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Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Anybody got experience with the inner workings of small UPS units or 12 V : 120 V inverters?
I've long wanted a source of 120 VAC with adjustable frequency. Power capacity about 25 watts. Frequency range 48 Hz to 120 Hz (400 Hz would be nice). Sinusoidal waveform not required but would be nice. Applications include:
demonstrating collection of frequency meters (vibrating reed and phase-shift indicators)
Demonstration of AC motors
Driving a HV-isolated filament transformer whose primary saturates at about 70 V at 60 Hz.
Building from scratch, I could use rectified mains switched with an H-bridge, or + and - rectified mains switched with a half bridge, or lower voltage DC switched into a step-up transformer.
Maybe it's easier to adapt the power path from an inexpensive inverter or small UPS. I got one of the latter for free, after its 24V battery pack was scrapped: an APC 1000VA unit.
whose block diagram is probably a lot like that in this document: Note the 4 or 5 magnetic components, not counting line input filter, and the 3 blocks of heat-sinked semiconductors. The biggest transformer is an iron-core coil with just 3 leads, perhaps an ouptut filter. The PCB and heatsinks are drilled to accept more transistors in parallel, for higher-VA models of the product. Control electronics are on the same PCB as the power path, in what looks like a high quality design. Maker does not provide service info, other than troubleshooting for dummies.
I'm qualified to reverse-engineer the power path, but would welcome hints that might save time or point me to a different solution. Thanks! -Rich
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Just take a 100 Watt (real watts) and use it in conjunction with a 4 : 600 ohm audio transformer (turns ratio 12:1) that will give you 25 watts at 0.2 amps at 120 volts with lots of headroom. If you want more wattage, wind several 600 ohm windings and connect them in parallel. 300 ohms will yield 50 watts. The best multi impedance audio transformers allowed 'strapping' connecting windings in series parralleltt optimize copper no matter what Z
Drive it with your audio oscillator. no need to worry about buzzy singing AC driving your motors.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Today was electronics flea market day & I kept an eye out for audio amplifier or receiver as Radiotech suggested. (Already have some audio transformers, including the kind to match 4 and 8 ohm speakers to standard "25 volt" and "70 volt" audio distribution lines).
I procured a couple of smaller gadgets that should be hackable more easily than the UPS in original post. For $1, a 200 VA UPS unit from APC, which switches a 12V battery into center-tapped 60 Hz step-up transformer. And for free, a nonworking 12V-to-120V inverter that probably incorporates a boost converter and output bridge -- the latter could run off rectified mains voltage.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
The original unit in the first post may be possible to hack quite easily. The best bet would be changing the clock frequency of the controller that runs the whole thing, if the controller handles higher frequencies than it's already running at. I would guess that the crystal in the upper middle of the board is the one to swap out. You can do a quick test with a crystal with a similar frequency from the junkbox and see if the output frequency changes accordingly. To make it adjustable you could replace it with a suitable adjustable oscillator. If it's a 4MHz crystal, this is not too hard, if it's a 40MHz one it might be a bit more tricky.
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120V VFD from hacked UPS?
