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.

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.

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.

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 Schmitt 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 Schmitt 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 Schmitt 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 & -3.5V voltage on the base being >VBE to turn the transistor on but less then BVEB to not put the junction in breakdown, the resistor in series with this output sets IB for the transistor, IB varying between approx.10uA to 40uA.

The longer the time constant of the differentiator the the closer the output resembles the input, as we are using visual observation a longer time constant is preferred.

This decaying pulse gives a range for IB allowing for greater device variation in gain rather than a constant value

T = RC = 100K * 10uF = 1000mS a ratio of On time to differentiator time constant of between 5 & 10 will suffice.

The ratio of the oscillator On period to the differentiator time constant is ~7 and within the range.

Subject to the gain of the transistor the intensity of the associated LED will vary, higher gain more intense, lower gain less intense.

Low current LED's (2mA), are used due to the low current output for the CD4093, approx. 5mA max at 5V.

The second inverter connects to the current sink NFET used in NPN mode whilst the third inverter connects to the current source NFET used in PNP mode.

The NFET (BS270), is a low voltage threshold device suitable for use with low voltage logic.

For an NPN transistor.

A positive voltage is applied at the base and simultaneously to the 2 resistors in series with the LED's. At the same time the current source NFET is switched off and the current sink NFET is switch on.

Therefore the current path is via the NPN indicating LED the collector emitter (CE), of the transistor and current sink NFET. Under normal circumstance only the NPN LED is on as the PNP LED anode is pulled low by the FET reverse biasing the LED and therefore switching it off.

For a PNP transistor.

A negative voltage is applied at the base and simultaneously to the 2 resistors in series with the LED's. At the same time the current source NFET is switched on and the current sink NFET is switch off.

Therefore the current path is via the current source NFET the CE of the transistor and PNP indicating LED. Under normal circumstance only the PNP LED is on as the NPN LED anode is pulled high by the FET reverse biasing the LED and therefore switching it off.

A polarity switch is provided to enable the transistor CE to be switched allowing a Forward or Reverse condition to be applied without removing the transistor.

For Transistors, see the table for typical LED modes subject to various transistor conditions.

FETs and diodes can both be checked with this project.

The circuit is assembled on Stripboard of 20 x 20 holes.

Identify the location where the socket will be mounted and cut the tracks between the pins using a track cutter or 3mm drill bit then solder the socket on place.

Cut the other tracks and fit the wire links

Fit the capacitors resistors and SIL pins.

The SIL pins will be used to mount the FET's and enable connection of the off board components.

The offboard components are the LED's the SPDT switch, power connections and transistor socket.

CN2 goes to the base and CN1 & CN3 go to the collector and the emitter.

The box only requires a few holes, the polarity switch (6mm hole), 2 * 5mm LED's and transistor socket.

In order to make changing a socket easy as a result of wear, 2 transistor sockets are used, one mounted in the other.

Holding the sockets together and securing them to the lid is accomplished by drilling a 2mm hole through both the sockets and the lid and fitting a screw and nut.

The LED's are held in place by the close tolerance of the 5mm holes.

Now its built its time to give it a test run.

Power the circuit and place an NPN transistor in the socket flip the switch in the Forward switch position the red LED will illuminate and with a PNP in the socket and the switch in the reverse position the green LED will illuminate.

With nothing in the socket no LED'S will illuminate and with a short circuit between Collector and Emitter (CE), both LED'S will illuminate.

Check the other fault conditions as per the supplied table.