power supply circuit

[sherazi]

hi friends...

i want to make a power supply unit for my project, i want 5vdc,12vac and 24v ac from it, i am using a 12+12V transformer, and attached is the schematic... is it going to be OK, or should i make some changes

 

[(*steve*)]

the 12VDC will almost certainly be significantly higher that 12V and will vary with load.

You don;t give any specs for the PTC thermistor nor for C4. the DC voltage should be taken from after the thermistor, not before it.

You would probably be better off with a 12V regulator if you need something close to 12V.

Your 0V and ground are not the same. Depending what you are going to use the 24VAC for, the unusual coupling of one end of it to the DC rails may cause problems.

 

[sherazi]

c4 is 1000uF, i have removed R3,and LED2 they arent required,


what i want is a supply that would be used for 5Volts for microcontroller and related circuits ... and a power source for photo sensors for obstruction detection that runs either on 12v Ac or 24VAc, i would hence use a slide switch to switch b/w 12V and 24V for the purpose...

 

[(*steve*)]

F1 will cause you to have more ripple on the 12V rail than you would otherwise have.

Does the 12V rail have to be regulated? It will vary up to about 17V depending on load and may have substantial ripple.

What current do you require from these rails?.

 

[sherazi]

the F1 is not even necessary i just added it for having a better result, but its not so, so may even remove it... i expect to have a small current on 5v, there is a microcontroller, a rs 485 driver, and a few leds, of which at-most three would be on at a time..and 4 relays and two MOC3022, thats it,

the 12vAc and 24Vac needs less than 200mA for photo senor, the 12V dc would be given to relays ..

 

[(*steve*)]

Well, like I said, the 12v is likely to be more like 17V (maybe even higher) so if this is beyond the spec for the relays then you may need another regulator.

 

[sherazi]

well i have used it with 12v relays and it didnt made any problem,... but i would switch to 24 V then for precaution...

 

[sherazi]

Your 0V and ground are not the same. Depending what you are going to use the 24VAC for, the unusual coupling of one end of it to the DC rails may cause problems.

so what should i do to solve this?.. i need 24V AC.[that has to be taken from 0V and 24VAc] for the photo sensor and i have to use the same transformer....

 

[(*steve*)]

well i have used it with 12v relays and it didnt made any problem,... but i would switch to 24 V then for precaution...

Just check that the relays and any associated components are OK for 18V. 24 volt relays would not be my choice. The specs for most relays allow them to operate over a range of voltages and these may be quite OK operating from the higher voltage, but it is best to make sure.

so what should i do to solve this?.. i need 24V AC.[that has to be taken from 0V and 24VAc] for the photo sensor and i have to use the same transformer....

As long as the device needing 24VAC is not connected electrically with any other device running from this power supply then you'll be fine. If it does then you need to carefully check that you don't end up shorting you power supply.

 

[sherazi]

what if i rectify this 24V by using a bridge rectifier, and then use it carefully...
after that both the GNd of 24 and 5V would be same and can be connected up isnt it?

by carefully as far as i understand you mean to say that i dont mess up with connecting 24V either to 12 or gnd....