[nickbatz]

I'm a few years into my second Radio Shack VOM, the $20 analog one. It's been perfectly fine for project studio and household use, which is 98% checking cables and connectors, 1% checking batteries, and 1% AC outlets.

Yesterday I tried to use it for troubleshooting an electrical issue on my motor scooter. The battery read about 12V DC...and also 12V AC!

Is that normal behavior, or do I just have a broken VOM?

No, the battery wasn't connected.

TIA

[William Sommerwerck]

"nickbatz" wrote in message
...

Yesterday I tried to use it for troubleshooting an electrical issue
on my motor scooter. The battery read about 12V DC... and also
12V AC!

Nothing wrong. The DC is simply passing through the rectifier in the AC
measurement circuitry.

[Scott Dorsey]

nickbatz wrote:
I'm a few years into my second Radio Shack VOM, the $20 analog one. It's been perfectly fine for project studio and household use, which is 98% checking cables and connectors, 1% checking batteries, and 1% AC outlets.

Yesterday I tried to use it for troubleshooting an electrical issue on my motor scooter. The battery read about 12V DC...and also 12V AC!

Is that normal behavior, or do I just have a broken VOM?

The AC metering setting just puts a diode in series with the meter and adds
a 1.4X correction factor to compensate for the fact that the average voltage
of a sine wave is 70% of the peak voltage.

So... if there is any DC, it will register voltage (but not accurately) and
if there is any waveform that isn't a sine wave it will register voltage
(but not accurately). The AC setting _only_ gives you a meaningful reading on
a pure AC sine wave with no DC component and no harmonics.... if you give it
something else, the meter will move but what it shows won't be valid.

A higher end meter would add a blocking capacitor so that you only see the
AC component when on the AC setting. A much higher end meter would have an
actual RMS detector so that it could give you a valid average level given a
waveform that isn't a sine wave.

There is a reason why the Fluke costs more than the Radio Shack product.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Paul[_13_]]

On 7/12/2014 6:36 AM, Scott Dorsey wrote:
nickbatz wrote:
I'm a few years into my second Radio Shack VOM, the $20 analog one. It's been perfectly fine for project studio and household use, which is 98% checking cables and connectors, 1% checking batteries, and 1% AC outlets.

Yesterday I tried to use it for troubleshooting an electrical issue on my motor scooter. The battery read about 12V DC...and also 12V AC!

Is that normal behavior, or do I just have a broken VOM?

The AC metering setting just puts a diode in series with the meter and adds
a 1.4X correction factor to compensate for the fact that the average voltage
of a sine wave is 70% of the peak voltage.


You meant to say, the Root-Mean-Square value is: Vrms = Vpeak * 0.707

The Average voltage is: Vave = Vpeak * 0.637

[Dave Plowman (News)]

In article ,
nickbatz wrote:
I'm a few years into my second Radio Shack VOM, the $20 analog one. It's
been perfectly fine for project studio and household use, which is 98%
checking cables and connectors, 1% checking batteries, and 1% AC outlets.

Yesterday I tried to use it for troubleshooting an electrical issue on
my motor scooter. The battery read about 12V DC...and also 12V AC!

Is that normal behavior, or do I just have a broken VOM?

No, the battery wasn't connected.

I'm not keen on using a cheap meter on mains - at least here in 240v land.
I've had one blow apart - due to brass dust from the selector switch
shorting it out. Wasn't an el cheapo, either. I now carry a proper mains
one now - and keep the DVM for low (ish) voltage stuff.

I'm not surprised an analogue one gives you that reading on an AC range -
it will simply have a rectifier added to measure AC, which will also pass
DC.

--
*It's a thankless job, but I've got a lot of Karma to burn off

Dave Plowman London SW
To e-mail, change noise into sound.

[Les Cargill[_4_]]

nickbatz wrote:
I'm a few years into my second Radio Shack VOM, the $20 analog one.
It's been perfectly fine for project studio and household use, which
is 98% checking cables and connectors, 1% checking batteries, and 1%
AC outlets.

Yesterday I tried to use it for troubleshooting an electrical issue
on my motor scooter. The battery read about 12V DC...and also 12V
AC!


