Fluke 289 DC Frequency measurements?

[Soertier]

The 289 simply does not measure 4KHz or 15KHz on 4V.
My cheap Uni-T UT61E can measure those two frequencies under the same setup.
No matter what I do I cannot get it to pick those frequencies, so unfortunately I get no duty cycle readings either.

Also does anyone know why the Lo Ohms function simply goes down in reading and eventually will even go to negative values?

[grenert]

The 289 simply does not measure 4KHz or 15KHz on 4V.
My cheap Uni-T UT61E can measure those two frequencies under the same setup.
No matter what I do I cannot get it to pick those frequencies, so unfortunately I get no duty cycle readings either.

I just read a 1V AC 4kHz square wave on top of a 4V DC offset with my 289, so I'm not sure what the problem is with your meter.  Are you on the ACV setting?

Also does anyone know why the Lo Ohms function simply goes down in reading and eventually will even go to negative values?

Is this with the leads open or shorted?  Are you sure you're not in the Low Impedance (LoZ) mode?

[Soertier]

grenert,

I thought that since everything works in the circuit on DC, I should be in DCV measuring the Frequency.
I just switched to AC and the Fluke 289 can pick that 15KHz only if I set the trigger on the falling edge.
My source is 4V DC so I tried reading the Frequency while on DC voltage measuring.  The 15KHz frequency is of a LED driver circuit board.
With the Fluke set on AC Frequency I cannot see the DC Voltage.


I am definitely in the Lo?  function not LoZ. Trying to measure small values that should be around 30m?-100m?.  The values definitely do down until negative.

[grenert]

If what you're trying to measure is pulsed DC voltage going to drive some LEDs, then you need to understand that pulsed DC is no longer DC; it is AC.  The peak function on ACV will show you the voltage of your DC pulse.

What exactly are you trying to measure with low ohms?

[Everett_Tom]

I am definitely in the Lo?  function not LoZ. Trying to measure small values that should be around 30m?-100m?.  The values definitely do down until negative.

It sounds like your trying to measure ohms on a circuit that still has power in it somewhere. That's the only time I've ever seen negative ohms. I had the same thing happen due to a capacitor in the circuit adding a little bit of charge our during my measurements. Once I unsoldered one end of the resistor, It stopped and the resistor read correctly.

[Soertier]

If what you're trying to measure is pulsed DC voltage going to drive some LEDs, then you need to understand that pulsed DC is no longer DC; it is AC.  The peak function on ACV will show you the voltage of your DC pulse.

What exactly are you trying to measure with low ohms?

Sorry for the late reply. What I am trying to measure is the resistance of switch for example low ohms, should be about 100mOhms to 30mOhms. Here a video showing how it decreases and shows negative values. http://youtu.be/MqnAuaTx1Jw

I was not trying to measure ohms in a circuit.

That LED has forward voltage of about 3.5VDC at 3Amps and am measuring at the driver input. Using the Peak function on the ACV I can only see 0.039V

[Soertier]

Does anyone know why the readings go to negative values?
http://youtu.be/MqnAuaTx1Jw

[DavidDLC]

If what you're trying to measure is pulsed DC voltage going to drive some LEDs, then you need to understand that pulsed DC is no longer DC; it is AC.  The peak function on ACV will show you the voltage of your DC pulse.

What exactly are you trying to measure with low ohms?

Pulsed DC is still DC, the name says it all.

If the current is flowing in one direction is DC, if the current goes in one direction for a period of time and then in the opposite direction on another period of time, then is A.C.

[Soertier]

I understand that my question may seem poor to some, but why are there Pulse width, Duty cycle and Frequency functions on DCV (on the Fluke 289) if they need to be measured on ACV for a full DC circuit (input and output)?

I really find that Low Ohms function strange going to negative values, it does not matter how the contact is, I used some alligator clips and it's the same.

[grenert]

Soertier, I realize now that using ACV peak measurement would only work for a 50% duty cycle (take the ACV peak reading and then double it).  For a different duty cycle, I'm not sure how I would measure that with a DMM.  Do you have access to an oscilloscope?  That would be what I would use to make this measurement.

