Simple Transistor Checker
Introduction: Simple Transistor Checker
Transistors (derived from transfer resistance), are the basic building blocks of our technological world.
You may have a box full of assorted transistors or you may have them segregated by type in individual labelled draws or boxes.
In either case you may want to determine the type and/or whether it is functional or not.
Unless, you have some type of dedicated component tester you may resort to using a DMM in diode or resistance mode unless a transistor test mode is provided.
However, if you have a large number of assorted transistors, purchased as a mixed bag or had your carefully labelled boxes end up as an unintentional mixed bag using a DMM with basic functions can be a long process.
Have only a DMM with basic functions than the following simple transistor checker will complement your tool set.
Supplies
Stripboard.
Suitable box - L105mm X W90mm x H35mm
CD4093
1uF/16V Electrolytic Capacitor.
10uF/16V Electrolytic Capacitor.
10nF Ceramic Capacitor.
100kR/125mW resistor.
68kR/125mW resistor.
1kR/125mW resistor.
220R/125mW resistor - Qty 2
5mm Red LED low current
5mm Green LED low current
BS270 NFET - Qty 2
14 pin DIP IC Socket.
SIL header strip.
18 AWG Tinned copper wire
SPDT (ON-OFF-ON) switch.
3 pin Transistor socket - Qty 2
6mm drill bit
5mm drill bit
2mm drill bit
Note: I have no affiliation to the links supplied they are merely provided for reference as a possible source, other sources more suitable to your needs be may be used.
Step 1: Function
The checker will simply indicate that the Transistor is functioning or not without any numerical or text based information by the use of LED's
These LED's will indicate the polarity type and condition of the transistor being checked.
The faults are likely to fall into 2 main categories low and high resistance.
Where:
Low resistance - a short circuit (SC), effectively 0R's.
High resistance - an open circuit (OC), effectively 100's MR.
However, low and high resistance is relative to the component construction and the type of fault.
A device may be resistive at some level that's neither short nor open and may of may not be faulty.
Under these circumstances differentiation between a good and bad unit may not be immediately obvious. Due to variations in parameter characteristics.
However, comparison between a know good component either verified under operational conditions or straight out of an unopened protective bag, could be applied to identify differences for a particular component type within the range of the checkers capability.
Usage would require placing the transistor into a socket and or connecting flying leads with clips to the leads and selecting the type with the polarity switch, N type or P type will be indicated by illumination of the appropriate polarity LED.
Faults will be indicated by variations in the LED indicators.
Step 2: How It Works
In use the circuit is powered at 5V, USB being an ideal source, taking ~0.5mA with the polarity switch in the off position.
The main component in the circuit is a CD4093 (Quad 2 input Schmitt trigger).
The <PERSON> trigger utilises a comparator circuit with dual thresholds and positive feedback, the output is triggered to change state when the input crosses the upper or lower thresholds and retains the current state when the input is between thresholds. This action re-enforces the change at the threshold discriminating against changes between the thresholds.
A standard gate does not have hysteresis and in the same configuration will likely settle at ~1/2VCC and therefore not guaranteed to oscillate predictably.
This action of the <PERSON> trigger is utilised to enable a single gate oscillator with the addition of a resistor and a capacitor.
T = RC ln ([VT+/VT-]*(VDD-VT-/VDD-VT+)) = ~291mS, F=1/T = approx. 3.5Hz
Where RC are the timing components, VT+/VT- are the <PERSON> trigger input thresholds.
The output of the oscillator is connected to the inputs of two other gates these and the other two gates are all configured as inverters by connecting together the 2 inputs on each gate.
One invertor drives both polarity indicating LED's whilst at the same time drives the base via a differentiator.
The capacitor/resistor differentiator applies an alternating +3.5V & -3. 134