DC-AC converter

[Mike]

if i wanted 2 buy somethin, we might as well close this site

[rshayes]

The simplest inverters put out a square wave. The lamp, radio, and TV set can all be operated off of square waves if the voltage is correct. The frequency can also be higher than 60 Hz if necessary, and this makes transformers smaller and easier to wind.<p>The radio and TV probably use bridge rectifiers with a capacitive filter. The power supply voltage (inside the radio or TV) probably depends on the peak voltage on the input wave form. For a 120 volt sine wave, this is really about 170 volts. The turns ratio needed is 170/12, or about 14 to 1. This is each side of a center tapped primary. The overall ratio would be about 7 to 1 for the secondary to full primary. If you are trying to use a 120 volt transformer, you would look for one with a secondary voltage of about 17 VCT, probably at abuot 5 amps. This is a rather unusual transformer, so I would plan on doing some transformer winding.<p>Moving the operating frequency up into the 3 to 5 KHz range both reduces the transformer size and the number of turns. This is still the region where laminated cores can be used. Much above this and you will need something lika a tape wound toroid, which is harder to find. Above about 20 KHz, you will probably need a ferrite core, which is also harder to find (you might salvage these out of a junked computer supply). However, working with power at these frequencies is rather tricky and usually results in a lot of blown parts until you get it right.<p>Somwhere around 3 to 5 KHz is probably best.<p>The simplest designs used a transformer, two transistors, and a few small parts. The frequency was controlled by transformer saturation, and was not very stable. These can be built up to around 100 watts with a tape wound core, probably less with a laminated core.<p>A slightly more complex design used two transformers, a non-saturating output transformer and a small saturating feedback transformer. This is more efficient and does not draw the high current pulses needed to saturate the output transformer. This is probably the best way to go, as it reduces stresses on the transistors.<p>Many designs of this type were published in QST in the 1960's. They were usually used to power mobile transmitters or receivers, which were not yet transistorized. Back issues or microfilm files of this magazine might be found in a public library.<p>Ap notes describing this type of supply might be on the web sites for either the transistor maunfacturers or the transformer lamination manufacturers.

[rshayes]

Try looking up Ap Note 222 on the ON Semiconductor site (www.onsemi.com). It discusses several types of power inverters and gives some design information. This is actually an old Motorola Ap Note, probably from the 1960's.

[Mike]

how much does the circuit draw? because i have soem 2n3904 transistors, which are NPN but are also very low powered. will they work?

[dyarker]

"how much does the circuit draw? because i have soem 2n3904 transistors, which are NPN but are also very low powered. will they work?"<p>You said you have a 12V, 2A transformer. I said you'll get less than 24W max out. That's feeding 2A thru the 12V side.<p>A 2N3904 has an Ic max of 200mA. Never design to max rating of semiconductors. So figure about 100mA into the 12V side of the transformer. 2N3904s will work, but the 120V output is about 1W max.<p>In a perfect transformer power in equals power out. So, if the Voltage increases, then current decreases by the same ratio to keep the product the same. if the voltage decreases, then the current increases by the same ratio to keep the product the same. The product is power (in Watts) equals EMF (in Volts) times current (in Amps).<p>Stephen mentioned radios and TVs using peak voltage of about 170V. That is correct. Items with rectifier/capacitor supply will operate the same on 120VACrms or 170VACpeak square wave power. However, lamps are resistive, and will not last long on 170VACpeak square waves. A way around this is pulses that are 70% on time, 30% off time. And still a lot of power is wasted in harmonic frequencies that disappear before getting to the load. That is why UPS manufacturers use "modified sine waves" or "synthesized sine waves"; efficiency.<p>A lot of math goes into building an inverter. If you want to build one to see how it works, take measurements, etc, that's a good thing. Do it! If you really want to power a couple lamps and a TV, buy! If you are serious about electronics (even hobby level), then pay attention in algebra and geometry classes. Some trig would be helpful too. Even imaginary numbers are useful.<p>Cheers,

[rshayes]

