AC failsafe.

Thread Starter

moorejohn90

Joined Dec 9, 2011
57
Hi everyone, I want to switch 40W AC light bulb from "Arduino".
There is the circuit diagram with attachment.
I draw that diagram base on diagram from three websites.
I want to know, my diagram is correct. I think it is correct.
But my dad is worry about my circuit(if it is wrong) of burning wire that are run inside ceiling and wall.
Am I need to add fuse, and if is it need where to add that in my diagram?
Tell me also the type(Amp or something else) of fuse to add.

John
 

Attachments

praondevou

Joined Jul 9, 2011
2,942
Are you going to use the wires that already are in the ceiling / walls? In this case there will be a fuse / circuit breaker in the electric panelof your house/apartement.

One thing you have to make sure when installing the circuit you proposed is that low voltage and high voltage parts are physically well separated, wires cannot become loose and touch the other part etc.

It would even be wiser to let an electrician give you the proper advise on how to do it.

As for the circuit itself does the Ardunio have a current limiting resistor onboard? (never worked with them) If it doesn't you need a base resistor. In this case it would be interesting to know the coil current of the relay.
 

Thread Starter

moorejohn90

Joined Dec 9, 2011
57
Hi, thanks to both of you of replying my post.

I live in Myanmar(Burma). Most of the electronic shop sell electronic component with model No(without give specification document). It is difficult to buy correct item if the device's model No doesn't know. Example, they don't know when I ask 1000 gain transistor, but they can give when I ask TIP120. So I don't know the current of relay which I bought. Is there any way to know current of relay(eg with multimeter).:)

John
 

praondevou

Joined Jul 9, 2011
2,942
Hi praondevou, Am I need to supply current(with transistor) more than relay current (measure with multimeter).
John

Just apply the rated voltage to the relay. Current is determined by voltage and relay coil resistance. Measure the DC current through the coil. No need to connect the transistor.
 

Thread Starter

moorejohn90

Joined Dec 9, 2011
57
Hi praondevou,
" Current is determined by voltage and relay coil resistance"
Voltage is 12 V.
Current is the same as current that are measure with multimeter.
I can't know the resistance cause of lacking document.

John
 

praondevou

Joined Jul 9, 2011
2,942
Hi praondevou,
" Current is determined by voltage and relay coil resistance"
Voltage is 12 V.
Current is the same as current that are measure with multimeter.
I can't know the resistance cause of lacking document.

John
Actually when you measure the current at a known voltage you will calculate the coil resistance with Ohms law.I guess I confused you. :)

Preferably one would need to know the current through the relay to determine the base resistor of the transistor. However, since this is probably a small relay with no more than e.g. 200mA coil current the base resistor value will not be so critical. Just throw in 10k and see if your relay works.

Do your tests without AC connected. In fact, test the whole AC light bulb circuit with a lower voltage first.
 

Thread Starter

moorejohn90

Joined Dec 9, 2011
57
Hi praondevou, Thanks for your explanation.
Yep, I will test my circuit with DC 12V computer's fan before with AC bulb.
Thanks
John
 

Audioguru

Joined Dec 20, 2007
11,249
Measure ther relay coil resistance with a multimeter. Divide the supply voltage by the resistance (ohm's Law) to find its current. Duh, simple.
 

Georacer

Joined Nov 25, 2009
5,182
Can we be sure that small arcing and sparking while the relay bounces while closing won't be a problem for any run-of-the-mill relay?
 

strantor

Joined Oct 3, 2010
5,333
I have used 1.2k base resistor when interfacting arduino with TIP120. it works. I would think that if the relay is rated for 7A 250V, it should be fine for a 40W bulb. Why would you suspect contacts bouncing? He's drawn the flyback diode correctly so it should work as its supposed to.
 

praondevou

Joined Jul 9, 2011
2,942
I have used 1.2k base resistor when interfacting arduino with TIP120. it works. I would think that if the relay is rated for 7A 250V, it should be fine for a 40W bulb. Why would you suspect contacts bouncing? He's drawn the flyback diode correctly so it should work as its supposed to.
Yeah that relay contact rating looks ok for a 40W light bulb.
However, bouncing does always occur as far as I know, doesn't it?
 

strantor

Joined Oct 3, 2010
5,333
However, bouncing does always occur as far as I know, doesn't it?
Yes, it is the natural behavior of relays as I understand it, but this is factored in to the rating. I'm not understanding why this contact bouncing is being mentioned as if it is a concern over and above the rating.
It sounded to me like Georacer was referring to relay chatter, which shouldnt be an issue if OP connects the diode as shown
 
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