| Introduction |
This page contains two calculators. One for creating a Precision Resistor, and the other for creating a Custom Resistor.
The Precision Resistor uses only two resistors and is useful when you need a very specific resistance, say for a active filter. Because its a low current circuit, your not really worried about the wattage, but the value you are looking for is not a standard resistor value. You might be able to find it in a 1% resistor. But do you really need to go to that expense when two common 5% resistors will get you just as close to the target value?
A Custom Resistor uses two to ten resistors, of the same value. It can server two purposes. Like the Precision Resistor, you might need a non-standard resistor value. Or, you might need a higher wattage resistor than is readily available. The correct combination of resistors could provide you with the value you need and the correct wattage.
| Precision Resistor |
This calculator determines the value of two standard value resistors (RA and RB) which, when connected in parallel, will result in a net resistance RT that will be within very close tolerances, of almost any value you want.
The program works by first selecting a standard resistor (RA) that is close, but larger, than the specified resistance. It then determines the value of another standard resistor (RB) that can be placed in parallel with the first one (RA) to obtain the resistance required. Because we are using standard resistors, the final value will not be exact, but it will be within 2 or 3 percent, even when you are using 10 percent resistors.
Using the select buttons and text areas to the left, enter the target resistance value, the tolerance of the resistors you wish to use, and the voltage across the two resistors. If you don't know the voltage right now, that's OK. It is not needed to calculate the final resistances. It is, however, a good idea to supply a value that is close to the expected voltage so that you can determine the power dissipated in each resistor and size your resistors properly.
Note: You can enter a value that is a standard resistor value. In that case the calculator will find the next highest standard resistor value, based on your Tolerance selection, for RA. It will then calculate an appropriate value for RB to meet your target requirement.
| Custom Resistor |
Custom Resistor from Parallel Resistors
Target: Ω
, Tolerance:
| # of Resistors | Resistors Value (Ω) | Net Value (Ω) | % Off Target | # of Resistors | Resistors Value (Ω) | Net Value (Ω) | % Off Target |
|---|---|---|---|---|---|---|---|
| 9 | 11,000 | 1,222 | 1.0% | 5 | 6,200 | 1,240 | 0.5% |
| 1 | 1,200 | 1,200 | 2.8% | 6 | 7,500 | 1,250 | 1.3% |
| 2 | 2,400 | 1,200 | 2.8% | 8 | 10,000 | 1,250 | 1.3% |
| 3 | 3,600 | 1,200 | 2.8% | 4 | 5,100 | 1,275 | 3.3% |
| 10 | 12,000 | 1,200 | 2.8% | 1 | 1,300 | 1,300 | 5.3% |
| 4 | 4,700 | 1,175 | 4.8% | 3 | 3,900 | 1,300 | 5.3% |
| 7 | 8,200 | 1,171 | 5.1% | 7 | 9,100 | 1,300 | 5.3% |
| 8 | 9,100 | 1,138 | 7.8% | 10 | 13,000 | 1,300 | 5.3% |
| 6 | 6,800 | 1,133 | 8.2% | 9 | 12,000 | 1,333 | 8% |
| 5 | 5,600 | 1,120 | 9.2% | 2 | 2,700 | 1,350 | 9.4% |
For example:
- if you need a 1 Watt, 1,234 Ω resistor, but only have 1/4 Watt resistors available. You could use 4 - 4,700 Ω 1/4 Watt, 5%, resistors in parallel, for 1 Watt, and be within 4.8% of 1,234 Ω.
- Or, you could use 4 - 5,100 Ω 1/4 Watt, 5%, resistors in parallel, again for 1 Watt, and be within 3.3% of 1,234 Ω.
- Of course, you could use 5 - 6,200 Ω 1/4 Watt, 5%, resistors, for 1-1/4 Watts, and be within 0.5% of 1,234 Ω.
Other combinations exist that may suit your resistance/power needs better.
The calculator takes the Target Resistance and calculates an array of resistors values that are 1, 2, 3, ... 10 times the Target Resistance. It then finds the closest standard 5% resistors above (≥), and below (≤), these values. The closest standard 5% resistor values are then divided by their multiplier (1, 2, 3, ... 10) and matched to the Target Resistance.
The table shows the percentage it is different from the Target Resistance. There are two sides to the table. The listings on the left uses resistor multiples that are less than, or equal. The listing on the left uses resistor multiples that are greater than, or equal. Combinations within 5% of the target value are high-lighted in BLUE. The default setting is to use 5% tolerance resistors, but that can be changed to 10%, 2%, or 1% tolerance. With 2% and 1% tolerance resistors, all of the selections in the table will generally be less than 5% of the Target Resistance.
Just enter the value of the custom resistor you want and click outside the box.