Electronics > Beginners

Electrician needs Rescue _ Tsunami of square waves ahead !!

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Kiriakos-GR:
 :D

I will get to the point ..  
I own one UNI-T DMM ( UT30D  year 2001)
On it , haves one fixed square wave output at 50 Hz with 100K Ohm resistance output.

My own (Japanese frequency counter 0-150MHz 4 digits)  Leader , measured the frequency at 00.52 = 52 Hz.
And also the Fluke 28II , confirmed the frequency at 51.87 Hz ( better resolution) .
I also measured the duty cycle of this signal , and was at 50% .

The UNI-T User's manual  says additionally :
1) The output voltage range will be over 3V when it is loaded 1M Ohm.
2) Use square wave output to repair audio equipment


So far so good .

And Now the problem description.
If i try to measure this square wave as voltage,
on the DC 600mV ( special range on the Fluke 28II)
I get as output  1.20 mV  

At min/max/ Peak i get :
Max = +29.3 mV
Min  = - 26.2 mV

And my questions :

1) How can I get the 3V output from it as reading ?

2) Only oscilloscopes can do that ? ( Read the wave form voltage )

3) How much effects the measurement, the impedance of the DMM it self. ?

And also, I had accidentally discovered, that the Low-Pass filter on the Fluke causes an extra burden ( load) ,
when it becomes active , that makes the DMM unstable at taking readings of Frequency and Duty cycle,
at those small in voltage signals.
I got  accurate readings in both Frequency and Duty cycle , but the LCD reading playing  like ON-OFF ON-OFF.
Example per second in Hz:   50...17...50....17  , and the same happens with the duty cycle.

With out the Low Pass filter , everything looks normal , and stable. 
I get 7.4 mV AC
With the Low Pass drops to 5.2 mV AC

Looks that I found the hot spot about the sensitivity of it ..

 
 
      



 

 

bilko:
Do you get the same readings on AC Voltage selection ?
Are you using a true RMS meter ?

metalphreak:
The 100k internal resistance is in series with whatever you connect at the output terminals. If you connect relatively low (to the 100k res) resistance, most of the square wave output voltage will drop over the 100k resistor, so you'll get a low voltage reading. If you connect a 1M ohm load then a majority of the voltage will drop over it instead of the 100k (basic voltage divider rules apply).

Kiriakos-GR:

--- Quote from: yachtronics on May 15, 2011, 02:59:22 pm ---Do you get the same readings on AC Voltage selection ?
Are you using a true RMS meter ?



--- End quote ---

Nice question ...

a) By setting the DMM at the specific mV DC range ( max 600 mV DC)  
I get an output of 0.3 mV ( yes it is dropped from before )
At min/max/ Peak mode :
Max = +29.3 mV
Min  = - 26.2 mV

b) On the standard  DC range  0-6 V range .
I do not get any voltage ..
But at min/max/ Peak mode :
Max =  + 0.091 V
Min  =  - 0.080 V

c) At the only one, AC V input ( with lowest range the 600mV AC ) .
I get about 8mV  AC
At min/max/ Peak mode :  
Max = + 0.085 V  ( +85 mV)
Min  = - 0.086 V   ( - 87 mV )


Well even if it struggles to make the measurement , it looks like a True RMS ..  ;)

vk6zgo:
I'm a bit confused by the values you quote.

Does this device put out a 3v p-p squarewave if you check it with an oscilloscope?

If you use a 1x probe,you should see 3v if that is the spec,10x will probably not be much different.

Does the most negative (or most positive) part of your square wave appear at zero volts on the scope? If so,you have a real square wave.

A DMM should read something less than 3v dc when placed across this output----If there was any justice in the world,it would be 1.5v dc.

It should certainly not be around the figures you quoted,unless the signal is really a clipped ac waveform.

If it is the latter,you would get a better reading on the ac range.

VK6ZGO
.

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