Author Topic: Fluke 752A 10:1 Input Resistance  (Read 1485 times)

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Offline LKM

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Fluke 752A 10:1 Input Resistance
« on: November 30, 2018, 11:13:26 pm »
Hi All,

    Does anyone know why the input resistance of the 10:1 divider is 380kOhm and not the expected 400k?

    In the 100:1 divider the input resistance is 2M instead of the expected 4M because there is a 4M divider chain in parallel to guard each of the 30R strings which are in metal cans.

    I'm assuming a similar situation is present in the 360k divider string of the 10:1 but so far I'm not seeing it described in the manual or schematic. There is mention of a passive guard but no resistor divider for the 10:1 section.

    I calculate 6.12Mohm in parallel with the 360k to give 340k but I'm not sure from where.

Thanks!
 

Offline dacman

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Re: Fluke 752A 10:1 Input Resistance
« Reply #1 on: December 01, 2018, 03:15:28 am »
Because the 100:1 divider resistors are still in the circuit.  Above the 10:1 divider is 3.6 MOhm from the 100:1 divider which is now in series with the 4 MOhm of guard resistors.  That is 7.6 MOhm in parallel with 400 kOhm, which is 380 kOhm.  This value is in a troubleshooting chart in the back of the manual as the correct value.
 
The following users thanked this post: acts238willy, LKM

Offline seoma

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Re: Fluke 752A 10:1 Input Resistance
« Reply #2 on: December 09, 2018, 08:18:35 pm »
Hello dacman
I think your studies on electronics are very high
Thank you for answering the question
Give more guidance
Thanks

Offline LKM

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Re: Fluke 752A 10:1 Input Resistance
« Reply #3 on: April 26, 2019, 12:47:33 am »
Found these notes cleaning my desk, figured I'd post for posterity. dacman's info checks out  :D

 


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