Actually I think the OP probably had a cheap instrument and never gave us a
make/model....so hard to say how it would react to rechargeables but if the
meter was cheap....it shouldn't make much difference since accuracy would
not be an issue with such an instrument.... :>) Ross
Very possible...I will check the site myself....might be one of those
situations where we are both correct by situation....not particularly
important as long as our fine meters get the job done...and fine they
are...take care Dick, Ross
In alt.home.repair on Sat, 26 Feb 2005 17:46:33 -0500 "Ross Mac"
I believe accuracy should be independent of whether the battery is
fully charged or not. They set it up so the measurement is in
proportion to a known resistance, so it should read correctly at any
battery voltage. (although eventually the battery is too weak to move
the needle or power the lcd's.)
BTW, everyone mentions alkaline batteries, but one of my digital
meters requires a 9-volt carbon zinc battery. An alkaline battery
won't fit in the space provided. (Unlike C, D, AA, and AAA batteries,
9 volt batteries are different in size between carbon-zinc and
If emailing, please let me know whether
or not you are posting the same letter.
Change domain to erols.com, if necessary.
Well, that way you need two batteries and a charger, and rechargable batteries
are more expensive than plain carbon zinc, and the meter may not work well on
1.2 vs 1.5 V, and this arrangement won't monitor house voltage, and it takes
more human attention, and it's less fun.
More like 1 mA, altho that's too much for an Eveready 1212 AAA with a 540 mAh
capacity that loses 10% of its energy over a year. That only needs 0.10x540
= 54 mAh over a year, or about 6 microamps on a continuous basis, if the meter
is never used for anything else, but the movement might require another 100
microamps. And why waste power in that resistor, vs a charge pump like this,
viewed in a fixed font?
| \ | | | 1.5 V
. | | | | |
120V . --- ---
. ^ -
------------ | |
| | |
| / | | |
- --- --- ---
- _ -
Q = 170C coulombs and I = 60Q = 106 x 10^-6 amps makes C = 0.01 microfarads.
The battery would be a fine smoother and voltage regulator. Harbor Freight
stores sell $2.99 digital multimeters. Maybe they need less than 100 uA.
My flashlight plugs into the wall. Very convenient, because I know where to
find it and don't have to change batteries. My CO and barn heat detectors
work that way too, by design, with audible and remote X10 alarms and "non-
rechargable" batteries that rarely need changing. I hate changing batteries.
I lose my cheap Casio watches with 7 year Li batteries before they go dead.
That might be an Insteon/X10 interface PIC...
The data sheet says it uses about 100 nanoamps at 3 V in sleep mode and
12 microamps at 32 kHz. If it's awake 1% of the time, say 1 second out of 100,
when it talks to a central controller, it would need 0.99x0.1+0.01x12 = 0.22
microamps. Q = 340C coulombs and I = 60Q = 0.22x10^-6 amps makes C = 10 pF,
If we needed more power and worried about overcharging the 2 AAAs, we might
use the PIC to measure the voltage and only enable the charge pump as needed.
Absolutely incorrect. That is not how a meter works.
And anyhow....a spare battery and a charger???....That is not expensive....
Not trying to fire ya up here but those are the facts.
My math error...E=IR 60PVDC/47Kohms = 1.2ma or so....The incorrect charging
rate for a rechargeable.
Those batteries are normally referred to as "Puny Duty" and typically never
make it to 10 recharges...
I don't think this is a good recomendation to the NG....
I cannot make heads nor tales out of your schematic, fixed font or not, but
it appears we now have a capacitor in the circuit...what happened to the
"diode and resistor" and you still need a regulator since you still have 60
volts across a 1.5 volt battery.
And please no, no....not that doorstop 3 buck meter...a good set of leads
will cost you more than that thing. That meter is more likely to get you
into more trouble than it ever gets you out of. That "THING" would only be
good for very crude troubleshooting.
This post is beginning to smell of TROLL.....well maybe not....but the bait
and switch characteristics are there!
