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Siglent SDM3045X mA current measurement - weird behaviour

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rf-loop:

--- Quote from: nctnico on November 12, 2018, 02:10:42 am ---Perhaps it is also a good idea to put two big diodes anti-parallel to the shunt. That is the recommended way to prevent an overcurrent (surge) from damaging a shunt resistor. Also looking at the size of the resistor it doesn't look like it has been properly sized for handling 10A continuously (= 1 Watt of heat dissipation). The resistor may be rated for 1W (or even more) but those ratings are always in ideal circumstances and running the resistor at the absolute maximum temperature. Derating 3 to 4 times is not a bad idea. A through hole current sense resistor made of thick wire is much more forgiving when it comes to abuse compared to an SMT resistor (AFAIK there are SMT current sensing resistors made entirely from metal but these aren't cheap).

--- End quote ---

Of course it can not handle 10A continuously. No one have made it for continuous 10A and no one have promised it can handle 10 continuously. So why this straw man argument?

Specifications for SDM3045X:
Current:
30 seconds OFF after 30 seconds ON is recommend for the continuous current that higher than DC 7 A or AC RMS 7 A.

Also in based to image image and other info it looks like 2W but also this same size some are rated for 1W. measuring from image without good reference for dimensions is just guessing.  But then @ahope: "Rough measurements puts it at 6.5x3.5mm or thereabouts." So perhaps size is 6432 metric size (2512).

Now if we look Siglent recommendation in data sheet and use 10A. 30sec on 30sec off. (1W 30sec and then cooling 30sec) average 500mW.   Then if look continuous 7A. It means 490mW. 

nctnico:

--- Quote from: rf-loop on November 12, 2018, 06:14:19 am ---
--- Quote from: nctnico on November 12, 2018, 02:10:42 am ---Perhaps it is also a good idea to put two big diodes anti-parallel to the shunt. That is the recommended way to prevent an overcurrent (surge) from damaging a shunt resistor. Also looking at the size of the resistor it doesn't look like it has been properly sized for handling 10A continuously (= 1 Watt of heat dissipation). The resistor may be rated for 1W (or even more) but those ratings are always in ideal circumstances and running the resistor at the absolute maximum temperature. Derating 3 to 4 times is not a bad idea. A through hole current sense resistor made of thick wire is much more forgiving when it comes to abuse compared to an SMT resistor (AFAIK there are SMT current sensing resistors made entirely from metal but these aren't cheap).

--- End quote ---

Of course it can not handle 10A continuously. No one have made it for continuous 10A and no one have promised it can handle 10 continuously. So why this straw man argument?

Specifications for SDM3045X:
Current:
30 seconds OFF after 30 seconds ON is recommend for the continuous current that higher than DC 7 A or AC RMS 7 A.

Also in based to image image and other info it looks like 2W but also this same size some are rated for 1W. measuring from image without good reference for dimensions is just guessing.  But then @ahope: "Rough measurements puts it at 6.5x3.5mm or thereabouts." So perhaps size is 6432 metric size (2512).

Now if we look Siglent recommendation in data sheet and use 10A. 30sec on 30sec off. (1W 30sec and then cooling 30sec) average 500mW.   Then if look continuous 7A. It means 490mW.

--- End quote ---
Well it says 10A on the spec sheet I pulled from a website quickly. And it also says 10A on the front of the SDM3055 DMM without a time limit (at least on the pictures I've been able to find). Usually when there is a time limit this is also printed on the front. Either way adding the two diodes is still a good suggestion because it would have prevented the damage seen by the OP. The SDM3055 isn't a particulary cheap instrument to begin with anyway so some protection against user errors would be nice.

tautech:
RTFM !

HKJ:

--- Quote from: nctnico on November 12, 2018, 08:53:04 am ---Perhaps it is also a good idea to put two big diodes anti-parallel to the shunt. That is the recommended way to prevent an overcurrent (surge) from damaging a shunt resistor.

--- End quote ---

You mean something like this (Multimeters often use a bridge rectifier to get 2 or 3 diodes in series):

nctnico:

--- Quote from: HKJ on November 12, 2018, 09:05:55 am ---
--- Quote from: nctnico on November 12, 2018, 08:53:04 am ---Perhaps it is also a good idea to put two big diodes anti-parallel to the shunt. That is the recommended way to prevent an overcurrent (surge) from damaging a shunt resistor.

--- End quote ---
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.

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