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ana aescrmea how to troubleshoot I I I I
transistorized circuits. Incorporated
into the text was a description of a
simple in-circuit transistor checker
that utilized the X-Y display
capabilities of an oscilloscope.
l a . NPN silicon transistor typical bias lb. PNP germanium transistor typical
We have received many requests for conditions bias conditions
copies of this article and especially Figure 1. Transistor bias examples
information about the transistor showing the "on" condition
checker. So, by popular demand, here
is a repeat of "How to Troubleshoot In a PNP transistor, the base must saturated germanium transistor
Transistorized Circuits Faster," by be more negative than the emitter in may have as low as 0.05 volts be-
George Stanley, based mostly on his order for current to flow. tween its emitter and collector,
book, Transistor Basics: A Short So "on" bias voltages for a transistor while a saturated silicon transistor
Course. Copyright @ 1967, 1975 by can be summed up by referring to may have 0.5 volts or less between
Hayden Book Company, Inc. All Figure 1 and the following two rules: these leads.
rights reserved. T h i s material is
printed w i t h the permission of - For a PNP, the base is negative, 0
Hayden Book Co., Inc., Rochelle the emitter is not quite as nega- n,
Park, N.J. tive, and the collector is far more 72
0
negative. -76
0
Fundamental Characteris- - For an NPN, the base is positive,
the emitter is not quite as posi-
tics of Transistors tive, and the collector is far more %
Before describing specific trouble- positive. 0 0
shooting tips, let's take a moment There is a distinct difference be-
and review several important tran- tween a transistor being turned "on" Figure 2. Transistor bias example
sistor characteristics. and being "saturated." When a tran- showing the "saturated" condition
1
Conventional PNP or NPN transis- sistor is saturated, it's generally
tors are basically ccofl"devices and thought of as being almost a short,
must be biased "on" to their operat- that is, the IR drop across the emit-
ing point. This is done by forward ter and collector resistors equals the
biasing the base-emitter diode to supply voltage as shown in Figure 2.
make the transistor conduct. Refer Naturally this means that there is
to Figure 1 for examples of forward practically no voltage drop between
bias on NPN/PNP silicon and ger- the collector and emitter of the tran-
nsistors. sistor. In this condition, both the
base-emitter and base-collector di-
Figure 1, an NPN tran- odes are forward biased (where in
have its base more posi- the "on" condition only the base-
le emitter in order for emitter is forward biased - the
ow. base-collector is reverse biased). A
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"Saturated" or "off" are the usual remove the forward bias as shown in amplifier stage, excessive leakage
conditions found in digital circuits. Figure 3. The collector voltage current can cause clipping distortion
In ac circuits where transistors are should then rise to the approximate because of the shift in the quiescent
used as amplifiers instead of level of the supply voltage. (Any dif- operating point.
switches, the amount the transistor ference is caused by ICO,the
is turned on depends upon current collector-to-base leakage current.) Tip #3
gain (beta) of the transistor, the re- The higher the collector voltage
In an amplifier with clipping dis-
sistors in series with the collector rises, the lower ICO,and the better
tortion, try cooling each transis-
and emitter, and the supply voltage. the transistor.
tor with spray coolant. Quite
Basic Troubleshooting Tips I 1 f IOV
1 likely you will find that when the
leaky transistor is cooled the
clipping distortion disappears.
In troubleshooting transistor cir- Conversely, heating a leaky tran-
cuits, the most important area to sistor will make the problem
examine is the base-emitter junction much worse by greatly increas-
as this is the control point of the ing the ICO leakage.
transistor.
Basic Circuit Analysis
If the base-emitter junction is for-
ward biased, the transistor would An interesting problem is illustrated
normally be "on." Figure 3. Amplifier with forward bias
removed in Figure 4. In this circuit, both
I I transistors are of the NPN type. Note
If the base-emitter junction has zero that Q2 has 0.8 V reverse bias on its
bias or reverse bias, it should be If the collector voltage doesn't rise as
expected, we've identified a bad emitter-base junction, but the 2.0
turned off. If it is not off under these volts on the emitter means that there
conditions, it is either shorted o r transistor. This technique is per- -
,
fectly safe in AC coupled circuits. is 2 mA of emitter current. Now, "p
leaky. since the emitter-base junction is not
However, in some DC coupled cir-
cuits, we could cause damage if shorted, this 2 mA of current also
Tip #I base-emitter shorts a r e applied flows through the 8K resistor in the
Measure the base-emitter volt- around high power levels (e.g., such collector of Q2. Therefore, t h e
age. From this decide how the as the output stage of a power collector voltage, Vcc, is:
transistor should be behaving. amplifier). 18V - (8K) x (2 mA) = 2V
Then look at the collector voltage Thus, it would appear that Q2 has a
and see if the transistor is behav- Now, back to ICO,the collector-to- short between collector and emitter.
