claytonjph avatar image
claytonjph asked

Ripple voltage shutdown and DC PSU


I wondered if I could please ask for some advice...

I live in a place with unreliable power supply, so I make use of a portable generator (not pure sine wave) when there is no power. Since I want to run sensitive electronics, I purchased a Phoenix 24/375 pure sinewave inverter. Instead of using a battery, I am using a switch-mode power supply unit connected to the generator's AC output, to provide the 24VDC required. I am aware that the instruction manual advises against this setup, however I have managed to run many different AC loads on the inverter like this - with no hassles (tested to around 160W load).

The problem arises when I connect just my TV, which draws 140W, to the inverter. All of a sudden, the inverter shuts down with a high DC ripple alarm. I see that the ripple is quoted to be 100mV on my DC power supply. I cannot find any data sheets regarding the unit's internals.

Any advice or notes, on this behaviour I have witnessed, will be hugely appreciated.

Many thanks

Clayton Pheiffer

Phoenix Inverter
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1 Answer
geomz avatar image
geomz answered ·

First thing that come to mind is that different loads generate different demands on a power supply. It's possible your PS is capable of providing a 160w resistive load, but at 140W inductive loads (which your presumably digital) TV's power supply needs). To produce that 140w, it might need 180VA, which, depending on the rated output of your PS, might overload it, and thus the ripple. 100mv ripple isn't much at all, but that might be an erroneous reading as the inverter will react to ripple much faster than you (or the PS' display) and shutdown, thus returning everything to "normal" again.

Of course that's all just guessing, Something else could be at play :)

In your situation (or at least as a test), I'd put in a pair of 12v lead-acid batteries (in series) to act as "shock absorbers" and see what happens. Plus that'll get you closer to the intended design of these units.

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claytonjph avatar image claytonjph commented ·

Thanks for the response! Yes I had wondered about the inductive vs resistive load, so I measured the current drawn by the TV (which works fine off the inverter running on lead acid batteries) and found it was drawing around 0.57A (230VAC) and drawing 7A DC from the Pb batteries. My 24VDC power supply is rated at around 15A. Would it help if I got a larger one - say 20A?

In any case, I like the idea of adding batteries as shock absorbers. Given that the majority of the current is provided using the PS, what would you say is the smallest (cheapest) battery set I could get away with? For instance is 10Ah too small?

Thanks again!

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geomz avatar image geomz claytonjph commented ·

Not sure how you measured the draw, but power factor measuring would be the only way to see for sure your VA vs W draw.

In the US, on an installation this small, I'd use one of those Killawatts units. They're rated to 15A Ac@120V. Plug it into the wall, plug your load into it, and easy peasy readout of VA, V, A, W, and PF. There are certainly more "pro" solutions that can be deployed, but that's a quick-win solution for me. Not sure what equivalent 230V options there are. A high-quality clamp meter would probably work, but that depends on if it's measuring RMS or realtime numbers and you'd need a way to measure both sides simultaneously to be certain. But it'd get you close-ish. Thus why I prefer the killawatt doodad :)

A bigger power supply might help. But no way to know for sure. Based on your numbers, that PS should be more than enough. But I know for a fact a lot of those PS are running at a PF of ~.70. So imagine the first power supply with a PF of .70, then the TV's PS with a PF (we'll be generous and call it) .85. Then the inverter will have some loss too. Not sure how efficient that unit is, but .90-ish? You're losing quite a bit before that first pixel fires up ;)

As for your batteries, you're really not gonna find much of a difference in price (at that price range) based on capacity. In fact you may pay more for a cutesy small battery, just for its cuteness :) And you'd be robbing from Peter to pay Paul, getting that small of batteries. Consider perhaps ones in the 30-50AH range. They don't even really need to be truly deep-discharge, since you're not using them for capacity but shock absorption. Heck, start batteries might work, but I'm not going to advise you to do that :)

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claytonjph avatar image claytonjph geomz commented ·

Update: I have sorted out the issue using two lead acid batteries as shock absorbers. Thanks for the time you took to guide me, much appreciated!

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geomz avatar image geomz claytonjph commented ·

You're very welcome. Glad it helped :)

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