Problems in measuring MOS tube temperature

shaorc

【Ask if you don't understand】 Now I want to measure the temperature of the MOS tube on the primary side of the single-ended flyback switching power supply in Figure 1 when it is working. The MOS tube is shown in Figure 2. The working condition is: the input PN bus of the primary side is 311VDC Prepare to use the thermal coupler in Figure 3 to measure the temperature range of the multimeter Clamp the measuring end of the thermal coupler between the MOS tube and the heat sink in Figure 2, and fix it with the screws of the MOS itself Now the question [1]The back of the MOS tube is not plastic-sealed. When it is working, the voltage of 311V directly passes through the DS ends of the MOS. If it is measured directly with a thermal coupler, will the thermal coupler be charged? [2]When the MOS tube is working, will the multimeter be damaged if the thermal coupler is inserted into the multimeter? Picture 1 Picture 2 Picture 3

maychang

【1】The back of the MOS tube is not plastic-sealed. When working, a voltage of 311V directly passes through the DS ends of the MOS. If it is measured directly with a thermocouple, will the thermocouple be charged? Yes. The metal plate on the "back" of the MOS tube is the drain of the MOS tube, and it is the same copper plate as the middle leg of the MOS tube in the second picture.

maychang

【2】Will the multimeter be damaged if the thermocouple is inserted into it while the MOS tube is working? If the human body does not touch any conductive part of the multimeter, and the mains power does not touch any conductive part of the multimeter, the multimeter will not be damaged.

maychang

"Clamp the measuring end of the thermocouple between the MOS tube and the heat sink in Figure 2, and fix it with the screws of the MOS itself." This installation will greatly reduce the contact area between the MOS tube and the heat sink, affecting the heat dissipation of the MOS tube.

shaorc

maychang posted on 2018-9-25 15:34 【1】The back of the MOS tube is not plastic-sealed. When working, the 311V voltage directly passes through the DS ends of the MOS. If it is measured directly with a thermocouple, will the thermocouple be charged...

Since the thermocouple wire is charged, but the outside of the wire is insulated, will a person get an electric shock if he touches it?

shaorc

maychang posted on 2018-9-25 15:35 [2] When the MOS tube is working, will the multimeter be damaged if the thermocouple is inserted into the multimeter? If the human body and any conductive part of the multimeter are not in contact, the mains...

Insert the two yellow plug ends of the thermocouple into the two jacks in the lower right corner of the multimeter, and turn it to the temperature range. How can you judge whether the mains and the live parts of the multimeter are in contact?

maychang

shaorc posted on 2018-9-25 15:45 Since the thermocouple wire is charged, but the outside of the wire is insulated, will people get electric shock if they touch it?

The thermocouple wire is charged, but the outside of the wire is insulated, will people get electric shock if they touch it? The human body will not get electric shock if it touches the outer skin of the two insulated wires of the thermocouple, or the insulated shell of the multimeter.

maychang

[quote]shaorc posted on 2018-9-25 15:46 Insert the two yellow plug ends of the thermocouple into the two jacks at the lower right corner of the multimeter, and turn it to the temperature range. How can you tell if the AC power and the live parts of the multimeter are in contact? It is impossible to tell. It is necessary to tell whether the thermocouple is electrically connected to the metal part of the MOS tube. A simpler method is to use another multimeter with an ohmmeter to measure the resistance between the thermocouple plug and the metal part of the MOS tube when the MOS tube is not powered on. If it is zero, the thermocouple and the MOS tube are in good electrical connection.

shaorc

maychang posted on 2018-9-25 15:52 Insert the two yellow plug ends of the thermocouple into the two jacks at the lower right corner of the multimeter, and then turn it to the temperature range. How can you tell whether the AC power and the live parts of the multimeter are...