As for connectiong the active and passive circuits in the meter....I want to
be there when you try to measure the 240vac coming into the house and you
put 120pvdc across that battery with a camera to catch the absolute look of
The "diode and resistor" have been replaced by a "charge pump" (you can
google this) for the reason of "why waste power in that resistor." This
is not a bait and switch, it is merely an improvement on an earlier
If you remove the battery from Nick's charge pump circuit proposal, you
have what's commonly called a "voltage doubler" (you can google this)
and the open-circuit output voltage will peak at twice the AC input
amplitude, which would be 340V for "normal" 120VAC-RMS input. Limiting
the size of the capacitor limits the current output capability. Placing
a battery across the output limits the output voltage by shunting the
current flowing through the capacitor. Also remember the
current=C*dV/dt, and dV/dt peaks at 170volts/sec. So to reiterate,
current and voltage have been limited: as long as the battery is there
and can accept the current, you do not have 60V, 120V, 340V or
whatever, you have 1.5V regulated by the battery and no additional
regulator is necessary.
I would modify this design by replacing the diode whose anode connects
to ground with a zener rated for 2.5 to 5 volts. That way, if the
battery should open-circuit, the downstream components won't see the
370V spike, it having been shunted by the zener. Similar
"transformerless stepdown" circuits are used to supply regulated 12VDC
to motion detectors.
I think Nick was off in his original math because he may not have
considered that the output diode (the one with the cathode connected to
the battery) begins conducting roughly as the AC input wave rises from
its -170V minimum, and continues to conduct roughly till the input
reaches its 170V maximum (we're ignoring .6V and 1.5V voltage drops
across diodes and batteries as insignificant compared to 170V).
Therefore the total charge moved per cycle is Q=C*340volts over the
full voltage swing of -170 to +170. (I now see and agree with his math
ammended in a subsequent post.) Current in amps (coulombs per second)
is Q*60 because there are 60 cycles per second. Substituting the
suggested value of .0047 uF for C, and current comes out to be 96
microamps. This is not meant to recharge a battery that has been run
flat, but rather to compensate for the self-discharge of the battery
such that it does not discharge during storage.
The question of whether the negative terminal of the battery is
connected to an identifiable "common" node in the VOM's internal
circuit is left for further consideration.
Very true. The really cheap meters will say something like 2 Kohm / volt or 3 K
ohm / volt on the face. A better quality one will say something like 20K ohm /
volt. These ratings are usually based on 1 milliamp giving full scale deflection
of the needle. The batteries for resistance readings only.
It was a pain trying to measure voltages across very high resistances. The meter
would be in parallel with the resistance and completely mess up the real values.
Getting my first Field Effect Transistor powered meter was a reall blessing. It
had something like 2 MEG ohms / volt sensitivity. Most of the digital stuff now
probably has 10 or 20 Meg ohms / volt. In some cases that's no good. With
almost no loading on a circuit/wire you can get voltages seemingly appearing out
of nowhere, giving rise to much head scratching.
On 2005-03-01 firstname.lastname@example.org said:
> email@example.com wrote:
> ... The really cheap meters will say something like 2 Kohm /
>volt or 3 K ohm / volt on the face. A better quality one will say
>something like 20K ohm / volt. These ratings are usually based on 1
>milliamp giving full scale deflection of the needle.
No, not 1 mA.
2 Kohm/volt = .5 mA
3 Kohm/volt = .33 mA
20 Kohm/volt = 50 microAmps
>Getting my first Field Effect Transistor powered meter was a reall
>blessing. It had something like 2 MEG ohms / volt sensitivity. Most
>of the digital stuff now probably has 10 or 20 Meg ohms / volt.
No, not "per volt". That's the input resistance for DC measurements.
My Beckman 3020 DMM is 22 megohms input resistance.
Fluke 27 and 73 read 11 megs on the Beckman
Fluke 36 DMM and clamp-on AC/DC ammeter reads 2 megs.
near Mountainair, (mid) New Mexico, USA
Net-Tamer V 1.12.0 - Registered
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