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Siglent SDM3045X mA current measurement - weird behaviour

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rf-loop:

--- Quote from: Qw3rtzuiop on November 13, 2018, 12:05:49 pm ---At my SDM3045X the internal Fuse is a ceramic LF T12AH250VP. The rear one is marked LF T10L250V.

--- End quote ---

I think that reason why there was a T speed class 10A fuse, it need investigate and I believe it is now under investigation.
Personally, I also do not understand why a 10A fuse was used there. I think the fuse selection is doubtful. Including speed and its current rate.

nctnico:
If Siglent is smart they fit a 7A fuse and fix current sensing circuitry so it cannot be damaged before the fuse blows.

ahope:
Hi all,

issue is now resolved.  Siglent provided new boards and everything now checks out fine and within specs again.  Verified against accurate voltage sources and the original test circuit where i discovered the problem in the first place.

One last thing: I ordered brand new 10A FAST fuses for the rear fuse panel! 
thanks to siglent for great service.

So to all you out there - I am still a big fan of Siglent. based on this experience i will continue to buy more instruments from them.  I am very happy with my 3045X and my 1202xe.


/andreas

Kleinstein:

--- Quote from: nctnico on November 14, 2018, 07:15:04 am ---If Siglent is smart they fit a 7A fuse and fix current sensing circuitry so it cannot be damaged before the fuse blows.

--- End quote ---
7 A fuses are not a common size, but there are 6.3 A and 8 A available.

So maybe downgrade the specs to 6 A and use a 6.3 A fuse in the back.

al777:

--- Quote from: tautech on November 13, 2018, 07:35:58 am ---
--- Quote from: rf-loop on November 13, 2018, 05:58:29 am ---
--- Quote from: tautech on November 12, 2018, 10:29:00 pm ---SDM3065X
Rear panel fuse: 5x20 glass LF (Littlefuse) marking: 10AL250VP  As per datasheet spec this should be FB fuse.
Internal PCB fuse: Later when more time available............ but already we know it's ceramic.

--- End quote ---

"10AL250VP"

Is it possible there is installed wrong fuses?
(Need ask because China and fuses - I do not wonder anything. I have seen there with my own eyes so much...)

Where is speed identifier? It need be before current.

Here is Littelfuse 5x20 cap marking

Littelfuse product catalog and design guide, small fuse "bible"

--- End quote ---
Rear panel fuse # corrected, it does indeed have T but maybe F or FF would be best.

--- End quote ---

Sorry folks... I know it's a 5-year old topic... I'm making a zombie I guess... :-)... But just so that the new info gets indexed by Google and someone like myself will save time on this topic in the future, here is what I've found.

First of all, on the new SDM3065X (non-SC, factory calibration is March 2023) the rear panel fuse is a glass-body one, marked as "T10AL250VP", with LittelFuse ("LF") logo, it's resistance (if anyone cares) is 4.575 milliohms, as measured with the RC3563 milliohm meter. There are 5 agency approval logos on the other cap.  My best guess is: it's the LittelFuse Series 218 5x20mm glass body time-lag fuse.

Second, I wanted to buy spares, just in case as incidents do happen - and I thought Fast Blow series would potentially be better (that would be LittelFuse Series 217). What I've found by reading datasheets is some new knowledge I wanted to share here.

The Time-Lag Series 218 outpaces the Fast Blow Series 217 for overload currents of 210% and only gets behind above that overload level.

218 series (slow) @ 210%: 120 seconds maximum; while 217 series (fast) @ 210%: 30 minutes maximum.

So, for DMM overload protection use, especially if there is an internal Time-Lag fuse for 12A - use of the external Time-Lag series fuse is well justified. Otherwise the "fast blowing" external fuse may allow the internal fuse to open first, at currents between 10A and 20A. Certainly, if you try to measure voltage of a low internal resistance source with DCA input :-) - then yeah, fast blow will be faster, but at 100's of amperes the damage may still happen... to both fuses and the DMM itself... Besides, slow characteristics may help in real life work, when your load (DUT) is capacitive for example... may save you few bucks on fuse replacements due to inrush current spikes...

So, in my opinion - the right replacement external fuse for SDM3065X is LittelFuse Series 218, glass body Time-Lag T10AL250V, here is the datasheet link:
https://www.littelfuse.com/~/media/electronics/datasheets/fuses/littelfuse_fuse_218_datasheet.pdf.pdf  (double .pdf at the end is not a typo).
They now go to my next Digikey order's list, $2.63CAD qty 1.

BTW, I've checked LittelFuse's ceramic body Time-Lag fuses as well - interestingly, while they do have way higher interruption capacity (not often a necessity for a bench meter) - they do what the label says, they lag behind, in a "mild overload" zone as well. So, Siglent engineers did choose the right fuse - that's my view of course :-). I'm not associated with Siglent in any way :-)!

Hope this will help someone to save time looking for the right replacement fuse for SDM30x5X DMMs. I'll now leave this old topic alone :-). All the best, folks!

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