Igniter Testers / Safety Ohmmeters..620A-4

 

620A Fail-safe Ohmmeter/ Igniter Circuit Tester
        

Download the Model 620A-4 datasheet (pdf format)

                                            

The 620A Fail-safe Ohmmeter NSN 6625-01-460-1499NM
Most versions of the 620A series have had the unit's circuitry reviewed and approved by U.S. and International Government Safety Boards as well as most branches of the US Armed Forces for specific military programs (i.e. U.S.A.F. AIM-7, AIM-9 Missiles - Sidewinder, Stinger, Army Standard Missile). The 620A series fail-safe ohmmeters are battery powered, ordnance test instrumentation which have been specifically designed for ultra-safe resistance testing on explosive devices (i.e. squibs, igniters, flares, detonators).

Safety Line Isolation Switch when Recharging the Batteries
The Rechargeable Ni-Cad batteries (4 heavy duty 5700 mAHR) provide the unit with hours of portable resistance measurement use before needing a recharge - most competitive models use the weaker 4000 mAHr Ni-Cad batteries. For added convenience and safety, the units feature an internal safety line isolation switching configuration which automatically disables the battery charging circuit when in the operate mode. Alternately, all power is removed from the output circuitry when in the off/charging mode.

Close-up Photo of the 620A-4 Resistance Ranges and Current Levels
USAF Nonnuclear Munitions Safety Board Approved
The 620A-4 Fail-safe Ohmmeter has been approved by the USAF for use on non-nuclear explosive devices (i.e. squibs, igniters, rocket motors, flares,and detonators) by US Air Force missile groups.

The 620A-4 Ohmmeter NSN 6625-01-460-1499NM with Accessory package includes:

The United States Department of the Air Force - Nonnuclear Munitions Safety Board approval notice for the 620A-4 is available for your review.

Polyurethane Epoxy Coated Circuit Board and More ...
A common question we get from evaluating safety engineers is "What sort of electrical and mechanical measures are taken with the 620A to prevent an internal loose wire, metal screw, or washer inside from floating around and shorting the battery output to the terminal output posts?". The ohmmeter's output posts are insulated inside the unit with heat shrink tubing covering any of it's conductive surfaces. The entire PCB (except for the calibration trim pots) is also nonconductive polyurethane epoxy coated, to make it extremely difficult to get an accidental conductive pathway. The ultimate electrical safety analysis for fail-safe current relies on "Ohms Law" for determining Imax, based upon a Vmax ( ~ 4.8 VDC battery value) divided by the equivalent circuit current limiting resistance (R-limit) for the particular version being considered. 620A circuit schematics are available upon request for reviewing the maximum fail-safe current level.

Fail-safe Design - Built-in 'Can't Fail' Life Saving Features
There are a variety of critical military, aerospace and industrial applications which utilize squibs and other blasting devices. In each case, it is extremely important that these devices detonate when required. It is equally important that these devices do not detonate while being tested for coil integrity. To this end, each instrument is equipped with redundant, dual fail-safe current limiting resistor network circuitry to ensure that the instrument test currents cannot exceed the specified amount. Even in the unlikely event of a worst case component failure, the output current limiting network is still in effect.

No Hazardous Mercury Batteries
The 620A does not use hazardous mercury batteries like those in older model igniter testers (i.e. Alinco 101BFG series). The 620A is battery powered with a heavy duty rechargeable Ni-Cad pack for portability purposes. A 120 VAC powered battery charger is standard.

 
Kelvin 4-Wire Error Compensation
Figure 1 illustrates how the Kelvin 4-wire Error Compensation principle is used to eliminate lead length and connection contact resistance as potential resistance measurement error sources. In many applications the contact resistance can exceed the value of the load by several orders of magnitude. The internal current source inherently overcomes all series resistance (within compliance voltage limits) and delivers stable constant current. This means it does not matter if the test leads are longer (higher resistance) and there is some contact resistance in the connection because the internal current source circuitry overcomes this resistance as part of a "Current Loop" (within compliance voltage limits) . There is no contact and lead resistance error created by the voltage measurement because there is essentially no current flow in the high impedance voltage sense leads. All Igniter Tester (Fail Safe Ohmmeter) probes, clips and cable accessories offered are 4-wire Kelvin configured, some to the probe tip end.


Specifications
Accuracy: (180 days 25°C ± 10°C) ± 0.02% of reading  ± 0.02% of range for 20 ohm through 200 Kohm ranges.
Display: 4½ digit (19999) L.E.D.  - 20,000 counts readable in daylight
Terminal Configuration: Four-wire Kelvin
Overload Indication: Display flashes
Maximum Input: 250 VDC or peak AC without damage
Operating Temperature Range: 0°C to 50°C
Temperature Coefficient: ±.002% per °C (from 0°C to 15°C and 35°C to 50°C)
Conversion Rate: Approximately 3 per second
Power: Rechargeable Ni-Cad batteries (4 each 5.7 AHr each) & Charger (115 VAC 50-60 Hz)
Max. Internal Voltage: 4.8 VDC - 4 ea. 1.2 V   Ni-Cads
Size: 235mm/9" D x 254mm/10" W x 76mm/3" H
Weight: 1.6 KG/3.5lbs NET 3 KG/6.5 lbs shipping

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