Components are typically installed on circuit boards in two forms: surface mount and through-hole. Surface mount components are directly adhered to the surface of the circuit board, while through-hole components require pre-drilled holes on the circuit board, into which the component pins are inserted. As electronic devices become more precise, there is a growing selection of surface mount components available. However, through-hole components are still necessary in certain situations, with a typical example being through-hole resistors. In this article, TechSparks will focus on introducing the concept and advantages of through-hole resistors to help you better understand them!
What are Through-Hole Resistors
Conventionally, let’s break down this term for better understanding: through-hole and resistor.
Firstly, a through-hole is a hole drilled by manufacturers during PCB production, passing through the entire PCB board. Its purpose is to establish electrical connections between different circuit layers to facilitate the transmission of current, signals, and data. Through-hole components typically require the use of through-holes because component pins need to pass through the circuit board and be soldered on the other side. This enhances mechanical stability and reduces the risk of damage in harsh environments. However, as mentioned earlier, an increasing number of components are being manufactured in surface mount form because through-holes result in lower PCB surface area utilization and require additional processing steps and costs.
Secondly, a resistor is a passive electronic component used to restrict the flow of current or adjust voltage and current in a circuit. Resistors can be classified into different types based on their characteristics and into SMD or THT types based on their mounting method.
Therefore, through-hole resistors are a type of resistor classified based on their mounting method. These resistors are typically made of carbon film, metal film, or metal oxide, and are usually cylindrical or rectangular in shape, with leads for connection. During installation, the leads of through-hole resistors are inserted into the holes on the circuit board and soldered in place.
Through-Hole Resistor Color Codes
You will find some colored bands on through-hole resistors, which are not just for decoration but represent a coding system for the resistance value. By these color band combinations, we can easily determine the resistance value of the resistor. For example:
For a 4-band, the 1 and 2 bands represent numerical constants, the 3 band represents the multiplier, and the 4 band represents the tolerance. For instance, if the 1 band is orange, the 2 band is yellow, the 3 band is red, and the 4 band is gold, then the resistance value of this through-hole resistor is 3.4K ohms with a tolerance of ±5%.
5-band provides greater precision, where the first 3 bands represent numerical constants, the 4 band represents the multiplier, and the 5 band represents the tolerance. 6-band, in addition to the 5-bands, also includes a band representing the temperature coefficient.
If you’re not familiar with these rules, TechSparks provides a 4-band through-hole resistor color code calculator for your convenience.
| Band 1 | Band 2 | Band 3 | Band 4 |
Through-Hole Resistor Footprint Dimensions
Through-hole resistors are typically housed in AXIAL-xx footprint, where xx denotes the center-to-center spacing of the solder pads, measured in inches. These footprint sizes are slightly larger than the resistor itself to ensure secure solder connections. Below are some common dimensions and their corresponding power ratings:
| Power | Body Length (I) (mm) | Body Diameter (d) (mm) | Lead Diameter (da) | Lead Length (a) (mm) | Imperial | Solder Pad Pitch |
|---|---|---|---|---|---|---|
| 1/8W | 3.5±1.0 | 1.8±0.5 | 0.40±0.05 | 28±3.0 | AXIAL-0.3 | 7.6mm |
| 1/6W | 3.5±1.0 | 1.8±0.5 | 0.40±0.05 | 28±3.0 | - | - |
| 1/4W | 6.0±1.0 | 2.3±0.5 | 0.40±0.05 | 28±3.0 | AXIAL-0.4 | 10.2mm |
| 1/2W | 9.0±1.0 | 3.2±0.5 | 0.50±0.05 | 28±3.0 | AXIAL-0.5 | 12.7mm |
| 1W | 11.0±1.0 | 4.5±0.5 | 0.78±0.05 | 35±3.0 | AXIAL-0.6 | 15.2mm |
| 2W | 15.0±1.0 | 5.0±0.5 | 0.78±0.05 | 35±3.0 | AXIAL-0.8 | 20.3mm |
| 3W | 17.0±1.0 | 6.0±0.5 | 0.78±0.05 | 35±3.0 | AXIAL-1.0 | 25.4mm |
| 5W | 24.0±1.0 | 8.0±0.5 | 0.78±0.05 | 30±3.0 | AXIAL-1.2 | 30.5mm |
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