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G8MNY  > TECH     16.05.19 08:15l 122 Lines 5610 Bytes #6 (0) @ WW
BID : 5032_GB7CIP
Read: GUEST
Subj: Edgcume Peebles Megger & Low R
Path: ED1ZAC<ED1ZAC<GB7CIP
Sent: 190516/0605Z @:GB7CIP.#32.GBR.EURO #:5032 [Caterham Surrey GBR] $:5032_GB
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                        (New Jan 08)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

This is an old tester the engineers used to use for testing wiring
(e.g. telephone).
 ___________ 
넬컴컴컴컴커
납 .'~'. 납  250V DC test voltage.
납\      邂  Range 20 k - 25 M (0.5M  scale)
납  \      납  Low Ohms 0.02 - 50 (1  scale)
냅컴컴컴컴켸  Often in a Leather case.
  [~~~~~]  
   ~~~~~     9V PP4 battery (or a good PP3!)
             LIVE circuit detector.
          
          
 _________

PRINCIPLES
The meter uses an inverter to provide the 250V megger supply. In the idle push
button position there is an un calibrated warning meter mode. The low ohms uses
a reverse current bridge circuit.                     _______
                                                     쿎urrent  LOW OHMS
       MEGGER                WARNING VOLTAGE         퀂ource 쳐컴컴컴컴커
                                                     읕컴쩡컴          
        /~~~\ +   +250V                                   +/~~~\    Battery
+Probe캑Meter쳐훀컴        +Probe훀컴>쳐커     +Probe컴탠퀾eter쳐    
        \___/ 旼컴컨컴커                                  \___/      
              쿔nterter혾att                            R             
              읕컴컫컴켸            /~~~\                            
-Probe컴컴컴컴컴컴켸        -Probe캑Meter쳐     -Probe컴좔컴컴컴컴좔컴켸
                                    \___/

METER CIRCUIT
The circuit is made quite complex due to the switching around the 3 modes.

           旼캑<쳐컴커                旼컴컴컫컴컴쩡컴컴
                      OHMS              100R  __    
           쳐컴>쳐컴  \/                Cal2  \_/   __
                       |旼+9V               __   /_\'
             /~~~\     |>o컴커           e\ \_/     
           쳐퀾eter쳐   |  o컴)컴컴켸   PNP3 쳐컨220캑
             \___/     |                   /        
                    쳐컴컴>o컴)16k<+250V           
           쳐47100컴   |  o컴)컴컴컴컴컴컴 CONSTANT 
               Cal1    |                    CURRENT 
                     旼컴>o컴)컴컴         SOURCE  
                         o커           22R         
+Probe훟-o)컴컴컴컴)컨3k6컴))커        Cal3       220
      fuse          읕컴컴컴캑   33K                
                       MEG      __      읕컴컴컴컴캑
                       \/旼컴)켸  \_/                 
                        |>o                        
                        |  o)컴)컴)컴>INV 9V        
                        |                           
           쳐컴컴컴컴컴컴컴>o)컴)컴)컴컴컴컴컴컴컴컴컴
                        |  o)컴켸                    
                        |旼컴)컴컴컴    +9V           
                         >o켸           __           
                           o                         
           읕컴컴컴컴컴컴컴컴                         
-Probe컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컨컴컴컴컴컴컴좔0V

                   INVERTER     __________
                           NPN/        
+250V<컴컴컴쩡컫컴2M2컴컴컴컴   3k9    
           ===               \e       
         3n3  윱<쳐커   旼컴컴)컴 1k2  +
                     )||(______      ===
                     )||旼컴컴)컴     
                     )||(___          
                     )||旼컴)컴)컴)컴   
                     )||(            
            쳐컴쩡컴켸 ||(            
              +2u2 旼컴            
           6k8 ===       \          
INV 9V>2R2캑        PNP2쳐 4k7      
            쳐컴       e/__        
                     \   \_/        
                  PNP1쳐좔컴좔켸       
                    e/                
            읕컫컴컴컨컴컴컴컴컴컴켸     
              ===47u                     
   0V >컴컴컴컴좔컴컴컴컴컴컴컴컴컴컴컨컴켸

CIRCUIT OPERATION
With the rocker switch idle the meter is connected to the probes via 33k & a
diode, so it warns of AC or +ve voltages connected, as well as providing a
discharge path to any voltage stored on the cable under test from meggering.

Pressing the M button puts 9V into the inverter, the 2 darlington connected
PNPs with the transformer form the invertor oscillator. The secondary is
rectified to provide the output DC voltage, which also turns on the NPN to
provide a voltage control feedback. The NPN turning on, takes base drive
current from PNP1 completing the control loop.

The 250V feeds an 18k to the meter to the +probe, a 3k6 in is the -probe lead.
Calibration is provided with Cal1 preset across the meter to set Zero & mid
scale ohms.

Pressing the  button puts 9V into the constant current source consisting of 2
voltage limiters on the supply a zener & then 2 diodes, feeding the PNP base
giving exactly 0.6V across its emitter resistor Cal2 & hence constant current
from the collector. Cal2 sets the current which gives the full meter indication
on no load. The mid scale calibration (@10R) is done with Cal3 preset that
takes half the supply current at mid scale.

N.B. Calibration must be done in Cal1 Cal2 Cal3 order!

MODIFICATION
I have modified mine to give 550V for mains wiring MEGER testing. To do that I
added a diode & cap to make voltage doubler on the secondary, & then changed
the 16k for a 121k to keep it all calibrated.


Why don't U send an interesting bul?

73 de John G8MNY @ GB7CIP


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