LED Ballast Resistor and Current Spread Calculator

Enter the supply, the string and your target current. The calculator gives the ballast resistor and, more usefully, the current spread once supply tolerance, V_f binning and temperature are stacked — and tells you when a resistor has stopped being the right part.

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A resistor only regulates as well as its headroom

A ballast resistor sets LED current by dropping whatever voltage the LEDs do not. Its ability to hold that current steady depends entirely on what fraction of the supply it drops. If the resistor takes half the supply, a 1% supply change moves the current by about 2%. If it takes 10%, the same 1% change moves the current by 10%.

This is the single fact that decides whether a resistor is acceptable. Stacking as many LEDs in series as the supply allows looks efficient — it minimises wasted power — and it simultaneously destroys regulation. The tool shows the headroom fraction and the resulting sensitivity, because the two are the same decision viewed from different ends.

Three things move V_f, and they stack

• Binning. LEDs are sorted into forward-voltage bins, and unless you buy a specific bin you get the full spread — often ±0.15 V or more per device. In a series string this multiplies by the count.
• Temperature. V_f has a negative coefficient, typically around −3 mV/°C per LED. A hot LED drops less, so the resistor sees more headroom and passes more current, which heats it further. It is a mild positive feedback, and it is why thermal runaway is a real failure mode for LEDs driven from a stiff supply with little headroom.
• Current itself. V_f rises with current, which is a weak negative feedback working in your favour, and is ignored here as a small conservative simplification.

The worst cases combine, and the tool takes them at their corners: minimum supply with maximum forward voltage gives the dimmest condition, maximum supply with hot, low-bin LEDs gives the brightest and the most dissipation. Those two rows are what your production spread will look like, not the nominal.

When to stop using a resistor

Three signals, all visible above:

• Headroom below about 15% of the supply. Regulation is then so poor that ordinary tolerances produce visible brightness differences between units.
• Efficiency that matters. The resistor's share of the power is heat. On a battery product, or anywhere with many LEDs, a switching constant-current driver pays for itself.
• Matched brightness across strings. Parallel strings each with their own resistor will not match, because their LEDs came from different points in the bin distribution. Constant-current drivers, or one driver per string, are the fix — never parallel LEDs on a single resistor, where the lowest-V_f device takes most of the current and gets hotter, which lowers its V_f further.

Also worth checking

• Peak versus average in PWM dimming. Dimming by PWM at full current means the resistor dissipation is duty-cycled but the LED's peak current is not — check the peak against the absolute maximum rating, not the average.
• Reverse voltage. LEDs have very low reverse breakdown ratings, and an antiparallel or series diode is cheap insurance where reverse polarity is possible.
• Resistor placement. High-side or low-side changes nothing electrically but matters for driving from a microcontroller pin, and for what floats when the driver is off.

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