HCNR200 Input Link Feedback

电子小趴菜

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Isn't this a photoelectric tube? The light from D1 affects the light from D2, which in turn affects the current of D2. Why can D1 in turn affect the driving IF? I'm confused.

maychang

[Isn't this a photoelectric tube? The light from D1 affects the light from D2, which in turn affects the current of D2. Why can D1 in turn affect the driving IF ? I'm confused.]

It is not that [D1 can in turn affect the driving IF ] , but that D2 adds feedback to the op amp, affecting the output current IF of the op amp .

maychang

In order for HCNR to work correctly and play a linear isolation role, the article also mentions op amps U2, U3, etc., but you did not include U2 and U3 in the diagram. This makes it impossible to understand the operation of linear optocouplers.

电子小趴菜

maychang posted on 2024-11-19 09:38 In order for HCNR to work correctly and play a linear isolation role, the article also mentions op amps U2, U3, etc., but you did not include U2 and U3 in the figure. This is incomprehensible...

U2 and U3 are both output related. I am a little confused about the input terminal.

maychang

Electronic Xiaopa Cai published on 2024-11-19 09:41 U2 and U3 are both output related. I am a little confused about the input terminal

[U2 and U3 are both output related, but I am confused about the input terminals]

Yes, U2 and U3 are both on the output side. But if you don’t analyze U2 and U1 together, you can’t understand the operation of the linear optocoupler.

U3 is secondary. U3 constitutes a voltage follower and is not important.

电子小趴菜

maychang posted on 2024-11-19 09:46 [U2 and U3 are both output-related input terminals. I am a little confused] Yes, U2 and U3 are both on the output side. But if U2 and U1 are not put together...

Why does the voltage drop of R1 become larger? Isn't the + terminal of the op amp grounded? Shouldn't it always be at 0?

maychang

Electronic Xiaopa Cai published on 2024-11-19 09:41 U2 and U3 are both output related. I am a little confused about the input terminal

U4 is a linear optocoupler HCNR200. The so-called linear optocoupler does not mean that there is a linear relationship between its output and input (in fact, it is very nonlinear), but that the two photodiodes D2 and D3 are the same, receiving the light emitted by the light-emitting tube D1 at the same time, and the relationship between the current in D2 and D3 and the current in D1 is also the same (nonlinear!).

In this figure, the arrow representing the light ray D2D3 is drawn outward, which is wrong. It should be inward, as shown in the following figure.

maychang

Electronic Xiaopa Cai published on 2024-11-19 09:52 Why does the voltage drop of R1 become larger? Isn't the + end of the op amp grounded? Shouldn't it always be 0?

See the HCNR basic circuit below (copied from the HCNR manual).

maychang

Electronic Xiaopa Cai published on 2024-11-19 09:52 Why does the voltage drop of R1 become larger? Isn't the + end of the op amp grounded? Shouldn't it always be 0?

The figure above is the basic circuit. In the figure, because the op amp A1 has a very large amplification factor, the output voltage always controls the current in the light-emitting tube, making the voltage across the phototube PD1 zero. And because PD2 is the same as PD1, the voltage across PD2 is also zero due to the action of A2.

Let's look at the actual circuit below. A1 here constitutes a current-voltage conversion circuit. Compared with the above figure, capacitor C1 and resistor R3 (for current limiting) are added. A2 is only different from the above figure and adds a capacitor C2. Capacitors C1C2 constitute a low-pass filter.

Therefore, due to the very large amplification factor of the op amp, the output of A2 becomes linear with the input on the left side of R1. This is the origin of the "linear optocoupler".

maychang

Electronic Xiaopa Cai published on 2024-11-19 09:52 Why does the voltage drop of R1 become larger? Isn't the + end of the op amp grounded? Shouldn't it always be 0?

[Why does the voltage drop of R1 become larger? Isn't the + terminal of the op amp grounded? Shouldn't it always be at 0?]

The op amp's non-inverting input is grounded, so of course the potential is always zero.

In this sentence, [thereby causing the “+” input voltage of A1 to drop back] is probably incorrectly written as [thereby causing the “-” input voltage of A1 to drop back].