Failsafe Design
The AMPTEC
630ES 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
. A
component failure that could occur in this circuit
would be a Q202 short
, which would effectively
connect the -5 volt supply through R218 to the cathode
of D202. The anode side of D202 would be limited by
D203. The
W
W
), which makes for significantly
lower (safer) current levels
.
The AMPTEC 630ES Igniter Tester measurement
circuitry is failsafe current limited, even under worst
case component failure.
For the 630ES the normal or typical operating current
level is less than 5 mA, and <8mA on 2 Ohm and the
20 ohm range(s) as a Failsafe Level
.
voltage limiting diode, D203, provides a 1.6
volts maximum across R227
~
112
W
(comprised of 2
each 56
W
carbon composition resistors in series
)
and
the 2 and 20 ohm range attenuation resistor R223 (100
W
wirewound resistor), which are a total of 212 Ohms
across the Igniter Testers output terminals.
All other
range attenuation resistors are much higher values
(900
and 999 K
Constant Current Circuit Operation
Assume that terminals I and I of Figure E-3 are
hi
lo
shorted, and 0.5 volt is applied to
E
so that
I
is
in
hi
positive. To equalize the 0.5 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 +0.5 volt.
Since the output of IC201-8 must be zero, the drop
across R213 is 0.25 volts, making the inverting
input 0.25 volts. The drops across R212, R221
and R222 will also be 0.25 volts. Since the inputs
to IC201 are essentially equal, its output is zero
(offset by the few microvolts required to drive
IC202 to +0.5 volt). Under these conditions the
sum of the voltages across R212, R213 , R221
and R222 equals the sum of
E
plus the output of
in
IC202.
Consider now that the short is removed from the
I
hi
and
I
terminals and a 100-ohm resistor (R ) is
lo
L
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 0.5 V.
The voltage drop
across the range resistor is 0.5 V, just as it was
when the output terminals were shorted. The
current through R is 5 mA, just as it was through
L
the jumper when the output terminals were shorted.
Figure E-3 Constant Current Circuit
E-7.B AMPTEC 630ES -
Failsafe Current Limiting Circuit
(I
max
)
Current Max. Calculation (2 or 20 Ohm range)
<
1.6V/212 ohms 0.008 Amperes (8mA) max
=
*
note -
Additional safety margin is provided by the
historic
failure mode for all wirewound resistors
(ie
R223A is a 50 Ohm wirewound)
and
carbon
composition resistors
is to
fail
"
open circuit
" (infinite
resistance)
not as a dead short (zero ohms)
.
R223B is
a different mfg. and type of precision resistor
(metal foil) that is also 50 ohms in series with R223A
(together they make a 100 ohm attenuation string).
Should either R223A or R223B fail as a "short"
there is still the other 50 ohms in series to act as a
failsafe current limiter. If either R223A or R223B
fails as an "Open" then the current goes to zero "0".
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 are
current limited themselves. The ±5 volt supplies can
only deliver 20 to 25 milliamperes before the DC/DC
converter disengages, dropping the -5 volt output to
zero due to the design of both supply isolation micro-
transformers T101 and T102 . See Section D-7 of the
operator manual for complete circuit diagram.
R212
100K
R213
100K
R222
100K
R221
100K
R215
100K
RANGE ATTENUATION
RESISTORS
RL
I high
E in
I Low
IC201
IC202
(50 ohms R223A)
(50 ohms R223B)
R = load resistance (i.e. squib)
L
