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Old 05-21-2019, 11:44 AM   #1
Dannyx
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Lightbulb Increasing input voltage of DC-AC inverter - possible ?

Good day folks. Some of you may remember that a while ago I wrote about a colleague of mine who likes tinkering with batteries and design all kinds of crazy devices using them. He reached out to me again and asked me if it would be possible to increase the maximum input threshold of a DC-AC inverter he's using for god knows what. The reason he wants to do this is because this inverter runs at 15v max according to the label, while the battery pack he built puts out 18v which causes the inverter to shut off - some sort of OVP on the input there.

Not really having the time and not wanting to ruin someone else's junk, I kindly brushed him off so I could think about it. What would the limitations be here or is such a hack even remotely possible ? Sounds like a window comparator to me. An IC of sorts (at the most basic level an op-amp) has a fixed reference voltage on its N/I input and a resistor divider on its inverting input (or vice versa depending on whether the output swings negative or positive when OVP is triggered). The "top" of the resistor divider goes to the input and the two resistors are calculated in such a way that when the input is under the threshold, the voltage is lower than the reference and the op-amp is not triggered. When the input causes the resistor divider's output to go higher than the reference voltage on the other input, the output swings low or high stopping the device one way or another - perhaps by cutting VCC to a switching IC. It's a fairly cheap thing so I doubt it's fancy enough to have a micro in there - THAT would be a problem since the threshold might be internally programmed !

Aside from the theory, an immediate practical issue would be the capacitors (and other components along the input for that matter) since they're very likely not rated for anything more than 16v, so those would require swapping for 25v or higher ones....

That's all I can think of TBH...doesn't sound too difficult on paper, but who knows what I'm faced with once I actually pop the cover off It's rather small in size, so I don't think it's rated for anything higher than 200w - maybe 300 pushing it...

Any thoughts ? Cheers and thanks
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Old 05-21-2019, 12:54 PM   #2
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Default Re: Increasing input voltage of DC-AC inverter - possible ?

I would find out what kind of chips are being used then look up spec sheet and application notes circuits.
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Old 05-21-2019, 01:40 PM   #3
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Default Re: Increasing input voltage of DC-AC inverter - possible ?

Correct. I must first see what's connected on that input and their maximum ratings.
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Old 05-21-2019, 03:26 PM   #4
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Default Re: Increasing input voltage of DC-AC inverter - possible ?

can you not use a regulator to drop the 18v to 15v ?
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Old 05-21-2019, 04:20 PM   #5
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Default Re: Increasing input voltage of DC-AC inverter - possible ?

It will be too much power wasted since it is DC-AC inverter so it probably draws lots of current.
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Old 05-21-2019, 10:20 PM   #6
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Default Re: Increasing input voltage of DC-AC inverter - possible ?

He's already using a DC-DC converter between the batteries and the inverter to make it somewhat functional, but it's not ideal, hence the idea to drive the inverter directly somehow.
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Old 05-22-2019, 06:53 AM   #7
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Default Re: Increasing input voltage of DC-AC inverter - possible ?

Could you use more batteries and regulate the batteries voltage to the desired input voltage??
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Old 05-22-2019, 09:00 AM   #8
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Default Re: Increasing input voltage of DC-AC inverter - possible ?

list the details - power source, wanted output,
parts he has.
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Old 05-22-2019, 09:43 AM   #9
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Default Re: Increasing input voltage of DC-AC inverter - possible ?

Quote:
Originally Posted by stj View Post
list the details - power source, wanted output,
parts he has.
I shall post pics and other details once I actually get my hands on the inverter, but what I do know for now is that he wants to power it with a 4S-4P battery pack, so around 16v input voltage - 230v out, modified sine wave most likely - don't even need to check, since all cheap inverters are like that...
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Old 05-22-2019, 11:42 AM   #10
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Default Re: Increasing input voltage of DC-AC inverter - possible ?

Yeah, pics and such would definitely be good. In particular, pic\s of the chips and close ups of surrounding circuitry between the switching mosfets and the chip would be helpful. Depending on the configuration and how cheap the inverter is, you may need to replace certain things like resistors or capacitors. (16v limit may be due to the use of 16v capacitors.)
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Old 05-22-2019, 11:52 AM   #11
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Default Re: Increasing input voltage of DC-AC inverter - possible ?

A while back I managed to repair a small inverter like this, though that one was designed for 24v and I had the opposite issue: it wouldn't run on my 12v input Part of the board was burned to a crisp and I had to redo traces and other things. I remember it had a linear regulator (I THINK a 7805) somewhere in there to power the various ICs and other small things, making it rather inefficient in the grand scheme of things, as an input of 24v means 19v are being dropped by this regulator !

I'm expecting this one to have something similar, so by increasing the input, we're also increasing the heat and dissipation...
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Old 05-22-2019, 01:46 PM   #12
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Default Re: Increasing input voltage of DC-AC inverter - possible ?

Oh, yes indeed. haha Well, you never know! Maybe it's actually a Gem and we are all just doubting it. :'D
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Old 05-22-2019, 05:14 PM   #13
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Default Re: Increasing input voltage of DC-AC inverter - possible ?

Quote:
Originally Posted by budm View Post
It will be too much power wasted since it is DC-AC inverter so it probably draws lots of current.
a good enough amount of heat to run my fridge i would think . although i thought it was only 150 watts inverter that should only draw a few amps .maybe 10 amps .
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Old 05-22-2019, 06:13 PM   #14
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Default Re: Increasing input voltage of DC-AC inverter - possible ?

the invertor is probably clamping the input because it thinks it's a surge - probably a big mov or zener diode.
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Old 05-23-2019, 01:31 AM   #15
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Talking Re: Increasing input voltage of DC-AC inverter - possible ?

This is the inverter he's using, though it's not the 24v version like I linked to here...I don't speak Polish so I have no idea what it says there - just wanted to show you guys at least what it looks like. I thought it looked a bit on the cheap side, but apparently it's 500w, though that's probably peak.

All well...let's crack it open now I'll keep you posted.
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