555 timer overheats

Thread Starter

dgogate07

Joined Mar 25, 2020
15
Hi,
I have attached the circuit diagram of my setup.
I want to drive the relay for about 3 seconds after about every half an hour, as per one of the online calculators I have selected components:
+VCC = 9V
R1 = 470K
R2/R4/R5 = 1K
R3 = 2.2K
C1 = 4700mf
Both Transistors are BC547 NPN Transistors.

Now, after I power the circuit, as soon as the IC triggers for the 1st time, it overheats and gets damaged. does anyone know why it happens?
 

Attachments

wayneh

Joined Sep 9, 2010
16,275
Hi,
I have attached the circuit diagram of my setup.
I want to drive the relay for about 3 seconds after about every half an hour, as per one of the online calculators I have selected components:
+VCC = 9V
R1 = 470K
R2/R4/R5 = 1K
R3 = 2.2K
C1 = 4700mf
Both Transistors are BC547 NPN Transistors.

Now, after I power the circuit, as soon as the IC triggers for the 1st time, it overheats and gets damaged. does anyone know why it happens?
Looks ok at first glance. I suggest you double and triple check the pin out. Maybe show us a picture - another set of eyes might see something.
 

Thread Starter

dgogate07

Joined Mar 25, 2020
15
Schematics are easier to read if the component values are given:
View attachment 202406
4700mF seemed too large, so I interpreted as 4700uF.

Maybe the lack of a snubber diode is damaging the timer??
I have not connected snubber diode across the relay, but do you really think that can be the reason? as I have connected it to the 2nd transistor not directly to the Pin-3 of the IC.
 

iimagine

Joined Dec 20, 2010
441
That circuit is very inefficient, Q1 spends half an hour doing nothing more but wasting power cutting of Q2. Why not use a PNP to trigger the relay instead?; Or use a low duty cycle scheme:
 

Attachments

Thread Starter

dgogate07

Joined Mar 25, 2020
15
That circuit is very inefficient, Q1 spends half an hour doing nothing more but wasting power cutting of Q2. Why not use a PNP to trigger the relay instead?; Or use a low duty cycle scheme:
Hi,
My requirement is of 15 to 30 min. of delay, so I cannot go any lower than that.
Initially there were no transistor in the circuit, I had directly connected the the relay between +Vcc and Pin-3, but I found that it is getting overheated and hence I introduced those 2 transistors but still issue persists.
I have checked, the relay takes 30mA current and as per the datasheets, 555 can sync about 200mA current.
 

iimagine

Joined Dec 20, 2010
441
My requirement is of 15 to 30 min. of delay, so I cannot go any lower than that.
The circuit that I posted in post # 8 does exactly what you want but is more efficient. The RC timing is still the same, pretty much, but the pulse is inverted so that a second NPN is not needed.
As for why your 555 over heat, i do not know
Does it over heat the instance when you apply the power or when it triggers the relay only?
 

Dodgydave

Joined Jun 22, 2012
8,866
Post pictures of your wiring pcb, firsy start by removing pin 3 resistor and put your Dvm or a led between pin 3 and ground, if all is well, move on to the transistors, use the circuit in post #8.
 

Thread Starter

dgogate07

Joined Mar 25, 2020
15
The circuit that I posted in post # 8 does exactly what you want but is more efficient. The RC timing is still the same, pretty much, but the pulse is inverted so that a second NPN is not needed.
As for why your 555 over heat, i do not know
Does it over heat the instance when you apply the power or when it triggers the relay only?
Hi,
Thanks for your reply.
it overheats when it triggers, not when I power it.
 

Alec_t

Joined Sep 17, 2013
10,918
Initially there were no transistor in the circuit, I had directly connected the the relay between +Vcc and Pin-3, but I found that it is getting overheated
You likely damaged the 555 if the relay coil current is >200mA and/or the coil had no snubber diode. A damaged 555 may well overheat. You should always use a snubber when driving coils (relays, motors) with transistors.
 

Thread Starter

dgogate07

Joined Mar 25, 2020
15
You likely damaged the 555 if the relay coil current is >200mA and/or the coil had no snubber diode. A damaged 555 may well overheat. You should always use a snubber when driving coils (relays, motors) with transistors.
Hello,
based on suggestions here, I connected a snubber diode, but still the same issue. it burnt my IC.
I checked current through the relay, its 30mA.
 

dl324

Joined Mar 30, 2015
10,072
based on suggestions here, I connected a snubber diode, but still the same issue. it burnt my IC.
Try operating the circuit without the relay. I know that may seem pointless, but it will eliminate or identify the relay as the problem.
I checked current through the relay, its 30mA.
How did you measure the current? The measurement could have affected circuit parameters.
 

Thread Starter

dgogate07

Joined Mar 25, 2020
15
Hi,
Thanks for your reply.
it overheats when it triggers, not when I power it.
Hello,

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 and no transistors, it works fine without any issue.
But if I increase the delay, it breaks the IC.
 

Thread Starter

dgogate07

Joined Mar 25, 2020
15
Try operating the circuit without the relay. I know that may seem pointless, but it will eliminate or identify the relay as the problem.
How did you measure the current? The measurement could have affected circuit parameters.
I connected a multi meter in series with the relay to measure the current going through the coil.
 

BobaMosfet

Joined Jul 1, 2009
1,012
Hi,
I have attached the circuit diagram of my setup.
I want to drive the relay for about 3 seconds after about every half an hour, as per one of the online calculators I have selected components:
+VCC = 9V
R1 = 470K
R2/R4/R5 = 1K
R3 = 2.2K
C1 = 4700mf
Both Transistors are BC547 NPN Transistors.

Now, after I power the circuit, as soon as the IC triggers for the 1st time, it overheats and gets damaged. does anyone know why it happens?
Probably because you're using a relay- which causes an inductive VOLTAGE spike. You need a snubber diode around your inductor.
 

dl324

Joined Mar 30, 2015
10,072
I connected a multi meter in series with the relay to measure the current going through the coil.
That can perturb the circuit and give you erroneous readings. DVM's use the voltage across a resistance to measure current. This is sometimes referred to as "burden voltage". The resistance from the meter depends on the current range used.

My Simpson 467 can insert up to 1000 ohms into the circuit.
1585330337927.png
 
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