Series Resistor: Preparation, Placement and Process Control

A light emitting diode needs its current limited, and the cheapest way to do that is a series resistor. Getting the value right is arithmetic, but choosing where to put the resistor and how to rate it takes a little more thought.

Why a Series Resistor

The forward voltage of a diode changes only slightly with the current through it, so a small increase in voltage produces a large increase in current. Connecting one directly across a supply is a way to destroy it.

A series resistor takes up the difference between the supply voltage and the total forward voltage of the string. It provides the current limiting, and its value is calculated from the operating point.

The Voltage Drop Calculation

The calculation starts with the supply voltage and the sum of the forward voltages of the diodes in the string. The difference is the voltage that appears across the resistor.

Dividing that difference by the required current gives the resistance. Both the supply and the forward voltage vary in practice, so the calculation is repeated for the worst case in each direction.

One Resistor per String or One for the Board

A single resistor can be used for a group of strings connected in parallel, but the current then divides according to the forward voltage of each string. A string that is slightly lower in voltage takes more than its share.

One resistor per string is the better arrangement. Each string is independent, the current sharing problem disappears, and the heat is spread across several components rather than concentrated in one.

Power Dissipation and Package

The power in the resistor is the voltage across it multiplied by the current. The result is compared with the rating of the package, and the rating is derated for the ambient inside the fixture.

A resistor that is run at half its rated power in open air may be well outside its rating inside a sealed luminaire. Choosing one size larger costs very little and removes a thermal failure mode.

<img src="https://www.gopcba.com/wp-content/uploads/2024/09/zuzhuang1-1.png" alt="Resistors and led string wiring on a lighting circuit board” />

A Worked Example at Twelve Volts

Three diodes of about two volts each in series give a forward voltage of six volts, leaving six volts across the resistor. At twenty milliamperes the resistance is three hundred ohms.

The dissipation is then about one hundred and twenty milliwatts, so a quarter watt part is comfortable. Where the supply is a nominal twelve volts but may reach fourteen, the same calculation gives a noticeably higher current.

A Worked Example at Twenty Four Volts

Six diodes at three point two volts give nineteen point two volts, leaving four point eight volts across the resistor at a supply of twenty four volts. At sixty milliamperes the resistance is eighty ohms.

The dissipation is about two hundred and ninety milliwatts, so a half watt part is used. Putting one resistor across the whole board instead would require about thirteen ohms and dissipate well over a watt, which is why the distributed arrangement is preferred.

Forward Voltage Spread and Matching

Diodes from the same production batch still differ in forward voltage, and the spread widens between batches. The calculation uses the minimum and maximum figures from the data sheet rather than a typical value.

Where several strings share one supply, the spread is what causes the brightness difference between them. Matching the strings, or giving each string its own resistor, keeps the appearance even.

Current Variation With Temperature

The forward voltage falls as the junction temperature rises, so the current through a resistor limited string rises with temperature. The effect is significant and it is one reason a resistor is not ideal for high power lighting.

The resistor itself also drifts, though far less. Sizing the resistor for the hot condition rather than the cold one keeps the diode inside its rating at the end of a long run.

Efficiency and Heat

A resistor converts the excess voltage into heat, so the efficiency of the arrangement falls as the supply rises above the string voltage. A large difference means most of the energy is wasted in the resistor.

That heat is also added to the fixture, which raises the temperature of the diodes. Where the difference is large, a switching driver is more efficient and cooler, as described in our notes on converter layout.

When a Constant Current Driver Is Better

A driver that regulates the current removes the sensitivity to forward voltage and to temperature, and it does not waste the excess voltage as heat. It is the normal choice for high power lighting.

The cost is complexity, a switching node and its own layout requirements. Where the current is small or the supply is close to the string voltage, the resistor remains the simpler answer.

Layout of the Resistor and the String

Place each resistor near the string it feeds, so that the heat is distributed and the wiring is short. Keep the return path of each string separate until it reaches the supply point.

Where the board is a metal core type, the resistor sits on the same insulating layer as the diodes. The thermal rules for that construction are described in our notes on the aluminium substrate.

Testing the String on the Bench

Measure the current with the supply at its minimum and maximum, and measure it again after the board has reached its working temperature. The two measurements bracket the real behaviour.

Then measure the resistor temperature and the diode temperature. A string that is bright and even at room temperature can drift within an hour if the current rises with temperature.

Common Mistakes

The usual errors are using a typical forward voltage instead of the worst case, ignoring the rise of supply voltage, and rating the resistor for open air operation. Each of them produces a string that is over driven.

Connecting strings in parallel to one resistor is the fourth. It works on the bench and produces uneven brightness in production, where the diodes come from more than one batch.

Choosing the Supply Voltage

The supply should be chosen so that the voltage across the resistor is a moderate fraction of the total. A small difference means the current changes sharply with supply variation, while a large one wastes energy.

Where the supply is fixed by the application, the string length is adjusted instead. Adding or removing a diode moves the operating point more effectively than changing the resistor, as described in our current calculation notes.

Documenting the Calculation

The calculation belongs in the design record, with the supply range, the forward voltages and the resulting resistance and dissipation. A note on the schematic saves the next engineer from repeating it.

Where the resistor is derated, the ambient it was derated for is part of the same note. A rating without the conditions it assumes is not a specification.

FAQ

How do I choose the resistor value? Subtract the total forward voltage of the string from the supply voltage, then divide by the required current. Repeat the calculation at the highest and lowest supply voltage.

Why not use one resistor for the whole board? Because the current divides according to the forward voltage of each string, so the strings are uneven and the single resistor has to dissipate all of the heat.

Is a resistor always good enough? For indicators and small strips it is. For high power lighting a current regulated driver is more efficient, cooler and more consistent.