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# My 555 IC overheats Answered

I am trying to construct astable multivibrator using 555 IC that will drive my relay. I am trying to make it trigger every 15 min. but when ever it triggers, the 555 IC get overheated, I have burnt almost 10 of them. any idea what could be the reason? If I don't connect anything to pin-3, it works very well.

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The values have helped a lot: R3 = 2 k 2

Here is the heuristic approach:
(Calculations are not for me).

Or if you have a potentiometer (i.e. variable resistor) lying aroud, try that (maybe 100k) linear instead of R3 and then turn it DOWN until the relay snaps.

Check what value you have and solder in the nearest value resistor.
Should work out at much more than 2.2 k, I think

Yeah. Josehf Murchison is right. Pls. post the values of R3 R4 R5 and the names of the Ts you used. Would really help.
You also might try this strategy:
Increase R3 100fold (i.e. if it was 1k brown-black-brown, make it 100k brown-black-yellow).
If the realy still works, the 555 should stay cold enough. That is the solution.
If it doesnt work, try 10fold (i.e. 10k brown-black-orange)...

You might also try a cooler on top of the 555.
Very unusual. But if it works, it's OK ;-)

Hi,
here are the values of resistors and capacitor:
R1 = 470K
R2/R4/R5 = 1k
R3 = 2.2K
C1 = 4700μf
Both transistors are BC547 NPN transistors.

Consider a Darlington output opto isolator and run the 555 timer on 5 volts via a regulator. The optoisolators output transistors the drives your relay drive transistor whose collector is connected through the relay coil which in turn is connected to the power supplies unregulated voltage.
another solution might be a diode in series with pin 3, there are also optoisolators with FET output stages which have surprising high current capacity because of the very low on resistance fets.

1 possibility I can think of. The relay coil maybe spike a back emp when the circuit turns off damaging the timer, some relays have a reverse bias diode or resistor to protect the rest of the circuit some do not.

Hi,
this time I connected the "flyback diode" as well, but still result was the same. it burnt the IC.

What is your voltage on Pins 8 & 4?
Some relais want 12V... 555 will die with 12V...

Hi,
It was connected to 12v DC supply which actually delivers 13 Volts.
my relay is a 12 volts relay but the pick up voltage of its coil is 9 Volts
Hence I bought another power supply of 9 volts which actually delivers 9.95 Volts. Now my IC and relay both were working on 9 Volts but still the same, The IC burns as soon as it triggers the relay.
Looks like the relay was a culprit, even after connecting a fly back diode, my IC was burning. I tried running it without the relay and it works well without the relay.
now I am considering to use a PNP transistor instead of 2 NPN transistors. but I need a PNP transistor which can support 1Amp. current.

Jupp. As Jack A Lopez already pointed out: the voltagespikes of a Relais could damage the 555 and needs to be clamped with a Flyback.
That would be my guess on why the 555 gets hot and becomes damaged...

Hi,
Thanks for the reply. To verify if that is the case, I changed R1 and R2 and to 1K each and C1 to 4700mf this gave me TH of about 7 seconds and TL of about 3 seconds and I connected the relay directly between +VCC and Pin-3 of the IC, and it works like a charm. it drives the relay very well without any issue, but it heats up when the delay is increased. dont know why.

My guess is that you have chosen a too small value for one of the resistors, R1, R2, or R3. How small is too small? Perhaps 1K =1000 ohms, is about the smallest any of those should be. Especially if the output, pin 3, is on, supplying current , most of the time, and only turns off for brief moments.

Hi,
Thanks for the reply. here are my components:
R1 = 470K
R2/R4/R5 = 1k
R3 = 2.2K
C1 = 4700mf

I have checked, the current going to the base of transistor from Pin-3 of the chip is fairly small.

I changed R1 and R2 and to 1K each and C1 to 4700mf this gave me TH of about 7 seconds and TL of about 3 seconds and I connected the relay directly between +VCC and Pin-3 of the IC, and it works like a charm. it drives the relay very well without any issue, but it heats up when the delay is increased. dont know why.

I hope C1 is not 4700mF but 4700uF / 4.7mF which is still HUGE.
Maybe 4700nF / 4.7uF? that would make more sense...

Hi,
yes, it is 4700uF and not 4700mF, sorry for that typo.

This all seemed reasonable, up until the point where I read the words, "I connected the relay directly between +VCC and Pin-3 of the IC,"

I think doing that would be bad, because pin 3 would have to sink all that relay coil current by itself. I mean, that network of two transistors is there so a transistor can switch the the big current to the relay coil, and the current pin 3 has to source or sink can be much smaller.

Hi,

I checked current through the relay and it is just 30mA and hence it works very well.
but if I increase the delay, it creates problem.

What is the duty cycle?
Which 555 IC, they are not all the same?
The schematic needs to have the values for a better answer however 15 minute charge on the capacitor and then all that discharge in one moment.
That is a lot of current.

Hello,
here are the component details:
R1 = 235K (2 470K resistors in parallel)
R2/R4/R5 = 1K
R3 = 2.2K
C1 = 4700μF
This gives me High Time of about 13 min. and Low Time of about 3 seconds.

As a test, I also tried one more combination,
I replaced both R1 and R1 with 1K resisters keeping the capacitor the same, this gave me a High Time of about 7 seconds and Low Time of about 3.5 seconds. and then even if I connect the Relay directly between +Vcc and Pin-3 of the IC, it works fine without any issue.
But if I increase the delay, it breaks the IC.

The Capacitor is discharging too much current through the 555 timer.
You could drop the capacitor value but you would need to up the resistors in the mega ohms.