I have some circuit design questions.

FYzj

I have some circuit design questions. [Copy link]

 

The company asked me to design a board for testing motors. It is mainly used to test brushless motors and carbon brush motors. It is required to be able to achieve a wide load power supply DC6-220V, with overcurrent protection and short circuit protection. There are also some other timing, display and so on.

I plan to use an external power supply for the DC voltage. I want to find a relatively powerful and adjustable DC power supply. I wonder if you guys have any recommendations. Now I don't know how to get the overcurrent protection, because different motor models have different overcurrent points, the large ones are about 10A, and the small ones are about 2A. I don't know how to distinguish them in the circuit. I have no experience in designing circuit boards before, and I hope you guys can give me some advice. Thank you very much.

maychang

“Requires wide load power supply DC6-220V”

The voltage range of this power supply is quite wide, and it is available on the market. If the power is relatively large, the price is probably relatively expensive.

maychang

"Because the motor models are different, the overcurrent points are also different. The large ones are about 10A, and the small ones are about 2A. I don't know how to distinguish them in the circuit."

This range is not too large, and the same shunt can be used to detect the current. If the overcurrent point is different, then the reference for comparison with the current can be made variable.

Current detection must be converted into voltage and then compared. The reference voltage used for comparison is divided by resistors, divided into several levels and then compared with the voltage converted from the detected current.

maychang

If the motor current is converted into a digital signal and then compared with the reference (the reference is a number in this case), it is even easier. Just modify the number representing the reference.

okhxyyo

Has the landlord's problem been solved?

chunyang

If a grading design is adopted, the cost will be much lower. For example, for the power supply, several groups of finished DC power supplies with different output ranges are used according to the working voltage distribution of the motor to be tested, and the power is based on the maximum power of the motor to be tested in the distribution area and an appropriate margin is reserved. The overcurrent protection is similar. The grading design is lower in cost than supporting continuous settings. After all, the types of motors to be tested are limited, and some power supplies themselves have overcurrent protection. Therefore, the comprehensive price is selected according to the principle of the best cost-effectiveness.

qwqwqw2088

The wide voltage range of this load is wide enough, DC6-220V

This DC power supply external power supply is estimated to need to be customized

For example, how many A is needed for overcurrent protection

It is also necessary to provide a stable voltage supply to the display circuit

FYzj

maychang posted on 2020-6-9 08:57 "Because the motor models are different, the overcurrent points are also different. The large ones are about 10A and the small ones are about 2A. I don't know how to distinguish them in the circuit." ...

I have used a shunt and LM358 to make an overcurrent protection. I don't know if it is correct. Could you help me check it? Can I achieve these functions? Thank you

maychang

"I have used a shunt and LM358 to implement overcurrent protection, but I don't know if it is correct. Could you please help me check it? Can I achieve these functions?"

Obviously not.

It is not allowed to connect the three op amp output terminals together.

maychang

FYzj posted on 2020-6-23 15:45 I used a shunt and LM358 to make overcurrent protection, but I don’t know if it is correct. Can you help me check it? Can these functions be realized? Thank you...

In addition, why do we need 9 op amps, three in parallel? The value of R33 is 1 kilo ohm, has it been calculated? What model is Q4?

FYzj

maychang posted on 2020-6-23 17:28 In addition, why do we need 9 op amps, three in parallel? The value of R33 is 1 kilo ohm, has it been calculated? What model is Q4?

Doesn't an LM358 have two independent op amps? I'm worried that one amplifier will not work well if it is amplified hundreds of times, so I use two amplifiers separately... Is this not okay? Q4 has not been selected yet, and the transistors in the figure have not been selected yet. I want to select them after the drawing is completed, so I also gave a value for R33 first.

maychang

FYzj posted on 2020-6-23 22:21 Doesn't an LM358 have two independent op amps? I'm worried that it will be bad if I amplify it a few hundred times, so I use two to amplify it separately... Is this not okay? Q4 ...