So you had it on AC volts, and it read 12V?

Is that normal behavior, or do I just have a broken VOM?

No, the battery wasn't connected.

TIA



Actually, it's the designers choice of how to deal with this.
Different instruments may behave differently.

I actually might expect it to show 12V times 1/sqrt(2). I would
not think the instrument is true RMS now. But maybe I forget; I
don't read AC with a meter much any more.

--
Les Cargill

[Mike Rivers[_2_]]

On 7/12/2014 9:36 AM, Scott Dorsey wrote:

The AC metering setting just puts a diode in series with the meter and adds
a 1.4X correction factor to compensate for the fact that the average voltage
of a sine wave is 70% of the peak voltage.

I just checked a 1.5v battery on the AC scale of my $9.95 Craftsman VOM.
It reads 3v, apparently a different crest factor estimate. There's
definitely a diode in series, however, as with the leads swapped, it
reads zero.

The Craftsman meter reads 121v across the AC power line however, same as
my $99.95 Fluke meter.

--
For a good time, visit http://mikeriversaudio.wordpress.com

[nickbatz]

Thanks everyone.

This is another one of those things I never stopped to think about.

[nickbatz]

Question: what value blocking capacitor would I use if I wanted to build something to filter DC from a 12V motorscooter system?

[Mike Rivers[_2_]]

On 7/16/2014 6:03 PM, nickbatz wrote:
Question: what value blocking capacitor would I use if I wanted to
build something to filter DC from a 12V motorscooter system?

Well, it depends on the frequency of the AC that you're trying to
measure in the presence of DC. But how about a little more information
about what you're trying to measure? Does it use an AC motor? Or is the
DC pulse width modulated to control the speed of a DC motor? Or what?


--
For a good time, visit http://mikeriversaudio.wordpress.com

[nickbatz]

Mike, I missed your post. Sorry.

The scooter has an AC stator coil (electromagnetic generator) that runs off the rear wheel (actually it's a dual coil - the other one triggers the spark plug). It runs its headlights and the instrument cluster off AC, but also sends AC to a rectifier/regulator that sends out DC: it charges the 12V battery and runs the rest of the 12VDC system - turn signals, brake lights, rear lights, horn, low oil light, plus a couple of things I don't use after having modified the bike.

I'm having issues with the DC side when the engine is running (only when it's running). The turn signals flash erratically and change with the engine RPMs, and the battery charging voltage is low.

You'd think the stator coil, especially the rectifier/regulator, or just the wiring are bad, but I have solid reasons to think each one of those isn't the culprit. It's probably a bad ground, or my latest suspect is that the spark plug wire - which is now old and has old insulation - is phaffing with me.

What I was trying to measure when I posted this is simply whether there's AC in the DC system, actually at the battery. I didn't find AC between the ground (which is the bike's frame) and another ground.

It's a simple electrical system that's now officially annoying the hell out of me.

[Mike Rivers[_2_]]

On 7/16/2014 8:13 PM, nickbatz wrote:
The scooter has an AC stator coil (electromagnetic generator) that
runs off the rear wheel (actually it's a dual coil - the other one
triggers the spark plug). It runs its headlights and the instrument
cluster off AC, but also sends AC to a rectifier/regulator that sends
out DC: it charges the 12V battery and runs the rest of the 12VDC
system - turn signals, brake lights, rear lights, horn, low oil
light, plus a couple of things I don't use after having modified the
bike.

I'm having issues with the DC side when the engine is running (only
when it's running). The turn signals flash erratically and change
with the engine RPMs, and the battery charging voltage is low.

I wonder if there's a short, or insulation breakdown, between the two
coils in the generator. The one that goes to the spark plug is likely a
high voltage (sort of like a magneto) and the other one is a low
voltage. If the insulation in that generator is breaking down, when the
engine is running, you could be getting spikes from the ignition
generator into the generator that runs the rest of the electrical system.

Got an oscilloscope?


