BC547 overheating

Thread Starter

haroontheengineer

Joined May 29, 2022
10
Hi guys! I'm trying to make a relay module to control appliances using NodeMCU but the problem I'm facing is overheating of the transistor and it stops working. Your help will be appreciated.
Image of my circuit is attached.
1653801873436.png
 

ag-123

Joined Apr 28, 2017
263
use a multimeter and measure the Vce voltage across BC547 when it is turned on, if it is rather high like closer or > 1 v, much higher currents is passing in the transistor. An 'easy' solution is to place a resistor like 10 ohms at the emitter.
 

bertus

Joined Apr 5, 2008
22,118
Hello,

As said, you need a suppression diode.
What is the output voltage of the MCU?
If it is lower as the mentioned 3.3 Volts, you might need to reduce the value of the 500 Ohms resistor.

Bertus
 

Thread Starter

haroontheengineer

Joined May 29, 2022
10
use a multimeter and measure the Vce voltage across BC547 when it is turned on, if it is rather high like closer or > 1 v, much higher currents is passing in the transistor. An 'easy' solution is to place a resistor like 10 ohms at the emitter.
I have replaced the transistor. Now the transistor sometimes switches on but the base current remains the same even though the transistor doesn't work i.e. almost 5 mA. Is this a transistor issue?
 

ericgibbs

Joined Jan 29, 2010
16,748
hi har,
The Gain of the BC547 is approx 100 at 100mA, so 5mA should switch On the transistor.
Can you post a photo of the layout of your circuit so that we can check the connections.?
E
 

Papabravo

Joined Feb 24, 2006
19,560
I estimate the power dissipation in the on state to be about 8 mW. The only thing that could cause heating of the magnitude that you are observing is an error of some type.

1653837034723.png
 
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