[PN7150] tag reading produces a voltage drop.

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[PN7150] tag reading produces a voltage drop.

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dominusdrr
Contributor IV

Hi.

I have designed a hardware with the PN7150 and managed to read an tag ISO14443_3A type.

It works fine, and I have decided to continue with other parts of the project. One of them is the use of a TFT screen.

When I present the tag to be read, I notice that the LCD resets.

Through an oscilloscope, I have concluded that there is a voltage drop that is what causes the device to restart.

The image shows the drop that occurs in the 5V power line that feeds both the PN7150 and the TFT screen.

 

dominusdrr_0-1663970390152.png

 

Initially I thought it was a problem with the power supply (which is an external 5V switching type), I have used others with a higher current capacity, but the problem is the same.

I also thought it might be some kind of antenna frequency issue with the switching source, so I did a test with one that uses an LDO regulator, but the problem persists.

Any suggestion or comment is welcome.
Regards.

 

 

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dominusdrr
Contributor IV

Hi.

 have discovered that the problem is the fuse. I have short-circuited it, the TFT doesn't reset and the voltage drops are small:

bvHjR

 

The fuse is soldered to the PCB, to avoid false contact resistance, even so it is the cause of the problem. According to the manufacturer, it has a cold resistance of 0.33 ohms.

 

The fuse has a fault current of 500 mA, and the current that is consumed (of all the hardware) when the NFC tag is read reaches about 450 mA, so, I think, when the current reaches the limit of the fuse , it begins to increase its resistance to produce enough heat to melt, but since it is not yet the maximum failure, it only increased the resistance and at the same time produces the voltage drop.

 

I have done the test with a 1A fuse and everything works without problems. It was definitely a mistake to use a fuse whose fault current was close to the maximum that the hardware draws.

Maybe you should test with fuses in ranges between 600mA to 700mA.

Regards

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1,159 Views
dominusdrr
Contributor IV

Hi.

 have discovered that the problem is the fuse. I have short-circuited it, the TFT doesn't reset and the voltage drops are small:

bvHjR

 

The fuse is soldered to the PCB, to avoid false contact resistance, even so it is the cause of the problem. According to the manufacturer, it has a cold resistance of 0.33 ohms.

 

The fuse has a fault current of 500 mA, and the current that is consumed (of all the hardware) when the NFC tag is read reaches about 450 mA, so, I think, when the current reaches the limit of the fuse , it begins to increase its resistance to produce enough heat to melt, but since it is not yet the maximum failure, it only increased the resistance and at the same time produces the voltage drop.

 

I have done the test with a 1A fuse and everything works without problems. It was definitely a mistake to use a fuse whose fault current was close to the maximum that the hardware draws.

Maybe you should test with fuses in ranges between 600mA to 700mA.

Regards