At currents over about 50 mA, the 2N3904 saturation voltage starts rising, and at 100 mA, the dissipation seems to be about 45 mW, which can probably be handled by the TO-92 package without too much difficulty.<p>If the supply voltage was 12 V, the input power would be 1.2 W. An 80% inverter would deliver an outputof about 1 W.<p>Building a small converter like this might be a good first step. It is cheaper and easier to build than a higher power converter and will give you an idea what to expect.<p>At this power level, and a frequency of about 5 KHz, you could probably use the parts from the driver and output transformers from an old transistor radio. The newer radios use IC audio sections that don't need transformers, but the older, six transistor type receivers used transformers with laminated cores. These are fairly easy to disassemble and rewind. New transformers are available from Mouser if you can't find an old junk radio. Radio Shack used to sell a package containing spools of #20, #24, and #28 wire. This would be about the right range of wire size.<p>At this power level, mistakes are cheap. A new set of transistors will cost about a quarter at the worst.<p>For a higher power converter, in the 100 W range, the input current will be about 10 A. This will require transistors in a TO-3 package, or one of the larger plastic packages (larger than TO-220). These will require substantial heat sinks.<p>Transformers will also be larger, and the wire sizes get harder to handle. Solid wire larger than #18 tends to have a mind of its own when being wound on a form. Fortunately, only a few turns will be required.<p>At this level, a new set of transistors may cost a couple of dollars, and the pain of remounting the new parts on their heat sinks.

[upsmaster]

design of even a small wattage inverter is not easy. Things u have to consider is transfer power,transformer loss, counter emf of transformer fields effect on switching transistors, bootstrap starting (inverter stall on start up only one side turns on and the other off in a static condition)
If u are intrested i have 10 sets of battery charger and inverter transformer sets out of working 1000va ups and i think i have a the electronics print and a bom that might help u get started if u want them just pay for shipping..i will look and see if i have all the information ..i am sure that i have the transformers

joe

[Mike]

just looking here, i have a bunch of samples of the on semiconductor MJ15003 and the MJ15004. i originally got them for power boosting in amps, but i have plenty left over for this.<p>these chips are in a TO-3 package.<p>according to http://www.onsemi.com/site/products/sum ... 04,00.html<p>they are 20A.

[Mike]

is this the right thing to build?<p>http://www.i4at.org/lib2/inverter.htm<p>just wondering, why cant i power a motor off of it??

[josmith]

It's simple enough so build it. YOu can expect stalling failure to start under load and unstable operation.

[dyarker]

At start-up and stall the resistance of a motor is only the copper wire of the winding. Nearly a dead short!<p>When stopping, any inductive load (like a motor) the current keeps flowing to dicipate the magnetic field. If the resistance is very high (like open switch or OFF transistor), the voltage is very high. Switches spark, but live; transistors get zapped, and die. (spikes)<p>In an inductive circuit, the phase of the current lags the phase of the voltage. The phase shift could stop the oscillator. Or, twice every cycle, a transistor will turn off, based on the voltage in the oscillator, while the current is flowing through the motor; spikes again.<p>With any wave form other than sine, the parts outside a sine at the same frequency are equivelent to more power at harmonic frequencies. The motor runs hotter, not faster, disipating the harmonics.<p>The brushes in the motor will probably wear faster.<p>It is possible to build an inverter to drive motors, but it's a "pro" task.<p>Cheers,

[Mike]

ok then i think i will try building that one. i have no need to drive a motor, just the warning not to confused me.<p>how about those transistors i said i have, will they work? and will i need heatsinks on them?

[dyarker]

With your 2A transformer, a pair of VMJ15003s will more than do. The very smallest TO-3 heatsink will be okay. Maybe none, because they'll each be dropping about 1W.

[Mike]

thanks, hopefully they won't need a heatsink, i don't have one that fits them.

[Mike]

ok, im about to build it, but do i need to use an HEP 154 silicon diode? i dont have that item, and was wonderind if i could substute something else for it. I have a bunch of 1N4001 and 1N4002 diodes. Would they work?