Front Panel Controls - Aim-TTI BS407 Bedienungsanleitung

The following paragraphs give a brief description of the controls. Fuller details of the operation of
the instrument are given in later sections.
Operate
The Operate switch connects the measurement circuits to the battery; it does not control the
battery charger which operates whenever the unit is connected to the supply mains.
Set Zero
Depressing the Set Zero switch diverts the test current away from the Force terminals so there is
no voltage drop across the resistance being measured. The reading can then be set to zero
taking account of the effect of external thermal emfs.
20mV Clamp
Depressing the 20mV Clamp switch limits the open circuit voltage of the force terminals to 20mV.
This is used when measuring the contact resistance of switches or relays as this low voltage
does not break down oxide films.
Polarity
Depressing the Polarity switch reverses the direction of the test current through the unknown. If
there are no thermal emfs in the measurement circuit the two readings should be the same (apart
from the minus sign). If a thermal emf is present it will add to one reading and subtract from the
other; the true resistance value is obtained by taking the numerical average.
Range switches
The legend above the range switches shows the display units (µΩ, mΩ, Ω or kΩ). The nominal
measurement current is shown below the switches for reference.
Charge lamp (red)
The Charge lamp will light while the unit is connected to the mains supply which causes the
battery to be charged. The unit should be disconnected from the supply when the battery is fully
charged.
Force On lamp (green)
The Force On lamp lights when the test current is flowing correctly. If the 20mV clamp is in use
the lamp will only light when the external voltage drop is less than the clamp threshold.
Display
The 3½ digit display shows the measurement result. It should only be taken as valid when the
Force On lamp is lit and the BAT indicator is off. When the resistance is too large for the range
selected (or the sense leads are open circuit) the overscale indication consists of a 1 and three
blank digits. A negative sign will show when reverse polarity readings are being taken. The zero
setting is signed; the display will alternate between –000 and +000 at true zero.
The signal BAT in the lower left corner of the display shows when the battery is nearly
discharged. At this point readings are still accurate, but there is no further indication when the
battery voltage falls to the point that errors arise; therefore when the BAT indication is displayed
the charger should be connected to the supply.

Front Panel Controls

7