- Blog entry posted in 'Uncategorised', June 21, 2009.
i see (and answer) a lot of posts both here and elsewhere about 1kW inverters. my immediate answer is usually something along the lines of "there's no such thing as a free lunch". first of all do the math. 1000W/12V=83A. assuming about 15% in losses, that's pretty close to 100A. not only are we talking "heavy iron" here for a transformer, we're talking heavy copper too, since the wire used in the primary circuit needs to have negligible copper losses at 100A. you also need about 20 MOSFETs per side (of a split winding on the transformer) assuming 10A devices, and a safety factor of 2:1. that also makes for a very large heat sink. so basically we're looking for a "welding" transformer with a 12Vct @100A winding, 00ga wire (or at the very very least 2ga kept very very short) 40 MOSFETs, and a very big heat sink.
like i said above, there are 1000W inverters that can be had for free, but it takes a bit of looking to find one that works from 12V. these would be computer UPS supplies. 90% of these run from 24 or 48V batteries, but once in a while you will see a 12V one. 99% of all UPS supplies get scrapped because of sulphated batteries, and the fact it's usually cheaper to buy a new one than pay somebody to replace the old batteries. these are a good "starter" for a 1kW inverter, since the heavy iron, wire, transistors, heatsink and driver board are already there, plus it's already in a chassis with AC outlets. however once you replace the batteries, you will soon find that you can't get it to run on demand. the reason for this is that the control logic wants to see the device pluggedin to line voltage when idle. you will usually want to find this with a schematic, but there is an AC detector circuit that wants AC present, then see the AC go away before it triggers the inverter into operation. the best thing to do here is find the actual logic line that starts the inverter and put a switch there, bypassing the AC detector. every UPS is different, so this is only a generality. you WILL need a schematic to find this, unless the signal is marked on the board somewhere (try looking for a signal marked something like START/STOP, or ACDET).
Menticol, October 24, 2009
Beautiful information! Thank you very much Some pictures would be great, to make your blog even more friendly Bad thing that people here tends to replace the batteries, instead of putting the whole unit into my claws
unclejed613, October 24, 2009
find a computer equipment recycler that sells the dead equipment to metal scrappers. find out how much they get per pound (or kilo), and offer them double. they will usually be more than happy to sell you one, or 2, or 10.
unclejed613, October 24, 2009
i do a lot of work, but rarely get pictures of it. i may ask the company i work for if i can get some pictures of the counterfeit transistors i have found (so i can post them here and at diyaudio.com). there has been a bit of activity in that area in the last couple of weeks, and my "database" of characteristics, both visual and electrical is growing rapidly.