--
For a good time, visit http://mikeriversaudio.wordpress.com

[Gray_Wolf]

On Sat, 12 Jul 2014 06:23:01 -0700 (PDT), nickbatz
wrote:

I'm a few years into my second Radio Shack VOM, the $20 analog one. It's been perfectly fine for project studio and household use, which is 98% checking cables and connectors, 1% checking batteries, and 1% AC outlets.

Yesterday I tried to use it for troubleshooting an electrical issue on my motor scooter. The battery read about 12V DC...and also 12V AC!

Is that normal behavior, or do I just have a broken VOM?

No, the battery wasn't connected.

TIA

Your meter measures 12vac on a battery? In my 50+ years I've never
seen it happen. Bad meter are you made a mistake reading it. In any
case the battery is probably okay. What are the symptoms of the
electrical problem?

[William Sommerwerck]

"nickbatz" wrote in message
...

Question: what value blocking capacitor would I use if I wanted
to build something to filter DC from a 12V motorscooter system?

No one has directly answered this, so I'm going to make a stab at it.

A cheap analog VOM probably has an AC "sensitivity" of 1000 ohms/volt. (The
user manual should list this.) Let's assume that's the case.

Let's also assume you're on the 20V scale. That gives a load resistance of
20K.

If you stick a capacitor //of any value// in series with the meter, you'll
block all DC. But if the capacitor is "too small", you'll attenuate AC you
want to measure.

The formula is

C = 1 / (2 * pi * R * f)

where f is the corner frequency. Response will be down 3dB at this frequency.

I don't know what the lowest frequency produced by an ignition system would
be, but 10Hz seems reasonable. Plug in the values, and you'll have the
capacitor size needed. (It comes to about 1 uF.)

A Mylar cap of 250V or higher should be okay. (Someone correct me on this if
I'm wrong.)

[nickbatz]

Okay, thanks very much for the help, everyone. And sorry for the diversion - this isn't exactly audio.

Mike, your analysis of how it works is 100% right, but I replaced the coil assembly and the problem persisted. It was a used one from a friend, but it was working on his bike beforehand (he's a moped collector and always has spare parts lying around).

There's almost certainly a short somewhere, but it seems unlikely to be between the two coils (although I am about to measure the coil resistance to make sure the charging coil is good). Still, now that I'm reasonably certain it's not a bad rectifier/regulator, the evidence is pointing to a short between the AC and DC systems after the R/R.

(If it's a bad R/R then three of them have the same problem; their DC sides are all passing current in one direction and blocking it when I reverse the test leads, which is what they're supposed to be doing.)

But I still want to measure whether there's AC hitting the battery, which is why I need to block DC from the meter. Ideally I'd be able to see whether there's current in the ground (aka the bike's frame); my two cheapo meters say there isn't, but I don't trust them.

Gray_, the battery is good. My cheap meters don't distinguish between AC and DC. Scott and other explained why earlier in this thread, which is why I want to block DC. Of course the battery itself isn't sending AC!

The symptoms are that the DC system on the scooter acts up - ONLY when the engine is running, i.e. it works fine off the battery, but something is going wrong when it's getting power from the stator. That's why I'm looking for stray AC.

[nickbatz]

Fixed!

The starter relay going nowhere was causing problems, or more likely the momentary contact switch going to it.

Thanks again for the help. It wasn't an audio issue, but I learned some new stuff about electronics that are applicable to audio.

Yeah baby.

[Shaun]

"nickbatz" wrote in message
...

I'm a few years into my second Radio Shack VOM, the $20 analog one. It's
been perfectly fine for project studio and household use, which is 98%
checking cables and connectors, 1% checking batteries, and 1% AC outlets.

Yesterday I tried to use it for troubleshooting an electrical issue on my
motor scooter. The battery read about 12V DC...and also 12V AC!

Is that normal behavior, or do I just have a broken VOM?

No, the battery wasn't connected.

TIA


Those cheap Radio Shack meters are very poor quality!! I know because many
years ago I used to work at a radio shack and customers would come in
regularly with blown meters.

[Mike Rivers[_2_]]

On 7/18/2014 12:22 AM, Shaun wrote:

Those cheap Radio Shack meters are very poor quality!! I know because
many years ago I used to work at a radio shack and customers would come
in regularly with blown meters.