This method is good. My question is that the thermocouple is connected to the drain of the MOS, so the resistance is zero at this time. If it is powered on, will the 311V voltage be brought into the multimeter and burn the multimeter? Moreover, why is there a 311V charge inside such a thin thermocouple wire, but it can be touched from the outside? The insulation of the outer skin is so strong?

maychang

[quote]shaorc posted on 2018-9-25 16:05 This method is good. My question is that the thermocouple is connected to the drain of the MOS. At this time, the resistance is zero. If it is powered on, will it bring 311V voltage into the multimeter and burn the multimeter? The 311V voltage refers to the N line in the first figure. As long as the internal conductive part of the multimeter has no electrical connection with the N line or AC 220V, when the thermocouple is electrically connected to the MOS tube, the internal conductive part of the multimeter automatically has the same potential as the drain of the MOS tube.

maychang

shaorc posted on 2018-9-25 16:05 This method is good. My question is that the thermocouple is connected to the drain of the MOS. At this time, the resistance is zero. If it is powered on, will it be 311...

Moreover, why is there 311V charge inside such a thin thermocouple wire, and it can be touched from the outside? The insulation of the outer skin is so strong? As long as the outer skin is not damaged, the withstand voltage is generally more than 1,000 volts. The current enameled wire, the paint is not damaged, the two strands are twisted, and the withstand voltage between the two strands is usually 3 to 4kV.

maychang

shaorc posted on 2018-9-25 16:05 This method is good. My question is that the thermocouple is connected to the drain of the MOS, so the resistance is zero at this time. If it is powered on, will it be 311 ...

My question is that if the thermocouple is connected to the drain of the MOS, then the resistance is zero. If the power is on, will the 311V voltage be brought into the multimeter and burn the multimeter? If your thermocouple is electrically connected to the drain of the MOS tube, because the drain of the MOS tube has a high-frequency AC voltage of more than 600V peak to peak to the N line (your figure is obviously a flyback switching power supply), then the conductive part inside the multimeter also has an AC voltage of more than 600V to the N line (also to the earth or the human body). This high-frequency AC voltage may not damage the multimeter, but it is likely to make the temperature measurement inaccurate.

qwqwqw2088

The thermocouple outputs millivolt voltage values, so the multimeter should use the millivolt range.

shaorc

maychang posted on 2018-9-25 16:09 My question is that the thermocouple is connected to the drain of the MOS, so the resistance is zero at this time. If the power is turned on, will the 311V voltage be brought into...

The question about withstand voltage is clear. Here, you mean that even if the MOS drain and the measuring end of the thermocouple are connected to the conductive part inside the multimeter and become the drain potential, the multimeter will not burn out as long as the ground and N line inside the multimeter are not connected to the same ground?

shaorc

maychang published on 2018-9-25 16:15 My question is that the thermocouple is connected to the drain of the MOS, so the resistance is zero at this time. If it is powered on, will the 311V voltage be brought into...

High-frequency peak-to-peak 600V AC power may not damage the multimeter, it is so resistant. Haha

zhuyebb

Thank you for your selfless sharing and dedication

maychang

shaorc posted on 2018-9-25 16:19 The question about withstand voltage is clear. Here, you mean that even if the MOS drain and the measuring end of the thermocouple are connected to the inside of the multimeter...

Not only the N line in the figure, there should be no electrical connection with the AC mains, the real earth, and the human body (usually connected to the earth).

maychang

shaorc posted on 2018-9-25 16:30 High-frequency peak-to-peak 600V AC may not damage the multimeter, it is so resistant. Haha

This is because the impedance of the thermocouple is very small, usually only a few ohms or even less. If the impedance is larger, it will be different.

maychang

shaorc posted on 2018-9-25 16:30 High-frequency peak-to-peak 600V AC may not damage the multimeter, it is so durable. Haha

If the wire connected to the multimeter's "ground" is broken, then the high-frequency AC voltage of 600V peak-to-peak may damage the multimeter if the protection circuit in the multimeter is imperfect or even absent.

chunyang

It is best to buy an infrared temperature measuring gun. I bought a dual-purpose temperature measuring gun on a certain website. One gear is for measuring body temperature, and the other gear is for measuring pan-temperature, ranging from 0-100 degrees. The accuracy of measuring body temperature is 0.2, and pan-temperature is 0.5. Although it cannot meet the high limit requirement of 125℃ for power semiconductor devices, it is sufficient in most cases, and the price is only a few dozen yuan (median). If you need to measure temperatures above 100℃, there are industrial infrared temperature measuring guns. Domestic ones cost more than 100 to several hundred, and imported ones cost hundreds to thousands. You can buy according to your needs.