Help! Circuit on ac/dc switching

I just recieve a project that required me to do ac/dc switching. The ac will be 230v, 50hz to power a application. When the ac power fail, a circuit from dc will pump in 12vdc to the application. There is 2 application connected thru the same ac and dc wiring (2 wire, AC/DC & GND), output (3 wire, AC, DC & GND). But the circuit i am going to design will switch to the ac application once the circuit detect ac and switch to the dc application once the circuit detect dc. I am trying to use a relay, but the problem is thats the switching time for a relay is not fast enough. The dc application cannot take in high voltage and high current from the mains and will blow in nano seconds when the mains power go in. Any recommendation to the circuit that will protect the dc application and will do a fast switching once it detect a change of power input(ac or dc).

Thanks, any help will be appreicated!

I am not sure I understand the problem correctly, maybe you can be more specific about what "application" means.

If you are referring to a sort of power backup system, then generally the circuit operates from a DC source. This DC voltage can be produced by either a battery or by an AC/DC power supply. Simply OR-ing together through a couple of diodes the battery and the output of the AC/DC power supply solves the problem. What remains is to detect the AC presence and turn on the charger to charge the battery.

Anyway, if this does not resemble even remotely your application, please post more details.

VVV said:

I am not sure I understand the problem correctly, maybe you can be more specific about what "application" means.

If you are referring to a sort of power backup system, then generally the circuit operates from a DC source. This DC voltage can be produced by either a battery or by an AC/DC power supply. Simply OR-ing together through a couple of diodes the battery and the output of the AC/DC power supply solves the problem. What remains is to detect the AC presence and turn on the charger to charge the battery.

Anyway, if this does not resemble even remotely your application, please post more details.

Click to expand...

Erm.. nope let me explain it more clearly.... It is for emergency use. During normal usuage, AC will kick in and power up the ac application. During power failure, the charged battery(DC) will kick in the same power line to power up the DC application(AC application will be off). When ac shut down, dc come in and power up the dc application is not a problem. I can use a relay to switch it. But when the ac main recovers and cut off dc, the relay will not be fast enough to switch to the ac application and thus kill the dc application instantly. Is there a way to sense and block ac power till the relay is been switch to ac application?

Can you explain what the "applications" are?

Why can't you use two relays? One connects the AC voltage to the "application", the other the DC voltage. When the AC fails you first diconnect the AC line from the application and then connect the DC. And when the AC recovers you first turn off the DC relay and then turn on the AC relay. Thus there is no need to "kill the DC application instantly", because in either case you have lots of time to switch the correct voltage to the "application".

Or, if the AC kills the DC, then why don't you use another set of contacts on the DC relay to actually turn on the AC relay. That way, when the AC recovers it turns off the DC relay, which then turns on the AC relay and so the two voltages will never be applied to the load at the same time. If needed, you can insert a little more delay with an electronic timer.

Whichever logical circuit you select, consider using solid state relay for switching AC and P-MOSFET for switching DC. This will ensure very short switching time and may make your logic circuit much simpler.

Same topic?

https://www.electro-tech-online.com/viewtopic.php?t=13623

E-design said:

Same topic?

h**p://www.electro-tech-online.com/viewtopic.php?t=13623

Click to expand...

Yup, same topic posted by me. Lol, just hope to get more help.

You might use a single transistor, or relay. The relay works when the ac mains is live, when the ac vanishes, the relay turns off, and connects the 12 DC supply.

Regards