[volt-nuts] Mechanical choppers for precision instruments
Dr. Frank Stellmach
drfrank.stellmach at freenet.de
Sun Mar 21 09:13:06 UTC 2010
I remeber HP and FLUKE meters and calibrators.
First there were the mechanical (coil) choppers, often defective by
mechanical degradation which led to unsymmetrical duty cycle, i.e. big
Later came the opto-choppers, their problem was the defective neon bulb
for the CdS LDR, which also had to be replaced often. Difficult /
impossible to find neon bulb today.
FLUKE had both techniques in their 332/335 calibrators, and also in the
The main coil chopper amp was later replaced with a MOSFET input
chopper, and a npn transistor at the output rectifier.
Those schematics with both versions you may find on the Fluke site, look
for service / manuals and search for the model. Download manuals for
models: 343 (MOSFET) , 332A, 332D, 335A, 335D, 845
The 335A_D manual contains the coil chopper for the calibrator
circuitry, (page 119/120) and the opto chopper version for the included
Nullmeter, same circuitry as of the 845A/B. Compare this with the 335_D
manual,(page 169) where you can find the MOS FET version for the
More information about the volt-nuts