ing as it should be. base leakage current mentioned
previously. As we implied, if the
For example, if the base-emitter transistor was perfect it would have
voltage is 0.6 volts forward biased no ICO leakage current. Look at Fig-
and the collector voltage is the same ure l a again. Note the collector volt-
as the supply voltage, something is age is more positive than the base
wrong. Probably the collector-base voltage. In this "on" condition the
junction is open. base-collector diode junction is re-
verse biased. This reverse biased
Expanding on the above idea leads diode should be off, but because we
to our second troubleshooting tip. have never been able to make a per-
fect diode, there is a very small cur- Fig. 4. Direct coupled two-stage
rent leaking across it. This leakage example circuit
Tip #2 current flows through the collector- ~~ ~
Modify the control signals pre- base junction and part of it goes Another interesting problem i n
sent and see if the circuit re- through the base-emitter (control troubleshooting illustrated i n
sponds accordingly. point) junction. Figure 5 . Although the emitter ` 3
current of Q1 is 1 mA, the collector
For example, if the transistor is for- Since leakage current is extremely current is only 0.52 mA (i.e.,
ward biased as shown in Figure 1, temperature sensitive, we can use 5.2V+10K). Stage Q2 shows 5 mA
see if it is behaving as an amplifier. this to our advantage i n trou- flowing in both the emitter and
Short the emitter to the base to bleshooting. For example, i n a n collector circuits, so Q2 is either
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associated resistance and capaci-
I I I
I 0
Figure 6 shows a simplified schema- tance. The loop is caused by the
tic of the transistor checker and the capacitance (probably a coupling
Fig. 5. Capacitive coupled two-stage ideal voltage vs. current waveforms capacitor), and the fact t h a t t h e
example circuit you can expect to see. waveform is not a perfect "right"
angle is because of the associated
shorted or saturated. The one voltage resistance (probably bias or load
that would answer this question is resistors).
not given; i.e., the voltage on the base
of Q2. If everything were working
correctly, this voltage would be
approximately 1.5V.
VB =
(15V) x (10K)
t H"rll
90K + 10K Figure 6. Transistor checker and ideal
waveforms
VB = 1.5v
Since the transistor checker puts out
What appears to have happened is Figure 8. Ideal waveforms for a good
a sine wave that has alternatively
that C3 is shorted. This would ex- diode
positive and negative half cycles, we'
plain why there is only 0.52 mA flow-
would expect a perfect diode to be-
ing through resistor R4. The other have as shown in Figure 7.
0.48 mA is flowing through C3 and
resistor R6. If C3 were shorted, it NOTE: All references to a diode also
would also explain the voltages on imply the base-emitter or base-
Q 2 . The 5.2 V on the base produces
collector diode junctions of a
5.0 volts on the emitter, which, in transistor.
turn, causes the 5 mA of d-c current
to flow and Q2 to saturate.
In actual practice, the waveforms
shown in Figure 7 are all possible
If capacitor C3 were replaced, the
because t h e test leads a r e not
base voltage of Q2 would be 1.5 V dc,
and the voltage on the emitter would
Figure 9. Typical in-circuit waveform
be about 1.3 V dc. This, in turn, for a good transistor
would cause about 1.3 mA of dc to
flow. The resultant collector voltage
would be 12.4 V dc. The schematic of the tester shows a
switch that shorts out a 5.6K resis-
In-CircuitTransistor Tester tor. This switch is primarily for cur-
rent limiting so you don't damage
Even though all the above tips are sensitive transistors. You can also
good ones, there is a transistor tester use it for in-circuit vs. out-of-circuit
that will speed up troubleshooting testing.
even more. This tester works on the
known fact that PNP and NPN tran- Figure 7. Ideal waveform of good This transistor tester leads to our
diode
sie;tors are made up of two diodes. next troubleshooting tip.
