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Siglent SDM3045X mA current measurement - weird behaviour

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HKJ:

--- Quote from: nctnico on November 12, 2018, 09:44:25 am ---
--- Quote from: HKJ on November 12, 2018, 09:05:55 am ---You mean something like this (Multimeters often use a bridge rectifier to get 2 or 3 diodes in series):


--- End quote ---
This looks more like a circuit to protect the input circuitry against an open current shunt and not to protect the shunt itself. But if the shunt resistor is already operating very near it's design limits then it won't work ofcourse.

--- End quote ---

Just about any multimeter uses a diode bridge across the uA and mA shunt, usual there is only fuse protection for the 10A shunt.

This meter also has another trick to protect the uA and mA ranges: It will auto range to higher range when it is overloaded, also when auto range is disabled.

nctnico:

--- Quote from: HKJ on November 12, 2018, 09:48:31 am ---Just about any multimeter uses a diode bridge across the uA and mA shunt, usual there is only fuse protection for the 10A shunt.

This meter also has another trick to protect the uA and mA ranges: It will auto range to higher range when it is overloaded, also when auto range is disabled.

--- End quote ---
Either way the current sensing circuit shouldn't be damaged by an overcurrent. Or put differently: the current sensing circuit should survive until the fuse blows (which in the case of the Siglent 3045X DMM means the resistor is underrated / insufficiently protected).

rf-loop:

--- Quote from: ahope on November 09, 2018, 11:30:34 pm ---Another thingvthat bothers me: why did the internal fuse blow and not the external? and why didi it not protect the internals?

--- End quote ---

Can you explain bit more this situation when this overload happen when Fuse blow. Also was it first time when any fuse blow in its history or have you previously changed back panel fuse. I mean, is is so that both fuses was original fuses after factory.

Then can you also explain exactly what happen in this case when this overload happen. What kind of current source. Its voltage and what kind of very short time peak current it can produce or some info for somehow estimate it.

Back panel Fuse is 10A and internal fuse is 12A.  (I do not know what kind of speed class they are and need be, but my opinion is that least back panel Fuse need be fast(F) or very fast (FF))

Because it is weird that internal blow first, really weird. It must not happen. Si it is good to know bit more about it.



Also I want bit criticize towards Siglent.

10A fuse in place what can not last continuous 10A load. Newer seen fuses characteristics (I2t, arcing time, clearing time)?
For enough protection it need be very special 10A fuse if it need open before damage in all circumstances including also small long time over current or fast very high short circuit peak. Also even if it is aka "ultra fast"  fuse (or what ever factory have installed (right or accidentally wrong)  and designed)  normal user without enough education and shared information may change wrong type fuse. Ampere and dimensions is not enough information.

For example, if a current of 20A was flowing through a fuse rated at 10A, a very fast acting fuse might open in 0.1 seconds, a fast acting fuse may open in one second, whilst a slow acting fuse may take tens of seconds to open.

FF   Very Fast Acting Fuse (Flink Flink)
F   Fast Acting Fuse (Flink)
M   Medium Acting Fuse (Mitteltrage)
T   Slow Acting Fuse  (Trage)
TT   Very Slow Acting Fuse  (Trage Trage)

BillB:

--- Quote from: tautech on November 12, 2018, 09:00:34 am ---RTFM !

--- End quote ---

To be fair, I looked over both the Quick Start Guides and the User's Manuals for the SDM3045X and SDM3055 and nowhere is the 7A/30sec recommendation to be found.  Only in the Datasheets are there tiny little footnotes describing this.  Something this important should probably be mentioned in those documents at least, and since no one really reads manuals any more, it might have been nice to have that on the front or back panel.

Honestly, knowing that there was a 10A fuse protecting the 10A range, I assumed 10A measurement was probably not a good idea.  But I certainly didn't remember the 7A/30sec recommendation until I went back through and found the footnote.

ahope:
A little background on the blown fuse.

The instrument was almost unused, bought new a couple of montgs before.
I was probing inside a chassis and accidentally touched the 220v and poof! it blew. quite a bang actually, those fuses dont go quietly,

The internal fuse is a 12A slow ceramic, the rear fuse a fast 10A. makes absolutley no sense that the internal should blow before the rear fuse.

This is the one and only fuse I have had to replace on this instrument.

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