Connectors 101

What is a connector?

A connector is an electrical component that links two or more components together. The major types of connectors are:

• Wire to wire (e.g., an in-line connector that connects a battery to a wire harness)
• Wire to board (e.g., a connector that attaches a sensor to a printed circuit board)
• Board to board (e.g., connectors used to mate a daughterboard with a motherboard)
• Power connectors, which handle higher currents
• Modular connectors (e.g., Ethernet plugs)
• Terminal blocks and barrier strips, which are useful for connecting and disconnecting wires with screw terminals (e.g., a connector used inside a sprinkler or security control box)

What companies manufacture connectors?

• Molex (free samples request)
• TE Connectivity (free samples instructions)

You can also order connectors directly from distributors (e.g., Newark, Digikey, Mouser)

How do I choose a connector?

Fuses 101

What are fuses and when do I need them?

A fuse is an electronic component that protects a circuit by creating an open circuit if it draws too much current. Fuses do not protect against voltages that are too high. 

Fuses are most useful around power supplies, both when power comes into a system and on individual power rails. For example, if you have a 12V rail that should only draw 750 mA and a 5V rail that should only draw 400 mA, you might put a 750 mA fuse in series with the 12V supply output and a 500 mA fuse in series with the 5V supply output.

A common mistake that fuses protect against is accidentally shorting the power rail to ground (e.g., with a DMM probe as you are trying to touch the correct pins). This causes an infinite amount of power to be drawn from the power supply, and can sometimes damage or destroy voltage regulators or power supplies.

What are the most common types of fuses?

• Standard fuses contain a small piece of wire made of an alloy that melts readily. If the current gets too high, it burns out and opens the circuit. Standard fuses come in many different shapes and sizes.
• Advantages: Inexpensive, replaceable
• Disadvantages: Must be replaced when blown, sometimes no visual indication that they are blown
• Examples from Littelfuse
• Resettable PTC fuses are tripped by heat caused by current above the rating of the fuse. If the current flowing through the fuse goes above its rating, the fuse changes from having very low resistance to having a high resistance (thereby stopping the flow of current through the circuit). It is self-resetting after power is removed and the fuse cools.
• Advantages: Reusable, useful when cause of overcurrent is a user failure (e.g., plugging a connector into the wrong pins on a PCB)
• Disadvantages: More expensive than standard fuses, small leakage current after breaking, significantly lower voltage ratings (60V) compared with non-resettable fuses (600V)
• Examples from Littelfuse
• Circuit breakers are typically used in AC power applications like homes.
• Advantages: Reusable and easy to reset by hand. This reduces downtime and repair costs. Easy-to-see indication of whether the circuit breaker is tripped. Useable as an on-off switch.
• Disadvantages: Very expensive, not well suited for low-voltage DC applications

DC Motors 101

Watch the video mini-lecture on DC motor control from Dr. Jordan

What types of DC motors are most common?

• Brushed DC motors. Current flowing in one direction makes the motor turn one direction, and current flowing in the opposite direction makes the motor turn the opposite direction.
• Stepper motors - provide absolute position control but require a special controller IC
• Servo motors - PWM signal controls the position of the motor

How do I connect a brushed DC motor to a microcontroller?

DC motors are inductive loads and therefore cannot be directly connected to a microcontroller without damaging the microcontroller. There are two protection mechanisms that go into a successful motor interface:

1. Back EMF / flyback diode - protects against current spikes that can damage a microcontroller. Use "rectifier" diodes (e.g., 1N400x series)
2. H-bridge - switches current on or off to the motor, and changes the direction of the motor

The LMD18200 is a common off-the-shelf H-Bridge IC.

How do I change the speed of the motor?

Generate a Pulse Width Modulation (PWM) signal from your microcontroller and use it to control the H-bridge.

How does a stepper motor work?

See the excellent animated GIFs in the Stepper motor entry on Wikipedia

How do I connect a stepper motor to a microcontroller?

Use an off-the-shelf driver IC instead of trying to build your own driver. The DRV8825 is a common stepper motor control chip.

Getting Started with Cadence

Introduction

A number of basic Cadence tutorial videos are available on YouTube.

