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| Siglent SDM3045X mA current measurement - weird behaviour |
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| tautech:
Just try and find/identify the problem......relay contacts/operation or damaged shunt. It will be good for the Siglent engineers and other members to know what it is so we know if to repair or it's a warranty claim. |
| ahope:
Did some more investigation. First, took the lid off and did a visual inspection. Then I tried to locate which relay was kicking in. It seems like all of them are kicking in simoultaneously, but at least the big white in the read box is kicking in. After looking around I found one resistor that looked bad - look inside the red box. In the next picture I have a close-up. Its R408 marked "SMT F0705 R010". From the picture it looks like a burn-mark on one side and you can see it is bulging up in the middle. Resistor R407 next to it looks like it may be a little non-flat, but not so easy to see. Both these resistors are close to the big white relay, and next to that is the 12A fuse that blew a while ago. Could this be the culprit? |
| ahope:
Another thingvthat bothers me: why did the internal fuse blow and not the external? and why didi it not protect the internals? |
| Performa01:
Nothing wrong with R408. It is the 0.01 Ohm shunt that works flawlessly in the amps range. That "burn mark" at the side comes from some milling for trimming its resistance value - I'm actually baffled that Siglent goes to the effort to do this on a low cost instrument like this. But the photos also show that my assumption was wrong and there are only two shunt resistors, just as stated (not very clearly) in the datasheet. The only other shunt resistor is R407 at 1 Ohm. According to the data sheet, this 1 Ohm resistor would be active for the 600mA range, but the very same data sheet also states a burden voltage of <0.5V in the 600mA range, which cannot be true because 600mA * 1 Ohm = 0.6V (plus voltage drop across cabling, fuse and connectors). Anyway, R407 looks good as well and also from the latest symptom description it is unlikely that the shunt is at a fault. If the meter works correctly in the 600mA range down to some 230mA, then the shunt resistor should be okay. The real problem is the relay(s) switching below 230mA - whichever they are, in any case throwing off the measurement. |
| bson:
Verify that it can measure low voltages correctly, like maybe 5-10mV down to whatever you can produce reliably (50µV maybe). |
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