1250 Frequency Response Analyzer Making Measurements: The Generator
AMK / 1250 / 4 4-7
4.5.5 GENERATOR OVERLOAD PROTECTION
Accidental Short-Circuit
The Generator is protected against accidental short-circuiting of its HI and LO terminals.
The 1250N (low source impedance) uses feedback within the power amplifier to limit the
current to ∼300mA into a short-circuit. The 1250 relies on its higher source impedance
(50 Ω) to limit the current to a similar value. No error message is displayed.
Excessive External Voltage at Generator Terminals
If an external voltage of between ∼ ± 18V (∼ ±22V for 1250N) and ∼ ±30V is applied
across the Generator HI and LO terminals, Error 31 'Generator Overload' is displayed
and the output is held open-circuit until the overload is removed.
For overloads greater than ∼ ±30V the 1250 and 1250N behave differently.
1250. An S.C.R. clamp is triggered which short-circuits the Generator terminals via 33Ω
for ∼ 5ms before open-circuiting them and displaying Error 31. Remove the overload to
restore normal operation.
1250N. An S.C.R. clamp is triggered which short-circuits the Generator terminals. Then:
a) If the overloading source can supply more than ∼ 800mA, an internal protection fuse
blows which open-circuits the Generator output. Error 31 is displayed. Replace the fuse
to restore normal operation.
The fuse is an 800mA anti-surge, 20mm X 5mm cartridge type, Part No. 360106110 and
is located on Option 12506D (Board 16). To access, remove the heat sink (4 screws).
b) If the overloading source cannot supply more than ∼ 800mA, the fuse does not blow
and the Generator output remains short-circuited even after the overload is removed.
Error 31 is not displayed. Switch the 1250N 'off then 'on' to restore normal operation.
During the short-circuit, the Generator output is fully protected.
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