Author Topic: Uni-T UT139C Hz measurement - defective?  (Read 72557 times)

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Offline mos6502

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Uni-T UT139C Hz measurement - defective?
« on: August 20, 2015, 03:21:47 pm »
Hey guys,

so I just bought a Uni-T UT139C. Seems actually like a nice multimeter. However, there seems to be one little snag: the frequency measurement does some weird things.  When I set the dial to 'Hz/%' and hook up a 1MHz signal from a can osciallator (i.e. about 5Vpp), the display just shows "000.0". But! As soon as I press the Hz/% button to switch to duty cycle measurement mode, the display shows the correct duty cycle (around 50%), but only for a second or two, and then goes to "00.0"! When I press the Hz/% button again to switch to frequency mode, the display shows "999.9 kHz", but again only for one or two seconds, then it goes back to "000.0"! So I can only measure frequency/duty cycle by pressing the Hz/% button all the time ...

Now for the really weird thing: when I run the oscillator signal through a 100n capacitor, and then to the meter, the frequency is displayed correctly, all the time!

What gives? Is my meter broken? Or does it really expect an AC coupled signal? I've attached the relevant page from the manual, I'm not quite sure what to make of the info.

I opened the meter up and at least optically, there is nothing obviously wrong. In fact, the quality of the soldering is excellent.
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Offline singapol

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Re: Uni-T UT139C Hz measurement - defective?
« Reply #1 on: August 20, 2015, 05:11:54 pm »
Quote
Posted by : mos6502

Now for the really weird thing: when I run the oscillator signal through a 100n capacitor, and then to the meter, the frequency is displayed correctly, all the time!

What gives? Is my meter broken? Or does it really expect an AC coupled signal? I've attached the relevant page from the manual, I'm not quite sure what to make of the info.

Yes it expects an ac signal. The specification you posted states clearly:

Input range a: ( DC level is zero). That means no DC + AC. Only AC. It's obvious your oscillator has DC super imposed with the clock signal.


 

Offline TheAmmoniacal

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Re: Uni-T UT139C Hz measurement - defective?
« Reply #2 on: August 20, 2015, 05:26:20 pm »
I don't know about this meter, but some can't autorange on frequency mode. So also try to toggle through the ranges?
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Offline bobcat

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Re: Uni-T UT139C Hz measurement - defective?
« Reply #3 on: August 20, 2015, 07:05:37 pm »
Are you measuring on AC mode?
 

Offline mos6502

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Re: Uni-T UT139C Hz measurement - defective?
« Reply #4 on: August 20, 2015, 07:11:17 pm »
Are you measuring on AC mode?

No, the dial is set to "Hz".

I don't know about this meter, but some can't autorange on frequency mode. So also try to toggle through the ranges?

The range button is inoperative when the dial is set to Hz.

Yes it expects an ac signal. The specification you posted states clearly:

Input range a: ( DC level is zero). That means no DC + AC. Only AC. It's obvious your oscillator has DC super imposed with the clock signal.

I think you  may be correct. Looks like it needs a zero crossing signal to measure Hz. What a nasty surprise. What is the point of this? Why not make the Hz mode AC coupled by itself? Do other multimeters work like this as well? I checked the UT61E manual and it has the same "DC level is zero" remark.

This makes the Hz measurement far less useful. Any sensor that outputs a frequency or duty cycle signal (e.g. a MAF sensor in a car) is DC referenced. So if you want to do frequency measurements with this meter, you have to carry a coupling cap with it and fiddle around with the cap while measuring.

I'm still somewhat in disbelief. Can more people weigh in on this? How is it possible this never came up in any of the DMM reviews?
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Offline Berni

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Re: Uni-T UT139C Hz measurement - defective?
« Reply #5 on: August 20, 2015, 08:24:08 pm »
Yup i got a little voltcraft meter that does the same thing. Kinda annoying.
 

Offline Lightages

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Re: Uni-T UT139C Hz measurement - defective?
« Reply #6 on: August 20, 2015, 08:49:59 pm »
Yes it is a bit annoying, but just stick a capacitor in series and you are set.
 

Offline mos6502

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Re: Uni-T UT139C Hz measurement - defective?
« Reply #7 on: August 20, 2015, 09:13:18 pm »
Yes it is a bit annoying, but just stick a capacitor in series and you are set.

I know the solution, I was just incredulous that it's the intended behavior.

It's really a great meter otherwise. Sturdy and heavy case with removable rubber holster with probe holders. Runs off AA batteries. Excellent display with beautiful backlight (as long as you look at it from the correct angle). High update rate.  Diode check outputs a whopping 3.25V. Excellent, super fast, latched buzzer. Well thought out mode switch. Oh well, I was really hoping mine was defective so I could get a replacement with "working" Hz measurement.
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Offline singapol

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Re: Uni-T UT139C Hz measurement - defective?
« Reply #8 on: August 21, 2015, 03:11:11 am »
It's a good meter for the price.I have one. Frequency measures up to 10Mhz.
provided the input signal is less than 20Vac. For duty ratio signal must be less than 100Khz.
This is in the specification data page.19 but on page 12 at the bottom it has a "reminder"that say
"To prevent personal injury, it is not allowed to input the voltage higher than DC 60V or AC 30V.

If in AC volt range frequency is less than 1Khz.

Other than that I don't think a low end Fluke can measure capacitance to 100K uf. or have non-contact AC voltage sensing.  ;)
« Last Edit: August 21, 2015, 03:17:54 am by singapol »
 

Offline Martini

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Re: Uni-T UT139C Hz measurement - defective?
« Reply #9 on: April 30, 2016, 02:32:24 pm »
"To prevent personal injury, it is not allowed to input the voltage higher than DC 60V or AC 30V.

If in AC volt range frequency is less than 1Khz.
Not sure I get it.
You can check high frequencies when they're under 30 VAC (ou 60 VDC, but why would they say that if it cannot measure DC frequency?) or high voltage if it's low frequency?
 


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