Archive: 12v Relay timer

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Søren

Joined Sep 2, 2006
472
Hi,

in the datasheet for the 555s I use, it says 200mA
Of course, must have been thinking of the CMOS version or whatever, I hadn't been sleeping for around 45 hours :rolleyes:
Sorry for the confusion.

It might be a bit less at 12v, i don't know I'm not an expert.
The max. is merely a sort of compromise, less current means less voltage drop. Some datasheets only shows the voltage drop curves to 100mA (pros don't use devices at their max. ratings anyway), but at 200mA, more than 2V will be dropped in each direction, so at 12V, the output will be around 2V to 10V (actually that's a bit optimistic).


The point is, you don't need a transistor to switch on a relay from a 555, which i think we're all in agreement.
Depends on the relay I'd say ;)
But yes, a small relay with a coil impedance of say 100 Ohms and up will be OK, but with inductive loads, never go to the limit, or you'll be sorry.
 

mozikluv

Joined Jan 22, 2004
1,437
hello everybody,

as a reminder, when you use a 555 timer on a 12vdc supply, just make sure that what you are buying has a maximum supply input of 18vdc. there are 555s that has maximum supply input of 12vdc only.

moz
 

Murod

Joined Dec 24, 2005
30
I don't know if some data sheet specify more than 12 V for the maximum supply input, but I agree with Moz, because I always get my 555 (from various manufacturer) hot when I apply a stabilized 12 Volt supply, I don't recommend 555 for more than 9V supply.
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bloguetronica

Joined Apr 27, 2007
1,423
You need to find out how much current the 555 can sink. If it is higher than the relay needs then you need to buffer it with a transistor. I have a habit of buffering the output anyway, probably a good idea even when the 555 can drive the relay directly.
Actually, the LM555, or NE555 can source or sink up to 200mA. You can use 100mA for safety.
 

bloguetronica

Joined Apr 27, 2007
1,423
One last sugestion. You should add a reversed rectifier or signal diode to the coil terminals of the relay. This diode is essencial to protect the 555 from reversed voltage spikes generated in the relay coil.
 
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