| Products > Test Equipment |
| Calling any Fluke 8808A owners |
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| AVGresponding:
--- Quote from: bdunham7 on June 03, 2022, 02:49:03 pm --- --- Quote from: diddly on June 03, 2022, 09:34:41 am ---Any advice on using the really low current ranges on the ammeter? I mainly wanted this meter for its decent low-burden ammeter when measuring sleep currents on devices. However, sometimes it is nice to let a unit cycle where it runs in the low mA range, then sleeps in the (hopefully) sub-uA range. Ideally I could set the 8808A up to autorange, but I feel it might sometimes hunt around and choose the 200mA range with a higher burden, resetting the device. Probably not a good idea to set it to 200uA range and then walk away and overload the meter, but what does happen in the case? Will it overload and damage the circuit, or simply shunt the current elsewhere safely? --- End quote --- It has a transimpedance-like circuit for the lowest two current ranges, thus the very low burden voltage. I don't know the details of it, but I would presume it is adequately protected against overloads. I don't know how high the voltage across it might get. As for autoranging, I don't see why that would create any significant burden--the shunt resistance only goes up to 1 ohm + the fuse, so a few mA would mean a few more mV drop. It specifies less than 50mV @ 20mA. --- End quote --- The problem you might get when it shifts range is the DUT might reset due to the circuit being open while the relay switches over. |
| gamalot:
Because I don't know what kind of device the OP's DUT is, I simply used a 50 ohm resistor as the load, the power supply voltage is 5V, and tested the current and burden voltage in 4 ranges from 200mA to 200uA. It can be seen from the test results that if the setting is at 200uA or 2mA, when the actual current exceeds the range, the burden voltage may be much larger than the 5mV given in the manual. |
| bdunham7:
--- Quote from: AVGresponding on June 03, 2022, 03:14:17 pm ---The problem you might get when it shifts range is the DUT might reset due to the circuit being open while the relay switches over. --- End quote --- I don't know for sure on this unit, but a fairly typical setup has the shunts arranged in series and the relay(s) bypass them as needed so there's never an open. |
| AVGresponding:
Yes, not to mention that any overrange "reading" is useless data in any case. |
| AVGresponding:
--- Quote from: bdunham7 on June 03, 2022, 03:46:41 pm --- --- Quote from: AVGresponding on June 03, 2022, 03:14:17 pm ---The problem you might get when it shifts range is the DUT might reset due to the circuit being open while the relay switches over. --- End quote --- I don't know for sure on this unit, but a fairly typical setup has the shunts arranged in series and the relay(s) bypass them as needed so there's never an open. --- End quote --- That makes sense. Should be fairly easy to test with a DSO. |
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