questions about a DC controlled AC switch

Hi all,
I am building a DC controlled AC-switch along the lines of this Controllable Power Outlet - SparkFun Electronics

see also my previous thread about the choice a relay here:

https://www.edaboard.com/threads/202070/#post862607

The major differences are that I am using a flanged-mount relay to keep the AC off the circuit board, and that I added a monostable-configured 555 to keep the relay on for about 30sec - 1Min.

I am wondering about two things.

1)
I am planning to use batteries driving a 7805 to power the DC control circuit. The power requirements of the relay coil are such that batteries would last long enough with standard usage.

The problem is: should I protect the batteries and or the 7805 with anything other than the diode in parallel with the relay coil? Do I need a fuse on the DC side or an additional diode to prevent backward current flow?

2)
I am thinking of enclosing everything into a plastic box and letting the AC come in and out using a standard AC extension cord. The neutral wire being bridged directly from input to output, and the phase wire going through the relay.

On the DC board, among the other things I would have a potentiometer to tune the 555 monostable period. I wonder if I can let the plastic rod on the potentiometer exit the box, or if I should leave it inside to avoid risks. The only way I see for AC reaching it is a failure of the relay.

Thanks to all for your help


Davide

I do not see the requirement for the 7805. If you use a 6V dry cell lantern battery or such it will soon run down, the relay should be able to handle the extra voltage. If you are using a 12V car battery, I would use a 12V relay. A 7805 or such like require more then 5V into them for them to provide 5V out. I would check this on its data sheet, else you might need a 8V battery! A 555 will work up to 15V.
Frank

Hi,
relay is quoted as being able to handle no more than 120% rated voltage. So that makes no more than 6V. Thus to be safe I thought of using 6 1.2v NiMh batteries to feed the 7805 (it needs 2V more than the output).

But maybe I could just put a diode in series with only 5 batteries to make sure I don't go over 6 and get rid of the regulator.

any ideas on my other questions?

chuckey said:

I do not see the requirement for the 7805. If you use a 6V dry cell lantern battery or such it will soon run down, the relay should be able to handle the extra voltage. If you are using a 12V car battery, I would use a 12V relay. A 7805 or such like require more then 5V into them for them to provide 5V out. I would check this on its data sheet, else you might need a 8V battery! A 555 will work up to 15V.
Frank

Click to expand...

You may have a problem with the NiMH batteries, you must use a circuit to protect them from Over-discharging or they can be damaged.

Nickel-metal hydride battery - Wikipedia, the free encyclopedia
"A complete discharge of a cell until it goes into polarity reversal can cause permanent damage to the cell. This situation can occur in the common arrangement of four AA cells in series in a digital camera, where one will be completely discharged before the others due to small differences in capacity among the cells. When this happens, the good cells will start to drive the discharged cell in reverse, which can cause permanent damage to that cell"

Alex

thanks alex, good point.
Do you have any pointers discussing how to protect them?

No I'm sorry, I have no links and i haven't found anything in Google that could help.

Alex

alternative would be to step down 220v to 5vdc, but I've never done that and I wanted to keep things simple enough.

It would make thinks simpler i think if you get a small pcb transformer,
you only need about 100mA , a small rectifier and cap and you wouldn't have to bother
with chargers batteries etc.

Alex

yep, as I said above I had been thinking of that. Batteries were more for simplicity than anything else, but if it gets to complex then I can power everything from the AC. However, this being my first real AC project, I wanna make sure I don't forget important stuff.

To start with. I was gonna use a simple prototyping board for the DC circuit and I chose a flanged relay to avoid having to bother about putting the mains voltage on the board.
Can you recommend a transformer to use and a circuit?
Even better if there was a prebuilt unit that I could plug into my circuit that wouldn't cost too much .

You would have to check your local stores for that, i usually go to local stores in my town for transformers
so I don''t have any specific type to recommend.
There are many online stores if you think they suite you, you can probably find many models in their pages.

Alex