Corrosion. 12.5VDC dont mean sh!t

[posplayr]

For the very same reason that you CANNOT use a digital to check HV (microwave) diodes, NOT ENOUGH CURRENT.
In this case, you are checking for Voltage; hence, the D'Arsonval Movement Meter will require/supply a lot more current than a digital when checking voltage/resistance.

And, if you don't like the looks of an analog multimeter, try getting a megger to blow things up.

Now, if you want to compare, try google-ing impedance/sensitivity on both.

The truth is, there is a voltage drop using a digital, but you "ain't" seeing it 'cause it will be in the -1.8 to -0.05V range and most that don't understand this are just happy to see 13.2V on a 14V system.
Which really boils down to knowing your toolz.

Aw, WTH.
For checking voltage using a digital VOM:
Input impedance=10M ohms
Circuit under test=12 Volts
Your E/IR; I=12/10,000,000=1.2uAmps (u as in MICRO, not Milli).

Analog voltmeters usually have a sensitivity of 20 to 30 kilohm per volt (kΩ/V), which varies with the voltmeter range setting.
12/25000=0.48mA

Comparing 0.48mA to 0.0012mA
400 times different.

Now you can see why using an incandescent bulb as mentioned b4 works great.
The truth is, you should be measuring "CONTINUITY" (I assume about 1.2mA on the digital) and not voltage.

Don't buy fancy/expensive tools you don't know how to use. The DIGITAL voltmeter was made to test ELECTRONICS with MINIMAL CURRENT so as to NOT DAMAGE components in circuit with too HIGH a CURRENT.

For automotive work, go buy your cheap "trouble-light", it's really all you need for those types of electrical diagnosis.

And if you were wondering, YES!
I have 4 digital multimeters, but know how to use them.
I also have 2 O'Scopes, 2 logic probes, 1 logic pulser, 1 variac, 1 analog multimeter, 1 megger, and $50,000.00 worth of other test sets/equipment. But that's getting into MY world, lol!

Well, look like you have a nice tool kit ; however your deduction that it is a test equipment issue doesn't appear to match the observations.

Given the confirmed fault was a corroded connection, implies that the connection must had sufficent resistance to drop most of the voltage across the connection. To keep things simple a 12W bulb at 12V is 1 amp which means it is 1 ohm of resistance in the bulb. To keep the light from coming on any resistance greater than 10 ohms in the connector would result in the observed fault measurements and the light not coming on.

If you do your circuit analysis, neither a 10M ohm input imedance of the DVM or the 20Kohm/V Analog meter will have any impact to those observations.

I provided the explaination in the other thread. It is really that simple and we dont need a $50K in test equipment to deduce that.

[bonanzadave]

Empirical or theoretical conclusions - Ask Mr. teddux (could you fix my TV ?)

Edit: Ask Jim

"It is really that simple and we dont need a $50K in test equipment to deduce that."

[mike_of_bbg]

What you should have done is check the voltage drop across the bulb in a closed circuit (i.e., when the bulb should have been lit). Let's say this is a 10W bulb, and you know your electrical system is pushing 12.5V. Ohm's law + definition of wattage says:

watts = V^2/R

So if you do the math with the W and V given, you get an R of 15.6 ohms for the bulb. If you get a voltage drop of only 6.2V across the bulb, you know there's a resistance of about 16 ohms elsewhere (just making the math easy - the bulb is putting up about 1/2 of the resistance, and the other 1/2 is elsewhere).

ANY VOLTmeter - digital or analog - should have very high input impedance so as not to distort the readings by its own parasitic effects. In the above scenario, ~16 ohms of resistance in the wiring sucked away 1/2 of your voltage. In a loop closed by a voltmeter with a ridiculously low impedance of say 1000 ohms, the voltmeter would still make up more than 95% of the voltage drop and read very close to 12V. Up that to the megaohms any respectable voltmeter should be rated, and that 15 ohms of resistance in the wiring is negligible.