That says more about the Radio Shack customers than the Radio Shack meters.

--
For a good time, visit http://mikeriversaudio.wordpress.com

[Dave Plowman (News)]

In article ,
Mike Rivers wrote:
On 7/18/2014 12:22 AM, Shaun wrote:

Those cheap Radio Shack meters are very poor quality!! I know because
many years ago I used to work at a radio shack and customers would come
in regularly with blown meters.

That says more about the Radio Shack customers than the Radio Shack
meters.

You've never gone to measure voltage just after current and blown the
fuse?

Hello, Superman. ;-)

--
*I don't know what your problem is, but I'll bet it's hard to pronounce

Dave Plowman London SW
To e-mail, change noise into sound.

[Scott Dorsey]

nickbatz wrote:

There's almost certainly a short somewhere, but it seems unlikely to be bet=
ween the two coils (although I am about to measure the coil resistance to m=
ake sure the charging coil is good). Still, now that I'm reasonably certain=
it's not a bad rectifier/regulator, the evidence is pointing to a short be=
tween the AC and DC systems after the R/R.

Watch out! A shorted turn in a coil won't show up on a resistance test, but
will make the coil totally useless.

The symptoms are that the DC system on the scooter acts up - ONLY when the =
engine is running, i.e. it works fine off the battery, but something is goi=
ng wrong when it's getting power from the stator. That's why I'm looking fo=
r stray AC.

I believe that with most of these things, the signal coming off the alternator
isn't really filtered, it's just rectified into pulses, and the battery has
the job of doing all the filtering. This is hard on the battery, but if you
are seeing alternator noise on the power buss it's apt to be either because
one or more phases of the alternator or rectifier is bad so some of the pulses
or missing, or because the battery isn't filtering well.

What you need is a Moto-Guzzi with a magneto and no electrics. Although I
have noticed that a lot of drivers can't figure out the hand signals anymore.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Scott Dorsey]

In article , Shaun wrote:

Those cheap Radio Shack meters are very poor quality!! I know because many
years ago I used to work at a radio shack and customers would come in
regularly with blown meters.

It's true that they are pretty poor quality but I think this reflects more
on the customers than the product...
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Mike Rivers[_2_]]

On 7/18/2014 7:10 AM, Dave Plowman (News) wrote:
You've never gone to measure voltage just after current and blown the
fuse?

Hello, Superman.

No, I've never done that, but I can believe that the typical Radio Shack
customer might do that. I have nothing against Radio Shack. I've bought
resistors there on Sunday. I have nothing against cheap multimeters - I
have a couple that I consider expendable, though I've never damaged one.
But I understand my tools and take care of them.

The fact that the meter is protected by a fuse from a certain degree of
stupidity or carelessness says that at least this has been given some
thought. They could save a dime by eliminating the fuse.



--
For a good time, visit http://mikeriversaudio.wordpress.com

[Dave Plowman (News)]

In article ,
Mike Rivers wrote:
The fact that the meter is protected by a fuse from a certain degree of
stupidity or carelessness says that at least this has been given some
thought. They could save a dime by eliminating the fuse.

Ever bought a Fluke fuse? ;-)

--
*Santa Claus has the right idea. Visit people only once a year.

Dave Plowman London SW
To e-mail, change noise into sound.

[Peter A. Stoll[_2_]]

"Dave Plowman (News)" wrote:
Ever bought a Fluke fuse? ;-)

Yes indeed. I'm not superman and did blow the fuse that protects the current circuitry
on my model 117. The 11A (very) slow blow fuse cost me $7.98-so a couple or more of
the Harbor Freight cheap meters if you catch the good sale.

[Ron C[_2_]]

On 7/18/2014 7:29 PM, Peter A. Stoll wrote:

"Dave Plowman (News)" wrote:
Ever bought a Fluke fuse? ;-)

Yes indeed. I'm not superman and did blow the fuse that protects the current circuitry
on my model 117. The 11A (very) slow blow fuse cost me $7.98-so a couple or more of
the Harbor Freight cheap meters if you catch the good sale.

