# Meter impedance

Discussion in 'Electronic Basics' started by James Harris, Oct 9, 2003.

1. ### James HarrisGuest

Beginner question: was trying to check voltages on a microphone
amplifier board last night but had more than a sneaking
suspicion that my meter was more of a load than the circuit.
Transistor base bias currents were set in a resistor ladder that
had over 1.5MOhms between 9v supply and ground. Query is, Would
I have been better to use a digital voltmeter due to its higher
impedance? - They do normally have higher imedances, don't they?
TIA
James

2. ### John FieldsGuest

---
You haven't said anything about the impedance of _your_ meter, but
most DMM's look like 10M ohms shunted by some 10's of picoFarads, so if
you're using a moving coil meter on a low voltage range it's not likely
that it'll be that high.

An easy way to find out what your meter looks like on any given voltage
range is to measure a voltage and then, without changing the voltage
range, to make the meter part of a voltage divider, like this:

+V
|
[POT]
|
[METER]
|
GND

Assuming the voltage you measured was +V, adjust the pot until the
meter reading is 1/2 of that, then disconnect the pot from +V and
measure its resistance. The resistance you measure will be the
impedance (well, the resistance, actually) of the meter on that range.
Easy, huh?-)

3. ### SpajkyGuest

yes, ... yes ...

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& visit site - http://www.spajky.vze.com
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4. ### John FortierGuest

Older analog multimeters generally had input impedances in the tens of
killohm range, which made them pretty useless when measuring high impedance
sources.

Digital meters have far higher input impedances and if the meter in question

I can well remember measuring the anode voltage of an amplifier tube at just
over thirty volts and receiving a significant belt from it when I tried to
pull the anode cap off! In fact, my experiences with transmitters have
generally been less than happy (ref; welding my elbow to a tuning coil!)

John

5. ### SteveGuest

Seems you already know the answer! Yes, DMM would be better. My
analogue meter has a 20kOhm/VDC sensitivity, by DMM has 10Mohm on all
ranges. Quite a difference when you're looking at 1.5Mohms!

10Mohm in parallel with 1.5Mohm and you get around 1.3Mohm. Looking
at a base bias voltage you'd be in a low range, say 2.5V, making for
50kohm meter impedance. Put this in parallel with 1.5Mohm and you get
around 48.4kohm!!!

Be aware that not ALL DMMs have 10Mohm impedance, the cheaper ones
tend to be more like 1Mohm. You can get a fairly decent one for
AUD\$40-\$60.

niftydog

6. ### JeffMGuest

They do normally have higher imedances, don't they?

As a rule, yes.

I really hate it when folks specify "DVM" instead of "high impedence meter"
--or even better, "meter with 10Mohm or higher input impedence".

anyone can build a DVM with a 1Mohm input (or even 1kohm) if he decides to.

7. ### Glenn GundlachGuest

That isn't quite right. I'm SURE somone will correct me if I'm in
error on this. A 20,000 ohm/volt meter will present a 60K load set to
3 volt scale. Set it to 100 volt scale and its 2 meg. So, on a 1000
volt setting its 20 meg. The load of the digital meter is 10-11 meg
all the time so it would load less on low voltage settings but more on
high voltage. My old analog was 50K ohm/volt.

As for your jolt, 30 volts was the DC component. What was the AC? I
trust you only made that mistake once (like I did).
GG

8. ### John FortierGuest

I was using a rugged old AVO, the military version, and the measured
voltages were well known to be inaccurate when measuring high impedance
source voltages. What the actual internal resistance was, I must admit I
have no idea, but I wasn't the only one to fall foul of this particular
error.

After this and the elbow welding incident, I became extremely cautious. As
to what the AC component was, the transmitter in question was capable of 2
KW VHF, so I think the answer there is HIGH!

John

9. ### James HarrisGuest

I appreciate all the help you guys have given. I'm now convinced
to look out for a digital meter - and will be checking the
impedance spec carefully!
Thanks, James

he decides to.

10. ### James HarrisGuest

John,

Not sure if this is very accurate but it's close enough. On
10VDC range, I read meter resistance as 150K, and on 2.5VDC
range resistance as 310K. Am convinced and have bought a DMM
with a 10MOhm impedance on DCV.

Thanks,
James