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| Brymen, difference in reading between mA and uA scale |
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| Monkeh:
--- Quote from: MarioBros69 on August 13, 2023, 09:57:23 pm --- --- Quote from: TimFox on August 13, 2023, 08:42:26 pm ---You still don’t understand. In a battery-resistor-meter circuit, the burden resistance of the meter reduces the actual current into the meter. What you measure is what is actually flowing, not what would flow into a short circuit. --- End quote --- Yes, I think I understand it, when making the measurement I have added a resistor to the circuit (which is the internal resistance of the meter) therefore the measured current is lower than the real current --- End quote --- No, the measured current is the real current. The current without the additional resistance in circuit is different. |
| EEVblog:
--- Quote from: MarioBros69 on August 13, 2023, 02:22:47 pm ---The truth is that I don't understand this very well..., if the measurements differ, which of the two is correct, the one on the mA scale or the one on the uA scale? --- End quote --- Both are correct. The act of putting your current meter in series changes (in this specific case) the currect because the meter has a shunt resistor in it. --- Quote ---How do we know then the current that circulates through the circuit when we are not measuring? --- End quote --- You always have to know what you meter is doing to your circuit and whether or not it is going to change the circuit current or voltage. |
| MarioBros69:
--- Quote from: TimFox on August 13, 2023, 10:18:35 pm ---What do you mean by "real current"? Your meter is reading the actual current through its terminals, which is lower than that which would flow into a short circuit, since your meter is not a short circuit. --- End quote --- Maybe I expressed myself wrong, by real current I meant the current that flows through the circuit when I am not measuring |
| MarioBros69:
--- Quote from: J-R on August 14, 2023, 01:00:17 am ---To be clear, typical bench and handheld DMMs don't technically measure current directly, they measure the voltage drop across a known precise resistance (shunt) and calculate the current using Ohm's law. The reason the burden voltage is a "problem" is because these DMMs have a limit to how low of a voltage they can detect. Since V=IR, the resistance must be made larger if the current is smaller in order to maintain a voltage range that can be detected/measured. The DMM manufacturers make a design choice for each current range depending on the capability of the DMM. That is why lower-end DMMs typically have a higher burden voltage. (The 121GW uses an amplifier in some ranges similar to what the uCurrent does in order to reduce the burden voltage as much as possible.) The absolute minimum voltage the BM869s can detect is 0.001mV (500,000 count mode). The lowest current it can measure is 0.001mA and 0.01uA. Based on the shunt values I provided earlier, we can see that 0.001mA results in a value of 0.001758mV and 0.01uA results in a value of 0.00101543mV. So both of these are clearly chosen to line up with the best the BM869s can do, 0.001mV. (10A shunt calculates to 0.00276mV.) Another way to measure current is with a Hall Effect sensor found in DC clamp meters, but most are going to bottom out in the low mA area. Forget uA or nA. Speaking of mA/uA/nA, I'll maintain there is no huge reason to need the uCurrent or similar since you can simply use your own known resistor as the shunt. For example, the uCurrent uses a 10k resistor for the nA range, so assuming you need to measure 1nA, the voltage drop across your shunt would be 0.01mV which the BM869s can manage well enough. It becomes trivial at say 50nA, which is 0.5mV. --- End quote --- In your previous post I see that you have both the BM869s and the BM789 according to your point of view, which of the two is more accurate when measuring current? Thank you for this excellent clarification |
| Fungus:
--- Quote from: MarioBros69 on August 14, 2023, 08:48:46 am ---In your previous post I see that you have both the BM869s and the BM789 according to your point of view, which of the two is more accurate when measuring current? --- End quote --- Do you mean which has the lowest burden voltage? It's specified in the manuals/datasheets of those meters. (One of them is better on the uA range and the other is better on mA and A - you can't win! :) ) |
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