I have noticed that high-end multimeters usually have four holes:

  • Common (COM)
  • Volts (V)
  • Milliamperes (mA)
  • Amperes (A)

While cheaper ones usually have three holes only:

  • Common (COM)
  • Volts (V) / milliamperes (mA) (shared)
  • Amperes (A)

Is there any clear advantage to have a dedicated hole for mA measurement?

  • Plenty of cheap meters omit the 10A range and the sockets, and just have two leads coming out (e.g. my Wavetek DM7 at home) – Chris H May 17 at 13:48
up vote 10 down vote accepted

Let's use the term "socket" rather than "hole".

The meter section of a multimeter usually consist of a millivolt meter. Full scale is typically ±199.9 mV (200 mV nominal) for what's now considered a low-quality meter and ±399.9 mV, etc., for better meters with higher resolution. All measurements including voltage, current and resistance have to be converted to mV in this range to get a meaningful reading.

From Ohm's law we can calculate the shunt resistance value required to generate the required voltage for various current ranges:

Range       Resistance
2.000 mA    100 Ω
20.00 mA     10 Ω
200.0 mA      1 Ω
2.000 A       0.1 Ω
20.00 A       0.01 Ω *

* Most meters will use this value for the 10 A shunt value but the power rating is only good for 10 A.

The idea here is that inserting the meter into a circuit to measure current will cause a maximum voltage drop of 200 mV and minimise the disturbance to the circuit under test.

enter image description here

Figure 1. Innards of a no-name multimeter. Source: Dismantle-It.

In the PCB of figure 1 notice the fine traces going to the range selector contacts. These won't take 10 A. Also notice the 10 A shunt resistor (a piece of resistance wire) mounted at the bottom of the board but standing off it for cooling. The manufacturers seem to calibrate it by attaching the brown voltage measurement lead at the appropriate position along the shunt - hopefully after a test measurement.

Any decent meter will use a dedicated socket for the high-current range to avoid running high currents through the selector switch.

  • 5
    I believe the question is about sharing a socket for V/mA, not for mA/A (which your answer seems to address). – Dmitry Grigoryev May 17 at 7:13
  • Thanks for pictures and explanation. I also found this drawing to be useful. – tigrou May 17 at 22:26

Meters are often left connected to voltage sources for purposes of monitoring them. Further, they often use the same knob for both mode selection and power on/off. If a meter uses the same knob to turn the meter on and off and control the mode, it may be very easy for someone turning the knob to accidentally turn it to a current-measurement mode while it is connected to a voltage source. Doing this will effectively short out the supply, putting as much current as it can source through the meter. Not good.

Accidentally switching the meter to a resistance-measurement mode could pose similar risks if the meter is connected to a particularly large voltage, but it's easier to protect resistance-measurement circuitry against moderate overvoltage than it is to protect current-measurement circuitry against severe overcurrent.

Using a separate connection for current measurement means that turning the knob to a current-measurement mode while a probe is connected to a voltage source (and fed to the voltage-input jack) will likely yield either a meaningless number or an error display, but will not allow massive amounts of current to flow through the meter. In some cases, it may also offer the benefit of allowing the meter to pass through current any time the current-measurement input is used, even when the power switch is "off", thus allowing a device under test to be left powered on without wasting out the meter's battery at times when nobody cares about its measurements.

It is cheaper for the manufacturer, who doesn't have to engineer a way to switch high currents to different shunts.

This was difficult to do well with mechanical switches.

Since the advent of power fets, it is just an economic choice. Tektronix TX1 and TX3 meters solved it in the '90s, and did away with it. e.g TX schematic pg 48. (You will see that the fets don't carry the 10A range, only the lower 100mA ranges)

Low cost meters save money, by only having a single shunt, no mA hole, but no mA range. By contrast the TX3 has no mA hole, but still has a 100nanoamp resolution.

It is hugely inconvenient for users, as the mA fuse is always blown, if you work anywhere meters are shared. Personally, I loathe the mA hole.

  • In my experience, cheap meters do have a mA range, provided via the shared V/mA socket. Or maybe the cheapest kind isn't sold where I live. – Dmitry Grigoryev May 17 at 7:25
  • @DmitryGrigoryev: There's probably a strong correlation between a shared V/mA socket and a blown mA fuse, though mA fuses can also get blown in some moderate over-current situations (e.g. measuring motor current and having the motor stall) where it would be helpful to e.g. put two-amp back-to-back diodes across the sense resistor so that the meter could tolerate currents above those where it could take useful measurements. – supercat May 17 at 15:31

The higher current range requires a lower resistance shunt, which will not provide much signal or even accuracy for low mA range currents. The signal/noise ratio and accuracy can be poor even when it's amplified.

The mA range can use a larger resistance shunt for better accuracy and noise performance.

  • 2
    If I understand correctly, you told me that mA/A measurements have different requirements (so they cannot share same hole). But why mA / V is better to be not shared ? When measuring volts there isn't any current flowing right ? – tigrou May 16 at 21:35
  • 1
    That's a different question, but when measuring current, you need the current to flow through a low, hopefully negligible resistance path (the shunt). So the meter has to be in series with the load. When measuring voltage, you want the meter to have a very high resistance to avoid loading the voltage you are trying to measure. In a voltage measurement you measure the potential BETWEEN two points, the meter is not in series with the circuit. – John D May 16 at 21:40
  • 1
    Unless I missed something, sharing a socket between mA and V is not a different question, it's exactly what the OP asked. – Dmitry Grigoryev May 17 at 7:17
  • @DmitryGrigoryev I think you're right, I may have read the question too quickly, or been confused by the wording. – John D May 17 at 14:50

Your Answer

 
discard

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.