top of a sine wave is pretty soft. And it wouldn't shock me if some people skimp on caps.

It's not hard to measure. Or just plug it into the converter and not worry. An 800 watt converter probably won't notice a 40 watt wart.

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.

On a sunny day (Sat, 25 Jun 2022 02:10:59 -0000 (UTC)) it happened bob prohaska snipped-for-privacy@www.zefox.net> wrote in <t95qrj$r99$ snipped-for-privacy@dont-email.me:

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... 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...

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** So when rectified is pure DC. ** See above.

...... Phil

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

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.

[snip]

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.

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

Thanks for writing,

bob prohaska

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

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)

bob prohaska wrote: ======================

** 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

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: =================

** 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 snipped-for-privacy@gmail.com wrote in snipped-for-privacy@googlegroups.com:

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...

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**Yep.

** Nonsense.

No charging pulses AT ALL. It's f****ng DC you fool.

..... Phil

They're talking about wallwarts loading the output of a UPS inverter that generates a 120VAC output with either a square or sine waveform.

Capacitive rectified input filters of the wallwarts' primary(SMPS), or capacitive rectified output filters of the wallwart's (linear) secondaries will produce a different 120VAC current crest factor, depending on whether they are fed by a square or sine wave.

If a square wave has it's simplest form (+-120V ~180degree, then crest factor is low, but the capacitive rectified output voltage will be only 70% of that produced by a 120V sine source.

If the square vave is tailored to produce a more similar output (peak to RMS ratio), with higher voltage at a reduced duty cycle, then crest factor of rectified current will increase, and can easily exceed that of the sinusoidal standard input. Input current will look more like an RC charge/discharge, than a haversine charge/RC discharge.

RL

On a sunny day (Sun, 26 Jun 2022 08:43:40 -0400) it happened legg snipped-for-privacy@nospam.magma.ca> wrote in snipped-for-privacy@4ax.com:

Here the waveform of my cheap UPS loaded with a normal 25 W lightbulb:

measured a while back when it was new,

230V AC 50Hz Europe, cannot read the Vpp from the picture but 230 * sqrt(2) = 325, *2 = 650 Vpp this looks like 5 to 6 divisions maybe around 500 to 600 Vpp (not sure). Waveform is interesting :-)

That's the modified square wave. It will produce quite peaky current waveforms in a capacitive rectified filter.

But, as you've said, if it works, it works. The OP was trying to predict performance, based on insufficient information.

RL