Transistor Tester
Circuit :  Andy Collinson
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Description:
A test circuit for BJT (Bipolar Junction Transistors). This circuit can measure both small signal hfe and DC current gain hFE of a low to medium power power transistor. In addition it can measure collector-base and collector-emitter leakage current. This circuit can also measure hFE at different operating points. A multimeter can be used at multiple test sockets to make all measurements, or two DC ammeters can be used.

BJT Tester
Notes
The circuit has two requirements: a variable DC power supply and a test and measuring circuit.

Starting with the power supply, the input source is two 9 Volt batteries, series connected to create an 18 Volt supply. If the tester is designed to be portable then batteries can be used, if used in a workshop, then any variable DC power supply can be used. The power supply in this circuit is a standard L200 regulator circuit. R2 and VR1 allow the supply to be varied from 2.85 Volts to almost the full 18 Volt supply, current is limited by R1 to 450mA.

The test circuit is a collection of switches and passive components. The main tests are performed by S3 a 3 Gang 6 way switch, see this page in the practical section for help on switches. A transistor socket is used to connect the transistor under test, a multimeter switched to DC Milliamps can be connected to terminals M1 and M2 to measure collector current (a wire link should be used to short M3 and M4). The same meter can then be set to DC Microamps and measure the base current at M3 and M4 (terminals M1 and M2 short be shorted with a link). Alternatively analogue meters can be used for both meters, however as a digital meter offers better precision and resolution, a multimeter is the preferred choice.

Function of Switches
S1 is a DPDT switch wired to reverse polarity, as drawn it is used to test NPN transistors, in the opposite state it reverses the power supply and used to test PNP transistors.

S2 is a normally open push to make switch. This is the general test switch and pressing this switch allows base current or collector current to be read on the multimeter.

S3. This is the selector switch and controls the different functions for the tester. S3 is a 3 gang, 6 way rotary switch. This means that one single shaft rotates arcs S3a, S3b and S3c simultaneously each turn.

S4. This switch reduces the base current and allows the small signal current gain hfe to be measured.


Tests Using Rotary Switch S3
Position of S3 Function Conditions Result
1 ICO VCB = VS Read Multimeter Direct
2 hFE IB = 20uA Set Vs to 6V hFE = Meter reading / 19.5uA
3 hFE IB = 100uA Set Vs to 6V hFE = Meter reading / 92.6uA
4 hFE IB variable Vs variable. hFE = Meter reading M1,M2 / M3,M4
5 ICEO VCEO = VS Read meter M1,M2 direct.
6 No Function


General Usage
Using the tester is easy, starting with power off, insert a transistor into the test socket. Set S1 for NPN or PNP and rotate S3 to the required test position. Rotate VR1 so the desired collector emitter voltage. Pressing S2 now allows the measurement of hFE to be made. Pressing S2 and S4 allows hfe to be measured. More detailed usage now follows.

Measuring Collector Base Leakage
With S3 in position 1, insert a transistor into the test socket and set S2 for NPN or PNP. M3 and M4 need to be shorted and a multimeter set to DC microamps between M1 and M2 now allows collector base leakage current to be measured. With silicon transistors, you may not see a reading at all, but germanium transistors have leakage current which can be measured.

Measuring DC Current Gain at 20uA
Set S1 for NPN or PNP and rotate S3 to position 2. Rotate VR1 so the power supply reads 6 Volt between terminal Vs and ground. Place a shorting link across M3 and M4 and a digital multimeter set to measure DC lamps across M1 and M2. Pressing S2 now allows the measurement of hFE to be made. This will be the meter reading / 20 uA.

Measuring DC Current Gain at 100uA
Set S1 for NPN or PNP and rotate S3 to position 3. Rotate VR1 so the power supply reads 6 Volt between terminal Vs and ground. Place a shorting link across M3 and M4 and a digital multimeter set to measure DC milliamps across M1 and M2. Pressing S2 now allows the measurement of hFE to be made. This will be the meter reading / 100 uA.

