Protecting op amp input using Diodes

Thread Starter

The Engineer

Joined Sep 3, 2014
35
I am currently building a metal detector, In one part of the circuit there are 6 diodes that are used in order to "protect" the op amp. Can somebody please explain to me how they are protecting the op amp ? and why they are arranged in the order that they are?

Here is a link to the PDF describing the metal detector circuit:-
http://www.geotech1.com/pages/metdet/projects/twinloop/twinloop.pdf

The area in question is labelled "Fig 4. Circuit diagram of reciever"

Any help is greatly appreciated !
 

MikeML

Joined Oct 2, 2009
5,444
During normal operation, none of the diodes conduct. At most, they add a little capacitance to the opamp inputs. However, if you touch the coil assembly to something that is highly statically charged, the diodes will conduct, clamping the voltage at the opamp inputs at ~+-1V. The 470 Ohm resistors are there to limit the peak current that can flow from the coil toward the circuit board.

Read this
 

Thread Starter

The Engineer

Joined Sep 3, 2014
35
During normal operation, none of the diodes conduct. At most, they add a little capacitance to the opamp inputs. However, if you touch the coil assembly to something that is highly statically charged, the diodes will conduct, clamping the voltage at the opamp inputs at ~+-1V. The 470 Ohm resistors are there to limit the peak current that can flow from the coil toward the circuit board.

Read this
Thankyou I am starting to understand, So would I be correct in saying D5 will protect the input two of the op amp from any surges during the positive half cycle, and D6 will protect from surges during the negative half cycle ? Also can you please explain why it clamps the inputs at +- 1V as opposed to another figure ? Thankyou for the link this looks very informative and I am in the process of reading it :) It looks as if these products may work as a substitute for the diodes.
 
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