MFJ MFJ-854, Manual

Range:  

The 

Range

 switch selects between five calibrated scales (from 30 mA to 3 

Amps).  The sixth range is uncalibrated and tied to the unit's 

Sensitivity

 control.  To 

prevent meter pegging, begin checks on a high range and work down.  To estimate 
unknown current levels, use the formula below (I = Unknown Current in Amps, P = 
Applied Power in Watts, and R = Load Resistance in Ohms). 

I

P

R

=

 
Sensitivity:  

The 

Sensitivity

 control varies meter drive when reading weak RF-current 

levels (below 30 mA).  At maximum setting (fully clockwise), RF currents of 1 mA or 
less will yield a usable meter deflection.   This function is especially useful for tracking 
RFI pickup on household wiring and cables.  Note that some residual meter deflection is 
present with the sensitivity control at maximum.  This is normal. 
 

Power Switch:

  Power is applied via a "press-to-test" momentary contact switch.  A 

built-in hold circuit keeps the unit powered for approximately 15-20 seconds after the 
switch is released.  This feature permits "hands-off" measurement on energized 
conductors. 

 
RF Hazard--Important Warnings:  
 
1.  RF-current measurements must be conducted with extreme caution.  Physical 
contact with conductors carrying even moderate RF-energy levels can result in severe 
RF burns, or injury.      
 
2.  Do not touch the metal meter case while measuring current with RF levels 
exceeding 5 Watts or RF voltages exceeding 50 Volts.  Power the meter, then apply RF 
and make measurement "hands-off" using the 15-second power-down feature.  
 
3.  Avoid measuring RF current on conductors where high RF voltages may be present 
(>500 Volts).  Damage to your meter may result.      

Theory of Operation 
 

<30mA

3A

Sensitivity

M

<30mA

3A

Calibrate

Q1   Timed Switch

T1

R2

R3

SW1

9V

Det
D2

Line Under 

Test

Sense Coil

U1a

U1b

SW2b

SW2a

Range