Selecting transformer for a power supply with UPS backup

[hamzahaider]

I am working to make a power supply with Ups backup. Im still learning different things about making a power supply. However currently i am trying to chose an appropriate transformer size.

my load would be

1) 5v, 2A
2) 12v, 2A.

Some one told me that a transformer 220vac to 18vac rated at 3A would be more than enough. However when I used Isis protues to simulate the current being drawn, the ac current rose to above 4A on a 15v 2A lamp which equals 30W. The DC current being drawn was however 1.6A after being rectified which makes sense at around 18v it almost equals 30W.

Now i am confused on how big a transformer should i use.

Ive attached a jpeg of the simulation for reference.

Any sort of help is well appreciated so thank you in advance.

 

[keith1200rs]

Power Supply Issue

I cannot tell from your simulation what you are actually measuring, but what matters to the transformer is average power. Your simulation could be showing peak current. You would need to do a transient analysis and plot average power to determine if there is a problem.

Keith

 

[IanP]

Re: Power Supply Issue

First, you have to decide whether you’ll use linear or switching voltage regulators ..
Switching regulators have roughly 75% efficiency, linear – depending on the difference [Vin-Vout] - much lower than that ..
For example, 5V linear voltage regulator at 2A load and Vin=15V will produce max 10W “load” power and >20W “heat”-wasted power – efficiency is below 33% ..

So, estimate power(s) from back-to-front, and I can assure you that a pencil and a piece of paper is more than enough for that exercise ..

Rgds,
IanP

 

[rajudp]

Power Supply Issue

in the design for 14.6 volt you are used 7815 ?
how it is getting regulated ?

 

[IanP]

Re: Power Supply Issue

14.6Vac after rectification (+smoothing cap) gives you roughly 20Vdc (unregulated) and that leaves around 5V (20-15) for regulation ..

Rgds,
IanP

 

[rajudp]

Power Supply Issue

but in design after rectification (+smoothing cap) gives only 14.6 volt

 

[IanP]

Re: Power Supply Issue

Are you sure you're talking about 7815? Where is it from?
I thought the voltages in question were 12V and 5V ..
:?:

 

[rajudp]

Power Supply Issue

check the circuit provided

 

[hamzahaider]

Re: Power Supply Issue

I have not completed the circuit yet. This was just a check simulation. However IanP thankx for reminding about swtiching or linear voltage regulators. I will see which regulators suitable for my operation are available in the market and chose the transformer accordingly.

However the voltage drops from 15vdc after the 7815 to 12vdc as soon as I introduce this load. Is there any solution to not letting this voltage drop for example a buffer or something similar?


Thank you for your time.

 

[IanP]

Re: Power Supply Issue

78XX voltage regulators require V[in] to be min 3V above V[out] ..
In this case V[in] is even below V[out], it's not possible to regulate the output voltage ..

 

[keith1200rs]

Power Supply Issue

One thing to bear in mind is you want a lot of current. With linear regulators you will have a lot of heat to get rid of. Switching regulators, while more complicated, are probably a better solution.

Keith.

 

[hamzahaider]

Re: Power Supply Issue

I am aware just less than 3v above Vin are required for the regulators and as long as the heat is conerned, i can use heat sinks. I am not looking to build this power supply with a lot of precision. I am sorry its not what most of you would like including myself but i am short of time for my project and just want to complete this supply as soon as possible as I have to move onto other parts in the project.

I have attached a DSN file for proteus of the complete circuit approximately. The lamps depict the loads ill will be using. Any adjustments and comments will be of great assistance.

I still dont understand why is there so much fluctuation?

Thank you for your time.

 

[hamzahaider]

Re: Power Supply Issue

I think this is even a better circuit with another 7815 limiting the current flowing through to the battery and keeping it below 900mA. However the fluctuation in voltage is killing me.

Any suggestions


Thank you for your time.

 

[rajudp]

Power Supply Issue

which version of proteus you used, mine is version 7.4 and is not opening