BK Precision 501A Instruction Manual Download

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for more detail).

VERTICAL SENSITIVITY-The setting of the VER

TICAL SENSITIVITY control is determined primarily

by  physical  size  of  the  transistor  or  semiconductor
being  tested. Most small signal  transistors,  including
most small  plastic  case  transistors,  should  be  tested

at the 1 ma/Div position. Some small signal transis

tors, including several metallic case transistors,
should be tested at the 2 mA/Div position The 5 mA/

Div and 10 mA/Div positions are primarily for power
transistors. The 5 mA/Div is for low to medium

power  transistors  and  include  most  plastic  case
power transistors  and  those with  self-contained  heat
radiating  cases.  High  power  transistors  should  be

tested at 10 mA/Div. Power transistors may  be tested
without  their  usual heat  sinks if testing  is  limited  to
a  few  seconds;  just  long  enough  to  make  the  read
ing.  It  is  especially  important

keep testing time

short  during  periods  of  voltage  breakdown  and
current  limiting to  prevent  overheating  and  thermal
runaway  damage.

The curve tracer employs current limiting in the

collector voltage supply to prevent damage to the
transistor being tested. However, the point of current

limiting is increased with each step of the VERTICAL

SENSITIVITY  control.  Thus,  the  lowest  acceptable
setting  of  the  control  must  be  used  to  protect  small
transistors. Start  with  the lowest  position  (1  mA/Div)
and  increase  to  a  higher  setting  only  as  necessary.

STEP SELECTOR-The STEP SELECTOR switch

selects the base current steps. The approximate set

ting  of  this  control  also  is  somewhat  related  to  the
physical  size  of  the  transistor.  The  fast  set-up  posi
tion (10 µ.A per step) is  a good starting point  for most

signal transistors. Very

High-gain

small signal tran

sistors will require a lower setting. Large signal and

power transistors will require a higher setting.

If transistor specification sheets are available, it

is  desirable  to  use  the  manufacturer's  data  and  set
the  STEP  SELECTOR  to  produce  the  specified  col
lector  current  where  Beta  is to  be  measured.  If  data

sheets are not available, the general rule is

adjust

the STEP SELECTOR for the most curves displayed

on the VERTICAL SENSITIVITY range being used.

If the setting is too high, some of the curves may

reach the current limiting value and be superim

posed on each other, causing less than five curves
to be displayed. When more than one position dis

plays all five curves, select the position that pro

duces the most even spacing between curves.

The Volts per Step positions are for testing FET's.

In  these  positions,  step  polarity  is  reversed  with
respect to  the  sweep  voltage.  The  method  of adjust
ment  is  the  same;  set  for  the  maximum  number  of
curves  and  the  most  even  spacing  between  curves
on  the vertical  range  being  used.

Sorting and Matching Transistors

A better method of oscilloscope set-up simplifies

sorting and matching of transistors when the types

are mixed or  unknown.  This technique  is  especially
helpful when  trying  to  match  the  gain  of  two  oppo
site  polarity  devices  (NPN  vs  PNP).

I. Calibrate the oscilloscope vertical axis for 1 volt

full scale and the horizontal axis for 30 volts
full scale.

2. Adjust the oscilloscope positioning controls to

place the CRT trace (dot, with no inputs) within
the center circle of the special graticule.

3. Adjust 501 SWEEP VOLTAGE until the trace

sweeps out to either edge of the graticule, de
pending on which position the POLARITY switch
is set.

4. Set VERTICAL SENSITIVITY to the 2 mA posi

tion.

5. Set STEP SELECTOR to the 10 ,uA position.

NPN transistors will now display curves in the

upper  right-hand  quarter  of  the  graticule,  and  PNP
in  lower  left-hand  quarter.  The  polarity  of  an  un
known  device  can  immediately  be  determined  by
inserting  it into either  socket, switching  to  that  sock
et, and throwing the SOI  POLARITY switch  back and

forth  until  curves  appear,  noting  its  position  (See
Figure  14).  Once  curves  are  displayed,  the  STEP
SELECTOR  may  need  readjustment  to  bring  all  6

into view on the screen. Note that when POLARITY
is set to PNP for an NPN device and vice versa, a

CRT trace positioned

ot center of graticule.

8) .......... ( ..... 1 .....

! ' . .

POLARITY switch in NPN

position (curves displayed) "'{

4) ........... ;, .. , ....... : ... , ... .

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