AC Voltage reading from a rectifier+voltageDivider issue

[Prince Vegeta]

hi, now I am trying to read the voltage (dc) from a rectifier + filtering cap + voltage divider circuit like the following:

**broken link removed**

the means voltage (or any 220v rms ac/50hz sinewave signal) is fed into the rectifier bridge (4 diodes here) then i connected a 100uF filtering cap in parallel to filter the signal... then put a voltage divider as shown... after all that i took that wire to the RA0_pin in PIC16F877A.

voltage divider is: 115k and 1.47k -----> Vout = Vin (1.47/116.47) and hence at 220 approximately 3.9v (the max of 5v happens @ 280v rms nearly so there's enough gap).

the problem is that the voltage divider isn't connected to ground because of the rectifier so when I took the reading between at the 1.47k resistor, it was correct.

BUT, when connected to PIC, it doesn't! it gives about 2.6v or so... also tried connecting that node (1.47k) to ground and it didn't change nor work.

PIC's voltage source comes from a 7805 regulator which is fed by a charger that gives about 9v dc, this charger's ground is the circuit ground.

I made a voltage follower like this:

**broken link removed**

Vin = 8-9v
ground = ground of the 9v charger.

this follower made a problem which is the output voltage of the charger (and the regulator) gets halved, thus PIC stopped working cuz low voltage is now connected at Vcc. AND, it didn't work as a follower which gave the same result as the node connected to ground.


the reason i chosen this voltage follower is to make an isolation between PIC and divider, and to enhance input impedance is it shouldn't be high.

can you tell me what to do to be able to deliver the correct voltage from the 1.47k resistor to PIC's ADC pin (or that voltage follower OP-AMP)?

thanks.

 

[jayanth.devarayanadurga]

Your circuit is very dangerous as it has DC voltage of 280V AC. It is better if you use a 230V AC to 9V DC transformer and then rectify the 9V AC to get DC unregulated and then use a potential divider to scale down that DC to 0 - 5V DC range and feed it to adc pin. Any change in 230V AC will also result in change in 0 - 5V DC.

 

[Prince Vegeta]

Your circuit is very dangerous as it has DC voltage of 280V AC. It is better if you use a 230V AC to 9V DC transformer and then rectify the 9V AC to get DC unregulated and then use a potential divider to scale down that DC to 0 - 5V DC range and feed it to adc pin. Any change in 230V AC will also result in change in 0 - 5V DC.

Aha, nice! that's what i thought it will be.

can you post a circuit schematic of your suggested circuit? cuz that will be handy.

plus, if the input AC rms voltage exceeded the nominal 230v rms input voltage of the transformer to a value say 300v... what will happen?

I will use a voltage divider of 15k and 100k which can limit the current to a good low value and the voltage will be good too.

Vout = 0.15 Vin

so if Vin was 9v (as your transformer gets 230v to 9v): Vout = 0.15*9 = 1.35v which is a fairly low voltage.

and if Vin got up to 12v it will be 0.15*12 = 1.8v.

so the rms value will be calculated as follows: 230/1.35 = 170 which is a compensating factor or so...

1.35 * 170 = 230v rms

1.8 * 170 = 306v rms

^ is that a good way to calculate stuff?

I will connect a rectifier bridge (will have 2 diodes drop voltage = 1.4v) and thus I will get the peak voltage of the secondary windings of the transformer.

if the secondary windings' voltage is 9v (rms right?) when 230v is input... 9v*1.414 = 12.17v peak.

this Vpeak is gonna be fed into ADC pin so should I base my calculations to deal with peak voltage THEN multiply it by sqrt(2) to get RMS (sinewave only here)?

is there a way to make the calculation suitable for ANY AC signal other than just sinewave?

 

[jayanth.devarayanadurga]

if the input AC rms voltage exceeded the nominal 230v rms input voltage of the transformer to a value say 300v... what will happen?

Transformer will burn

Another alternative is to use a potential divider to drop AC 300V to AC 12V and then rectify it and scale down it to 0 to 5V DC and feed it to adc pin. You can use so called transformerless powersupply circuit to get 12V. See this link https://www.electroschematics.com/3752/transformer-less-power-supply/

The current will not be sufficient for MCU circuit but you can use this circuit for adc input after scaling down the 12V to 5V.

 

[Prince Vegeta]

i want to use something that can be grounded. My previous circuit didn't have a ground but a neutral which is typical for these kinda circuits.

I read that transformer followed by rectifier can be grounded easily and thus can be used.

even if i fed the measuring node of my 1.47k resistor into the ADC pin with or without grounding, voltage measured will be false. it was about 2.6v instead of 3.9v.

___

Isn't transformers capable of handling overvoltage?

your solution is some how similar to mine, i used a rectifier and a potential divider with filtering.

 

[jayanth.devarayanadurga]

Transformer can't handle overvoltage. There will be a limit. Maybe a 230V trf can withstand 280V but not 300V. In the link I gave 230V is dropped to 12 or 15V AC and then rectified.

 

[Prince Vegeta]

Transformer can't handle overvoltage. There will be a limit. Maybe a 230V trf can withstand 280V but not 300V. In the link I gave 230V is dropped to 12 or 15V AC and then rectified.

But the same thing in your link and my design is that there is no true isolation right?


it's the reason I want a transformer PLUS the grounding thing you never told me about ^_^... I have a 9v DC charger which is fed into the 7805 regulator to get 5v to power PIC and other stuff... here, can I connect the ground of the transformer + bridge + divider circuit to the ground of that charger (no other choice)?

___

280v seems nice and big enough! but, is there any other transformer rating bigger than 230v input? one that can withstand more? i.e, typical rating.

___

if you can, post a schematic of your circuit.

 

[jayanth.devarayanadurga]

can I connect the ground of the transformer + bridge + divider circuit to the ground of that charger

Yes.

No. 230V and 110V trf are common. If you want 300V to 15 or 18 or 20V trf then calculate the no. of primary and secondary winding and get it manufactured.

 

[Prince Vegeta]

I didn't say that it should be 300v. But, any rated standard specs higher than 230v. secondary voltage isn't that much important ad 15k and 100k res can handle pretty much very high voltage.

so, I connect the -ve of the bridge to that ground and the +ve of the bridge to the ADC pin, right?

there will be a 1.4v drop voltage because of the 2 diodes in the bridge, so how to handle them?