Message 25963 in TECH.WW
From: G8MNY
Date: Thu, 16 Jul 26 08:46:00 Z
Newsgroups: TECH.WW
Subject: Lafayete 57 Range Multimeter
Message-ID: <65055_GB7CIP>
Path: N2MH4|N3HYM|K5DAT|GB7CIP
R:260716/0851Z 33765@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
R:260716/0850Z 9985@N3HYM.#FRDK.MD.USA.NOAM BPQ6.0.25
R:260716/0849Z 45793@K5DAT.#NEWI.WI.USA.NOAM LinBPQ6.0.25
R:260716/0846Z @:GB7CIP.#32.GBR.EURO #:65055 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Corrections Jan 16)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Model 99-5076
MULTI METER PRINCIPLES
DC VOLTS 1000 100 10 1 Volts
sensitivity to do many o o o o
the voltage drop across /~~~\
it will affect the lowest 90uA\___/
Series resistors are used
to protect the switch more Sensitivity is 11K1/V
from high voltages.
DC CURRENT 100uA 1mA 10mA 100mA
meter load.
AC VOLTS
For AC a rectifier diode is
used, and normally this alters 1000 100 10 Volts
multiplier arm from the DC is used. o o o
diode D1, so the low voltage
range can take this into account
on the scale. D2 can be any type Sensitivity is 4K5/V.
and protects the low voltage D1.
OHMS
Here a voltage source is put
in series with the meter and a
known resistance that set the
half scale resistance and includes (actually -ve!)
voltage the higher resistance /~~~\ o o o o
ZERO reading on shorting the
probes depending the battery state.
Note that the battery internal resistance is added to the unknown resistance,
and this affects the low range scale calibration! Also the probe polarity is
reversed to that indicated and it is different than for digital meters!
ACTUAL MULTIMETER CIRCUIT
0.25 2.5 10 50 250 1k R/5 R 10 100 1k 1k 250 50 10 3 500 50 5 50u
\________DC_VOLTS_________/ \_____OHMS____/ \___AC_VOLTS___/ \____DC_mA____/
& dB Scales
The configuration switching between modes and all the ranges is all done on
just 1 large rotary switch with 3 bridging contacts (I) and 5 wipers of 2
circuits.
Only 4 probe sockets are used, 2 extra for the high current (10A) & the DC
isolated AC measurement.
Extra switches provide a meter reverse feature and a double sensitivity that
doubles the number of meter ranges. (has no meaning on ohms!)
This meter's movement is a extremely sensitive @ 15uA for FSD, but not very
accurate, so it is calibrated with an ICal & VCal presets at manufacture.
The AC ranges are calibrated with select on test resistors 2x "4K7 Low" & the
"5K Hi" that accurately allows for the diode forward resistance loss & leakage.
Most of the resistors are special values made for the meter, and of course they
are very accurate and stable 0.5% types.
See my tech buls on "Meter Movement Types", "Maplin Mains Meter 2000MU-UK",
"Edgcume Peebles Megger & Low R", "Edgcume Peebles Earth Loop Z", "Car Dwell
Angle & RPM Meter", & "Kelvin Varley Voltage Divider".
Why don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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