Multimeter selection...

[Paul Schreiber]

The DVM in question is no longer made. The RS meters over $49 all use the ASIC that Doug Curtis and I
designed. If you buy the $69-$99 meters with true RMS, then any one will work. I like the $89 hand held that has
the cap meter and the transistor checker. This is a good way to match beta in transistors, and it will
automatically identify the leads (E B C) if you don't have a data sheet. RS likes to change the cosmetics every year,
but I still call them by price!

They are not as accurate as a Fluke 87 ($280 meter) but certainly good enough for 99% of use. The only real difference in the RS and the Fluke is absolute accuracy, and the RS meters are generally 0.2%, which is 30mv
out of a 15V signal. A Fluke is like 0.05% or 7mv error in a 15V signal. The main reason is the cost of the 14
precision resistors used in the divider network. The 0.05% resistors, which are in a laser-trimmered array
(you trim the ratio of them, not the exact value) cost about $2ea (a whopping $28) and the 0.2% resistors
in the RS meter are around 11 cents each. In fact, I have my RS meter 'hopped-up' with 0.05% discrete resistors.

The other limit is the temperature compensation of the bandgap voltage reference. The RS meter uses a statistical
averaging (using the on-board uP, a screaming 4bit Samsung part) with 2 trims. The Fluke uses a 'Zener zapped'
(laser trimmed) reference that has about the same overall % drift as mine, but the initial accuracy is much
tighter: so over the 0-50C range, it drifts 'closer to home'.

Voltmeters are almost as fun to design as synths. Don't miss the UL testing, though ("Let's put it on the OHM scale
and apply 1000 VAC for 1 minute!!!")

Paul Schreiber
Synthesis Technology
www.synthtech.com


----------
From: 	Haible Juergen[SMTP:Juergen.Haible at nbgm.siemens.de]
Sent: 	Monday, April 06, 1998 12:02 PM
To: 	synth-diy list; dr john tuffen
Subject: 	AW: Multimeter selection...



	>I remember a few months back, there was a discussion about which
tools
	>should be in a basic toolbox - and multimeters were among the
things
	>discussed. In particular there was a Radio Shack meter that seemed
to be
	>recommended..... anyone tell me which it was? (It was (co-)designed
by a
	>listmember IIRC)

Be sure you have a frequency counter, a capacitance meter, and a
transistor beta tester built in.

JH.