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

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.

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" ( snipped-for-privacy@comcast.net) writes:

Since capacitors add when in parallel, then all you have to do is note the capacitance reading "at rest" and then subtract that from the value you get with the unknown capacitor, and the result is the value of the capacitor.

But, those meters are better at higher value capacitors. Fancier ones will have better resolution at low value capacitors, and will have a means of nulling out the stray capacitance.

Even without the stray capacitance, such general purpose meters have limitations. If you're reading a 10uF capacitor, a little bit off won't mean much, but a 10pF capacitor, a little bit off will mean a big difference. Yet the meters aren't really good at reading low value capacitors.

My DMM has a couple of contacts for capacitor testing. If your's has them using them (and removing the test probes) may help a bit, at least in terms of the stability of the stray capacitance. The regular test leads don't just add capacitance to the reading, they change the reading depending on their placement, which makes reading the capacitance even harder.

Practically from the time I got mine a decade ago, I made up a little jig to make better use of those contacts. Since they are placed somewhat below the panel, they aren't so useful for components with short leads. So I took a piece of copper clad circuit board, put a cut through the foil in the middle, and soldered a couple of leads to the board. Then I plug it into the contacts intended for component testing, and that provides an easier surface for short lead components. And it is easier than using the probes for such components.

Michael

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?

measuring,

is

have

any

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

1. Don't top post.

1. 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.

Huh? No RS product is going to be noted for accuracy. Low price, maybe, low quality probably, but not high accuracy.

I'll agree with that... The OP is lookng for the impossible.

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.

Hello

No it's an auto ranging.

Why ?

Thank

Gaetan

No, that's not the question.

The question is what's the maximum value that can be measured on the lowest capacitance range. On my Radio Shack DMM, it's 4nF, ie .004uF, ie 4000pf. Small values don't show up very well on that, since 5pf can only use up one digit, the least significant, so if it's off it will either show 5pF or 4pF (if it's lower in value) or 6pF (if it's higher in value). That doesn't make for reading low value capacitors accurately, since you cannot know it's exact value.

This is precisely why one is supposed to use a range that puts the reading up towards the maximum. You put the range down to the 4v range if you are reading a 1.5V battery, since you get the most precise reading. Put the range on a higher range, and the voltage will only "use up" the least significant digit, or at best the 2 least significant digits.

Autoranging means nothing in this case. All it does is automatically kick the range up if the value being measured overflows the range, so if you try to read 5v on the 4v range, it will automatically shift it up to the 40v range. This is exactly what you'd do manually when you see the voltage is overflowing the range, change the range switch to the next highest voltage. Autoranging just does it automatically.

Micahel

An auto-ranging meter still has ranges. What is the lowest capacitance range?

"accuracy",

If you understood these things, you probably wouldn't be asking this question.

is

have

any

With that I am sure we are well beyond the capabilities of the OP!

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        dave@davenoise.co.uk           London SW

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  

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
_____

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

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