When a schematic worksheet, exam, or CAD prompt asks you to draw the symbol for the following components, mixing up regional drafting standards is the most common way to lose points or miswire a board. North American engineers typically use ANSI/IEEE 315, while the rest of the world relies on IEC 60617. Below is the exact reference you need to draft, read, and verify these symbols correctly, followed by the specific pitfalls that trip up most hobbyists and trade students.
The Master Schematic Symbol Reference Table
This table covers the fundamental passive, active, and electromechanical components you will encounter on any basic schematic. Use this as your direct answer key when prompted to draft these shapes.
| Component | ANSI/IEEE 315 (North America) | IEC 60617 (International) | Practical Circuit Meaning |
|---|---|---|---|
| Resistor | Zigzag line (usually 4-6 peaks) | Hollow rectangle | Restricts current flow; dissipates power as heat. |
| Capacitor (Non-Polarized) | Two parallel straight lines | Two parallel straight lines | Stores energy in an electric field; blocks DC, passes AC. |
| Capacitor (Polarized) | One straight line, one curved line (+ on straight) | One straight line, one curved line, or hollow rectangle with + | Electrolytic/Tantalum; stores higher capacitance; explodes if reverse-biased. |
| Inductor | Series of 3 or 4 connected semi-circles (humps) | Series of 3 or 4 connected semi-circles | Stores energy in a magnetic field; resists changes in current. |
| Diode | Triangle pointing to a perpendicular line | Triangle pointing to a perpendicular line | Allows current in one direction (anode to cathode); drops ~0.7V (Silicon). |
| NPN Transistor (BJT) | Circle with vertical bar; arrow pointing OUT on emitter | No circle; vertical bar; arrow pointing OUT | Current-controlled switch; arrow indicates conventional current flow direction. |
| Relay Coil | Circle or rectangle with diagonal lines | Rectangle (often labeled 'K' or 'KA') | Electromagnet that actuates isolated switch contacts when energized. |
| Normally Open (NO) Contact | Two parallel lines with a diagonal slash bridging them | Two parallel lines with a diagonal slash | Switch is open at rest; closes when the coil is energized. |
| Normally Closed (NC) Contact | Two parallel lines with a diagonal slash and a cross-line | Two parallel lines with a diagonal slash and cross-line | Switch is closed at rest; opens when the coil is energized. |
Regional Variants: ANSI/IEEE vs. IEC vs. Old UK
Before you put pen to paper or place a part in KiCad, you must know which standard your region or company mandates. Mixing them on a single drawing is a major drafting error.
- ANSI/IEEE 315 (North America): Dominant in the US and Canada. Recognized by its use of the zigzag resistor and circular enclosures for transistors and relay coils. If you are working with IEEE standard documentation or older US industrial panels (NEMA style), use this.
- IEC 60617 (International): The standard across Europe, Asia, and modern global CAD libraries. It favors rectangular boxes for almost all passive components and removes the outer circles from transistors to reduce visual clutter. Modern PLC schematics almost exclusively use IEC ladder logic symbols.
- Legacy BS 3939 (Old UK): Withdrawn in favor of IEC 60617, but you will still find these symbols in older British industrial plants and legacy marine wiring. It used unique graphical representations for things like variable resistors (a rectangle with an arrow through it at an angle) that differ slightly from modern IEC drafting.
The 'Rows People Get Wrong' Notes Section
When grading exams or reviewing junior engineer schematics, these specific symbol variations are consistently drawn incorrectly. Pay close attention to the geometry.
1. NPN vs. PNP Transistor Arrows
The arrow on a Bipolar Junction Transistor (BJT) is always on the emitter leg. For an NPN transistor, the arrow points outward (away from the base). For a PNP, it points inward (toward the base). The standard mnemonic is that NPN stands for 'Not Pointing iN'. If you draw the arrow on the collector leg, the schematic is invalid.
2. Polarized vs. Non-Polarized Capacitors
Drawing two straight parallel lines for an electrolytic capacitor is a critical error that can lead to reverse-polarity wiring on the bench. The negative plate must be drawn as a curved line in ANSI/IEEE, or explicitly marked with a '+' on the positive plate in IEC. Furthermore, the curved side always represents the outer foil in non-polarized film caps, which should be connected to the lower-impedance (usually ground) side of the circuit to act as a shield.
3. NO vs. NC Switch Contacts
The difference between a Normally Open and Normally Closed contact is a single perpendicular tick mark crossing the diagonal slash. People frequently draw the NC tick mark parallel to the contact lines, which accidentally creates a symbol for a capacitor. The tick mark must intersect the diagonal bridging line at a 90-degree angle.
Safe Interpretation When Markings Are Faded or Missing
On the bench, you will frequently encounter PCBs with degraded silkscreen or schematics where the ink has faded, leaving you to guess the component type. Here is how to safely deduce the component without relying solely on the drawing.
- Set your DMM to Diode Test Mode: If you are looking at a faded 3-pin semiconductor, use a multimeter (like a Fluke 117) in diode test mode. If you read a ~0.6V drop between two pins in one direction and an open circuit ('OL') in the other, you are looking at a BJT. The pin that triggers the drop with both other pins is the Base.
- Check for Gate Capacitance: If the 3-pin device reads 'OL' in both directions across all pins, it is likely a MOSFET. You can confirm this by touching the positive probe to the Gate and the negative to the Source, then checking for continuity between Drain and Source.
- Look for Physical Asymmetry: Faded polarized capacitors often have a physical indicator. Look for a crimped base, a longer positive leg, or a subtle stripe indicating the negative terminal. Never apply power to a deduced polarized cap without verifying the orientation with the circuit's ground plane.
Frequently Asked Questions
How do I draw the symbol for the following: a relay versus a contactor?
In IEC 60617, both use a rectangle for the coil, but the alphanumeric designation differs. A relay coil is labeled with a 'K' (e.g., K1, K2), while a contactor coil is labeled with 'KM' (e.g., KM1). When drawing the contacts, relay contacts are typically drawn as standard NO/NC switch symbols, whereas contactor power poles are often drawn with a specialized arc-chute symbol (a small loop next to the contact) to indicate they are rated to break high inductive motor loads. For a deep dive on contactor drafting, refer to All About Circuits' schematic guide.
What is the correct way to draw the symbol for the following: an N-channel MOSFET?
Draw a vertical channel line. Draw the Gate pin as a line that comes close to the channel but does not touch it (indicating the insulated gate). Draw the Drain and Source pins connecting to the channel. For an enhancement-mode N-channel MOSFET, the channel line must be broken into three distinct segments. If you draw a solid, unbroken channel line, you have drawn a depletion-mode MOSFET, which is normally ON and requires a negative gate voltage to turn off—a very different component.
If a test says 'draw the symbol for the following ground types,' what are the differences?
There are three primary ground symbols you must distinguish. Earth Ground is drawn as a vertical line with three descending horizontal lines (representing a physical rod in the dirt). Chassis Ground is drawn as a vertical line with three diagonal lines fanning out to the left (representating connection to a metal enclosure). Signal/Common Ground is drawn as a simple solid triangle pointing down, or a single horizontal line. Mixing these up in a mixed-signal PCB design will result in noisy ADC readings and ground loops.
