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Wheatstone Bridge

The Wheatstone Bridge example extends the single resistor to a network of four resistors with connectivity. This introduces how AP 210 models circuits and connections.

The Circuit

A Wheatstone Bridge consists of:

  • Four resistors (R1, R2, R3, Rx) in a diamond arrangement

  • A voltage source across one diagonal

  • A galvanometer across the other diagonal

Product Structure

In the STEP file, this appears as:

#1 = PRODUCT('Wheatstone Bridge', '', '', (#2));
...
#100 = NEXT_ASSEMBLY_USAGE_OCCURRENCE('R1', 'R1', '', #1_form, #R1_def, $);
#101 = NEXT_ASSEMBLY_USAGE_OCCURRENCE('R2', 'R2', '', #1_form, #R2_def, $);
#102 = NEXT_ASSEMBLY_USAGE_OCCURRENCE('R3', 'R3', '', #1_form, #R3_def, $);
#103 = NEXT_ASSEMBLY_USAGE_OCCURRENCE('Rx', 'Rx', '', #1_form, #Rx_def, $);

Connectivity

The bridge’s connections are defined as nets:

// Net A: R1.1 - R3.1 - V+
#200 = PHYSICAL_CONNECTIVITY_DEFINITION('Net A', '', #1_design);
#201 = PHYSICAL_CONNECTIVITY_ELEMENT('R1.T1', '', #200, #R1_T1);
#202 = PHYSICAL_CONNECTIVITY_ELEMENT('R3.T1', '', #200, #R3_T1);

// Net B: R1.2 - R2.1 - Galvo+
#210 = PHYSICAL_CONNECTIVITY_DEFINITION('Net B', '', #1_design);
...

Functional Decomposition

The functional view separates logical function from physical implementation:

#300 = FUNCTIONAL_UNIT('Wheatstone Bridge', '', #1_design);
#301 = FUNCTIONAL_UNIT_USAGE('R1 function', '', #300, #R1_func);
#302 = FUNCTIONAL_UNIT_USAGE('R2 function', '', #300, #R2_func);
...

// Connect functional units
#400 = CONNECTIVITY('R1-R2', '', #300, #R1_func_T2, #R2_func_T1);

Mapping Function to Physical

The mapping links functional terminals to physical terminals:

#500 = DESIGN_FUNCTIONAL_UNIT_ASSIGNMENT('R1 mapping', '', #301, #100);

This tells us that the "R1 function" usage is implemented by the physical occurrence of R1 in the assembly.

Reading the File

When reading the Wheatstone Bridge file:

  1. Identify the four resistor products and their definitions

  2. Find the assembly relationships (NEXT_ASSEMBLY_USAGE_OCCURRENCE)

  3. Trace the connectivity definitions (PHYSICAL_CONNECTIVITY_DEFINITION)

  4. Map the functional units to physical occurrences

  5. Verify the network topology matches the expected diamond arrangement