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Old 13th November 2004, 01:48 AM   #1
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Default moddifying power inverter for amp

I am looking at a 100W power inverter on sale for 5 bucks at officemax this week.

I was wondering if I woud be able to modify it to provide only about 36VCT, without the use of a step down transformer at the output. This would power 2 chipamps powering a sub.

I would think it has something to do with the transformer inside it. would I just have to rewind or buy a different one fore it?

If this would work, i'll be off to the store to pick one up tomorrow.

Thanks!
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Old 13th November 2004, 09:35 AM   #2
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2 choices:

1. Replace/rewind the existing transformer. This would require that you obtain one of adequate VA to supply your amplifier(s) in suitable turns ratio.

2. Have a second transformer at the output, or power your amp from the AC output of the inverter. (This gives you an amp that you can use at home, as well.)

Remember that audio amps tend not to be very efficient, so 100W at input is NOT going to translate to 100W of amplifier. I'd consider it lucky if you got 50W without the inverter crapping out, however, if you're replacing the transformer, using a bigger core (more VA) might make it doable. You could also consider reverse engineering the oscillator section, and increasing the switching frequency. This will probably require some component changes other than the oscillator timing components as well.
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Old 13th November 2004, 11:22 AM   #3
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the internal SMPS of an inverter converts 12V DC into 110-120V DC output REGULATED then converted into modified square wave output. so if you tinker with the transformer, you have to change the regulator and bypass the H bridge. I'd say it is easiest to make an ordinary step down PSU for your amp and wire it to the inverter or make a real car amp SMPS.
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Old 13th November 2004, 01:27 PM   #4
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OK, well thanks for the replies. Just thought that if it would work, you can't beat the price.
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Old 13th November 2004, 02:56 PM   #5
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Default not strictly true...

Hi,

If you are electronically competent you can do this, assuming a lot of other things are OK...

There are a couple of different strategies. The easiest by far will be to remove some turns from the smps transformer secondary. This will make the feedback circuit unhappy, so you either have to add another set of what are to be very low current windings with the same number of turns as originally present, or after removing some turns and soldering new connections then rewind them and reconnect them to the same place on the board. If you add a whole new 110v secondary no real current will flow here, so this can be very small gauge wire.

The number of turns to remove will be proportional to the voltage you want to achieve, if there were 40 turns and you want 1/3 of the output voltage, you need to end up with 2 x 13 for your output windings...I'd unwind 26 turns, cut, solder new leads, rewind 13 and solder new leads. This option gives you independent secondaries, but then you probably require the new 40turn winding to satisfy the feedback need.

Then you'd have a pair of approx 36v secondaries. You'd also need rectifiers, ideally 8 diodes to make 2 bridges and output caps, none of which will be present.

At the price I'm gonna go see what they've got and try it myself. If you want to get one and work on it too, I'll keep updating what I'm doing, between the two of us we should be able to do some real damage.

There are a lot of things that can stop this working, but again for the price, who cares...

Stuart
PS If you decide to try and do this anyway let me know, I can probably send you most of what you need, I have an extensive collection of bits and pieces...
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Old 14th November 2004, 04:38 AM   #6
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That sounds like a great idea.

Unfortunatly, I went there but they were out of stock of them, so I couldn't get one. Were you able to?

Hopefully they will be on sale next week too.
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Old 14th November 2004, 05:20 AM   #7
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Default nearly...

Hi,

They were out of stock, but gave me a raincheck. Apparently they also have a 300w unit for $20, didn't know if you'd be able to get that one, so I figured I'd just wait 'til tuesday, when they claim they'd have them in again.

Stuart
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Old 17th November 2004, 04:27 AM   #8
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Default a different inverter...

I bought another inverter that I'm gonna hack up, I may still be on for the officemax one, but this one had a couple of cool features that were just begging to be abused. You can get yours for $20 at costco, prolly other places too...

The xpower 400, from xantrex. It is rated at 400w continuous, 700 surge, internal tunnel heatsink, 12v thermal controlled fan. And get this it has a 3 digit display for input voltage, output voltage and output power, you can cycle through the 3 options, and the voltages at least are accurate...

It has 2x 110v outlets, and a pair of terminal posts for the input. There are 2 cable sets supplied, 1) 3ft long 8 gauge, rings on one end and crocodile clips on the other, 2) 3ft long, 10 gauge, cigarette lighter plug...like this could supply 45 amps...oh wait they warn you in the manual to only use it for upto 180w.

So far all I've done is opened the case. small board, simple layout, unfortunately it's not using a toroid, but I've not popped the heatsink tunnel away from the board yet to check the way it's wound.

More later

Stuart
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Old 7th December 2004, 05:05 AM   #9
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Default finally got one...

Hi,

The local officemax finally got the 100 watt inverters back in stock and I actually got mine for the $5 price...thats the good news. The bad news is that it does NOT use a toroid for the output transformer, so modifying the output voltage will be tricky. Also the case is very lightweight, not really a good heatsink for a reasonable amp.

I also got around to dismantling the 300w inverter from xantrex, still no toroid, and no fun, though it works very well as an inverter. It's not until we dismantle a coleman 300w inverter that we get a toroid, plus a nice extruded aluminum case with a little fan that could house a couple of gainclones no problem.

At this point I'm going to play with the coleman, since it looks easiest to mod.

Have fun

Stuart
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Old 5th June 2005, 12:36 AM   #10
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I ended up picking up a 60W continous/75W peak PowerLine brand inverter. It's a nice, small inverter and it was on clearence for 6 bucks, so I picked it up.

Looking inside it, there is no torroid, or any transformer for that matter. It has a choke right where the power comes in, and has two chips in it and four transistors.

The chips are SG3524 and TL494, both of which I know are common car amp power supply chips. The transistors are made by ST, and it's hard to read the numbers. It looks like they are W1B221.

Since there is no transformer, there must be some way that the chips can be controlled with a resistor or something to regulate the power to put out 60V rather than 120V. If there is a way to center tap that is even better.

Any ideas on what to do to regulate the voltage down?

Thanks!
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