Basic Circuit Building Blocks

These are circuits and parts of circuits that we see over and over again in larger projects. Understanding a complex circuit is much easier if you understand these building blocks.

Current Sense Resistor ( Shunt Resistance )

A current Sense Resistor is a low value of resistor that is placed in series with some other circuit. We can then measure the voltage across the resistor to compute the current. If the resistor has a low value compared to other components we can ignore the effect on the circuit. We use the word shunt when the voltage is measured by a device that has a fairly low resistance itself. We then have to do a more careful calculation of how the current is shared by the two devices.

More information:

1. Scienceshareware.com's How A Precision Resistor Is Used to Measure / Calculate Current and Power in an Electrical Circuit.
2. High-Side Current-Sense Measurement: Circuits and Principles
3. Shunt (electrical) From Wikipedia, the free encyclopedia

• Other ways to measure current: Motor_driver#current_sense

Transistor Low Side Switch

Use this circuit when you wish to turn a load on and off with both a low voltage and a low current. Note that neither side of the load is grounded.

A low side switch is one which switches a circuit on and off at the ground or low side of the circuit. The advantage of a low side switch is that when using a transistor as the switch the voltage to drive the transistor is itself a low voltage. It is often the easy way to drive leds motors and other high current devices from such low power devices as PIC output ports. Low side switche are popular and there are many integrated circuits for them as well as this circuit.

Circuit:

Transistor Low Side Switch

Where

• LED is a low power LED
• R_LED is a current limiting resistor for the LED
• Q is a bipolar transistor
• R_1 is a current limiting resistor transistor base current
• VPLUS_VDD is the power supply for the LED

The voltage at the collector of the circuit should fall to a fraction of a volt when the input is high. To acomplish this:

• Compute the value of R_LED using ohms law and the specifications for the LED.
• Compute the current through the LED.
• The transistor must supply the current, it should be equal approxtely to the input voltage divided by R1 times the beta of the transistor.

An example calculation would be nice, and will appear later.