On Fri, 21 Mar 2014, Trevor Wilson wrote:


**I started using digital meters back in 1975 (Fluke). I didn't much care for
them. It took me many years to wean myself off an analogue meter. Once weaned
off them, I've never looked back. Analogue meters are cool, but for day to
day use, nothing beats a digital meter.

The interesting thing here is that the original post was about replacing
the battery in a VTVM, and the battery is only used for measuring
resistance.

The loading effect doesn't come into play. And it's probably easier to
read ohms on a digital meter than an analog meter.

I would argue that digital meters haven't been as forward looking as they
could. I got one of those Intersil development boards for their famous
DVM IC in 1978. I had big plans for it, never actually done. But I was
going to add a buffer using a mosfet or jfet input op-amp, and feed that
to a cheap meter that had been a signal strength meter in a radio. That
would give the analog movement for tracking peaks and trends, all in one
box. You dn't need to know voltages in that case, just that something has
gone up or down. I hope I would have remembered to put the 1M resistor in
at the tip of the DC meter, just like a VTVM. I can't remember what else
I'd planned.

Yes, one of my DMMs has an "analog" display, but it's sluggish, works off
the DMM rather than directly off the input voltage. I know at one point
there was at least one DMM that had a small analog meter built in for
reading those peaks, a really good solution.

There is no reason we couldn't see multiple test leads (or a lead with a
switch to bypass that 1meg resistor in the tip) in DMMs, it just simply
didn't happen. When I used my HP 410B a lot, I'd always be tangling (and
untangling) the leads, since it had one for AC, one for DC, and one for
ohms. So some consideration probably was given to that, and likely a
blindspot, "who really needs the isolation resistor in the tip in this day
of digital?".

I wish that cheap DMM actually was useful as a frequency counter, but at
400KHz tops, it can't be. And I wouldn't trust the test leads for even
those upper frequencies.

I wish they didn't use the test leads for the capacitance measuring
section, they have a place on the meter to put the leads in, and the test
leads just cause problems with extra and variable capacitance. I remember
one construction articel in Radio Electronics for a digital capacitance
meter that had an adjustment to compensate for the circuity, so it really
was 0 with no capacitor, useful for those low value capacitors.

I did replace the battery in my RCA VTVM with some circuitry, long
forgotten. I found another RCA VTVM just like it a couple of years ago, a
stray item in a box on a busy street, no sign of other garbage. And DMMs
have become so cheap that last year I got another reasonably fancy DMM for
not much, and a pen DMM, which has it's uses, for ten dollars. We seem to
have passed the point of readily available meters the size of a calculator
It reminds me of that mid-seventies pen DMM from HP, the one with an LED
readout and a switch so you could turn the readout upside down, depending
on how you were viewing it.

The early days of building your own digital meters were much more
cmplicated, endless 7400 series TTL, but it gave flexibility. The same
counter section could be used for a decent frequency counter, a decent
capacitance meter, whatever else you needed to measure. We've lost that,
I can't think immediately of some IC that is readily available and cheap
and provides that counter function, while DMMs have become so cheap that
one can get them for ten dollars, free in some places in the US
apparently. Once we started buying them, we lost the ability to control
how we wanted them.

Michael