Difference between revisions of "Basic Circuit Building Blocks"

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== Transistor High Side Switch ==
 
== Transistor High 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.
+
Use this circuit when you wish to turn a load on and off with a voltage at a low current.  Note that low  side of the load is grounded. The voltage to turn on the switch is equal to the supply voltage ( or perhaps just a bit larger )
  
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.
+
A high side switch is one which switches a circuit on and off at the supply voltage or high side of the circuit.  The advantage of a high side switch is that the load is grounded on one side.  Compared to the low side switch it needs a higher voltage to drive it, but it also eliminates one resistor of that circuit.  It the voltage to drive it is available it may be the circuit of choice.  It is often the easy way to drive leds motors and other high current devices from such low power devices as PIC output ports.  
  
 
Circuit:
 
Circuit:
  
[[Image:tran_lss.png | Transistor Low Side Switch ]]
+
[[Image:tran_hss.png | Transistor High Side Switch ]]
  
 
Where
 
Where
 
*LED is a low power LED
 
*LED is a low power LED
 
*R_LED is a current limiting resistor for the LED
 
*R_LED is a current limiting resistor for the LED
*Q is a bipolar transistor  
+
*Q is a bipolar transistor  
*R_1 is a current limiting resistor transistor base current
 
 
*VPLUS_VDD  is the power supply for the LED
 
*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 compute the values in the circuit:
  
 +
*Compute the value of R_LED using ohms law and the specifications for the LED.
 +
*The current to drive the circuit is approximately the current to drive the load divided by the beta of the transistor.
  
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.
+
No resistor is needed into the base of the transistor because as the load draws current the voltage at the base will rise and limit the base current.  The input voltage should be about equal to VPLUS_VDD, high compared to that needed for the low side switch.
  
 
== Transistor Emitter Follower ==  
 
== Transistor Emitter Follower ==  

Revision as of 07:02, 16 January 2008

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.

Series Circuit

Parallel Circuit

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

Filter Capacitor / Decoupling Capacitor / Low Pass Filter

Pull Up and Switch

Use this circuit when you want to feed a user input to a digital circuit, for example a PIC input pin.

A pull up is a fairly high value resistor ( say 1 to 100 K ohms ) that is connected to the positive side of the power supply. This makes the other end of the resistor the same voltage as the power supply ( as long as it is connected to a high impedance. The other end of the resistor is connected to a switch that is then connected to ground. When the switch is connected current flow through the resistor dropps the entire power supply voltage and the input voltage for the circuit is now 0 (sometimes called active low, since when the switch is active the output is low). Pull up is sometimes used without the switch to keep a signal high all the time.

Circuit:

Pull Up and Switch

Where

  • PUSH_BUTTON_SWITCH is a push button switch
  • R_LED is a current limiting resistor for the LE
  • R_PULLUP is the pull up resistor
  • VPLUS_VDD is the power supply voltage

More information: Very Basic Circuits

Pull Down and Switch

Op amp Non Inverting Amplifier

Op amp Unity Gain Buffer

Use this circuit when you have a signal of high impedance ( can supply only a little current ) that you want to connect to another circuit that draws a significant current ( up to about 10 ma for the typical op amp. ). For example if you wish to measure the out put of a voltage divider with a 0 to 1 ma meter a unity gain buffer might be just what you need. This circuit is also know as a voltage follower.

The unity gain buffer has an output voltage just the same as the input voltage. The advantage is that the input circuit does not “feel” the output. That is the input acts pretty much like a very large resistor ( many mega ohms or more ) connected to ground, and the output supply's whatever current ( up to about 10 ma ) is necessary to maintain the output voltage. Here is the circuit:

Circuit:

Op Amp Unity Gain Buffer

Where

  • INPUT the input signal you wish to buffer
  • RIN the input resistor, often 0 ohms.
  • OPAMPA the op amp
  • RFB the feed back resistor
  • OUTPUT the output

The values of RIN and RFB are not very critical and are normally 0 ohms, just a straight connection. The op amp here is a quad or 4 op amp part, we are using just one section of it. Power needs to be supplied to pin 8 and 4 in the usual way for op amps.

More information:

Buffer amplifier From Wikipedia, the free encyclopedia

Operational Amplifiers

Op-Amp Buffer

Light Emitting Diode ( with current limiting resistor )

Three Terminal Regulator

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.

Transistor High Side Switch

Use this circuit when you wish to turn a load on and off with a voltage at a low current. Note that low side of the load is grounded. The voltage to turn on the switch is equal to the supply voltage ( or perhaps just a bit larger )

A high side switch is one which switches a circuit on and off at the supply voltage or high side of the circuit. The advantage of a high side switch is that the load is grounded on one side. Compared to the low side switch it needs a higher voltage to drive it, but it also eliminates one resistor of that circuit. It the voltage to drive it is available it may be the circuit of choice. It is often the easy way to drive leds motors and other high current devices from such low power devices as PIC output ports.

Circuit:

Transistor High Side Switch

Where

  • LED is a low power LED
  • R_LED is a current limiting resistor for the LED
  • Q is a bipolar transistor
  • 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 compute the values in the circuit:

  • Compute the value of R_LED using ohms law and the specifications for the LED.
  • The current to drive the circuit is approximately the current to drive the load divided by the beta of the transistor.


No resistor is needed into the base of the transistor because as the load draws current the voltage at the base will rise and limit the base current. The input voltage should be about equal to VPLUS_VDD, high compared to that needed for the low side switch.

Transistor Emitter Follower

Voltage Divider

Light Emitting Diode ( with current limiting resistor )

Push Pull Circuit ( with transistor emitter follower )

Darlington connected Transistors

Schmitt Trigger