AC measures 27volts

[Doug Miller]

Perhaps we're actually arguing over the word "load". If by "load" you mean "current draw", then I have no disagreement with you.

[Mark Lloyd]

Would you be saying you don't know what "impedance" means?

A high impedance source (such as that 27VAC probably is), is essentially a voltage source in series with a resistor. The effect of this is that any attempt to draw current from this source will lower the voltage, potentially to near zero. Phone lines are like this.

A high impedance load (such as a digital meter or VTVOM/FETVOM) will draw very little current from a source (not lowering the voltage very much). Note that such a meter is necessary for some sensitive electronic circuits (that would be disturbed by a low impedance load).

An analog meter on a high impedance source will give inconsistent readings on different ranges, since it has a different impedance on each range, loading the circuit differently. Adding a (120V 60W) light bulb in parallel will make the impedance MUCH lower.

[Doug Miller]

Not at all -- and if by "a high impedance ... loads the circuit less" you mean that a high impedance draws less current than a low impedance, I have no disagreement with you.

[tnom]

Precisely backwards...a _high_ impedance meter can't load a circuit and read "phantom" voltages.

[Mark Lloyd]

That's what I meant.

[Mark Lloyd]

Ambiguous language.

Can't load a circuit, and can't read "phantom" voltages.

OR

Can't load a circuit, and CAN read "phantom" voltages.

[Stormin Mormon]

My shop teacher used to have a sign "Danger, 100,000 ohms".

[The Other Funk]

Finding the keyboard operational Stormin Mormon entered:

We used to keep the cleaning crew away from the benches with a sign that said "Danger high impedance". Bob

--

-- Coffee worth staying up for - NY Times

[David Combs]

I've had only HORRIBLE experience with digital voltmeters (plural).

Please do say more about the problems with them.

And if some are OK, or even EXCELLENT -- what is it that makes them that way -- as compared with the el-cheapo ones that seem reliable only on DC (batteries, etc). AC -- forget it.

(At least that's my experience.)

Thanks,

David

[David Combs]

... ...

---------------- question about *this* post:

#1:

#2:

#3:

About #1, please explain what effect the impedance of the "source" has on this problem. (I'm sure it has one, just not sure what it is.)

About #2: You're trying to measure volts (as opposed to current); thus you're putting the two leads "across" the load.

eg, you have an extension-cord onto which is plugged three floor-lamps, a stereo, etc. So mentally you'd like to rub off the outside insulation on the two wires in the extension cord and touch your two leads there.

Obviously you want to NOT disturb the situation (by merely measuring it), you want your meter to have HIGH impedance -- so that only a TINY bit of current runs through it.

Whereas one with low impedance (resistance here) would get more (much more?) current running through it, "disturbing" the situation.

So that sort of "proves" (well, restates) #2.

About #3: Here you mix in the source-concept with the meter.

Could you perhaps explain this part a bit more?

Thanks!

David

(Way, WAY back when, I recall the name "Thevenin's Theorem", something to do with the source, maybe, but I surely have no concept now (not sure I ever did back then!).)

[marika]

I was in Lowe's recently and heard this over the loud speaker:

"mr smith to aisle 6, wirecutting emergency"

mk5000

"The question is this: What is the best (or at least a good) way to search all the machines on a LAN for running instances of this COM object? (I've seen SQL Server's Enterprise Manager do something like this when adding a server registration and hitting the "..." button to select the server.)"--John Fisher

[Mark Lloyd]

I forgot what they CALL that, but any real voltage source can be considered as an ideal voltage source (one that never changes voltage under any load) in series with some impedance. That impedance determines how much the voltage will drop when a load is applied.

BTW, one high impedance source people have is a phone line. 48VDC or so, dropping to 6-10V when a phone is in use.

Yes, that's how you measure voltage.

Right.

An effect almost unnoticable when the voltages source has low impedance.

I found the idea of source impedance difficult to understand once. Think of a 12V battery with an internal series resistor, where you have NO access to the point between the battery and resistor. You just measure the voltage at the terminals.

1. Measure the voltage with no load.

1. Measure the voltage with a load. This voltage will be lower than the no-load voltage. The amount of this drop depends on the resistance of that hidden resistor.

The load has resistance, which is in series with than internal resistance. Voltage is divided between the series resistors, according to resistance.

Real batteries aren't normally made with resistors, but they do have some internal resistance and will show a voltage drop with load.

Impedance is like resistance, but with AC. It considers that some components react differently to AC.

Where you read 120V with a DIGITAL meter, but 27V with an ANALOG meter, you've found a high impedance source. When a wire that's not connected is physically close to one with current in it, these wires act as a transformer (not a very good one).