Re: 555 timer -> Relay -> Light problem


default wrote:
On 6 Sep 2006 08:43:23 -0700, "Echinos" <echinos@xxxxxxxxx> wrote:

The lamp draws 4A when connected directly to the 12V power source, and
the relay is rated for 5A. The relay is still OK, it works fine if I
remove the lamp.

I guess I'm going on the assumption that the 555 is sitting there fat
dumb and happy until a negative trigger comes in to pin 2 (if my
memory is right) then the relay pulls in and light comes on for what
should be a short period of time - but just stays on. Have I got that
right? Relay and 4 amp light is working on normally open contacts?

Correct-a-mundo.


What initiates the monostable timing cycle? A switch or some other
circuitry?

For now, just a momentary pushbutton. Later on it will be some sort of
sensor or limit-swith type of thing. All this is on a breadboard atm.


I dunno about that 5 amp relay. I was referring to a 120 vac 100 watt
lamp - reads about 6 ohms until it heats up, then it is over 100 ohms
- halogen lamps are worse in that respect (smaller thicker filaments
designed to run hotter).

Aha - this is a halogen lamp, I believe. Could be making the problem
worse.

I really don't understand it not at least clicking - drop out or try
to, and the 555 re trigger immediately on the noise. That's what
prompts me to ask about the relay.

A good thing to try would be to connect the light - trigger the
monostable then physically disconnect the light, while the circuit is
powered up and see if the relay drops out.

Yes, indeed, that's what happens. If I trigger the light, it turns on
and stays on, and if I disconnect the lead going from the relay to the
light, I hear the relay turn off immediately. I can then re-attach the
light, and retrigger it, with the same results.

Depending on the state of your parts supply . . . Why not just
eliminate the relay and see what happens? Use a power transistor to
control the lamp. With 12 volts and a standard 555 you might have
enough drive to saturate a switching transistor directly - you'd sure
have enough to turn on a mosfet.

Basically, I'm working with what I have atm. I have no problem going
and getting parts if I need them, but I had seen this described as a
way to control higher current from a 555, so it just where I've
started. I have tried a power transistor, but it did not seem to work.
I think it might have to do with the fact that it is a halogen light.
The power transistors I happen to have are the TIP31 and TIP32. I don't
think they can handle 4A, but I might be able to use a different lamp
with less current.

I built a solid state flasher for my motorcycle - can flash from 200
ma to over 10 amps using a single 30 amp mosfet with no heatsink - I
cut the tab off the TO220 case to save room.

Impressive. I imagine such a mosfet might be a little pricey?

Cheers

.