OK, I blew the fuse on my meter once ...under rather odd conditions.
My students were measuring very low currents on a several K-volt
experiment. [Google "antigravity lifter experiment" ] An unexpected
arc over blew the fuse. Um, does that count?

==
Later...
Ron Capik
--

[nickbatz]

I've replaced the fuse on my Radio Shack meter several times.

"What you need is a Moto-Guzzi with a magneto and no electrics. Although I
have noticed that a lot of drivers can't figure out the hand signals anymore."

Things like brake lights and headlights come in handy too.

[Gray_Wolf]

On Fri, 18 Jul 2014 09:28:03 -0400, Mike Rivers
wrote:

On 7/18/2014 7:10 AM, Dave Plowman (News) wrote:
You've never gone to measure voltage just after current and blown the
fuse?

Hello, Superman.

No, I've never done that, but I can believe that the typical Radio Shack
customer might do that. I have nothing against Radio Shack. I've bought
resistors there on Sunday. I have nothing against cheap multimeters - I
have a couple that I consider expendable, though I've never damaged one.
But I understand my tools and take care of them.

The fact that the meter is protected by a fuse from a certain degree of
stupidity or carelessness says that at least this has been given some
thought. They could save a dime by eliminating the fuse.

I've had a pair of Radio Shack's better quality Micronta digital
multimeters for the last twenty five years that still work. The
accuracy is pleanty good for what I do. I did find out that one should
take their maximum rated input of 750 volts seriously. Microwave power
supply killed it.

[TimR]

I've killed the radio shack fuse once too.

Hard to remember details, but I think I'd been measuring resistance and then used it on line voltage without changing.

It's still working fine after 25 years or so. I do reach for the Simpson for some applications but the little RS is usually enough.

[Dave Plowman (News)]

In article ,
TimR wrote:
I've killed the radio shack fuse once too.

Hard to remember details, but I think I'd been measuring resistance and
then used it on line voltage without changing.

Usual one is measuring current then going to voltage without re-plugging
the leads. The shunt used for current measurement then draws excessive
current.

--
*When blondes have more fun, do they know it?

Dave Plowman London SW
To e-mail, change noise into sound.

[Mike Rivers[_2_]]

On 7/21/2014 6:50 PM, Dave Plowman (News) wrote:

Hard to remember details, but I think I'd been measuring resistance and
then used it on line voltage without changing.

Usual one is measuring current then going to voltage without re-plugging
the leads. The shunt used for current measurement then draws excessive
current.

I don't understand why everyone thinks that blowing a fuse is such a big
deal The fuse is there to protect the meter when you don't connect it
properly. The fuse is doing it's job. If you don't want to have to
replace it. DO YOUR JOB!!!


--
For a good time, visit http://mikeriversaudio.wordpress.com

[Dave Plowman (News)]

In article ,
Mike Rivers wrote:
On 7/21/2014 6:50 PM, Dave Plowman (News) wrote:

Hard to remember details, but I think I'd been measuring resistance
and then used it on line voltage without changing.

Usual one is measuring current then going to voltage without
re-plugging the leads. The shunt used for current measurement then
draws excessive current.

I don't understand why everyone thinks that blowing a fuse is such a big
deal

Don't think most do. But with the cost of Fluke ones, best avoided.

The fuse is there to protect the meter when you don't connect it
properly.

Only if you don't connect it properly?

The fuse is doing it's job. If you don't want to have to
replace it. DO YOUR JOB!!!

Hmm. ;-)

--
*Give me ambiguity or give me something else.

Dave Plowman London SW
To e-mail, change noise into sound.

[William Sommerwerck]

The fuse is there to protect the meter when
you don't connect it properly.

Only if you don't connect it properly?

In the case of an autoranging unit, yes.

[Mike Rivers[_2_]]

On 7/22/2014 8:28 AM, William Sommerwerck wrote:
Only if you don't connect it properly?

In the case of an autoranging unit, yes.

Even one with a manual range switch, it sounds like if you have the
probes in the "current" jacks and connect it to a voltage source (or is
it the other way around?) a fuse blows. I don't know, never having blown
a fuse in a meter because I'm careful how I use it.