Measuring DC Current Gain at an Operating Point
Set S1 for NPN or PNP and rotate S3 to position 4. The parameter hFE varies with different collector currents and temperatures. VR1 and VR2 allow you to set up different operating points. Suppose you have a circuit where a transistor is run from a 15 Vdc supply and base current is 15 uA. First set VR1 so the power supply reads 15 Volt between terminal Vs and ground. Place a shorting link across M1 and M2 and a digital multimeter set to measure DC microamps across M3 and M4. Press S2 and adjust VR2 until 15 uA is measured between M3 and M4. Now release S2, short terminals M3 and M4, remove the link across M1 and M2 and set the meter to read DC milliamps. Pressing S2 now allows the measurement of hFE to be made. This will be the meter reading / 15 uA (or whatever base current you choose).

Measuring Collector Emitter Leakage Current
With S3 in position 5, insert a transistor into the test socket and set S2 for NPN or PNP. M3 and M4 need to be shorted and a multimeter set to DC microamps between M1 and M2 now allows collector emitter leakage current to be measured. With silicon transistors, you may not see a reading at all, but germanium transistors have leakage current which can be measured.

Measuring Small Signal AC Current Gain
The value of the small signal current gain hfe can also be measured with this circuit, for base currents of approximately 20uA, 100uA or any particular operating point. Proceed as in the previous steps for measuring DC current gain and with S3 at position 2, 3, or 4. The voltage Vs should be set to 6 Volt, a short across meter terminals M3 and M4 is required then press S2 and read the current on the meter across terminals 1 and 2. This reading will be called IC1. Now keeping S2 pressed, also press S4, record the reading, this is measurement IC2.

hfe is calculated as follows:


This is for S3 in postion 3 (20uA base current).


This is for S3 in postion 2 (100uA base current).

Measuring Small Signal AC Current Gain at a Particular Operating Point
You can also measure hfe at any operating point within the voltage and current range of the power supply. The power supply can deliver 18 Volts at up to 500mA. Larger currents will drain the batteries so a bench power supply would be recommended.

To measure hfe at a VCE of 12 Volts and collector current 1 mA. First adjust VR1 so that the supply Vs is 12 Volt. Short M3 and M4, press S2 and connect a multimeter to M1 and M2 and adjust VR2 to read 1 mA. Now release S2, short M1 and M2 and remove the short on M3 and M4 and set your meter to microamps and measure the current. Record both values of base and collector current. Now press S2 and S4 and measure both collector and base currents again. The value of hfe is the difference in collector current divided by the difference in base current.




How the Signal AC Current Gain is Calculated
The small signal AC current gain is achieved by changing the value of the base current. When S4 is pressed, the input voltage is reduced by the fraction R7 / (R4+R7) which reduces the input base current. hfe is the change in collector current divided by the change in base current. The base current which is calculated as follows:



With S3 in position 2, the base current Ib is:




With S3 in position 2, and when S4 is pressed the base current Ib becomes:


The difference in base current is therefore 92.6uA - 72.8uA = 19.8uA This value is then used as the denominator for the larger change in collector current, as in the previous section.

hFE versus hfe
In practice, the difference between hFE and hfe is often so small that one value can be substituted for the other. Data sheets invariably quote the value for the dc current gain hFE, the parameter hfe is the ac quantity and decreases also at higher frequency. As this circuit measures the change in base current at dc the value of hfe will only be approximate at low frequencies up to 1kHz. To measure hfe at a particular frequency, then a signal generator would be required and the meter set to measure ac base and collector currents.

Testing on a Breadboard
Although simple, the wiring of the switch can be troublesome, and if you already have a variable power supply, multimeter and a breadboard, then you can set up the circuit as shown below:



In my test transistor, a BC109C Base current was 94.6 uA. The power supply was set at 6V and a 56k and 1.2k resistor wired in series. Next the collector current was measured, see below:



My sample BC109C produced a collector current of 41.2 mA. The hFE was therefore 41.2/0.0946 = 436.
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