Input impedance of Dick Smith Q-1420 multimeter (cheap)

[Matthew Kirkcaldie]

Silly question, probably. I have a cheap DSE multimeter, model Q-1420.
Documentation's long gone, but I wanted to know its input impedance when
measuring DCV. I'm setting the idle current on an amplifier and the
service manual specifies a VTVM (tube voltmeter) to make the measurement
- presumably because of high input impedance. I assume if the DMM has
10 meg input impedance it'll be OK - but I need to know!

Cheers, MK.

 

[David L. Jones]

Matthew Kirkcaldie wrote:

Silly question, probably. I have a cheap DSE multimeter, model Q-1420.
Documentation's long gone, but I wanted to know its input impedance when
measuring DCV. I'm setting the idle current on an amplifier and the
service manual specifies a VTVM (tube voltmeter) to make the measurement
- presumably because of high input impedance. I assume if the DMM has
10 meg input impedance it'll be OK - but I need to know!

Cheers, MK.

If you had a second meter you'd be able to measure it!
The DMM most likely has in the order of 10M so it should do just fine
for your application.
Yes, the VTVM was specified because of its higher input impedance than
a normal analog meter (10-50Kohm/volt typical).
Incidentally, some DMMs have a selectable "infinite input impedance"
mode (in the order of 10's Gohms) for specilised high impedance circuit
measurments. I think I saw a cheapie meter once that had no input
impedance on the mV range and the usual 10Mohm on all others.

Dave

 

[Andy Wood]

"David L. Jones" <altzone@gmail.com> wrote:

Matthew Kirkcaldie wrote:
Silly question, probably. I have a cheap DSE multimeter, model Q-1420.
Documentation's long gone, but I wanted to know its input impedance when
measuring DCV. I'm setting the idle current on an amplifier and the
service manual specifies a VTVM (tube voltmeter) to make the measurement
- presumably because of high input impedance. I assume if the DMM has
10 meg input impedance it'll be OK - but I need to know!

Cheers, MK.

If you had a second meter you'd be able to measure it!

How about using the meter in question to measure it:

Say you have a DC power supply set to 10V. You measure this with the
meter and (hopefully!) it shows 10.0V. Then you measure it again with
a high value resistor, of say 10M in series and, for example if the
reading is now 5.0V the impedance of the meter is 10M. You might have
to try a few different resistors if you start out with no clue as to
the answer.


Andy Wood
woodag@trap.ozemail.com.au

 

[Phil Allison]

"Andy Wood" ..

"David L. Jones"

If you had a second meter you'd be able to measure it!

How about using the meter in question to measure it:

Say you have a DC power supply set to 10V. You measure this with the
meter and (hopefully!) it shows 10.0V. Then you measure it again with
a high value resistor, of say 10M in series and, for example if the
reading is now 5.0V the impedance of the meter is 10M. You might have
to try a few different resistors if you start out with no clue as to
the answer.

** To see if a DMM is 10M or 1M impedance on the DC volts range - you only
need is *any* battery and a 1M resistor.

The answer is irrelevant to the OP in any case.



......... Phil

 

[Phil Allison]

"Jasen Betts" jasen@free.net.nz>

connect it in series with a known resistance....

** Fuck off - copy cat idiot.




....... Phil

 

[Phil Allison]

"Matthew Kirkcaldie"

Silly question, probably.

** Thats right - Doc.

I have a cheap DSE multimeter, model Q-1420.
Documentation's long gone, but I wanted to know its input impedance when
measuring DCV.

** Nearly always, DMMs have 10 or 11 Megohms impedance on DC voltage ranges.

Very cheap ones might have 1 Megohms.

I'm setting the idle current on an amplifier and the
service manual specifies a VTVM (tube voltmeter)

** Err - must be a vacuum tube amp, right Doc ?

to make the measurement
- presumably because of high input impedance. I assume if the DMM has
10 meg input impedance it'll be OK - but I need to know!

** Any DMM will do the job just fine.

1M or 10 M.


BTW:

Old VTVMs are just vaguely interesting relics nowadays.

DMMs are guilty of planet wide, VTVM genocide.





........ Phil

 

[Mr.T]

"Phil Allison" <philallison@tpg.com.au> wrote in message
news:431j6hF1l0s6gU1@individual.net...

Old VTVMs are just vaguely interesting relics nowadays.

DMMs are guilty of planet wide, VTVM genocide.

Yes, I doubt anyone could build a VTVM and sell it for $10

I still have a FET-VOM floating around though, along with an AVO 8. They are
still useful for peaking and nulling. Much better than a digital read out or
LCD bar graph for that purpose IMO.

MrT.

 

[Jasen Betts]

On 2006-01-16, Matthew Kirkcaldie <m.kirkcaldie@removethis.unsw.edu.au> wrote:

Silly question, probably. I have a cheap DSE multimeter, model Q-1420.
Documentation's long gone, but I wanted to know its input impedance when
measuring DCV. I'm setting the idle current on an amplifier and the
service manual specifies a VTVM (tube voltmeter) to make the measurement
- presumably because of high input impedance. I assume if the DMM has
10 meg input impedance it'll be OK - but I need to know!

connect it in series with a known resistance....

get a 9V battery and a 10M resistor,
measure the battery - call this number A, then disconnect one probe from the
battery and connect the resistor across the gap, call the reading you get B

B
impedance is 10M x ---
A-B


Bye.
Jasen