# Bridge Rectifier and control circuit

#### MisterFisk

Joined Jun 8, 2020
18
Good day,

I was hoping someone could help me understand this circuit.

As far as I understand the circuit its a Bridge rectifier, where C1 acts as a smoothing capacitor. X2 and X3 are OP amps and they serve to raise the Max voltage of the circuit of which the load is R5.

I am uncertain what the circuit around X1 is used for.

Any help would be appreciated.

#### wayneh

Joined Sep 9, 2010
17,172
... C1 acts as a smoothing capacitor.
Agreed. Steady power for the op-amps.
X2 and X3 are OP amps and they serve to raise the Max voltage of the circuit of which the load is R5.
No voltage increase. X3 is configured as a comparator to switch the MOSFET full on or full off.
I am uncertain what the circuit around X1 is used for.
Me too. I suspect it oscillates and then the frequency (or duty cycle?) at the MOSFET is controlled by that. You could look over a list of standard op-amp circuits and identify how X1 is configured. I've seen it before but don't recall the purpose.

An LTspice simulation would be interesting.

• MisterFisk

#### LesJones

Joined Jan 8, 2017
3,773
I think I understand what the circuit does but as this is homework you would learn nothing if I told you. Instead I will ask you what you expect the waveform to be on the output of X2 and to draw this below the incoming sine wave. I think this should make you realise the function of X1 and Q2.
Thinking about the purpose of diode D1 should help.
Les.

• MisterFisk

#### MisterFisk

Joined Jun 8, 2020
18
Agreed. Steady power for the op-amps.
No voltage increase. X3 is configured as a comparator to switch the MOSFET full on or full off.

Me too. I suspect it oscillates and then the frequency (or duty cycle?) at the MOSFET is controlled by that. You could look over a list of standard op-amp circuits and identify how X1 is configured. I've seen it before but don't recall the purpose.

An LTspice simulation would be interesting.
You wouldn't happen have a link to a list where I can research this?

#### Ian0

Joined Aug 7, 2020
5,925
I hope X2 is the type of op-amp that will withstand its negative input being taken to a voltage higher than its positive supply.

• MisterFisk

#### MrAl

Joined Jun 17, 2014
9,355
Good day,

I was hoping someone could help me understand this circuit.

As far as I understand the circuit its a Bridge rectifier, where C1 acts as a smoothing capacitor. X2 and X3 are OP amps and they serve to raise the Max voltage of the circuit of which the load is R5.

I am uncertain what the circuit around X1 is used for.

Any help would be appreciated.
Hi,

Here is a hint in case you did not realize this yet.
One or more of those op amps is being used as a voltage comparator rather than an actual linear op amp. That tells you something about their output too. Ask the question why would the designer want to do that.

• MisterFisk

#### jean_neau

Joined Apr 7, 2020
2
Please find some hints...

1- On the anode side of D1 we find a full-wave rectified sinewave, an always positive waveshape with a peak value equal to V1 minus 2 diode forward voltage drops.
2- On the cathode side of D1, given a good sized C1, we find a full wave rectified AND smoothed voltage Vcc1 providing power supply to the Op Amps and relatively stable voltage references to their respective inputs.. Note that power supply Vcc1 has some ripple given the absence of any voltage regulating device: we must assume a stable AC input...
3- Between X2 output and inverting (-) input we find no feedback path (neither resistor or connection) and given the fact that Op Amp have very high open loop gain, X2 can not operate in linear mode and thus must rather operate in «switched mode», e.g. as a comparator (note that so called hysteresis comparators do have a resistor between output and the non-inverting (+) input).
4- R1 and R8 form a voltage divider providing a reference voltage for X2 (Vref = R8/R8+R1) corresponding to less than one percent of Vcc1. Thus X2 will change state when it's input voltage (across R6) is either below or above about one percent of the peak value of the rectified waveform. From that point we can already suspect this whole circuit is probably a «zero crossing detector»... VOILÀ !
5- At the output of X2 we find a transistor connected across a capacitor C2. This transistor can only be either saturated (Vce = 0) or non conducting (Vce = C2 voltage), following X2 «binary» output behaviour. Now, assuming that X1 output is low, R3 will charge C2 until it reaches Vcc1... unless it is discharged when Q2 is conducting. From that point we can suspect some sort of timing circuit ... In fact, the zero crossing event detected by X2 is very steep and short (remember: one percent...)...Thus X1, combined with R3/C3 timing circuit, must be acting as a single shot monostable (triggered by X2 through Q2) thus extending the «zero crossing pulse event» to a more usable waveform. Just my guess... (I have not simulated the circuit...)
6- Further down we find an other comparator X3 with a reference voltage equal to about half of Vcc1 (R4 = R10). Looks like a buffer circuit providing proper logic states for the output pulsed waveform...
P.S. : If you want to learn (and understand) by yourself, I strongly suggest using LTSpice, a simple, intuitive and powerfull learning tool... «Big problems» generally can be splitted into a bunch of smaller ones...» Otherwise, this circuit can only be analysed (by a beginner) with a scope and breadboard...