LED Series Resistor Calculator
Size the series resistor for one LED string from supply voltage, forward voltage and current, then pick a nearby E-series value and check resistor power.
Electronics design calculators
electronics.tools is a small set of design calculators and learning notes for circuit work. Every tool runs in your browser. Nothing is stored, and there are no accounts.
Try a value
Enter a target resistance and see the nearest standard E24 value before you open a full tool.
Nearest E24 value
Featured calculators
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Voltage Divider Calculator
Find the tap voltage of a two-resistor divider, see how a load pulls that tap down, and check the current and power in each resistor.
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Resistor Color Code Decoder
Decode 4-, 5- and 6-band through-hole resistor colour codes to resistance, tolerance and optional TCR.
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Parallel Resistor Calculator
Combine resistors in parallel or series. See the equivalent resistance and the conductance of the network.
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AMS1117-ADJ Calculator – Resistors, Dropout & Heat Check
Pick AMS1117-ADJ resistors with standard values, then check dropout, the 10 mA minimum load and junction temperature before you build.
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Battery Life Calculator
Estimate run-time from battery capacity in mAh, average load current and an optional derating factor.
How it works
- Enter your circuit values in the calculator at the top of each tool page.
- Read the answer block for the recommended E-series part and the checks applied.
- Follow the formula and parameter notes when you need to verify against a datasheet.
All calculators
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LED Series Resistor Calculator
Size the series resistor for one LED string from supply voltage, forward voltage and current, then pick a nearby E-series value and check resistor power.
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Voltage Divider Calculator
Find the tap voltage of a two-resistor divider, see how a load pulls that tap down, and check the current and power in each resistor.
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Resistor Color Code Decoder
Decode 4-, 5- and 6-band through-hole resistor colour codes to resistance, tolerance and optional TCR.
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Parallel Resistor Calculator
Combine resistors in parallel or series. See the equivalent resistance and the conductance of the network.
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AMS1117-ADJ Calculator – Resistors, Dropout & Heat Check
Pick AMS1117-ADJ resistors with standard values, then check dropout, the 10 mA minimum load and junction temperature before you build.
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Battery Life Calculator
Estimate run-time from battery capacity in mAh, average load current and an optional derating factor.
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Capacitor Calculator
Capacitive reactance Xc at a frequency and RC time constant τ from resistance and capacitance.
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CH340 Selector – CH340G vs C vs E vs B, Pinout & 3.3 V Use
Pick the right CH340 variant by crystal, package, modem lines, temperature and 3.3 V supply, with the WCH pin differences and baud-error figures for each.
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LM2596-ADJ Calculator – Feedback Resistors, Inductor E·T, Min VIN
Set LM2596-ADJ output with standard resistors, get the inductor E·T value TI uses, and check minimum input voltage for TI, onsemi or XL2596 parts.
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MP1584 Calculator (NRFND) – Resistors, RFREQ, Inductor + MP2338 Notes
Set MP1584 output and frequency resistors, size the inductor and check the 100 ns minimum on-time. MPS lists MP1584 as not recommended for new designs; see what changes with MP2338.
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Ohm's Law Calculator
Solve V = I × R for voltage, current or resistance when you know the other two.
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Electrical Power Calculator
Compute DC power and the third quantity from any two of voltage, current and resistance using P = VI, I²R and V²/R.
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Reactance Calculator
Capacitive Xc and inductive Xl from frequency and capacitance or inductance.
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TL431 Calculator – Output Voltage & Cathode Resistor Window
Calculate TL431 divider resistors with Iref error, the valid cathode-resistor range for your input and load, and SOT-23 power and temperature rise.
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TP4056 Charge Current Calculator – RPROG, Heat & Cutoff
Set TP4056 charge current from RPROG, check chip temperature at your battery voltage, and see why "1.2k = 1A" tables disagree with the datasheet.
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Electronics Unit Converter
Convert resistance, voltage, current, capacitance, inductance, frequency, time, power and energy with SI prefixes.
Cross-reference
Datasheet-checked equivalents and SOT-23 marking notes. A verdict only applies under the conditions listed with it.
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AMS1117 Equivalents – Datasheet-Checked Pin-Compatible Parts
TI LM1117, ST LD1117 and JSCJ CJT1117B checked line by line against the AMS1117 datasheet: SOT-223 pinout, dropout, min load, capacitor and heat.
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CH340G Alternatives – CH340C, CH340E, CP2102, FT232RL Compared
What changes when you swap a CH340G: pinout, crystal, supply, baud rate, temperature and USB IDs for CH340C, CH340E, CP2102 and FT232RL, from each datasheet.
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LM2596 Equivalents – TI vs onsemi vs XL2596 Checked
TI LM2596, onsemi LM2596 and XLSEMI XL2596 compared pin by pin: saturation voltage, duty cycle, current limit and ON/OFF thresholds. XL1509 needs a new layout.
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TL431 SOT-23 Marking Codes (T3C3, TAC3, T3G3) & Pinout
Decode TI TL431/TL432 SOT-23 (DBZ) markings into grade (0.5/1/2%) and temperature range, from TI's package addendum, and avoid the TL431 vs TL432 pin swap.
Learning guides
Short notes on formulas, passive values, and common footprints — each linked to a calculator where it helps.
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Battery capacity and runtime
Convert mAh to runtime with load current and efficiency factors.
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Capacitive and inductive reactance
Xc and Xl at a frequency with links to the reactance calculator.
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Capacitor types for bypass and bulk
Ceramic, electrolytic, and film capacitors — where each belongs in a board.
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Common SMD package sizes
Footprint codes for resistors, capacitors, and small ICs — facts only, no substitution guarantees.
Component categories
Selection criteria and widely used part numbers to orient a BOM — not a catalog or storefront.
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Buck (step-down) converters
Switching regulators for efficient step-down rails.
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Ceramic capacitors (MLCC)
Dielectrics, voltage derating, and bypass placement.
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Electrolytic capacitors
Bulk input and output capacitors — ripple, ESR, and lifetime.
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General-purpose diodes
Rectifiers and switching diodes for protection and ORing.