How Do You Choose The Right Resistor For Your Circuit?

Table of Contents

Introduction

As one type of passive electrical component, resistors do something very simple but very important: they stop the flow of an electric current. Have you ever seen an LED shine? That was possible because of the reliable resistor. Putting a resistor in front of an LED in a circuit lets all the lights shine without burning anything.

Resistance Material

You can also separate resistors into the parts that make them up. The material they are made of greatly impacts how well they block current. Among these things are:

  • Metal Film
  • Carbon Composition
  • Foil Wire Wound
  • Thick And Thin Film
  • Carbon Film

How Do I Pick the Correct Resistor?

Figuring Out Your Resistance

You will use Ohm’s Law to figure out your resistance here. If you know the voltage (V) and current (I), use one of the formulas below.

Rating For Power Or Size Of The Resistor

As long as the wattage stays the same, a resistor will reach a steady temperature that depends on its size (surface area) and the power it loses. When the heat input rate (relating to wattage) equals the rate of heat loss (by radiation, convection, and conduction), the temperature stays the same. The heat loss rate is higher when there is more resistor area per watt to be lost. This means that the temperature rise is lower. For each type of resistor, the relationship between the losses is different.

Junction Transistor With Two Poles

We will first look at a bipolar junction transistor or BJT. In general, transistors are three-terminal parts that work like switches in electronics. If you apply a small current to a BJT’s base pin, the “switch” closes, letting a much larger current flow between the other two pins. At the base pin, we only need a small amount of current; any more is wasted to use a high resistor.

Most circuits work with 3.3V to 12V; a BJT can handle 5mA to 15mA of current. We should be fine if we add a resistor in the 1k© to 10k© range. Before making any changes, I would design the circuit with a 10k? resistor and then use Multisim to see what was happening. This is helpful if your current source is a microcontroller since they are very sensitive to output pins that drain more than 40mA of current.

Timer IC

One of the oldest and most popular ICs on the market is the 555 timer. As an astable multivibrator, or oscillator, it produces a square waveform, and you can change the duty cycle and period by changing the values of the resistors and capacitors. A big resistor, for instance, will slow down the rate at which the capacitor discharges, which will make the duty cycle and period longer.

Tolerance (%)

Tolerance in a resistor tells you the biggest difference from the given resistance value. Pick a resistor with the right amount of error to meet the circuit’s accuracy needs. Let us look again at the LED current-limiting resistor example to see how important resistor tolerance is, especially in critical situations where matched resistances must be exact.

Remember that in the last example, we figured out that the current-limiting resistor needed a resistance value of about 450? to power an LED with a 9V battery that was drawing 20mA. You can usually see resistor tolerance as a percentage. This shows how far the resistor’s real resistance can be from its stated nominal value. For example, a resistor with a 5% tolerance means that its real resistance can be ±5% different from the value given.

Conclusion

When choosing a resistor for your circuit, you must understand Ohm’s Law, consider power rating, and choose the right material and tolerance. Remember to calculate resistance, size-dependent heat dissipation, and circuit tolerance. These steps ensure your circuit works properly and efficiently.

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