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I have a 555 timer set up in a standard monostable oscillator configuration. Nothing fancy, no modifications whatsoever. I'm driving it with about 12V. If I trigger the 555 with the ground directly, everything works as planned. However. if I connect the collector of a transistor to pin 2, the emitter of that transistor to ground, and the base to a positive pulse, the 555 triggers, but then it quickly overheats and dies.

Of note, the circuit triggered by the number 3 pin is an RC circuit where C=940uf and R=10k, and when the timer switches off, pin 3 drains this fairly large capacitor. However, with a 9V battery, there's no overheating, and no dead chip. The overheating only happens with 12V, which I'm getting from a hobby power supply normally used for a train. My meter reads ~11.5V on the supply.

Why is my 555 chip overheating and burning out? Everything looks within specs, and it doesn't happen if I use a 9V battery. circuit diagram

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  • \$\begingroup\$ Do your resistor values read correctly? I.e., verify the resistances of the actual resistors. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 19 '14 at 16:06
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    \$\begingroup\$ Please draw better schematics in the future: Label the pins with their functions (!), signal flow from left (input) to right (output), positive power supply at the top, negative supply (ground) at the bottom. Don't use the physical pin layout of the IC in the schematics, place the pins according to function (inputs left, outputs right, power top and bottom). \$\endgroup\$ – starblue Oct 20 '14 at 14:32
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The overheating only happens with 12V, which I'm getting from a hobby power supply normally used for a train. My meter reads ~11.5V on the supply.

I have a suspicion that the sort of power supply used for a model tranin may be un-smoothed rectified DC. You meter reads 11.5 VDC but the peak voltage may be above the 18 volts specified as the maximum for the NE555DR.

If you used an electrolytic capacitor on the output of the power supply, you could check this fact out. My suspicion is that the DC voltage would rise to something above 18 volts.

The average value of a full wave rectified sinewave is 0.637 x peak value therefore, the peak value for an average of 11.5 volts is 18.05 volts.

It's worth checking.

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I'm with Andy on this one. To fix it, I'd recommend a 5V regulator powering the 555 circuit. You could replace the 12v train supply with a regulated supply, but once you start running trains on the track you'll find significant spikes and noise which might also damage the CMOS 555.

Use a 7805 or similar linear regulator with an input diode and capacitor and output capacitor to give your circuit clean 5v regulated voltage. The input diode will prevent running the regulator backward during troughs in the input supply, and allow the capacitors to fill those troughs just for this circuit.

The transistors you are driving are current based, so they don't have to be on the same power supply as your circuit, if you need them to drive 12v LEDs that's fine. When the output of the 555 is high, they will turn on to saturation with the correct resistors. When it's low, they will turn off. If, at some future time, you drive other items with them, particularly inductive items such as motors, sole pods, or relays, you'll want to follow the guidelines for driving inductive loads with transistors (fly back diodes, etc). LEDs won't pose a problem though.

If the PIR sensor needs 5v, you are powering it incorrectly. As-is you are giving it 7v, and the power is as noisy as the 12v supply. I'd expect this sensor to have problems sensing correctly when trains are running. Attach it to the regulated 5v as well.

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  • \$\begingroup\$ Yeah you're right about the sensor. It's actually not in the circuit yet so I'll fix that. I will try a regulator. I was attempting to keep the parts count down since it's a one off but oh well. \$\endgroup\$ – Fadecomic Oct 19 '14 at 17:57
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You are forgetting that a 555 can sink current and outsource it. Look at your output line pin 3. You are driving a large load and when your output goes low, those 2 big 470 uF capacitors are feeding current back to your output. Put a diode in line with pin 3 to stop that and maybe your overheating problem will go away. But as the others said, running a 555 on 12 volts is running it near its maximum capabilities and thats not good engineering practice.

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