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About the Radio-Shack (Tandy) 22-175b multimeter

G

Gaetan Mailloux

Jan 1, 1970
0
Hello

I have a Radio-Shack (Tandy) 22-175b digital multimeter.

Wen I use it to measure capacitors it do not do it's 0 before measuring,
even without any wires plug to it there is alway a .1 nf reading, there is
only two trimpots on the pc board, how can we reajust to have 0 ?

Any body would have the schematic diagram for that meter ?

Thank

Bye

Gaetan
 
W

William Sommerwerck

Jan 1, 1970
0
Wen I use it to measure capacitors it do not do it's 0 before measuring,
even without any wires plug to it there is alway a .1 nf reading, there is
only two trimpots on the pc board, how can we reajust to have 0 ?

On the lower capacitance scales, it _shouldn't_ be zero. The test leads have
a certain amount of capacitance.

If you've removed the leads, a residual reading might be due to internal
capacitance, or the converter's LSB flopping.

In any case, if you're reading microfarads, 0.1nF (100pF) is hardly of any
concern.
 
G

Gaetan Mailloux

Jan 1, 1970
0
Hello

Even without tests leads there is a .1 nf reading, I want to use it for
low value cap testing (5 to 50 pf), so it's kind of problematic.

Thank

Bye

Gaetan


"William Sommerwerck" ([email protected])
writes:
 
W

William Sommerwerck

Jan 1, 1970
0
Okay. Does it have a capacitance scale _intended_ to read 5 to 50 pF? I
doubt it.

Do you have a basic understanding of the terms "resolution" and "accuracy",
and how they relate to measurements?
 
G

GregS

Jan 1, 1970
0
Hello

Even without tests leads there is a .1 nf reading, I want to use it for
low value cap testing (5 to 50 pf), so it's kind of problematic.

My Fluke 189 is .065 without leads. Of course I can zero
it by pushing a button. The leads add 20-30 pf depending on the leads.
You can't use leads for that low of a value. You need to insert the caps
directly into a stable fixture. Make thing to plug into the meter
and have a clip or something.


greg
 
P

PeterD

Jan 1, 1970
0
Hello

Even without tests leads there is a .1 nf reading, I want to use it for
low value cap testing (5 to 50 pf), so it's kind of problematic.

Thank

Bye

1. Don't top post.

2. You cannot use a Radio Shack meter for any precision measurement.
Get a proper meter and you'll not have this problem.
 
Hello

Even without tests leads there is a .1 nf reading, I want to use it for
low value cap testing (5 to 50 pf), so it's kind of problematic.

Thank

Bye

Gaetan

"William Sommerwerck" ([email protected])
writes:


Hi
For such low capacitances, there are is only one method that I've
found
that works well. Find or make a coil that when used with you caps to
be measure, will resonate in the 2 to 10 MHz range some place.
Use a few reference caps of known value and tolerance. These should
be close to the values being checked. Use a grid dip meter to measure
the frequency and back calculate the values of the caps.
Do remember that smaller values can be made by stacking ( or series
wiring ).
If you wind your own coil, try to leave at lease one or two wire
widths
between each turn. This will minimize the effects of internal
capacitance
of the coil. This is easiest done by winding wire two or three wires
and then removing the extras while a fixing glue is setting.
I do have a bridge that I get reasonable measurements for around 50
pf
but anything in the 5pf range is not reliable.
 
W

William Sommerwerck

Jan 1, 1970
0
2. You cannot use a Radio Shack meter for any precision measurement.
Get a proper meter and you'll not have this problem.

This is not true. It has nothing to do with "precision".

Using a simple digital meter, you can't make accurate measurements of
capacitors whose values are comparable to the lead capacitance. You need
more-specialized test equipment.
 
P

PeterD

Jan 1, 1970
0
This is not true.

Huh? No RS product is going to be noted for accuracy. Low price,
maybe, low quality probably, but not high accuracy.
It has nothing to do with "precision".

Using a simple digital meter, you can't make accurate measurements of
capacitors whose values are comparable to the lead capacitance. You need
more-specialized test equipment.

I'll agree with that... <g> The OP is lookng for the impossible.
 
P

PeterD

Jan 1, 1970
0
Hi
For such low capacitances, there are is only one method that I've
found
that works well. Find or make a coil that when used with you caps to
be measure, will resonate in the 2 to 10 MHz range some place.
Use a few reference caps of known value and tolerance. These should
be close to the values being checked. Use a grid dip meter to measure
the frequency

A chain is only as strong as its weakest link... That grid dip meter
needs to be accurate, and you need to know the inductor's value
accurately as well.
 
A chain is only as strong as its weakest link... That grid dip meter
needs to be accurate, and you need to know the inductor's value
accurately as well.

