HP 3487A current measurements

[IvoS]

I just did some simple current test measurements with HP3478A multimeter compared to my fairly new Fluke 87 V. I used low noise lab power supply loaded with 3ohm 1% 20W resistor as a load. The resistor was spot on, measured 3.0041ohm with my BK precision LCR meter.

HP measurements: power supply output: 5.495V
                            resistor voltage: 4.999V
                            current: 1.658A

Fluke 87V measurements:  power supply output: 5.4906V
                                        resistor voltage: 5.187
                                        current: 1.723A

Here is my question: Given the numbers I can get about 0.176ohm shunt with Fluke 87V. Is the internal shunt resistor 0.1 ohm actually? (+ the resistance of the leads connected would be the 0.176ohm)

HP 3487A shows approx. 0.3ohm shunt with leads connected. The manufacturer specification say 0.1 ohm? I have no idea why the shunt is so high. I was playing with the front/rear terminals switch, it's kinda dodgy but I don't believe that could be the problem. The current difference between these two multimeters is 65mA, which is very high number. Any idea why?
         
                             

[ve7xen]

Lower total resistance = higher current, so the smaller shunt makes a smaller total resistance, allowing a larger current with the same input voltage.

Not sure where you're getting the 0.1 ohm specification, the spec says 0.3 ohm maximum, not including leads of course.

[IvoS]

I have service manual for HP 3468 and it says 0.1 ohm shunt. Aren't those two units basically the same?

[ve7xen]

Apparently they have different current shunts 

Agilent usually posts full service and operation manuals for their older equipment on their own site, and this case is no different. You can get full manuals for your instrument here.

[Dr. Frank]

I have service manual for HP 3468 and it says 0.1 ohm shunt. Aren't those two units basically the same?

Hi,

Page 149 of the 3478A service manual shows the 3A shunt, R107 = 0.1 Ohm, but in series there's a 3A fuse which will increase the resistance of the current circuit. This will make the difference. And also, a maximum burden of 1V, equivalent to 0.3 Ohm,  is specified.

Small changes in the overall serial resistance in current mode will lead to big differences in that kind of circuitry you use.

Frank

[Dave]

Forget about the shunt resistances and burden voltages. Connect both meter in series and put a current through them. The same current will be flowing through them, no matter what shunt resistance.

KISS

[IvoS]

I thought about the same thing yesterday night so I had to try it before bed time  .And yes, both units measured within 1mA difference if all connected in series, reading some 1.6xxA. But the HP3478A is absolutelly unusable for higher current precise measurements due to higher burden voltage. Thank all of you guys for an effort of clarifying this thing.

[alm]

The effect of burden voltage depends on the circuit voltage. For a 1.8 V system it's unusable, but for a 48 V system it doesn't really matter.

[baoshi]

Think this was mentioned in EEVblog #2. The 3478A burden voltage "sucks". The same reason I bought a uCurrent immediately after I bought the 3478A. But uCurrent is not able to measure too high current. Heard uCurrent 2 will have amp range.