BASIC CIRCUIT – All of our instruments use the same fundamental concept, illustrated for the 20 ohm range in Figure 1. The voltage drop across the unknown resistor (R) is measured via a true differential amplifier. Current through the unknown resistor is determined by measuring the voltage drop across a precision resistor in series with the unknown. The Ohm’s Law calculation is then performed by a ratio circuit and the result displayed on a LCD digital panel meter.
Figure 1 – Basic Concept for 20 Ohm Range
FAIL-SAFE CURRENT – Figure 2 illustrates what the instrument circuit is reduced to in the event of the worst case failure of all active components. A failure of the regulator applies the full battery voltage across the series combination of 1.24K resistor and (R). Assuming the part to be less than 1 ohm, the fail-safe current is 8 mA.
Figure 2 – Circuit After Worst Case Failure
FAIL-SAFE MODULE – To prevent anyone from bypassing the critical fail-safe circuitry, it is sealed in a potted module attached directly to the output connector. This module will continue to limit test current even if the instrument is dropped on a concrete surface from a height of 100 ft.
CHARGER INTERLOCK — The AC charger must be unplugged in order to connect the test leads, making it impossible for a test to be run with the AC still connected.
SEALED BATTERY – We use a special re-chargeable nickel metal hydride battery pack which is totally sealed in a thick plastic case to guard against leakage and has an output connector which does not match any off-the-shelf battery of higher voltage. A fuse inside the case protects the instrument in the event of a short circuit.
EXTENSIVE TESTING – The instruments are subject to a series of environmental and performance tests, ensuring both accuracy and reliability.
BUILT-IN mA METER — A milliamp meter mounted in the cover of the instrument verifies that test current is within safe limits before attaching the leads to the part.
MILITARY APPROVAL – Our instruments have been approved by the military as safe for testing electro-explosive devices.
OTHER INSTRUMENTS – We make custom and computer programmed production versions of these testers. Contact our sales department.