As long as your reference parts are on the same scale of the grid dip
meter,
you should be able to ignore the coils actual value. One should be
able
to interpolate the grid dip scale. If more accuracy is needed, a
frequency
counter can be used. In which case, your readings would most likely be
as good as the reference parts.
Your not depending on the absolute accuracy of the meter or knowing
the
actual value of the coil, that is why you use reference parts.
Dwight
 
G

Gaetan Mailloux

Jan 1, 1970
0
William Sommerwerck" ([email protected]) said:
Okay. Does it have a capacitance scale _intended_ to read 5 to 50 pF? I
doubt it.

Hello

No it's an auto ranging.
Do you have a basic understanding of the terms "resolution" and "accuracy",
and how they relate to measurements?

Why ?

Thank

Gaetan
 
P

PeterD

Jan 1, 1970
0
As long as your reference parts are on the same scale of the grid dip
meter,
you should be able to ignore the coils actual value. One should be
able
to interpolate the grid dip scale. If more accuracy is needed, a
frequency
counter can be used. In which case, your readings would most likely be
as good as the reference parts.
Your not depending on the absolute accuracy of the meter or knowing
the
actual value of the coil, that is why you use reference parts.
Dwight


With that I am sure we are well beyond the capabilities of the OP!
<bg>
 
D

Dave Plowman (News)

Jan 1, 1970
0
I have a Radio-Shack (Tandy) 22-175b digital multimeter.
Wen I use it to measure capacitors it do not do it's 0 before measuring,
even without any wires plug to it there is alway a .1 nf reading, there
is only two trimpots on the pc board, how can we reajust to have 0 ?
Any body would have the schematic diagram for that meter ?

What is the spec for measuring capacitance?

With a decent ;-) general purpose meter like say a Fluke 110 the maximum
resolution on capacitance is 1 nF, so a 0.1 nF error is neither here nor
there.
 
G

Gaetan Mailloux

Jan 1, 1970
0
Dave Plowman " ([email protected]) said:
What is the spec for measuring capacitance?

With a decent ;-) general purpose meter like say a Fluke 110 the maximum
resolution on capacitance is 1 nF, so a 0.1 nF error is neither here nor
there.

--
*Why is it that most nudists are people you don't want to see naked?*

Dave Plowman [email protected] London SW
To e-mail, change noise into sound.


Hello

They say in the booklet a resulution of 1 pf, well it seem that it's not true.

I will look for another meter for real 1 pf resolution.

Thank everybody.

Bye

Gaetan
 
P

Peter Bennett

Jan 1, 1970
0
Hello

They say in the booklet a resulution of 1 pf, well it seem that it's not true.

I will look for another meter for real 1 pf resolution.

Thank everybody.

Bye

Gaetan

That spec means that on the lowest range, the least significant digit
is pF. Even if the meter reads 0.1 nF with no capacitor connected,
you may still be able to measure capacitors down to 10 pF or so. Just
note the "no capacitor" reading, and subtract that from the reading
with a capacitor connected to get the actual value of the capacitor.


--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca
 
J

JANA

Jan 1, 1970
0
This is normal for your meter. There will always be some residue
capacitance.

Take the reading with the leads opened up. Then when taking a reading
subtract the difference. The problem will be that even the physical position
of the leads will change the readings a little.

For capacitors in the value range that you want to read, you really need a
high end cap meter. Check out the higher end of the Fluke DVM's with the
capacitor option. With these meters, you will have a much higher resolving
and stability. But, when reading caps below 200 pF, you have to consider the
wires. It is best to use very short wires something less than 4 inches with
small clips in them.

For example, with a Fluke 89 series, they have excellent capacitance
performance. There are also dedicated capacitor meters which are the best.

You can also set up a cap tester using a signal generator and a scope. If
you want to do the math, you can work out a an RC network where you feed in
a frequency and amplitude calibrated signal. You then use a dual trace scope
and plot out the phase and amplitude differential. With some calculations
you can then have a very precise reading of the cap.

--

JANA
_____



Hello

Even without tests leads there is a .1 nf reading, I want to use it for
low value cap testing (5 to 50 pf), so it's kind of problematic.

Thank

Bye

Gaetan


"William Sommerwerck" ([email protected])
writes:
 
W

William Sommerwerck

Jan 1, 1970
0
They say in the booklet a resulution of 1 pF, well it seem
that it's not true. I will look for another meter for real 1 pF
resolution. Thank everybody.

What you don't understand is that resolution has nothing to do with
accuracy.
 
M

msg

Jan 1, 1970
0
JANA wrote:

You can also set up a cap tester using a signal generator and a scope. If
you want to do the math, you can work out a an RC network where you feed in
a frequency and amplitude calibrated signal. You then use a dual trace scope
and plot out the phase and amplitude differential. With some calculations
you can then have a very precise reading of the cap.

One can also make a capacitance bridge from easily obtained components; as
a kid I made one in a cookie tin that served me well even for values in the
10s of pf for many years. Now I rely on a Sprague Tel-Ohm-Mike for the
single digit pf measurements (a better capacitance bridge).

Regards,

Michael
 
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