First, the output terminals of the three op amps cannot be connected together. The output voltages of the three op amps cannot be absolutely the same, if one is high and the other is low, what should be done with Q4?

maychang

FYzj posted on 2020-6-23 22:21 Doesn't an LM358 have two independent op amps? I'm worried that it will be bad if I amplify it a few hundred times, so I use two to amplify it separately... Is this not okay? Q4 ...

To detect currents of 2A to 10A, a shunt is usually used, and the rated voltage of the shunt is 75mV. However, the input offset of the LM358 can reach 5mV or higher. In other words, the error caused by the offset voltage may be 6-7%. Although overcurrent protection usually does not require high accuracy, is an error of 6-7% acceptable?

If this is unacceptable, use a precision op amp or precision comparator instead.

maychang

FYzj posted on 2020-6-23 22:21 Doesn't an LM358 have two independent op amps? I'm worried that it will be bad if I amplify it a few hundred times, so I use two to amplify it separately... Is this not okay? Q4 ...

How does your overcurrent protection require to work? Does it stop increasing after the current reaches a certain value? Or does it shut down the power supply after the current exceeds a certain value? If it is the latter, do not use an op amp, but a precision comparator.

maychang

FYzj posted on 2020-6-23 22:21 Doesn't an LM358 have two independent op amps? I'm worried that it will be bad if I amplify it a few hundred times, so I use two to amplify it separately... Is this not okay? Q4 ...

Your digital tube display uses 12 digital tubes for scanning display. If the average current per segment is 1mA (this is a very small value, the digital tube is very dim), the peak current is 12mA, and the peak current per bit is 12mA*7=84mA. The 74HCXXXX chip cannot provide such a large current and needs additional driver.

FYzj

maychang posted on 2020-6-24 07:13 How do you require the overcurrent protection to work? Does it stop increasing after the current reaches a certain value? Or does it turn off the power supply after the current exceeds a certain value? If it is the latter...

6-7% error is acceptable. I wanted to use Q4 to lower the voltage of the motor control pin to stop it from working when the current exceeds a certain value. Now it seems that I was not thoughtful enough. Thank you for your advice. I will modify the circuit diagram.

maychang

FYzj posted on 2020-6-24 08:18 6-7% error is acceptable. I want to use Q4 to lower the voltage of the motor control pin when the current exceeds a certain value to stop it from working. Now it seems that I don't consider...

"I want to use Q4 to lower the voltage on the motor control pin when the current exceeds a certain value, so that it stops working."

Your motor has a control foot? What kind of motor is that?

maychang

FYzj posted on 2020-6-24 08:18 6-7% error is acceptable. I want to use Q4 to lower the voltage of the motor control pin when the current exceeds a certain value to stop it from working. Now it seems that I don't consider...

"I want to use Q4 to lower the voltage on the motor control pin when the current exceeds a certain value, so that it stops working."

To stop the motor from working, just cut off the power. Then, it is still switch control.

FYzj

maychang published on 2020-6-24 09:52 "I want to use Q4 to lower the voltage of the motor control pin when the current exceeds a certain value to stop it from working." Your motor has a control pin...

No, I connected a transistor to the front section to control it, because I need to time it, so it will be convenient for the program, right? Another question I want to ask is about the 74HC595. I saw in the data sheet that the maximum output of each pin is 35mA. I control 12 segments with one pin, so each segment can have about 3mA of current, right? Can it be calculated this way? Or can I add a transistor driver behind each pin?

maychang

FYzj posted on 2020-6-24 10:26 No, I connected a triode to the front part to control it, because I need to time it, so it will be convenient for the program, right? Another thing I want to ask about 74HC595...

The matrix scanning display digital tube is that each digital tube (including 7 or 8 segments) is displayed in turn. If 1mA is required for each segment, then 7 segments are fully lit with 7mA. But because 12 bits are displayed in turn, each bit is only displayed for 1/12 of the time. Then the current passing through each digital tube should be multiplied by 12, which is 84mA, so that the average current can reach 7mA.