Voltdivider

Switchable Voltage Divider PCB — Circuit Overview and Build Guide

This PCB implements a switchable resistive voltage divider intended to derive accurate, repeatable fractions of a stable DC reference (nominally 10 V). Each divider tap produces a fixed ratio of the input voltage, selectable via a 1-pole, 5-position rotary switch.

The design is optimized for trimming resistance values to achieve (relatively) precise divider values.

Circuit Description

There are five taps on the PCB. Each tap on the PCB has space for two resistors and a trim pot (Bourns 3362).

Each tap is a two-resistance voltage divider:

Vin ── Rtop ──┬── Vout
              |
            Rbottom
              |
             GND

Rtop is a fixed value for all taps. (I used 100k.)

Rbottom is implemented as:

• A fixed resistor
• A small trim potentiometer (Bourns 3362)
• Optional second series resistor or jumper

This arrangement allows:

• Coarse ratio setting with fixed resistors
• Fine trimming with a low-value trimmer
• Good long-term stability by minimizing the trimmer’s contribution to total resistance

The rotary switch selects which divider output is routed to the output jack.

Divider Equation

The fundamental voltage divider relationship is:

Vout = Vin * (Rbottom / (Rtop + Rbottom))

To design a divider for a desired ratio:

k = Vout / Vin

Solve for the shunt resistor:

Rbottom = (k / (1 - k)) * Rtop

Example Divider Values

Exact values matter. Let's say Rtop = 100,468 Ω.

In practice, these are realized using nearby standard values plus the trim pot to dial in the exact ratio.

You can, of course, use whatever ratios you like.

Build Notes

The PCB is designed around a 1-pole 5-position (using 1P/8-pos form factor) like the one available from pedal parts supplier Love My Switches or PedalPCB.

The PCB is also designed to fit within a standard 125B aluminum enclosure with room to install your choice of inputs and outputs (banana jacks, RCA, etc.).

Build Instructions (Summary)

1. Populate fixed resistors first

   • Use precision metal film resistors where possible.
   • Keep resistor types consistent across all taps to improve ratio tempco matching.

2. Install the trimmer (Bourns 3362)

   • Use the trimmer only for fine adjustment.
   • Aim to keep it near mid-range once trimmed.

3. Install jumpers or series resistors

   • If a second resistor footprint is unused, install a jumper.
   • Otherwise, use it to reduce the trim range and improve stability.

4. Clean the PCB

   • Thoroughly remove flux residue, especially around the 0.01 tap.
   • Leakage dominates errors at low-voltage, high-impedance nodes.

5. Trim each divider

   • Apply a stable input voltage.
   • Measure Vin and Vout with the same DMM.
   • Adjust the trimmer until the ratio (Vout / Vin) matches the target value.

6. Final assembly

   • Mount the PCB and switch in a metal enclosure.
   • Keep wiring short and thermally uniform.

Notes on Use

• Divider accuracy should be evaluated ratiometrically, not as an absolute voltage.
• The 0.01 tap is most sensitive to leakage, temperature gradients, and contamination.
• A metal enclosure significantly improves short-term stability and repeatability.

This design is intentionally simple, adjustable, and reproducible.

License

Switchable Voltage Divider PCB © 2026 by Terrence Dorsey

is licensed under Creative Commons Attribution-ShareAlike 4.0 International