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--
Tip #4 and RxlO scales, VOMs often have a resistance scales. These voltages
Use the transistor checker for very high short circuit. This current also can damage delicate emitter-
rapid testing. Make sure to test may be as high as several hundred base junctions. Usually the RxlK
both the base-emitter and base- mA and can damage small delicate scales are safe for most meters but it
collector diodes. transistors. On t h e other hand, is best to measure your own. Table 1
VOMs often have high open circuit shows the characteristics of several
A little experimenting with a voltages (22.5V) on t h e i r high common ohmmeters.
printed circuit board containing
many transistors will rapidly show
you the various waveforms you will
encounter for'good transistors. The
main point to look for is whether or The working technician is quite produce discrete pulses at several
not the waveform has a "break" in it likely to encounter tunnel diodes in hundreds of MHz.
(Pt. A in Figure 9). If it does, the the trigger circuits of scopes, fre-
Due to very high impurity levels, the
transistor diode is good. Remember, quency counter front ends, and
diode's quiescent forward voltage
the lower the resistance of the bias elsewhere.
drop is very low and its reverse
resistors, t h e less defined t h e leakage current very high. This
"break" (Pt. A Figure 9), and the In theory, these diodes have a nega- would lead your ohmmeter to con-
more the waveform appears like a tive resistance slope in one portion of clude that the diode is shorted in
"short". Of course, when testing their characteristic curve, making both directions. A first glance with
out-of-circuit, the "break" will be them capable of amplification and the transistor tester will give the
very sharp -just like a true diode. oscillation. See Figure A. In actual same appearance. However, a little
practice, however, we have a prob- extra effort and a closer look may
This tester can also be used for test- lem if we try to look at this slope. reveal that at or near its rated cur-
ing tunnel diodes. The waveform is Any simple circuit that we can de- rent the diode does, in fact, switch
shown in Figure 10. vise to gradually increase the cur- states. If the transistor tester has a
rent through the diode will have 100 ohm current limiting resistor,
some internal resistance. Therefore, then 1 volt vertical deflection will
it's almost impossible to arrive at correspond to 10 mA of junction cur-
point B because the diode will ab- rent. Any reasonable facsimile will
ruptly switch from A to C and vice work so long as you can display
versa on the decreasing swing. This about 0 to 30 mA vertically. The
switch action results in about 0.5 curve on a good diode will be similar
volt change across the diode and oc- to Figure 10 in the main article
curs at nominally 5 to 15 mA cur- allowing you to discern the switch
rent. The voltage change occurs very points and get a fair idea of the cur-
rapidly. Circuits like Figure B can rent magnitude.
Figure 10. Tunnel diode waveform
1
I
When testing tunnel diodes, make i
T
Integrator
sure the switch is in the In-Circuit
position as you need the higher
current.
Transistor Tests with a
VOM I
Another way to test transistors is to
perform a forward and reverse
ohmmeter check on the two transis-
t o r diodes. It's much slower than E
with the transistor checker. Also you Figure A. Figure 0.
have to be careful about the short-
circuit current and open-circuit
voltage of your ohmmeter. On Rxl
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_
~ _ _- - _I----
I Tip #5
Measure the short-circuit current
Ii TABLE 1. CHARACTERISTICS OF COMMON
OHMMETERS and open-circuit voltage for each
resistance scale on your VOM's
and VTVM's. Keep this informa-
Ii Make, Model,
and Range
Open Circuit Short Circuit
Voltage Current Polarity tion along with the polarity of the
leads on a chart on the back of
i HP 412A (VTVM)
the ohmmeter.
I R x !-
R y: 1U
0.01 v
0.1 v
8.0 m A
10.0 rnA R --
Tn -c Tip #6
R x 100 1.0 v io.0 rnA
+R x 1K 1.0 v 1.0 rnA BLACK - If you are using a VTVM, make
R x 10K 1.0 v 100.0 pA
sure the range you are using has
R x 100K 1.0 v 10.0 pA
R x 1M 1.0 v 1.0 PA enough open-circuit voltage to
R x 10M 1.0 v 0.1 pA
overcome the 0.2V for ger-
HP 4108 (VTVM)
manium and 0.6V for silicon.
Otherwise you will get an un-
R x 1 1.1 v 120 m A satisfactory reading.
