What's wrong with the circuit?

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Sunny55

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Hi, i am working on this circuit and the problem i faced is the relay does not shift and sometimes the relay also get 'confused'(Not working like it used to be). Here is the circuit. **broken link removed**

The relay i am using is a 12VDC 5 pins relay and power supply to the circuit is around 13VDC. The circuit works perfectly sometimes and then it does not work.

Can anyone please help me ?

Thanks
Sunny55.

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I am using just R1 and R4( Pots R2 and R3 not used). Regarding the relay's switching problem, could it be the problem with the right side of the circuit?
 

some time work perfect ?
if so, it might not connected your connection .
 

What is the actual problem:
  • Transistor switches on, but relay contacts do not close?
  • Transistor does not switch on when it should?
  • Something else?
 

godfreyl said:
What is the actual problem:
  • Transistor switches on, but relay contacts do not close?
  • Transistor does not switch on when it should?
  • Something else?
Click to expand...

The relay works nicely sometimes and sometimes does not switch. How to verify if a transistor switches nicely?
 

You can check two things with a multimeter or voltmeter:
  • Output from the cmos chip at pin 11 should go to high voltage (about 10V) to switch transistor on.
  • Collector of transistor should go down to low voltage (about 1V) to switch relay on.
 

Ok. I will measure and see. Today, i have tested the board again and this time the relay got stucked. It made sounds as if itd trying to switch but stuck . Any idea how can can test the components which are soldered on a stripboard?
 

Hi how can i do on circuit testing of the components ? Meaning testing each components that soldered on board. Pls helpUrgent.

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Hi i got voltage at pin 11 > collector voltage but not same value. Pin 11 gives 8.2v while collector is 7.45v.

godfreyl said:
You can check two things with a multimeter or voltmeter:
  • Output from the cmos chip at pin 11 should go to high voltage (about 10V) to switch transistor on.
  • Collector of transistor should go down to low voltage (about 1V) to switch relay on.
Click to expand...
 

Sunny55 said:
while collector is 7.45v.
Click to expand...
That's the problem. Supply voltage is 13V, so voltage across relay coil = 13V - 7.45V = 5.55V. That's not enough to reliably turn on the relay.

I'm surprised. If R5 is 2.2K and pin 11 is at 8.2V, then there should be about 3.5mA base current for the transistor. That should be enough to turn it on hard.

Are you sure you didn't use 22K by mistake?
What transistor is it? (maybe beta is too low)
What's the resistance of the relay coil? (maybe resistance is too low)
 

Hi, thanks alot for the answer. The relay according to the circuit construction shows that it receives also 13V. It shows that the power supply is given to both timer circuit and the relay.

As for the resistor's value, i am sure it 2.2K, transistor model is BC547 and coil's resistance is around 360ohms.

Hope that helps.


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I start to worry that component(s) might not be working. How can i do on-circuit testing for each components ?
 

Sunny55 said:
As for the resistor's value, i am sure it 2.2K, transistor model is BC547 and coil's resistance is around 360ohms.
Click to expand...
That should work fine.

Sunny55 said:
I start to worry that component(s) might not be working. How can i do on-circuit testing for each components ?
Click to expand...
From the measurements you already made, it looks like the transistor isn't working properly. Maybe it's connected the wrong way around, with the collector and emitter swapped. If that's the case, it still works as a transistor, but with very low current gain.

BTW: when there's 8.2V on pin11, what's the voltage on the base of the transistor?
 

BTW: when there's 8.2V on pin11, what's the voltage on the base of the transistor?[/QUOTE]

Base is connected to the 2.2K resistor. Should be the same voltage of pin 11.
 

Nope, it should be about 0.7V, so the voltage across the resistor = 8.2 - 0.7 = 7.5V and the current through the resistor to the base of the transistor = 7.5V/2.2K = 3.4mA.
 

Sorry for that answer. I will measure it. What is the possibility of damaged ic and other parts like diodes or relay? Caps and resistors should be strong enough for handling 13v.
 

godfreyl said:
Nope, it should be about 0.7V, so the voltage across the resistor = 8.2 - 0.7 = 7.5V and the current through the resistor to the base of the transistor = 7.5V/2.2K = 3.4mA.
Click to expand...

Hi, just want to ask regarding the protection diode in parallel with the relay, should the diode be placed as close as possible to the relay? Does the position makes any difference in operation and performance?

Thanks
Sunny55.
 

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