reading ac voltage through microcontroller pic16f887 for stabilizer

[jit_singh_tara]

dear friends ,
PLease suggest how to measure ac voltage for stabilizer application....need to instantly switch transformer tapping when voltage drops.

I tried to convert 220v ac to around 12 v ac ...using transformer....then i rectified it with the full wave rectifier...then i used potential divider to bring it to 5v for adc ...and then i had to put a big cap to remove ripples....
But by doing so my response to change in input voltage becomes very slow ....as voltage across changes slowly.....
But if i remove the cap , the voltage is not pure dc as it was converted to full wave rectified .




What should be done in order to measure ac voltage for stabilizer application.any practical ckt or idea is welcome..........

thx

 

[shatruddha]

you can use a sine wave mapping for your case, but I feel like thats going to take more number of instruction cycles on your controller. I think you should check how these SMPS controller ICs work. That will give you a very good idea about the implementation. They use a threshold voltage and a resister devider for an input and using comparators swich on and off the pwm.
To start with you can check UC3845

 

[nandhu015]

Look here

https://www.edaboard.com/threads/162880/

 

[jit_singh_tara]

I am able to read the ac voltage by clipping the lower cycle and using resistor divider to scale the 300Vac to Round 5 v positive half cycle peak......


...



Now i am able to read the peak of this cycle after sensing the interrupt on zero crossing :i.e the moment i detect a high at transistor bc547 collector...i start taking samples and read the peak of readings in upper half cycle....Now i am instantly able to read the voltage ......

By using other method of converting the ac to dc using a cap to filter the wave ,decreased the response time.....which could not be used in stabilizer....coz i have to switch the relay in the very next cycle of detecting the fluctuating ac voltage .....


now the sensing part is fine . but i m facing problem in switching the relay 25A 24V dc Relay.I am trying to switch it as follows :
1. there is a tapping from the autotransformer which is around 18v ac , which i am using for two purposes :
a. Driving my mcu card which senses voltage and displays it on seven segment ,does the processing for switching relays .
b. Using the same supply and converting it to 24V dc using a diode bridge and 1000uf cap 35V .for driving the uln2003 for relays....

The moment the relays switches on and off i face problem as:
a. There is spike on my mcu card 5V power supply....
b. in some cases when a relay switched for switching higher winding tapping of the autotransformer , The mcu card stops working the display gets stuck ,,all the relays show 0V across them and there is humming in the auto transformer .

PLease suggest what to do for this........

 

[paulfjujo]

hello,


Could you synchronise the commande of relay to zero volt detection of the AC voltage measure ?
to avoid spike on coil of relais
and add some RC filter(snubber) across the contact ie: 0,1µF + 1K.

and maybe use a full bridge of diodes instead one 1N4007 to get double alternance measure...

 

[jit_singh_tara]

how can i remove the spike on the 5V supply ......when the relay switches the tapping of transformer.......here is the image......


I am using 24V relay with reverse 1n4007 diode across the coil of the relay.....


i am using 5 such relays ....every time the relay switches a winding of the transformer ...there is a spike on the 5V line ....what should i do......

 

[FvM]

The voltage measurement doesn't look real.

Apparently you have connected your circuit ground to mains neutral (using autotransformer for uC supply). I wonder how you can measure voltages with a regular oscilloscope in this situation?

 

[jayanth.devarayanadurga]

Can't you use a 12V transformerless powersupply and convert 12 V AC to DC and scale it to 5V DC then filter it and feed it to ADC? In code calibrate so that it shows Vrms AC or Vm AC value.

 

[paulfjujo]

hello,

Try to use intermediate relay wich need low energy to give a dry contact ,
and drive your 24V relay outside , to remove the spike energy far away of the MCU.

Do you have a load on your 24V relay ?..
maybe need also (snubberç RC circuit accross the contact .