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Circuits designed by David Johnson, P.E.
Last Updated on:
Thursday, March 22, 2012 05:26 AM
Master Category List
- Dave's Circuits
Text & Graphics
Copyright © David A. Johnson, PE - ALL Rights Reserved.
LINKING to Dave's circuits is permitted but DO NOT COPY to your WEB SITE server! |
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Transistors Circuits |
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CHARGE COUPLED BI-DIRECTIONAL POWER MOSFET RELAY
The circuit uses an inexpensive C-MOS inverter package and a few small
capacitors to drive two power MOS transistors from a 12v to 15v supply. Since
the coupling capacitor values used to drive the FETs are small, the leakage
current from the power line into the control circuit is a tiny 4uA. Only about
1.5mA of DC is needed to turn on and off 400 watts of AC or DC power to a
load.
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CIRCUIT
PROTECTS FROM BATTERY POLARITY REVERSAL
This simple circuit can protect a sensitive electronic circuit from an accidental
connection of a battery with a reversed polarity. The N-channel FET connects the
electronic device to the battery only when the polarity is correct. The circuit
shown was designed for a device powered from a single 1.5 volts button cell battery.
However, the circuit will operate with higher voltages as well.
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FET INPUT
HIGH SPEED LIGHT DETECTOR
This circuit is yet another design that converts current from a PIN photo diode to a
voltage. It has a bandwidth that extends beyond 50MHz.
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FINGER
TOUCH ACTIVATED SWITCH
It does not get any easier if you want a solid-state switch that is activated by the
touch of a finger. Two small metal pins route voltage through the finger skin to a
MOSFET switch. The circuit is great for situations where a membrane type mechanical
switch is not desired.
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High
Impedance JFET Buffers
I have used these circuits many times. They are great when you need a low gain AC
signal amplifier with a very high input impedance. It is good to beyond 50MHz.
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N-CH AND
P-CH TRANSISTORS FORM PUSH-PULL DRIVER
This circuit can produce high speed output signals with fast rise and full times. The
unique change pump action allows the voltage of the upper P-ch device to range from
millivolts to hundreds of volts. The output current is only limited by the rating of the
transistors. I have used this circuit beyond 2MHz.
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NPN/PNP
TRANSISTOR TESTER
I designed this circuit about 30 years ago and still use it from time to time. It is
powered from a 3v battery. It makes a tone when the transistor tests good. It can be
used to determine device type (NPN or PNP) and the base, collector or emitter transistor
pins. It can also test most transistors will still connected to a circuit.
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