{"id":52227,"date":"2026-11-01T10:50:42","date_gmt":"2026-11-01T10:50:42","guid":{"rendered":"http:\/\/siratek.net\/index.php\/tutorials\/electricity-for-makers\/"},"modified":"2026-11-01T10:50:42","modified_gmt":"2026-11-01T10:50:42","slug":"electricity-for-makers","status":"publish","type":"siratek_tutorial","link":"https:\/\/siratek.net\/index.php\/tutorials\/electricity-for-makers\/","title":{"rendered":"Voltage, Current and Resistance"},"content":{"rendered":"<body><p><\/p>\n<h2>What this covers<\/h2>\n<p>Enough electrical theory to stop you destroying parts, and no more. Three quantities, one equation, and the two calculations you will actually do: choosing a resistor for an LED, and working out whether a pin can drive something.<\/p>\n<h2>Before you begin<\/h2>\n<p>Nothing to wire. A calculator helps. If you have a multimeter, keep it nearby, because every quantity here is one you can measure rather than take on trust.<\/p>\n<h2>The three quantities<\/h2>\n<table>\n<thead>\n<tr>\n<th>Quantity<\/th>\n<th>Unit<\/th>\n<th>A rough picture<\/th>\n<th>What it does to parts<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Voltage<\/td>\n<td>volts, V<\/td>\n<td>The pressure pushing electricity along<\/td>\n<td>Too much punches through insulation and destroys a chip<\/td>\n<\/tr>\n<tr>\n<td>Current<\/td>\n<td>amps, A<\/td>\n<td>How much is actually flowing<\/td>\n<td>Too much heats things until they fail<\/td>\n<\/tr>\n<tr>\n<td>Resistance<\/td>\n<td>ohms, R<\/td>\n<td>How hard it is to push through<\/td>\n<td>Sets how much current a given voltage produces<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>The distinction that matters: <strong>you choose the voltage, the circuit decides the current.<\/strong> You connect something to 5 V, and what flows is whatever the resistance allows. This is why a part rated for 5 V can still be destroyed in a second if nothing limits the current.<\/p>\n<p>Most beginner components are measured in thousandths: milliamps, written mA. An LED is happy at about 20 mA, which is 0.02 A.<\/p>\n<h2>Ohm\u2019s law<\/h2>\n<p>One equation ties the three together.<\/p>\n<pre><code>V = I * R\n\nvoltage = current * resistance<\/code><\/pre>\n<p>Rearranged for the two forms you will use:<\/p>\n<pre><code>I = V \/ R     to find the current a resistor allows\nR = V \/ I     to find the resistor you need for a target current<\/code><\/pre>\n<p>That is the whole of the theory. Everything below is this equation applied to a real part.<\/p>\n<h2>Why an LED needs a resistor<\/h2>\n<p>An LED is not a resistor. It has a roughly fixed <strong>forward voltage<\/strong>, which it drops across itself, and almost no resistance beyond that. Connect one directly to 5 V and the current is limited only by the wiring, which means it is not limited at all. The LED will be very bright for a moment and then permanently dark.<\/p>\n<p>Forward voltage depends on colour, and varies between parts.<\/p>\n<table>\n<thead>\n<tr>\n<th>LED colour<\/th>\n<th>Typical forward voltage<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Red<\/td>\n<td>around 1.8 to 2.2 V<\/td>\n<\/tr>\n<tr>\n<td>Yellow or amber<\/td>\n<td>around 2.0 to 2.2 V<\/td>\n<\/tr>\n<tr>\n<td>Green<\/td>\n<td>around 2.0 to 3.2 V<\/td>\n<\/tr>\n<tr>\n<td>Blue or white<\/td>\n<td>around 2.8 to 3.4 V<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Treat these as a starting point and check the part you bought where the exact figure matters.<\/p>\n<h3>The calculation<\/h3>\n<figure class=\"siratek-tutorial-figure\"><img decoding=\"async\" src=\"http:\/\/siratek.net\/wp-content\/uploads\/2026\/11\/electricity-for-makers-led-resistor.svg\" alt=\"A five volt pin feeding a 220 ohm resistor, then a red LED, then ground, with the working showing why 220 ohms is chosen\" loading=\"lazy\"><figcaption>The one calculation you will do over and over. The resistor drops whatever the LED does not.<\/figcaption><\/figure>\n<p>The resistor has to drop whatever the LED does not, at the current you want.<\/p>\n<pre><code>R = (supply voltage - LED forward voltage) \/ target current<\/code><\/pre>\n<p>For a red LED on a 5 V pin at 20 mA:<\/p>\n<pre><code>R = (5 - 2) \/ 0.02 = 150 ohms<\/code><\/pre>\n<p>Resistors come in fixed values, so round <strong>up<\/strong> to the next one you have. A 220 ohm resistor is the usual choice, gives about 13 mA, and lasts longer than a 150 ohm one. Going higher makes the LED dimmer, not faulty, so when in doubt use the larger resistor.<\/p>\n<p>This is the reason a 220 ohm resistor appears in nearly every beginner kit.<\/p>\n<h2>What a pin can drive<\/h2>\n<p>An Arduino Uno pin can supply about <strong>20 mA<\/strong>, with an absolute maximum of 40 mA at which damage becomes likely. Treat 20 mA as the limit.