Getting Started

1. In Windows, find and open the application Capture CIS (see Cadence Schematic Tutorials)

2. Create and name a new project and add existing part libraries (see Creating a New Project in Cadence)

3. Add parts to your schematic. First, check to see if the parts you need already exist in a library. For example, schematic symbols for header pins (which are useful for connecting to components external to your custom printed circuit board, like batteries, PSoC® boards, and sensors, see Figure 1) can be found by searching for "Header" in the Place Part dialog box (see Figure 2)

Figure 1: Header

Figure 2: Place Part dialog box

4. Create a custom library to store custom schematic symbols for your design that are not included in the built-in libraries (very common), and add it to your project (see Creating a Custom Library in Cadence)

5. Create and save custom schematic symbols in your custom library. If you later need to edit a custom schematic symbol, make sure to replace the edited symbol in the cache (see Creating a Custom Schematic Symbol in Cadence)

6. Create printed circuit board footprints for all components on your printed circuit board

1. For electronic devices with DIP/SOIC/PLCC/QFP/PGA/SIP/ZIP packages (which are typically integrated circuits and plug-in modules with evenly-spaced pins), it is easiest to use the "Package Symbol Wizard" (See Creating a Custom PCB Footprint using Package Designer in Cadence)
2. For all other components, create custom footprints manually (see Creating a Custom PCB Footprint Manually in Cadence)

7. Create custom padstacks for the pads of each custom footprint (See Creating a Custom Padstack in Cadence)

8. Link all footprints to schematic symbols in Design Entry CIS by updating the library search path (see Changing the Library Search Path in Cadence Design Entry CIS)

8. Link all footprints to schematic symbols in PCB Editor by updating the library search path (see Changing the Library Search Path in Cadence PCB Editor)

9. Transfer the schematic to PCB Editor for the PCB layout and design (see Transferring a Cadence Schematic to PCB Editor)

10. Prepare and submit your PCB Layout for fabrication

Based on a tutorial by Cody Van Cleve

Power Supplies 101

What are the components of a basic power supply subsystem?

1. Source of power - battery, AC adapter, solar
2. Input electronic noise filtering circuit
3. Voltage regulator to provide a constant supply voltage
4. Output electronic noise filtering circuit

What are the design considerations for a battery-operated circuit?

• How many Amp-Hours of capacity are needed?
• Create a power budget to determine
• What type of batteries?
• See Battery University for more information
• Are the batteries rechargeable?
• NiCd, NiMH, LIB, LiPoly? Different tradeoffs of cost, memory, lifetime, weight
• Need a charging circuit. See Maxim Integrated Circuits
• Discharge Rate
• Batteries can only discharge up to a certain limit to remain safe.  Different chemistries have different limitations.

What are the design considerations for an AC-powered circuit?

The language "AC adapter" and "AC power supply" is used inconsistently. Sometimes it refers to regulated power supplies with a fixed voltage output, and sometimes it refers to unregulated power supplies with a promised voltage output that is inaccurate when measured with a DMM. More information on AC power adapters is available here: http://www.dxing.info/equipment/wall_warts_bryant.dx

Linear Power Supplies

Linear power supplies are made up of a transformer, diodes, and sometimes a filter capacitor. They provide clean power, but waste lots of energy in the form of heat. See http://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/rectifier-circuits/ for more information on how a linear power supply works.

Switching Power Supplies

Switching power supplies are most commonly used in PCs and other digital applications. They have a "noisy" power output because of the switching frequency, and are not ideal for inductive loads. They also tend to be more expensive, but are much more energy-efficient and generate less heat.

How do I choose a voltage regulator?

There are three major types of voltage regulators that you will encounter in this program:

• Linear voltage regulator - burns off extra power in the form of heat
• Advantage: High current capacity
• Advantage: Low cost
• Disadvantage: Low precision
• Disadvantage: High dropout voltage (the voltage difference between the input and output)
• Low dropout voltage regulator - burns off extra power in the form of heat, but continues to regulate even when the input voltage is near the output voltage
• Advantage: Excellent for battery-powered applications
• Advantage: Higher precision
• Disadvantage: Lower current capacity
• Disadvantage: Higher cost
• Switching voltage regulator - generates as much power as is needed by the circuit, up to the maximum allowed by the regulator. Similar to Switching Power Supplies above
• Advantage: Highly energy efficient
• Disadvantage: High cost
• Disadvantage: External components (some difficult to find) may be necessary
• Disadvantage: Printed circuit board layout can be difficult

A highly detailed guide to linear and switching regulators is available here: http://www.ti.com/lit/an/snva558/snva558.pdf

What capacitor values should I use for input and output noise filtering?

First, check the data sheet for the voltage regulator and see if it specifies values for the input and output filtering capacitors. If it does not, the rule of thumb is to use 0.33 µF ceramic non-polarized capacitors for input filtering and 0.1 µF ceramic non-polarized capacitors for output filtering.

What capacitor values should I use for bypass filtering near each integrated circuit?

Digital integrated circuits (ICs) need "bypass capacitors" to both filter the power going into the IC and provide a nearby reservoir of energy that the IC can draw from instantaneously. The rule of thumb is to put one 0.1 µF ceramic non-polarized capacitor between each power pin on an integrated circuit and ground. This means a microcontroller might have 2 - 8 or more capacitors, one for each power pin on the IC.

Where can I find additional information about power supplies?

• Wikipedia

Where can I find companies that manufacture power supply components?

• Raw components: Component Manufacturers
• Component distributors: Dr. Jordan's Favorite Distributors