Calculate resistance, Thevenin Rth, and thermal heatsink requirements from voltage, current, load, and temperature inputs for circuit design.

Use existing circuit data; never short a live source to obtain Isc.

Rth Formula

Rth = Vtest / Itest

Variables:

  • Rth is the equivalent resistance (Thevenin resistance) (ohms, ฮฉ)
  • Vtest is the test voltage (volts)
  • Itest is the test current (amps)

For a linear network, Rth = Vth/Isc when Isc is nonzero and both references are consistent. A test-source method instead applies a test voltage/current with independent sources deactivated and dependent sources retained. Never short a live source to obtain Isc.

How to Calculate Rth?

The following steps outline how to calculate the Rth.

  1. First, determine the test voltage (volts). 
  2. Next, determine the test current (amps). 
  3. Next, gather the formula from above = Rth = Vtest/Itest.
  4. Finally, calculate the Rth.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

test voltage (volts) = 75

test current (amps) = 100

FAQ

What is Rth in electrical terms?

Rth most commonly refers to Thevenin resistance: the equivalent resistance โ€œseenโ€ looking into a two-terminal network from its output terminals (in ohms, ฮฉ). In thermal calculations, thermal resistance is typically written as Rฮธ (theta) and has units of ยฐC/W (or K/W).

Why is it important to calculate Rth in electrical circuits?

Thevenin resistance helps you simplify a circuit to a Thevenin equivalent, predict how the terminal voltage changes with different loads, and analyze power transfer and source impedance effects.

Can Rth vary with temperature?

Yes. Since Rth is based on the resistances of the components in the circuit, and many componentsโ€™ electrical resistance varies with temperature (for example, most metal conductors increase in resistance as temperature rises), the resulting Rth can change with temperature.

How does test voltage and test current affect the calculation of Rth?

For a linear network, Rth = Vth / Isc uses open-circuit voltage and nonzero short-circuit current with consistent references. The separate test-source method uses a test source with independent sources deactivated. Zero divisors or indistinguishable two-load currents do not determine a finite unique resistance.