Electronics > Beginners
Single shot 555 timer getting hot!!
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ArthurDent:
Might be a couple of things. First, there appear to be many variations of the ‘555’ and although they should functionally work the same, the circuitry inside is different and can cause many different types of problems. A lot of 555 chips may have what is known as a totem pole output where there are 2 transistors, stacked one on top of the other, between the + and – supply with the output from their common junction. This configuration allows the 555 to either source or sink current. There is an extremely small moment of time when switching that both transistors could be on, producing a short and having a large current flowing through the transistors. The way around this is to wire a, say 22 ohm, bypassed resistor in series with the + for the 555 to limit the current to a safe value.

The second problem is somewhat related to the first and relates to the NE555 you are using. Here is an answer to a question I found on line.   
“You've discovered one of the best kept secrets of the 555. That is, they aren't all the same. In fact, I know which one you have. It's the NE555 from Texas Instrument (silver top, right?), because that's what RadioShack stocks even though the catalog says LM555. The difference is that the LM555 is a bipolar chip and the NE555 is a CMOS chip. And I bet that pin 2 of this circuit is waving in the air — picking up static electricity. When the first CMOS 555s showed up on my workbench, I had the same problem until somebody told me they were CMOS, then the light went on. When working with CMOS technology, every pin has to go somewhere, used or not. What you need to do is tie pin 2 high through a 10k resistor, that's pretty close to its bipolar equivalent, and the circuit will work.”
Zero999:
Perhaps the 555 or MOSFET are bad?

The circuit is more complex than it needs to be.

How much current does the relay's coil need? Can it be powered directly off the 555 timer?

The relay's coil will need a back-EMF suppression diode in reverse parallel with it.

Try this circuit. The 555 can be powered from a voltage regulator, if required and the relay switched using a transistor, if the 555 can't source enough current.
StillTrying:
There's no decoupling very near the 555, so it could be oscillating due to the breadboard coupling of pins 2 and 3.

It's only used as a comparator to detect when C3 reaches 2/3V so Reset and CV decoupling are not needed, use them caps to decouple the 555's supply pins instead. :)
radar_macgyver:
I think the problem is highlighted in the attached image. You've taken care to run the +12V for the lamp through a separate breadboard's power rail, but the ground return for the lamp follows the same rail as the 555. When the lamp draws current, the voltage drop on the ground rail would likely cause the 555 to misbehave. If instead the MOSFET's source is connected to the left breadboard's ground rail, this wouldn't be a problem. Also, the source node of the FET is being shared with the gate pull-down resistor - have that go straight to the ground rail.

Also, consider moving the power input barrel connector to the bottom of the board (near the lamp) instead. Now the load current and the 555's supply share a much smaller path. As @sourcecharge already pointed out, the layout is way too complicated for the circuit you're trying to implement. Simplify the number of connections and it will likely work a lot better.
Unordung:

--- Quote from: ArthurDent on July 28, 2018, 01:05:30 pm ---What you need to do is tie pin 2 high through a 10k resistor, that's pretty close to its bipolar equivalent, and the circuit will work.”


--- End quote ---

Sadly this didn't solve my problem, it was, however, very interesting. I have tied the Discharge pin to vcc via a 10K just as good practice.


--- Quote from: mikerj on July 28, 2018, 11:07:54 am ---It's quite possible the MOSFET is oscillating due to the all the stray inductance in your breadboard layout.  You could try adding a resistor between the 555 output and the MOSFET gate, something of the order of 100 ohms or so would be fine as you aren't switching anything fast.

--- End quote ---

Sadly I thought of this and the same fault happened without the mosfet connected.


--- Quote from: StillTrying on July 28, 2018, 03:32:46 pm ---There's no decoupling very near the 555, so it could be oscillating due to the breadboard coupling of pins 2 and 3.

It's only used as a comparator to detect when C3 reaches 2/3V so Reset and CV decoupling are not needed, use them caps to decouple the 555's supply pins instead. :)

--- End quote ---

I have now added a .1uf cap across the VCC input to the 555 still no dice.



--- Quote from: radar_macgyver on July 28, 2018, 03:51:35 pm ---I think the problem is highlighted in the attached image. You've taken care to run the +12V for the lamp through a separate breadboard's power rail, but the ground return for the lamp follows the same rail as the 555. When the lamp draws current, the voltage drop on the ground rail would likely cause the 555 to misbehave. If instead the MOSFET's source is connected to the left breadboard's ground rail, this wouldn't be a problem. Also, the source node of the FET is being shared with the gate pull-down resistor - have that go straight to the ground rail.

--- End quote ---

I gave this a shot, and no dice sadly. The advice, however, was very good and I will keep it in mind for future projects.



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