555 timer -> Relay -> Light problem
- From: "Echinos" <echinos@xxxxxxxxx>
- Date: 5 Sep 2006 20:59:30 -0700
I have set up a one-shot 555 timer to trigger a relay for a few
seconds. With just that set up, it works fine; I can hear the relay
turn on and off. With an LED connected to the relay output, it turns
on and off as expected, along with the relay.
Finally, I hooked up a car fog light that draws 4A, and the light and
relay will turn on, but not off. When I disconnect the fog light from
the relay, then I hear the relay switch off. I can then reconnect the
fog light, and retrigger it again, but the relay again does not
deactivate.
Notes: I have no transistor between the 555 output and the relay. I do
have a 'kickback' diode on the coil side of the relay. Both the 555 and
the fog light are on the same power supply, which happens to be the 12V
output of a PC power supply. The 555 is triggered by a momentary switch
between GND and the trigger pin, and the trigger pin is tied high
through a 100K resistor. The relay is a 12V relay that is rated to
handle 5A on the contact side.
My guess is that the 555 is being retriggered somehow, as if the fog
light is causing a trigger signal to the 555, but I'm not sure why the
200mA of the LED isn't doing anything, but the 4A of the fog light is.
Any ideas?
.
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