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G8MNY > TECH 06.09.25 18:24l 122 Lines 5632 Bytes #3 (0) @ WW BID : 40260_GB7CIP Subj: Edgcume Peebles Megger & Low R Path: JH4XSY<N3HYM<VE3CGR<PD0LPM<PI8ZTM<CX2SA<ED1ZAC<GB7CIP Sent: 250906/0918Z @:GB7CIP.#32.GBR.EURO #:40260 [Caterham Surrey GBR] 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). ___________ ｳﾚﾄﾄﾄﾄﾄﾄﾄﾄﾄｿｳｳｳ .�'~'�. ｳｳ 250V DC test voltage. ｳｳ纊嵭ｳ Range 20K - 25M (0.5M ｫ scale) ｳｳ \ ｳｳ Low Ohms 0R02 - 50R (1R ｫ scale) ｳﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙｳ Often in a Leather case. ｳ [~~~~~] ｳﾞ ~~~~~ ﾝ 9V PP4 battery (or a good PP3!) ﾞﾝ LIVE circuit detector. ﾝﾞﾝﾞｳ_________ｳ 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. _______ ｳCurrentｳ LOW OHMS MEGGER WARNING VOLTAGE ｳsource ﾃﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄｿﾀﾄﾄﾄﾂﾄﾄﾄﾙｳ /~~~\ + +250V ｳ +/~~~\ Battery +ProbeﾄｴMeterﾃﾄﾄRﾄﾄｿ +ProbeﾄRﾄﾄｴ>ﾃﾄﾄｿ +ProbeﾄﾄﾅﾄｴMeterﾃﾄｿｳ \___/ ﾚﾄﾄﾄﾄﾁﾄﾄﾄｿｳｳ \___/ ｳｳｳInterterﾃBatt ｳ R ｳｳﾀﾄﾄﾄﾄﾂﾄﾄﾄﾙ /~~~\ ｳｳｳｳ -Probeﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ -ProbeﾄｴMeterﾃﾄﾙ -Probeﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾙ \___/ METER CIRCUIT The circuit is made quite complex due to the switching around the 3 modes. ﾚﾄﾄｴ<ﾃﾄﾄﾄﾄｿﾚﾄﾄﾄﾄﾄﾄﾂﾄﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄｿｳｳ OHMS ｳ 100R _ｳ_ ｳﾃﾄﾄﾄｴ>ﾃﾄﾄﾄｴ \/ ｳ Cal2 \_/ _ｳ_ ｳｳ |ﾚﾄﾄ+9V ｳｳ _ｳ_ /_\' ｳ /~~~\ ｳ |ﾀ>oﾄﾄﾄｿｳ e\ｳ \_/ ｳﾃﾄｴMeterﾃﾄｴ | oﾄﾄﾄ)ﾄﾄﾄﾄﾄﾙ PNP3 ﾃﾄﾄﾁﾄ220Rﾄｴｳ \___/ ｳ | ｳ /ｳｳｳﾃﾄﾄﾄﾄﾄ>oﾄﾄﾄ)ﾄ16Kﾄ<+250V ｳｳﾃ47Rﾄ100Rﾄｴ | oﾄﾄﾄ)ﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄｴ CONSTANT ｳｳ Cal1 ｳ | ｳｳ CURRENT ｳｳｳﾚﾄﾄﾄ>oﾄﾄﾄ)ﾄﾄﾄﾄｿｳ SOURCE ｳｳｳｳ oﾄｿｳｳ 22R ｳ +Probeﾄo-oﾄ)ﾄﾄﾄﾄﾄﾄﾄﾄﾄ)ﾄﾁﾄ3K6ﾄﾄ)ﾄ)ﾄｿｳ Cal3 220R fuse ｳﾀﾄﾄﾄﾄﾄﾄﾄﾄｴｳｳ 33K ｳｳｳ MEG ｳｳｳ _ｳ_ ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄｴｳ \/ﾚﾄﾄﾄ)ﾄﾙｳ \_/ ｳｳ |ﾀ>o ｳｳｳｳｳ | oﾄ)ﾄﾄﾄ)ﾄﾄ)ﾄﾄﾄ>INV 9V ｳｳ | ｳｳｳｳﾃﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄ>oﾄ)ﾄﾄﾄ)ﾄﾄ)ﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄｴｳ | oﾄ)ﾄﾄﾄﾙｳｳｳ |ﾚﾄﾄﾄ)ﾄﾄﾄﾄﾄﾄﾙ +9V ｳｳﾀ>oﾄﾙ _ｳ_ ｳｳ oｿﾜｳﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙｳｳ -Probeﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄ0V INVERTER __________ NPNｳ/ ｳｳｳ +250V<ﾄﾄﾄﾄﾄﾄﾂﾄﾄﾂﾄﾄﾄ2M2ﾄﾄﾄﾄﾄﾄﾄﾄｴ 3K9 ｳｳ === ｳｳ\e ｳｳｳ 3n3ｳﾀｴ<ﾃﾄﾄｿﾚﾄﾄﾄﾄﾄﾄ)ﾄﾄｴ 1K2 ｳ+ ｳ )||(______ｳｳｳ === ｳ )||ﾚﾄﾄﾄﾄﾄﾄ)ﾄﾄｴｳｳｳ )||(___ ｳｳｳｳｳ )||ﾚﾄﾄﾄ)ﾄﾄ)ﾄﾄ)ﾄﾄｴｳｳ )||( ｳｳｳｳｳﾃﾄﾄﾄﾂﾄﾄﾄﾄﾙ ||( ｳｳｳｳｳｳ +ｳ2u2 ﾚﾄﾄﾄｴｳｳｳｳｳ 6K8 === ｳ \ｳｳｳｳｳｳ INV 9V>ﾄ2R2ﾄｴｳｳ PNP2ﾃﾄｴ 4K7 ｳｳｳﾃﾄﾄﾄﾙｳ e/ｳ_ｳ_ ｳｳｳｳｳ \ｳｳ \_/ ｳｳｳｳｳ PNP1ﾃﾄﾁﾄﾄﾄﾁﾄﾄﾙｳｳｳｳ e/ｳｳｳｳﾀﾄﾄﾂﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙｳｳ ===47u ｳｳ 0V >ﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾙ 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 16K 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 Ohm 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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