Voltage Divider Calculator with Load, and Wheatstone Bridge Calculator

Find the output of a resistor divider, with the effect of a load.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Enter the input voltage and the two resistors. The calculator gives the output Vo = Vin R2 over (R1 + R2).

Enter a load resistance to see how it pulls the output down, since it is in parallel with R2. Enter an output you want to find the R2 needed.

For a Wheatstone bridge, enter the four arms. It gives the balance value of the fourth arm and the output voltage.

Formulas used

  • V_o = V_{in}\frac{R_2}{R_1 + R_2}
  • V_L = V_{in}\frac{R_2\parallel R_L}{R_1 + R_2\parallel R_L}
  • R_x = \frac{R_2R_3}{R_1}

Worked example

With the default numbers in the calculator above:

  1. Output with no load: 6.00 V
  2. Output with the load connected (the load is in parallel with R2): 4.00 V, which is 33.33 percent below the no-load value
  3. Current in the load and the power in the divider: 0.40 mA; the supply delivers 9.60 mW

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: What do you want? Wheatstone bridge.

  1. Value of the fourth arm that would balance the bridge: 150.00 ohm
  2. Voltage at each side of the bridge: 6.00 V and 6.00 V
  3. Output voltage of the bridge with the value of Rx you gave: 0.00 V (balanced)

Common questions

Why does a load change the output?

It is in parallel with the lower resistor, which lowers it, so the output falls. Keep the load at least ten times the lower resistor.

Can a divider power a load?

Only a very small one. It wastes power, and the output changes with the load.

What is a balanced bridge?

One whose output is zero. It happens when the ratio of the resistors in one side equals that of the other side.

What is a bridge used for?

Measuring a resistance very accurately, and reading strain gauges and sensors.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Horowitz and Hill, The Art of Electronics; any circuit analysis text.

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