For low ohms, the only thing I can suggest is to first short the leads together and then zero out the reading.  Then take your measurement.

While I do not know how to measure the voltage, my 289 definitely will "see" a 4 kHz or 15 kHz pulse.  Do you know your meter is working correctly?  Why not try contacting Fluke?

[Soertier]

Soertier, I realize now that using ACV peak measurement would only work for a 50% duty cycle (take the ACV peak reading and then double it).  For a different duty cycle, I'm not sure how I would measure that with a DMM.  Do you have access to an oscilloscope?  That would be what I would use to make this measurement.

For low ohms, the only thing I can suggest is to first short the leads together and then zero out the reading.  Then take your measurement.

While I do not know how to measure the voltage, my 289 definitely will "see" a 4 kHz or 15 kHz pulse.  Do you know your meter is working correctly?  Why not try contacting Fluke?

Unfortunately I do not have access to an oscilloscope.

If I short the leads the same thing happens.  So let's say you take your leads and shot them then measure something with a low resistance like 30mOhms, your reading will stabilize and not drop until you get negative values?
I have attached an example of measuring a 0.1OHM wire-wound resistor (non-capacitive, 1% tolerance) with normal Ohms function and LoOhms function. Now with the LoOhms function if I use some leads with very low resistance it will be impossible zero them out so I used the standard leads. You can see there's some difference in what the normal Ohms shows as a resistance for the leads vs the LoOhms resistance for the leads. The leads resistance will not stabilize fully on the LoOhms and I have to "choose" a moment when to zero them out.
http://youtu.be/eKS8kx9UdtY

Ambient temperature is 21Celsius, Humidity 34%

[M. András]

i tried my 289 on low ohms via a single patch cord which shorted the inputs it settles at 0.025ohms in 5 secs and stayed there for hours now its a meter long 1mm² silicon wire with 2 hirschmann plugs on the ends, this came with a test hook kit (welded wire connections to the plug btw) so i dont have any idea why its goes below zero in your case maybe worth a call for a fluke service center?

[Everett_Tom]

So, you piqued my curiosity.

As far the the low ohms going negative --
In the Fluke 289 Datasheet, (I found it here :  http://assets.fluke.com/manuals/287_289_umeng0200.pdf ) page 72 in the document ( 82 in the PDF), that for the 50 Ohms , 500 ohms, 5K Ohms and 50 k Ohms you need to use REL to compensate for offsets. I think this is because in these ranges you have to get rid of the leads resistance. I think you see negative ohms because when the unit was first calibrated, the leads had about 15 milli ohms more resistance in them. Page 29 of the document ( 39 of the PDF) says about the same thing -- leads can add 0.1 to 0.2 ohms of error.

As for the measurement moving, it looks like it didn't move too much. The spec for this range is 0.15% + 20 counts, so for 0.100 error is just 20 base count. (0.15% of 0.100 is less then 1 count). So.. Did your measurement move more then 20 Milli ohms?

[Soertier]

i tried my 289 on low ohms via a single patch cord which shorted the inputs it settles at 0.025ohms in 5 secs and stayed there for hours now its a meter long 1mm² silicon wire with 2 hirschmann plugs on the ends, this came with a test hook kit (welded wire connections to the plug btw) so i dont have any idea why its goes below zero in your case maybe worth a call for a fluke service center?

I have some Hirschmann 1m long cables 1mm2 with 4mm plugs on each end, they are PVC labeled but are one of the best PVCs outthere.  I also tried a single cord, and here it's how it behaves.  http://youtu.be/h-sLW9cCPls

After 15minutes the resistance was still as at the video end.  Ambient temperature 21C , Humidity 35%

[Soertier]

So, you piqued my curiosity.

As far the the low ohms going negative --
In the Fluke 289 Datasheet, (I found it here :  http://assets.fluke.com/manuals/287_289_umeng0200.pdf ) page 72 in the document ( 82 in the PDF), that for the 50 Ohms , 500 ohms, 5K Ohms and 50 k Ohms you need to use REL to compensate for offsets. I think this is because in these ranges you have to get rid of the leads resistance. I think you see negative ohms because when the unit was first calibrated, the leads had about 15 milli ohms more resistance in them. Page 29 of the document ( 39 of the PDF) says about the same thing -- leads can add 0.1 to 0.2 ohms of error.

