5v and 12 v required.. i/p 100v-250v ac

[georgek86]

hi.....
this is for my mini project automatic stepup transformer..
need......
+5v for microcontroller
+12v for relay driver......

device have to work for a line voltage range...100v t0 250volts.. .
can any one suggest some circuit ideas for this requirement...

i know that some smps circuit will do... but i don't need that kind of load regulation..... just need the line regulation........

 

[Hero999]

What current do you require?

A PC power supply will do the job.

 

[palesha]

or just transformer to stepdown the voltage & use regulator ic as per the current required.
or find some PC power supply from any scrapped computer if noise is no problem.

 

[georgek86]

5v 12v

well basically i have to drive 8 relays and power 8 leds... then the power required by microcontroller tiny 15... current requirement is low ......

 

[Hero999]

Which voltage is to do what?

For example, a typical specification would be:
5V 100mA
12V 500mA

But it depends on how much current do those relays take and how big are those LEDs?

You can get large relays that take over 2A and large LEDs over 1A or small relays that just use 10mA or small LEDs that just use 3mA.
What about regulation?

Do both 12V and 5V supplies need to be regulated?

A transformer with a twin 120V primary and 12V secondary can be used, the LM7812 and LM7805 can be used on the output after a bridge rectifier and smoothing capacitor. However this depends on your current requirements and which supply needs to be regulated, for example if regulation isn't required on the 12V supply you could use a 9V transformer and omit the LM7812, if you need more current than 1A on the 12V supply and it needs to be regulated then you'll require a booster transistor.

For 120V operation the primaries need to be connected in parallel and for 240V operation they need to be connected in series.

 

[georgek86]

5v 100 ma
12v 800 ma

the problem is that.. as the project is automatic step up transformer..... it has to work in conditions where a 230v power line voltage has decreased to 100volts...as the dc voltages sre meant for micro and driver ic they should not decrease.........

 

[Hero999]

You can buy ready made 12V and 5V switching regultors that will work from 90VAC to 260VAC.

If you insist on making it yourself you could use the LM7812 and LM7805 or LM78L05, if you want to save space or you could go for the LM317.

Also put 5V regulator on the output of the 12V regulator because it will mean you can use a smaller heat sink.

Does it have to be automatic? Can't you have a switch to manually adjust the voltage?

You could easilly use a 24V transformer then it won't matter if the voltage drops to 100V because it will still work providing you can ensure the regulator doesn't go into dropout.

 

[georgek86]

re

i am making the thing automatic is the main part of my mini project... through this i'm intending to learn few things about microcontrollers.....
well i will try to make a design using 7805 or lm317... will the thing over heat if the thing work at above 200 voilt for a considerable period of time due to drop in regulators?????

 

[Hero999]

You don't use the regulator to reduce the voltage all the way down from the mains input; you use a transfromer for that.

The problem is that if you used a higher voltage transformer then when the input voltage is high the regulator will be dissipating a lot of heat.

A switching regulator is probably your best bet, but building one that works directly from the mains isn't easy so I'd go for one that will run directly from the output of the transformer.
https://www.romanblack.com/smps/a03.htm