R x 10 1.1 v
v
11 rnA RED -
R x 100 1.1 1.1 rnA
R x 1K 1.1 v 110.0 pA BLACK + Since leakage does not show up well
R x 10K 1.1 v 11.0 flA
on the transistor checker of Figure 6,
R x lOOK 1.1 v 1.1 p A
R x 1M 1.1 v 0.11 pA nor on the ohmmeter tests, it is best
to have an inexpensive beta/leakage
tester on hand. There a r e many
HP 410C (VTVM)
available and some of the best are in
R x 10 1.3 V 55 m A
kit form. If a leakage current tester
R x 100 1.3 V 5.7 rnA RED +
R x 1K 1.3 V 0.57 rnA BLACK - is unavailable, you can try shorting
R x 10K 1.3 V 57 PA
out the emitter-base junction while
R x lOOK 1.3 V 5.7 pA
R x 1M 1.3 V 0.5 pA simultaneously measuring the volt-
R x 10M 1.3 V 0.05 PA age drop across the collector load
resistor.
SIMPSON 260 (VOM)
Tip #7
R x 1 1.5 V 125 m A RED +
R x 100 1.5 V 1 rnA Measure ICBO by shorting the
R x 10K 7.5 v 60 pA BLACK - emitter-base junction and
monitoringthe voltage across the
SIMPSON 269 (VOM) collector lead resistor.
R X 1 1.5 V 74 rnA
R x 10 1.5 V 8 rnA RED -
R x 100 1.5 V 8 mA BLACK +
R x 1K 1.5 V 0.82 rnA
R x 10K 24 V 1.3 m A
R x lOOK 30 V 13 /*A For example, if you measured 30 mV
across a 10K load resistor, your
leakage current would be
TRIPLEll 630 (VOM)
R x 1 1.5 V 320 rnA RED -
R x 10 1.5 V 32 rnA BLACK +
R x 100 1.5 V 3.25 rnA (Varies with
R x 1K 1.5 V 325 p A serial
This would be about right for a ger-
R x lOOK 22.5 V 70 pA nurn ber)
manium transistor a t room tempera-
ture, but a little high for a silicon
TRlPLElT 310 (VOM) surface-passivated transistor.
R x 1 1.5 V 7.5 rnA RED -
R x 10 1.5 V 750 p A BLACK + One of the most common mistakes in
R x 100 1.5 V 75 pA (Varies with
R x 10K 1.5 V 75 PA serial analyzing transistor circuits is t o
number) miscalculate the gain of one stage in
a multi-stage amplifier. The error
O Numbers in bold type indicate safe range.
usually occurs in miscalculating the
real value of the load resistor for
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that stage. Figure 11 shows a two- here is a list of important points re- leakage current then equals the /"1
stage amplifier. The correct value lating to the troubleshooting tips voltage across the load resistor
for RL1 is not the actual listed value and characteristics previously divided by its resistance. (Make
of the resistor, but rather the paral- described. sure t h e collector is not DC
lel combination of RL1, Ra, Rb and - NPN and PNP transistors are coupled to the next stage.)
Rin of Q2. Usually the Rin of Q2 is basically "off' devices while vac- - Abnormal increases i n room
the most dominant factor in this uum tubes a r e basically "on" temperature leakage current
combination. devices. (e.g., 10 times normal) often in-
- Transistors are made up of two dicate contamination of t h e
diodes: a base-emitter diode and base-collector junction (possibly
a base-collector diode. In normal due t o a cracked or broken
(amplifier) operation, the base- hermetic seal). The result is a
emitter diode is forward biased shift in the normal bias operat-
and the base-collector diode is ing point. Trouble will only be
reverse biased. experienced if the driving signal
drives the transistor to or near
- Shorting t h e base t o emitter
*
Figure 11. Two-stage amplifier
turns off transistors while for-
ward biasing base-emitter junc-
tions turns on transistors.
cutoff. The transistor will not
properly turn off and the result
may be clipping or distortion due
to the residual leakage current
- All transistors have leakage cur- flowing through the external re-
rent across their reverse biased sistors. Heating and cooling a
Tip #8 base-collector diodes. For surface transistor aggravates this condi-
passivated silicon transistors, tion and sometimes shows up
When calculating the gain of a
stage, be sure and include the this current is usually no more marginal operation.
parallel loading effects of the than several nanoamperes. Since
germanium transistors cannot be
- Shorting collector t o emitter ")
next stage bias resistors and simulates saturation as the tran-
input impedance. surface passivated, this leakage sistor behaves like a closed
current normally may be several switch.
Summary microamperes.
Much information on transistors is
- Leakage current increases with available from HP on video tape in
All of the above tips relate back to heat (a law of physics) and dou- the Practical Transistor Series, HP
the fundamental characteristics of bles about every 10