Author Topic: Impedance looks fine but 3.3V is dropping to 0.9V  (Read 1007 times)

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Offline muthukural001Topic starter

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Impedance looks fine but 3.3V is dropping to 0.9V
« on: February 02, 2020, 06:43:07 am »
Hello All,

In my board, 3.3V rail going down 0.9V. I have one more board that is working fine and impedance of 3.3V on both the boards is same. I am wondering which is causing the voltage drop for 3.3V rail.


Thanks and Regards,
Muthu
 

Offline aix

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Re: Impedance looks fine but 3.3V is dropping to 0.9V
« Reply #1 on: February 02, 2020, 06:56:26 am »
Maybe this is useful?

 

Online moffy

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Re: Impedance looks fine but 3.3V is dropping to 0.9V
« Reply #2 on: February 02, 2020, 08:59:20 am »
You need to measure the current load. Active devices will only switch on above certain voltages. Feel the components on the board, if one is realy hot then have a look.
 

Offline muthukural001Topic starter

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Re: Impedance looks fine but 3.3V is dropping to 0.9V
« Reply #3 on: February 03, 2020, 04:24:56 pm »
Hi,

I probed 3.3V rail with the oscilloscope and attached the same. As we can see, it is not dropping to 0.9V but going between 3.3V and 2.4V. Also,I have noticed a strange behaviour in my board, I removed the 3.3V sense resistor and connected my multimeter in 10A current mode  across the sense resistor. With this setup, I do not see anything like in the image attached, it was a stable 3.3V.  The sense resistor is just 20mili ohms and I hav no idea why my board is not working as expected with the sense resistor connected instead of multimeter in current mode. Is there any logic behind this?


Thanks,
Muthu
 

Offline thinkfat

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Re: Impedance looks fine but 3.3V is dropping to 0.9V
« Reply #4 on: February 03, 2020, 08:12:10 pm »
What is that sense resistor doing?And how do you think people here will be able to help you if you keep information like this for yourself?
Everybody likes gadgets. Until they try to make them.
 

Online moffy

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Re: Impedance looks fine but 3.3V is dropping to 0.9V
« Reply #5 on: February 05, 2020, 08:05:08 am »
The multimeter is just shorting out the sense resistor. So 1: Bad sense resistor or 2: Bad solder joint to sense resistor or 3: partial break in track. My money is on 2.
 

Offline Prehistoricman

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Re: Impedance looks fine but 3.3V is dropping to 0.9V
« Reply #6 on: February 05, 2020, 11:42:43 am »
Did you use the milliamps range on your meter before adding the sense resistor? This range has a fair amount of resistance and could cause that much voltage drop. The 10A range is basically a short-circuit.
« Last Edit: February 05, 2020, 11:44:40 am by Prehistoricman »
 


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