<\/p>\n<p>That is enough for one LED. It is not enough for a motor, a relay coil, a filament lamp, a string of LEDs, or a buzzer that needs real volume. Those need the pin to switch a separate supply through a transistor, a MOSFET module or a relay, rather than powering the load directly.<\/p>\n<p>There are two further limits worth knowing before you build something large.<\/p>\n<ul>\n<li>The <code>3.3V<\/code> pin on an Uno supplies about <strong>50 mA<\/strong> in total.<\/li>\n<li>The whole board has a total current limit across all pins, so twelve LEDs at 20 mA each is a problem even though each pin is individually within its rating.<\/li>\n<\/ul>\n<h2>Ground, and why every circuit needs it<\/h2>\n<p>Voltage is always a difference between two points. \u201c5 V\u201d means five volts <em>higher than ground<\/em>, and ground is simply the point everything else is measured against.<\/p>\n<p>So a circuit needs a complete loop: out through the component and back to ground. A module connected to <code>5V<\/code> with no <code>GND<\/code> wire has no return path, and will either do nothing or behave erratically. When two boards talk to each other, their grounds must be joined as well, or neither has a shared idea of what zero means.<\/p>\n<p>If something is behaving strangely, check the ground connection first. It is the most common fault in beginner wiring, and it rarely announces itself clearly.<\/p>\n<h2>Series and parallel, briefly<\/h2>\n<ul>\n<li><strong>Series<\/strong>, one after another: the same current flows through each, and the voltages add up. This is why the resistor and the LED in series share the 5 V between them.<\/li>\n<li><strong>Parallel<\/strong>, side by side: each sees the same voltage, and the currents add up. This is why four LEDs in parallel on one pin draw four times the current and exceed the pin\u2019s rating.<\/li>\n<\/ul>\n<h2>Power and heat<\/h2>\n<pre><code>P = V * I     power in watts<\/code><\/pre>\n<p>A resistor turns the voltage it drops into heat. In the LED example, 3 V across 220 ohms at roughly 13 mA is about 0.04 W, comfortably inside the quarter-watt rating of an ordinary resistor.<\/p>\n<p>It is worth calculating when you switch bigger loads. A resistor running near or above its rating gets hot enough to discolour the board, and heat is usually the first sign that something in the design is wrong.<\/p>\n<h2>Check yourself<\/h2>\n<ol>\n<li>You have a blue LED and a 5 V pin, and want about 15 mA. What resistor?<\/li>\n<li>Why does using 470 ohms instead of 220 ohms not damage anything?<\/li>\n<li>A relay coil needs 70 mA. Can a pin drive it directly?<\/li>\n<li>A sensor is wired to <code>5V<\/code> and its signal pin, but not to <code>GND<\/code>. What will the readings do?<\/li>\n<\/ol>\n<p>Answers in order: about 150 ohms, since (5 \u2212 3) \/ 0.015 is 133 and you round up. The LED is dimmer because less current flows, which harms nothing. No, 70 mA is well over the 20 mA limit and needs a transistor or relay module. The readings will be meaningless, because without a return path there is no reference for the measurement.<\/p>\n<h2>Next steps<\/h2>\n<ul>\n<li><strong>Easier first:<\/strong> <em>How an Arduino Actually Works<\/em>.<\/li>\n<li><strong>Next:<\/strong> <em>Using a Breadboard<\/em>, where these connections get made.<\/li>\n<li><strong>Then:<\/strong> <em>Your First Circuit, an LED and a Resistor<\/em>, which is this page applied to real parts.<\/li>\n<\/ul>\n<h2>Sources and attribution<\/h2>\n<p>All text and calculations in this tutorial are original Siratek work.<\/p>\n<ul>\n<li>Arduino Uno R3 documentation and the ATmega328P datasheet, for per-pin and total current limits.<\/li>\n<li>LED forward voltages are given as typical ranges across common parts; they are not taken from any single manufacturer\u2019s document and should be checked against the component in hand.<\/li>\n<\/ul>\n<p>No third-party tutorial text, diagrams, photographs or code has been reproduced here.<\/p>\n<p><\/p>\n<\/body>","protected":false},"excerpt":{"rendered":"<p>Enough electrical theory to stop you destroying parts, and no more. Three quantities, one equation, and the two calculations you will actually do: choosing a resistor for an LED, and working out whether a pin can drive something.<\/p>\n","protected":false},"featured_media":0,"template":"","siratek_tutorial_level":[],"class_list":["post-52227","siratek_tutorial","type-siratek_tutorial","status-publish","hentry"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v28.8 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Voltage, Current and Resistance - siratek<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"http:\/\/siratek.net\/index.php\/tutorials\/electricity-for-makers\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Voltage, Current and Resistance - siratek\" \/>\n<meta property=\"og:description\" content=\"Enough electrical theory to stop you destroying parts, and no more. 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