Ground interference problem? (Attached circuit diagram, powered by USB 5V 1A)

hau30729

Ground interference problem? (Attached circuit diagram, powered by USB 5V 1A) [Copy link]

 

Ground interference problem?
I am currently doing an experiment to test water quality (pH, dissolved oxygen, etc.). The water quality test equipment is powered by a USB adapter (not connected to the ground), and the circuit itself has a reserved isolation circuit. But the test is not installed yet. I
encountered a very strange situation during the test. When the sensor was measuring, it was interfered by the small motor in the water body, causing the sensor reading to be abnormal. The situation is as follows:

When the motor is in the water, the sensor cannot read any value (ADC reading is close to 0).
When the motor is removed from the water, the sensor reading will return to normal.
When the motor is removed from the ground and placed in the water, the sensor reading will also be normal.

1. I think connecting an isolation circuit should solve this problem?
2. I want to ask why removing the ground can also solve this problem?

maychang

"Why would removing the earth also solve this problem?"

It is estimated that the current flowing through the distributed capacitance between the power supply of the small motor and the ground wire of the motor (casing and core) also flows through the ground wire of the sensor.

qwqwqw2088

It depends on the installation instructions of the water quality test equipment to see how the grounding is required.

How to connect the isolation circuit? You need to take a closer look

hau30729

qwqwqw2088 posted on 2019-11-19 11:05 You still need to look at the equipment installation instructions of the water quality testing equipment to see how the grounding and isolation circuits are connected. You need to take a closer look.

I plan to develop the test equipment myself, and then use it in a regular household fish tank, but many regular household water quality equipment do not seem to have grounding.
The isolation circuit is currently intended to separate the sensor from other circuits.

hau30729

maychang posted on 2019-11-19 10:21 "Why can removing the earth solve this problem?" It is probably because of the connection between the power supply of the small motor and the ground wire of the motor (casing and core)...

The motor is placed in water. So it means that the motor (leakage?) current flows through the water to the sensor of the device, causing the impact?
In this case, if the sensor is isolated, is it possible that it will be useless?

tj1931

Generally, household water storage equipment is not grounded.
The isolation circuit is currently intended to separate the sensor from other circuits.

maychang

hau30729 posted on 2019-11-19 11:15 The motor is placed in the water. So it means that the motor (leakage?) current flows through the water to the sensor of the device, so it causes an impact? In this way, if the sensor is made...

There is naturally no such thing as grounding for home fish tanks. They can be considered to be suspended, with distributed capacitance between them and the AC mains.

If your motor housing is connected to the ground, the AC mains voltage will flow into the ground through the sensor (connected to the water in the fish tank) and the motor, and then through the distributed capacitance between the motor and the AC mains. This current will cause interference in the voltage drop of the connecting line between the sensor and the motor.

maychang

hau30729 posted on 2019-11-19 11:15 The motor is placed in the water. So it means that the motor (leakage?) current flows through the water to the sensor of the device, so it causes an impact? In this way, if the sensor is made...

You may try connecting the water in the fish tank to the earth. If the interference changes, it proves that the interference is caused by the suspension of the fish tank.

maychang

hau30729 posted on 2019-11-19 11:15 The motor is placed in the water. So it means that the motor (leakage?) current flows through the water to the sensor of the device, so it causes an impact? In this way, if the sensor is made...

"So if we make an isolation circuit for the sensor, is it possible that it will be useless?"

It is probably useless. Besides, the water quality testing of home fish tanks should be very sensitive to price. Adding an isolation circuit will definitely increase the cost, and it is likely that users will not accept it.

hau30729

maychang published on 2019-11-19 11:50 You might as well try connecting the water in the fish tank to the earth. If the interference changes, it proves that the interference is caused by the suspension of the fish tank.

Does it mean that both the fish tank water and the motor
should be connected to the ground at the same time? It seems a bit difficult to ground the fish tank water... Find a regular computer 3-pin plug and leave only the ground wire, cut off the other two wires and try again, thank you.

hau30729

maychang posted on 2019-11-19 11:52 "If we make an isolation circuit for the sensor, is it possible that it will be useless?" It is probably useless. Besides, the water quality of the home fish tank...

If the problem cannot be solved, I feel the product cannot be sold. There should be many wired water quality detectors on the market, but I don’t know how to make them undisturbed and low-cost.

hau30729

maychang published on 2019-11-19 11:49 There is no such thing as grounding in a home fish tank. It can be regarded as suspended and has distributed capacitance with the AC mains. If your motor housing is connected to a large...

I may not have explained the interference clearly.
When the motor is in the water, the sensor cannot read any value (ADC reading is close to 0).
When the motor is removed from the water, the sensor reading will return
to normal. When the motor is removed from the ground and placed in the water, the sensor will also read normally.

chunyang

According to the original poster's description, the earth circuit connected to the sensor and the motor is short-circuited through the water. The solution depends on what sensor you use and the specific circuit. The electrical circuit relationship between the earth, power supply, and system reference ground must be clarified, so the circuit diagram must be complete, at least the electrical diagram must be complete.

gmchen

I agree with the 13th thread. When asking questions on a forum, it is best to state the problem in full, otherwise it will be difficult for others to obtain complete information, and not only will the problem not be solved, but it may also lead them into a ditch.

hau30729

gmchen posted on 2019-11-19 20:45 I agree with the 13th post. When asking questions on the forum, it is best to state the problem in detail. Otherwise, it will be difficult for others to obtain complete information. Not only will the problem not be solved, but it may also be...

Got it, thanks for the reminder. Added the circuit diagram, powered by USB 5V 1A

hau30729

chunyang posted on 2019-11-19 18:40 According to the description of the OP, it should be caused by the earth loop connected to the sensor and the motor being short-circuited through the water. The solution depends on what sensor you are using and the specific circuit...

Got it, thanks for the reminder. Added the circuit diagram, powered by USB 5V 1A

hau30729

maychang posted on 2019-11-19 11:52 "If we make an isolation circuit for the sensor, is it possible that it will be useless?" It is probably useless. Besides, the water quality of the home fish tank...

Added circuit diagram, powered by USB 5V 1A

maychang

hau30729 posted on 2019-11-21 13:57 Add the circuit diagram, use USB 5V 1A power supply

Is your motor, along with the measurement circuit and sensor, powered by the USB port? The picture does not show where the motor is powered.

hau30729

maychang posted on 2019-11-21 14:49 Is your motor, along with the measurement circuit and sensor, powered by the USB port? There is no indication in the picture where the motor is powered.

The motor is purchased from outside, and the adapter is provided by the manufacturer. It is a large charger similar to a laptop.

https://www.bulkreefsupply.com/vectra-s1-dc-1400-gph-ecotech-marine.html

maychang

hau30729 posted on 2019-11-21 15:28 maychang posted on 2019-11-21 14:49 Are your motors, measuring circuits and sensors all powered by the USB port? Not shown in the picture...

I'd like to know: for a motor like this, how do you connect its casing to the ground?