I was looking at schematic and noticed a design choice I’m curious about.
Is there any specific reason to use two resistors instead of one equivalent resistor? Are there any benefits of doing this?
If you’re strictly clinical, then using two resistors is only needed to:
- Share power disapation (e.g. 5W on R1, 5W on R2, lets you reach 10 Watts
- Achieving a non-standard resistance value (e.g 1k parallel with 2.7k gives you 729 ohms)
- Accommodate several circuits as options on one PCB
- Use existing inventory
The downside of having two when one will do:
- Additional cost
- Additional PCB real estate used up
In the real world, especially on prototypes and new designs:
- You may want to allow for changes
- You may be using whatever is on hand or cheap
However, the true engineering afficionado will quickly realize almost all of the above doesn’t apply. Why? Well look at the numbers next to the resistor terminals. The 4,5 and 3,6 are the clue. This would appear to be a resistor network, most likely a package of four isolated resistors in one component and not individual resistors. In that case they are using what is available, i.e. what they “can” use.
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In the past, combining two resistors in parallel was a method to achieve resistance values not readily available in standard packages. For instance, pairing two 1K resistors could yield a total resistance of 500 ohms, which was closer to the desired value than the nearest standard single resistor, typically 470 ohms.
While this practice may not be as common today, it was once a practical means to obtain highly specific resistance values for electronic circuits.
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In addition to the valid points already mentioned, another reason for using two resistors instead of one could improve heat dissipation.
As these resistors limit the current on an LED, they can heat up due to their small size. In such cases, the power they absorb is typically around 20mW. Having two resistors will help in better heat dissipation and improve reliability.