I'm designing UPS these days.
I found a circuit for the inverter inside the UPS. But the problem with that is that it generates and square wave. So I thought of filtering it to get the Sine wave.

So my question is that is it ok to filter the square wave after amplifing it to 240V/ 50Hz Ac or is it better to filter it before amplifing and amply the sine wave after that .

 

How many watts? Just curious, have you thought what the component values would be to filter a square wave? It would need to be a 2nd or 3rd order filter, and you would generate a lot of heat.

AC/DC Inverters

 

So my question is that is it ok to filter the square wave after amplifing it to 240V/ 50Hz Ac

No,and in UPS no body does that as far I know ...
You should filter it before amplifing i.e.. before driving the signal to driver section.And their are many different ways to create a pure sine wave inverter,as per me I would use a mcu to do that

 

How many watts? Just curious, have you thought what the component values would be to filter a square wave? It would need to be a 2nd or 3rd order filter, and you would generate a lot of heat.

AC/DC Inverters

Is there any other solution for that Bill? If filtering is not practicable, what shall I do?

How can get the sinusoidal??

 

@ debjit625 : DO you have any circuit diagram for UPS using a MCU.?

 

Basically you make a simple sine wave oscillator and use it to modulate a PWM circuit. Basically a simple Class D audio amplifier. I will eventually discuss this in my inverter article, but not today.

We get a lot of requests here for AC inverters. Truthfully it is one of those devices that aren't very practical to build, they truly are cheaper to buy as an off the shelf unit than to make one. However, some allowance is made for people wanting to learn the basics.

Which brings me back to my first question, how much wattage, and what is the battery voltage or source? I would like to walk you through some simple math to explain my point of view.

 

I'm sorry Bill, I forgot to mention the values above.

I want to design a 650W UPS. The battery is 12V normal UPS battery. The UPS is to be used to protect a normal desktop computer.

 

The PSU of a desktop PC rectifies the AC mains to a DC voltage in its primary.

Most recent PC PSUs are also designed to work from a very wide range of input voltages, so they can work from 110vAC to 240vAC without needing the switch that the old style had. As 110vAC rectifies to 160vDC you can run the PC PSU from 160v DC, maybe less.

It REALLY does not care about the "sine" shape of the incoming waveform, it doesn't even care if it is AC or DC. It is also fully isolated SMPS primary:secondary so you have a number of options for how you generate the 160vDC.

The simplest solution would be a 12vDC to 160vDC converter, optimised for efficiency and/or reliability, not for "sine" issues.

 

Assuming you are using the full 650W (a bad assumption) at 12V this would work out to 54.2 amps. Most batteries balk at this, it is enough current that you will need braid, not conventional wire. You could use 4 SLA (sealed lead acid) batteries in series for 48 volts, this will bring the current down to a more reasonable 13.6A, but that is still a lot of current. This is the primary reason I think inverters in general is a bad idea. Also, no conversion is perfect. If you loose 10% you are doing well, so you will need to over design the system (think 60 amps or more instead of 54 amps)

I use a backup UPS for my BBS (bulletin board system), batteries in general have lifespan of several years, then need replaced. It can be quite expensive, but it is the cost of doing business. A UPS (uninterpretable power supply) is probably what you are after, it is an inverter with batteries built in, along with a built in charger.

Given your requirements you will need multiple batterys, the more the better, and you will also need to charge them continuously. Generally a standby UPS converts the AC into DC, uses it to drive the inverter circuits while simultaneously charging the batteries. That way if power drops out the batteries should flawlessly take over. This will be a physically large system.

 

Or you could use 14 smaller batteries in series to get 168V and feed the supply straight.

 

I remember seeing one of those "hippie" alternative energy pages where they rewound the ferrite transformer in the PC PSU with different primary turns so it would take a 48v DC input (from solar batteries) and generate the PC voltages, so basically they made a 48v PC.

And I would be fairly confident you could buy commercial replacement PC PSUs that are designed to run from battery bank voltages.

Of course, a small energy efficient laptop etc will already run from 17v or so and use a LOT less current than a PC... Or one of those netbook things designed for battery use.