Voltmeters are meant to be used in parallel with active components - ammeters are meant to be used in series

[mike_of_bbg]

To keep things simple a 12W bulb at 12V is 1 amp which means it is 1 ohm of resistance in the bulb.

It would actually be 12 ohms of resistance. V=IR, 12!=1*1.

If you do your circuit analysis, neither a 10M ohm input imedance of the DVM or the 20Kohm/V Analog meter will have any impact to those observations.

Quite right.

[Steve]

Time to get the umbrellas and galoshes out.

This appears to be turning into a pi$$ing match.

.

[posplayr]

[QUOTE=mike_of_bbg;1174465]It would actually be 12 ohms of resistance. V=IR, 12!=1*1.

QUOTE]

Whoops Yea I got in a hurry ; as you mentioned, still no way the meter impedance is going to affect the measurements.

Funny how different people with all the same "tools" can come up with the exact opposite conclusions.

[bonanzadave]

Since I'm being bashed for my field of work/business/equipment:

Not by me. I have nothing but respect for your knowledge. Plus I still need help fixin my TV.

[mottyl]

"Plus I still need help fixin my TV"

Bonanzadave,

No promises but I was fairly current with televisions unitll about 3 years ago.

If you want to pm with the make, model and fault I will let you know if I have any thoughts.

Now! Back to the discussion.

Chris

[850GT_Rider]

Test equipment

Electric and electronic (yes, Virginia, there IS a difference) test equipment and tools are no different than all our mechanical tools. 40 years in electronics has taught me that none are suitable for every purpose. I've been playing with this stuff since TV's had vacuum tubes, fer chrissakes.

As stated earlier, for the test in question (the lights don't light), a test light is the best tool, or any VOM used to check resistance, not voltage. To check charging systems, a good analog VOM is the best choice. Neither will get fooled by trickle voltages that won't even light a dome light.

Didn't mean to start a p*ssing contest over the whole thing......

[bonanzadave]

Not by me. I have nothing but respect for your knowledge. Plus I still need help fixin my TV.

Where did Mr. teddux go ? And all his posts ?

[teddux]

Whoops Yea I got in a hurry ; as you mentioned, still no way the meter impedance is going to affect the measurements.

Funny how different people with all the same "tools" can come up with the exact opposite conclusions.

Here is a similar problem on one of my bikes,
Draw your own conclusions:

Battery Voltage (Red & Black)


Analog meter testing battery voltage


Digital Meter testing battery voltage

Key-Switch Voltage Lead (Yellow)


Analog Testing Switch Voltage


Digital Testing Switch Voltage

[teddux]

Where did Mr. teddux go ? And all his posts ?

This starts below this post.

Other foolish/"debatable" crap:
"Input Impedance" of Analog

Modified "posplayr impedance"


Value of Added resistor


Adding Resistor to Analog


Modified to "posplayr impedance"

Analog Testing Modified "posplayr impedance"
Key-Switch Voltage

Modified to "posplayr impedance"
Direct battery voltage.

[850GT_Rider]

ok...is all that proving you agree with the premise or not?

[teddux]

Watch out when checking for voltages with a digital voltmeter. The input impedance is so high that a mere trickle voltage is enough for them to register.
You'll have better luck using one of the 12vdc trouble lights. I've added a really long clip lead to the negative side of mine. Clip it to the -side of the battery to check for 12 volts. Clip it to the + side and you can check for ground continuity. And I still use an analog VOM a lot. Much more dependable than a digital unit sometimes.

Yes, I agree with you 1,000,000%
I also state that an analog is satisfactory for the job.
The point is that the amount of current required to DEFLECT the analog needle is proportionally equivalent to the current required to "light" a bulb.

To move the needle further "full-scale" will require MORE current.

[teddux]

Where did Mr. teddux go ? And all his posts ?

Forgot to "take meds", sry.
This is now libel...