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relays getting automatically "clicked" when connected to AC loads

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buch11

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Dear Sir,
The PDF contains the circuit for device control and we have used the same circuit without any changes.The only addition we have done are the flyback diodes across each relay coil.

The issue we are facing is,when we try to turn on a "tube light" using a relay, more than one relay gets "clicked" and which in turn switches on more than one device.This happens only with tube lights(old pattern ones with choke and starter).When we tested the circuit with bulbs and CFLs ,the circuit performed properly.We have also tested our source program by attaching LED to the parallel port, and we are pretty much sure that there is nothing wrong in the bits being sent to the parallel port.Our circuit has also been tested separately,and the results are positive.

So what possibly might be the cause of this abnormal functioning of the relay when the tube lights are connected as the load instead of bulbs or CFLs?

Regards.
 
The ULN2803 has flyback diodes inside. It is OK that you added mores diodes.
I understand you tested the PC by using LEDs on the parallel port. Have you used the LEDs while the "tube lights" are in use?
 
nope that has not been tested, but i am getting a suggestion to put a resistor between my uln input and ground.will that make a differance
 
IMO RF interference from the lights is getting to the inputs of the 2803. If the suggested R doesn't cure the problem you could try a cap (say 10nF) from each input to ground.
 
IMO RF interference from the lights is getting to the inputs of the 2803. If the suggested R doesn't cure the problem you could try a cap (say 10nF) from each input to ground.
so the configuration would look something like this:
=====================================
from parallel port ------------------to uln
.................................|
.......................|10 nF capacitor|
.................................|
...............Ground -------------------

=====================================
right?And the distance between light and the control circuit is at least 6 feet so RF interference dosen't seem possible to me...
 
Last edited:
Yes.
the control circuit is at least 6 feet so RF interference dosen't seem possible to me...
RF signals can go thousands of miles. 6 ft is a doddle!
 
I would suggest that you put a resistor and capacitor in parallel between pins 1 and 2 of each of the opto-isolators.

A very small amount of noise could trigger the opto isolator. The turn-on voltage on the opto isolator is smaller than that for the red LEDs, so you wouldn't see the LEDs flash on. The resistor and capacitor that I suggest would stop the opto isolator turning on so easily and quickly.
 
What about the transformer rating and the coil currents? Did you make any substitutions?

Check for power supply droop. It would be a good idea to use a scope.

Now, you could also put a ZNR on your power supply.

The other thing I would consider is an RFI input filter for the power line. If you used a torroidal transformer, that better at coupling RFI which is a bad thing.
 
Already tried a snubber network at the relay contacts?
 
I would try a pull up resistor on each output pin of the 2803 to prevent noise from triggering the relay.
 
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