As for the measurement moving, it looks like it didn't move too much. The spec for this range is 0.15% + 20 counts, so for 0.100 error is just 20 base count. (0.15% of 0.100 is less then 1 count). So.. Did your measurement move more then 20 Milli ohms?

My Fluke standard leads leads were measured in Normal Ohms function as 0.09 OHM. But in the LoOhms function they will go down to 0.054Ohms and stabilize there.



I tried measuring the resistance of that switch but now with the standard Fluke leads. Apparently if I wait some time the reading will get stable at 0.013Ohms. I do not know if measuring something this low can be  accurate dues to that 20counts specification by Fluke.



Apparently the negative values how up when the leads resistance is very very low. Example, each lead 35cm, 4mm2 (about AWG 11.2), shorted they give negative values.
http://youtu.be/z4mNz9jah1U

[KMSL]

My 289 (year 2012) in LoOhm (50 Ohm) works similar to yours. Takes a minute to stabilize the measure. Bridging short the input -0,002 Ohm

[Soertier]

My 289 (year 2012) in LoZ works similar to yours. Takes a minute to stabilize the measure. Bridging short the input -0,002 Ohm

You mean in Lo??

[Soertier]

I am back again with Fluke 289 frequency measurements. I know someone must have some knowledge on this.
You can see the current and the supposed frequency the LED driver circuit board is working at.  Fluke seems not to measure anything in two instances (8mA and 3062mA) while UT61E measures totally something else, but still measures some frequencies where the Fluke does not.

Still I do not understand why I have to go on ACV to get any Frequency measurement my FLuke 289 for a DC circuit  since there are Hz on DCV too. I really can't see the DC voltage that way.

[AndyC_772]

I'm sitting here in my lab with a 289, a signal generator and an oscilloscope.

It's not totally clear from your description, but I'm guessing that what you're trying to do is measure the switching frequency of the load connected to your power supply by measuring the frequency of the switching noise that appears superimposed on the 4V dc supply.

With the 289 set in DCV mode and the frequency display enabled, it seems that the 289 requires a very large signal in order to register a frequency measurement. A 4 kHz square wave oscillating between -2V and +2V does the trick, though there's also a dependency on the dc voltage offset, and the duty cycle measurement is still way off. I suspect there's a low pass filter in the meter which is used to give a steady DC reading in the presence of noise, and this has the effect of dramatically reducing the sensitivity of the frequency measurement too. I actually find the idea of measuring frequency when in DC mode a bit odd.

However, what you're trying to measure is the AC component of the signal coming from the power supply, so you want the meter on ACV. In this mode there's a high pass filter instead of a low pass filter, so the AC signal you're looking to measure isn't attenuated.

In ACV mode the meter's response is better. I can measure a 4kHz square wave all the way down to 0.5Vpp @ 4kHz, though this is probably still a much larger signal than the amount of noise you'll have on your power supply.

It's still not much use for measuring pulses with a short duty cycle, though. At 4kHz and 1Vpp it doesn't register anything nearly correct until the duty cycle is 23% or more, and it's similarly useless if the duty cycle is greater than 80%. Larger signals result in better performance, but your signal will be small.

In short, I'm afraid the 289 isn't a good tool for the job when used in this way, and you'd really need an oscilloscope. But there's a better way to use the meter to make the measurement you're after.

Put it in current mode and wire the 289 in series with your load. In current mode the 289 records the correct switching frequency at only a couple of mA, and duty cycle measurements work fine too. Moreover, you're actually measuring the duty cycle of your load rather than just the switching spikes which it imposes on the power supply.

[Soertier]

AndyC_772, thank a lot for you reply.

Basically what I want to measure is the switching frequency that the LED circuit driver. Different brightness levels on 4V input.
I've put the Fluke 289 in series  in the current mode to measure the frequency, now on 975mA is 30Khz, like the UT61E measures, buy before while in ACV parallel with the load it was 15Khz on the Fluke 289.
Still does not read anything on 8mA or 3062mA like when it was in parallel.