help me explain this filter

[Quoc Viet]

Hi all,

I have a problem with this circuit, i don't understand the function of C26 and R20. I attached full document below, that circuit lie on page 27.
Who can help me to understand that, thank you so much

source: **broken link removed**

Q.Viet

 

[MrAl]

Hi,

I think you meant C20 and R28 right?

Those two are used to reduce the voltage to the circuit without requiring a transformer. That kind of power supply is often referred to as an "offline" power supply. The capacitor reduces the AC level without dissipating too much power while a single resistor would dissipate much more power and get hot. The resistor R20 limits the input surge current when the power supply is first plugged into the main AC supply. Without that resistor the surge current through the cap could be very high.

The resistor value is chosen to reduce the surge yet not dissipate too much power during normal operation. The capacitor value is chosen to allow enough current to pass so the circuit being powered gets enough voltage but not too much.

 

[jjw]

Hi,

I think you meant C20 and R28 right?

R20 and C28

 

[Quoc Viet]

Hi,

I capture a picture, C26 and R20.
Rewiew it agiant and help me.

Thank

 

[ronsimpson]

I thought R19 should equal R20. See next example.

 

[MrAl]

Hi,

I capture a picture, C26 and R20.
Rewiew it agiant and help me.

Thank

Hi,

Looks like they dont want to connect the output lower terminal directly to the analog ground,so they want a little isolation there. That could help if the input and output grounds are different.

 

[Quoc Viet]

Hi,

Looks like they dont want to connect the output lower terminal directly to the analog ground,so they want a little isolation there. That could help if the input and output grounds are different.

Yes, that is thier purpose, but i can't understand how R20 and C26 work, both R20 and C26 are connected to the AGND.

 

[Quoc Viet]

I thought R19 should equal R20. See next example.
View attachment 85389

In your circuit i have a problem, can you explain how R14 and R18 work, if there wasn't R14 and R18, can the circuit work right ?

 

[MrAl]

Yes, that is thier purpose, but i can't understand how R20 and C26 work, both R20 and C26 are connected to the AGND.

Hi,

Yes probably for a better Common Mode Rejection Ratio.