Amptec Research 620A 4R, Operation And Maintenance Manual

E-5-2 

Constant Current Circuit Operation

Assume that terminals I  and I  of Figure E-3 

hi

lo

are shorted, and 1 volt is applied to 

E

 so that 

I

in

hi

is positive.  To equalize the 1 volt applied to 

E

in

, the inputs of IC202,  IC201 must be driven to 
zero.  This condition occurs only when the 
voltage drops across R212 and R222 are equal 
to the drops across R213   and R221.  For these 
voltage drops to be equal, the output of IC202 
must be at +1 volt.  Since the output of IC201-8 
must be zero, the drop across R213   is 0.5 volts, 
making the inverting input 0.5 volts.  The drops 
across R212,   R221 and R222 will also be 0.5 
volts.  Since the inputs to IC201 are essentially 
equal, its output is zero (offset by the few 
microvolts required to drive IC202 to +1 volt).  
Under these conditions the sum of the voltages 
across R212,   R213  , R221   and R222 equals 
the sum of 

E

 plus the output of IC202.

in

Consider now that the short is removed from the 

I

 and 

I

 terminals and a 100-ohm resistor (R ) 

hi

lo

L

is connected in its place.  The current through R  

L

increases the voltage at the input to IC201.  A 
balanced condition will be reached when the 
output of IC201 is equal to the non-inverting 
input of IC202.  Again, this condition occurs 
when the voltage drops across R212  and R222  
are equal to the voltage drops across R213   and 
R221.  At this time the output of IC202 is 1.1 
volts. 

The voltage drop across the range resistor 

is 1 volt, just as it was when the output terminals 
were shorted.  The current through R  is 10 

L

milliamperes, just as it was through the jumper 
when the output terminals were shorted.

E-6.

Failsafe Design

Reference to the AMPTEC 620A Tester Igniter 
Tester schematic will show that the output of 
IC202-6 is actually applied to the base of 
transistor Q202, 

which acts as a current limiter

.  

The worst-case component failure that could 
occur in this circuit would be a Q202 short

, 

which would effectively connect the -5 volt 
supply directly across R218, D202, the range 
resistor and R . 

L

D203, however, acts as a 1.6 volt zener diode, 
limiting the voltage that can appear across these 
components.  Even if every component in the 
amplifier circuit shorted, the current through the 

igniter could not exceed safe limits, because the 
-5 volt and +5V supplies includes inherent 
current limiting. Because of the design of both 
supply isolation transformers T101 and T102, 
the ±5 volt supplies can only deliver 20 to 25 
milliamperes before the DC/DC converter 
disengages, dropping the -5 volt output to zero.  
See Section D-7.

The AMPTEC 620A Tester is powered by a 
rechargeable internal battery pack and cannot be 
operated directly from the battery charging 
adapter.  This is to 

eliminate the possibility

 of 

an electrical short to/from the AC line.  

Only 

when

 the 620 POWER switch is in the “

OFF/ 

CHARGING

” position are the batteries  

connected to front panel charging jack.  

When 

the POWER switch is in the ON position

, the 

batteries are disconnected from the battery 
charger and connected to the internal circuits of 
the AMPTEC 620 Igniter Tester.