Burden Voltage is a BIG trap with Multimeters, don't get caught out!
A short video demonstrating the impact of Burden Voltage / Shunt Resistance on current measurement.
Mutlimeter Shunt Tutorial Part1:
Part 2:
Yes, I still see too many people getting bitten by this. Especially when they want to measure current on low-power devices with a large dynamic range.They'll use the lowest range for measuring sleep currents, then when the device wakes up, it will brown out due to the shunt resistor, and believe me, many will scratch their head before figuring it out.
That said, isn't Kirchhoff's loop rule for the birds?
Yes, I still see too many people getting bitten by this.
What prompted this video was a returned Amazon meter buyer claiming they retunred it faulty and got another one and it had the same "fault", so it must be the calibration of my meters.
Then I posted that on Twitter and some people said "what's burden voltage?".
Yes, and on some the lower current ranges it can be pretty high as well. Some analogue meters used the lowest voltage range, with a shunt resistor, as the most sensitive current range, so you could actually have 3V burden voltage at the lowest current range or around 50uA full scale. Head scratch time, till I looked at the circuit, conveniently given in the manual, and saw that the 50uA range was the exact same circuit as the 3V range, just a different portion of the range switch used to make it. But what do you expect from a 1980's era $1 analogue meter, which was the one I as a scholar could afford. Yes I did blow it up a few times on resistance range, though on current range there was (actually on all the ranges) a 1A fuse in series with the input. fixed those cooked resistors by looking for "close enough" values on scrap PCB's, and soldered them in. Then checked, and resistance was close enough, or at least not too different from before with that non linear scale, and anyway hard to read anything over half of the range of resistance, as the scale got really cramped at that left side. But good enough for continuity, and checking if a lamp was 6V, 12V or 48V, before blowing up the 6V lamp on 12V.
Never did find that fluorescent lamp starter I connected across the mains, to see it glow for longer than the normal half second though. But good news was that the new to the house earth leakage breaker, installed in the laundry, which powered the plugs in the garage (and as a side note this was from Australia, as it had 2 Aus socket outlets, with matching non sleeved plugs as well, used to connect the wiring, and actual 20A circuit breaker as well) and laundry, so as not to shock mom when the iron cord got leaky, did work, along with the 20A breaker. Reset after my ears had recovered from the bang, but before the dog came back from where he departed to at high speed.
Rest of the house still had rewireable fuses, and I was generally the one to climb up on top of the Defy Range, open the panel, and pull out the fuses to find the blown one, and wire it up again using the little screwdriver and paper cards of fuse wire, there on top of the wooden case for this purpose. Fuse wire, 5A, 10A and 15A, normally 4 turns of whatever was left on the card, so the sockets and stove would work again. Old house, so the lights were 2.5mm, the plugs 4mm and the geyser and stove 6mm copper wire. But at least it was PVC sleeved, even if the sleeve dripped green goo, not the older brittle DCC rubber wire. That was the wire from the meter to the board.
Many of the older analog meter are actually quite good (e.g. 50 mV) when it comes to the burden voltage. This way they could save a bit on the power rating for the resitors. This is at least for DC - the AC part was tricky as a passive recifier needs a relatively high voltage and a current transformer is quite some effort, though found in some older analog meters. There are also quite some analog meter that just don't have AC current ranges.
The burden voltage can be expecially bad with the autoraging meters in some of the lower ranges, like the example shown with the Bryman meters. It is just easier to use the same shunt and switch voltage gain than switching the actual shunt.
For the higher current ranges it is not just the shunt resistor, but also the fuse. A change in the fuse can also change the burden voltage.
With some benchtop meters one can find low current ranges with very low burden, that is done via a transimpedance amplifier.
The inverse of this, selecting the shunt resistor when you know the current range and maximum acceptable voltage drop, has been very useful to me as a hobbyist.
(I think all development-oriented single-board computers powered from a DC jack should be able to measure their input voltage and current, plus thermal sensors at least on the main SoC. These are not only useful for statistics and in the development phase, but if regularly monitored at end-use time, can also detect problems like sagging input voltage and overcurrent draw. Helps a lot when trying to root out instability issues.)
I think good DMMs should display the current value of the shunt when on a current measurement range. It would be very helpful, and avoid people either not thinking of the shunt, or having to dig back the info about shunt values hidden somewhere in the DMM's manual. It's not like modern DMMs don't have those large graphic displays with ample room for displaying *useful* information rather than cute icons.
I wish more meters had transimpedance current measuring systems on the lower ranges. Fluke 8505A and it's relatives have that as does the 8808A, but other newer, better models skip it entirely. Some better bench meters do have a larger number of shunts so if you have 6.5 digits you can just go up a range for low burden, but for the lower ranges nothing beats a TIA.
For the 8808A:
Shunt Resistance.............................................. 0.01 Ω for 2 A and 10 A ranges
1 Ω for 20 mA and 200 mA
Burden voltage < 5 mV for 200 μA and 2 mA range.
So it's great for the lowest two ranges, OK for 20mA (appears to use 10X additional gain) but sort of sucks on the 200mA range.
I think good DMMs should display the current value of the shunt when on a current measurement range. It would be very helpful, and avoid people either not thinking of the shunt, or having to dig back the info about shunt values hidden somewhere in the DMM's manual. It's not like modern DMMs don't have those large graphic displays with ample room for displaying *useful* information rather than cute icons.
Protek 608 displays shunt resistance. Also displays: dBm (AC V), input impedance (DCV), forward current (diode mode), output voltage (Ohms mode). Super useful stuff on a secondary display.
121GW lets you measure the burden voltage with a third lead, but does not tell you the shunt resistance. And its a bit convoluted to set it up.
It would be easy to show resistance in the top but there is no "Ohm" symbol there for it to make sense.
As you say on graphical meters this stuff should just be a given.