--
For a good time, visit http://mikeriversaudio.wordpress.com

[William Sommerwerck]

"Mike Rivers" wrote in message ...
On 7/22/2014 8:28 AM, William Sommerwerck wrote:

Only if you don't connect it properly?

In the case of an autoranging unit, yes.

Even one with a manual range switch, it sounds like if you have
the probes in the "current" jacks and connect it to a voltage source
(or is it the other way around?) a fuse blows. I don't know, never
having blown a fuse in a meter because I'm careful how I use it.

On most (likely all) Flukes, turning the knob to read voltage while the probes
are in the current jacks causes the meter to beep. You're being warned that
you're about to place a low resistance across a circuit that usually objects
to being shorted out. Whether the fuse blows depends on whether the circuit
can supply enough current, but at the very least, you're likely to damage the
Device Under Test.

A Fluke person once told me that half the cost of their DVMs comprised
components and circuits that protected the unit against damage (such as when
it was set to ohms and the probes shoved into the wall outlet).

[Dave Plowman (News)]

In article ,
William Sommerwerck wrote:
A Fluke person once told me that half the cost of their DVMs comprised
components and circuits that protected the unit against damage (such as
when it was set to ohms and the probes shoved into the wall outlet).

Possible to buy a reasonable DVM for the price of a Fluke fuse. ;-)

--
*How does Moses make his tea? Hebrews it.*

Dave Plowman London SW
To e-mail, change noise into sound.

[Mike Rivers[_2_]]

On 7/22/2014 12:48 PM, Dave Plowman (News) wrote:
Possible to buy a reasonable DVM for the price of a Fluke fuse.

I'm not sure what fuse you're talking about, but my Fluke 77 has two.
The cheap one is 63 cents. The 600v 3A fast-blow fuse is about 13 bucks
for which, I'll concede, you can buy a pretty cheap multimeter. But it
won't be as good a meter as the Fluke after you've replaced the blown fuse.

--
For a good time, visit http://mikeriversaudio.wordpress.com

[Mike Rivers[_2_]]

On 7/22/2014 12:48 PM, Dave Plowman (News) wrote:
Possible to buy a reasonable DVM for the price of a Fluke fuse.

Hmmm . . here's even a better deal: The 600v 3A fuse for my Fluke 77 for
$2.10. Maybe they have a good price on whatever one you're comparing to
a "reasonable DVM." I wouldn't trust a multimeter that costs two bucks,
though the one I got free from Harbor Freight lives in my garage and is
fine for checking the car's electrical system.

http://www.testpath.com/Categories/Fluke-Fuses-3127.htm

--
For a good time, visit http://mikeriversaudio.wordpress.com

[Dave Plowman (News)]

In article ,
Mike Rivers wrote:
On 7/22/2014 12:48 PM, Dave Plowman (News) wrote:
Possible to buy a reasonable DVM for the price of a Fluke fuse.

I'm not sure what fuse you're talking about, but my Fluke 77 has two.
The cheap one is 63 cents. The 600v 3A fast-blow fuse is about 13 bucks
for which, I'll concede, you can buy a pretty cheap multimeter. But it
won't be as good a meter as the Fluke after you've replaced the blown
fuse.

What you have to remember is in the UK most things from the US tend to
cost the dollar figure in GPB, or more, whereas stuff from China more or
less the same in real money.

So your 13 dollars becomes approx 22 dollars.

Even then I like my Fluke. Other thing that left me gasping for breath was
a new element for my Pace desolder iron - an SX80. 90 GBP.

--
*If you tell the truth, you don't have to remember anything.*

Dave Plowman London SW
To e-mail, change noise into sound.

[TimR]

I'm 61. I've blown one multimeter fuse in my entire life (and it was completely my fault, I had a momentary brain cramp.)

I've done some other things dumber than that though.

[TimR]

On Tuesday, July 22, 2014 3:44:05 PM UTC-4, Jeff Henig wrote:


I work with roughly 1300 men who've done much dumber things than that.



Ever been to prison, Tim? grin


Worked in one. Four most miserable years of my life.