Strange Problem with Energized Relay

[Mehmood Ahmed]

I have developed a PIC16F877A MCU based circuit. that works on +24VDC input supply which is split further into 2 power supplies a +12VDC with the help of 7812 and +5VDC with the help of LM317T. I needed two reference voltages for ADC that is 1008mV and 3030mV, they are made with the help of LM317L and they are fed to opamp buffers and then from buffers to ADC reference inputs of MCU . I have used a 12VDC relay for Alarm condition which is connected to MCU port pin with the help of transistor switch and is protected with the help of free wheeler diode. Now the problem is when that relay is energized due to Alarm condition the reference voltage changes from 1012mvolts that is 4mV added to the reference the 3030mv might also change but I cannot check that because my multimeter can read up to 0~2000mV on mV volts resolution. can anyone suggest what is causing this problem?

Thanks

 

[ericgibbs]

hi,
A LM317L is not good enough as a voltage reference for mVolts, it only a voltage regulator.
If the input to the LM317L falls due to the Relay drawing current from the 12V line, the output of the LM317 will change by a mV or so.

Look at the TL431 or equivalent to provide a stable reference.

The problem could also be caused by the way you have connected the 0V lines to the different parts of the circuit.

example. If the LM317 0V is connected so that the relay current flows via the LM317L 0V line, every time the relay is energised the LM317L 0V will be raise by a few mV,

EDIT:
This image shows the effect upon the mV source when the 0V lines are incorrectly terminated

 

[Mehmood Ahmed]

YOU ROCK !!! I provided relay a separate ground path and tested the circuit the reference remained UNCHANGED! THANK YOU SIR

 

[Boncuk]

What should I say?

Are you unable to wire an LM317L as constant current source?

Other than wired the classic way as regulated voltage source the output current is stable within a wide range of input voltage. If a burden resistor is connected to the constant current source it will drop a very constant voltage which might serve as reference voltage.

I demonstrated that to you already providing the formula how to calculate the circuit.

Even if the data sheet says it requires a minimum output current of 4mA you might use that current (worst case) to calculate for your requirements.

We have a saying in Germany: Who rejects help doesn't really need it.

Boncuk

 

[Mehmood Ahmed]

Are you unable to wire an LM317L as constant current source?

I did not disclaim your help nor I intended that, I would not make changes in my circuit accordingly as you suggested me to. I have already mentioned that I respect you as you are far more senior than me in electronics I do admit that constant current source would be better reference. I posted my problem here before I asked you for help. It seems that you have taken it personally while I have no bad intentions at all.

One last thing, I am very thankful for your help and I hope that you will continue helping me in future and forget whatever bad feelings you have.

Regards