Re: 555 timer -> Relay -> Light problem

On 5 Sep 2006 20:59:30 -0700, "Echinos" <echinos@xxxxxxxxx> wrote:

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?

If the fog light is causing the 555 to trigger (and it might well do
so, on the same supply and all) you should hear the relay click
periodically. Do you?

What is happening to the power supply voltage while this is going on?

Are the contacts of the relay good for the current? Incandescent
lamps have a very low resistance when the filament is cold, so they
can draw lots of current until they heat - the relay contacts may be
welding themselves shut (a series inrush current limiter would fix
that) for instance a 100 watt bulb may suck 20 amps (>2000 watts) for
a few milliseconds.

If the light is pulling down the power supply, put a large
electrolytic (>1000 ufd/16 volts) on the 555 circuit and a forward
biased diode just supplying current to the monostable (not the fog
light).

555's need stable power - with no electrical noise to work properly

If this is going in a car - the problem might be better with a low
impedance power supply (battery) or worse - more electrical noise.

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