Please help me find the current size of this resistor.

kal9623287

Please help me find the current size of this resistor. [Copy link]

 

This is a constant current source unit circuit. What is the correct current for R12 resistor 20R?

maychang

"What is the correct current for R12 resistor 20R?"

The current in R12 is 1.005mA.

davidzhu210

Curious about what the drain of Q6 is connected to on the right?

maychang

davidzhu210 posted on 2022-11-9 16:43 I am curious about what the Q6 drain is connected to on the right?

[Curious about what the drain of Q6 is connected to on the right? ]

Is that a Q5? There is no Q6 in the picture.

maychang

davidzhu210 posted on 2022-11-9 16:43 I am curious about what the Q6 drain is connected to on the right?

[Curious about what the drain of Q6 is connected to on the right? ]

The current output from the drain of Q5 to the right is equal to the current flowing through R2, which is equal to the current flowing through R12, which is equal to the current flowing through R3.

davidzhu210

I think this voltage-controlled current source solution is too complicated, there should be a simpler way

kal9623287

davidzhu210 posted on 2022-11-10 11:38 I think this voltage-controlled current source solution is too complicated, there should be a simpler way

What's the simpler way to tell me?

kal9623287

13.72-13.52

0.2/20=10mA

kal9623287

You wanted a current of 1mA but ended up measuring 10mA?

The resistance value is too large

Found the reason, the resistance value is wrong, not 20R, but 200R

Thank you all

davidzhu210

The current of R2 = Vr/R3*R12/R2.

The advantage of this circuit is that the output current is independent of the power supply voltage, but it requires that the resistors R2, R2, and R12 are very accurate and the bias current of the op amp is small.

maychang

kal9623287 posted on 2022-11-10 14:46 Tell me what is the simpler way

[Tell me what is the simpler way]

A simpler way is to use only half of the first circuit. The first circuit, the upper part (U4B, R3, Q3, etc.) is already a complete constant current circuit. The current in R12 is constant, determined by the input voltage and R3, but one end of the load is connected to the positive power supply instead of grounding.

davidzhu210

I agree with the opinion of the 11th floor. Most constant current sources do this, but the disadvantage is that power supply voltage fluctuations will cause output current changes. The circuit on the first floor makes up for this disadvantage.

maychang

davidzhu210 posted on 2022-11-10 17:59 I agree with the opinion of the 11th floor. Most constant current sources do this, but the disadvantage is that power supply voltage fluctuations will cause output current changes. The circuit on the first floor makes up for this...

If you remove the lower half of the circuit in the first post, the output current will not change with the power supply voltage. If the op amp is ideal, the output current is only determined by the input voltage and resistor R3.

davidzhu210

In that case, it will float. The requirement on the first floor seems to be to get a low-side current

maychang

davidzhu210 posted on 2022-11-10 18:33 In this way, it will float. The requirement on the first floor seems to be to get a low-side current

[The requirement on the first floor seems to be to obtain a low-side current]

To achieve low-side current, one end of the load must be grounded, that is, the upper half of the first circuit must be replaced with a P-channel MOS tube.

maychang

davidzhu210 posted on 2022-11-10 18:33 In this way, it will float. The requirement on the first floor seems to be to get a low-side current

【Then it will float to the ground.】

That cannot be called "floating ground". One end of the load is connected to the positive end of the power supply, and the positive end of the power supply is the AC ground potential.

davidzhu210

Is that so?

maychang

davidzhu210 posted on 2022-11-10 22:07 Is this true?

It's not like that.

As for Figure 17, a P-channel MOS tube is used, the resistor R1 that determines the current size should be moved to the source of the MOS tube, and the load should be placed on the drain.

davidzhu210

maychang posted on 2022-11-11 09:57 It is not like this. As far as the 17th floor diagram is concerned, using a P-channel MOS tube, the resistor R1 that determines the current size should be moved to the source of the MOS tube, and the load should be placed at the drain...

I don't understand. Please give me some advice by posting a picture.

maychang

davidzhu210 posted on 2022-11-11 12:33 I don't understand. Please give me some advice with the picture

[Please give me some advice on the picture]

Drawing a picture takes some time. Let's talk about it after I finish drawing it.