Two Transistor DC Motor Driver Circuit

Posted by Micro System

on Monday, September 27, 2010

This is a circuit for controlling an ordinary DC motor using a pair of transistors (1 NPN and 1 PNP). Note the dual supply of this circuit (+6V and -6V). A DC motor runs in one direction if the required voltage is applied across its winding and runs in the opposite direction if the polarity of the applied voltage is reversed. This is the figure of the circuit;

 

In this circuit, a positive voltage at the control input turns on Q1 (an NPN transistor) but turns off Q2 (a PNP transistor).  This causes current from the +6V supply to flow through the motor from node A to B (ground), making it turn in the forward direction. On the other hand, a negative voltage turns off Q1 and turns on Q2, causing current to flow from node B to node A of the motor, then to the negative supply through Q2, making it turn in the opposite direction.  An input of 0 V stops the motor.

Battery Powered Voltage Reference

Posted by Micro System

on

This is a design circuit for a circuit of a battery-powered voltage reference that draws only 17uA of supply current. Two AA cells can power this reference over 18 months at this level. Drift of the reference is only at 5.5 uV/°C over the industrial temperature range at an output voltage of 1.23 V @25°C. The load regulation is 85 uV/mA and the line regulation is 120 uV/V. This is the figure of the circuit;

The bandgap technique is based of the design reference. With scaling resistors R1 and R2, we can produces unequal currents in Q1 and 2. Temperature proportional voltage across R3 is created by the resulting VBE mismatch which, in turn, produces a larger temperature-proportional voltage across R4 and R5. This voltage appears at the output added to the VBE of Q1, which has an opposite temperature coefficient. To produce minimum drift over temperature, we can adjust the output to 1.23 V at 25′C. Bandgap references can have start-up problems. The OP90 is beyond its positive input range limit and has an undefined output state if there is no current in R1 and R2. Shorting Pin 5 (an offset adjust pin) to ground, forces the output high under these conditions and ensures reliable start-up without significantly degrading the OP90’s offset drift. [Circuit's schematic diagram source: analog.com].

Battery Disconnect Switch Circuit

Posted by Micro System

on Tuesday, September 14, 2010

This is a circuit battery disconnect switch or cut-off (breaker circuit). This circuit is designed to automatically turn off any battery operated device when its user has forgotten to switch it off within a preset period of time and so it will preserve the battery’s life. This is the figure of the circuit;

How does the battery cut-off switch work? When the device connected to this circuit is turned on (S1), the capacitor C1 gets a voltage of +9 volts through the diode D1. The connected device is now turned on as in a normal case. C1 slowly charges via R2. After two, three minutes, the gate voltage of T3 is much reduced to a level that T3 turns off. At this time, T2 turns off the power output to the connected device. Using this battery disconnect switch circuit is simple. To build it in the device, the positive line of the power (battery) to the device must be cut and the circuit (S1 and output) connected in series to this positive line. The ground line of the circuit must be connected to the minus line.

LVDT Signal Conditioner Design Procedure (AD589-Single Supply)

Posted by Micro System

on Sunday, September 5, 2010

This is the circuit for LVDT (Linear Voltage Differential Transformer) need a complex excitation circuit and signal conditioning to work. Fortunately, Analog Devices provide AD598 for complete solution of LVDT signal excitation and conditioner to ease your life. AD598 can be operated in single supply mode, great for battery-powered LVDT Application. This is the figure of the circuit;

 

The design procedure for determining the components values can be obtained in the datasheet of AD598 from Analog Devices.

On-Off Temperature Control Circuit

Posted by Micro System

on Tuesday, August 17, 2010

This is a circuit for controls a load (in this case a dc brushless fan) based on a temperature compared with a set point. The transducer is a diode in the forward polarization regime. In fact when forward biased, the forward voltage drop across a diode has a temperature depends, in particular has a negative linear slope. This is the figure of the circuit;

Anyway this circuit comparator a precise voltage reference (zener) with the forward voltage drop of the diode forward biased with 11mA of current. The comparator is simply a LM158/258/358 working in open-loop mode, the inverting input is connected to the diode sensor, and the non-inverting to the reference voltage. Se when the temperature rises above the set point, the forward voltage drops under the voltage reference and the comparator output is vcc turning on the transistor and so the fan.

Higher power transistor can be substituted for bigger fans, or you can substitute a relay, IGBT, mosfet etc to control higher loads (and higher voltages). The set point is adjusted with the potentiometer, and you can use a LM3914 led driver to make a temperature set point indicator (needs careful calibrations and the use of excel to calculate slope and intercept). Many modifications can be done, but the circuit works very well in its basic form. The comparator can distinguish 10uV differences so approx 0.01°C differences (carefully adjusting the potentiometer can allow to feel body heat from 1/2 cm from the sensor, or feel ambient heat, making to turn the fan on and off continue). You can control temperatures up to 140°C (150 max diode temperature), but linearity is not ensured.

TTL Audible Logic Probe Circuit

Posted by Micro System

on Tuesday, July 27, 2010

This is a circuit for audible logic probe. This circuit based on LM339 for control the operation circuit. This is the figure of the circuit;


When testing circuits with a logic probe, it is sometimes difficult to watch the LEDS on the probe to determine the logic state. With this probe the logic states are audible. This probe is designed for TTL circuits only but could be modified for CMOS. The way it works is as follows. The 5 volt power source will be the circuit under test. Clip the ground input of the probe to the ground of the circuit being tested. The other input lead is used to probe the different chips of the circuit being tested. Any input greater then 2 volts will be high and output a high tone through the speaker. Any input less then .8 volts will be low and produce a low tone through the speaker.