how to flip a relay by digital signal

[saurabh9]

Hi,
What would be an easy way to turn on a relay by a digital signal. The
relay would be hooked to a 220 V AC device.

 

[Rich Grise]

On Thu, 10 Jul 2008 04:25:22 -0700, saurabh9 wrote:

Hi,
What would be an easy way to turn on a relay by a digital signal. The
relay would be hooked to a 220 V AC device.

Drive the coil with a suitable transistor. Don't forget the catch diode!

Cheers!
Rich

 

[Bill Bowden]

On Jul 15, 10:21 am, Rich Grise <r...@example.net> wrote:

On Thu, 10 Jul 2008 04:25:22 -0700, saurabh9 wrote:
Hi,
What would be an easy way to turn on a relay by a digital signal. The
relay would be hooked to a 220 V AC device.

Drive the coil with a suitable transistor. Don't forget the catch diode!

Cheers!
Rich

Actually, the catch diode might not be needed. There's lots of
capacitance in the windings of the relay coil which charges up when
the transistor switches off and limits the voltage to whatever the
capacitance is combined with the energy put into it. A 1 millihenry
coil at 12 volts and 120 ohms will store 1/2 L* I^2 = 5 microjoules.
If the winding capacitance is say 2000pF, the voltage will be 1/2
CE^2= .000005 = 70 volts.

Did I do that right?

-Bill