uCurrent blown by overcurrent?

[dmg]

Hi,

I have an uCurrent which I bought to test a design which should consume very very little power while in stand by (low uA), but takes bursts of several hundred mA  (up to 600mA) while running. I wanted to test the device's stand by current so I hooked it up to the uCurrent and connected power. Previously I checked that the device was well behaved on power-up and started the ucurrent in SHORT mode. However when I flipped the switch to ON murphy striked and the device started running, trying to take bursts of high current but being unable to do because of the shunt resistor, and started to power-on oscillate until I disconnected the thing. (normally you have to press a button to enter high power state, but I discovered a bug in that circuitery after this).

This was the first time I used the uCurrent. I think I blew it because it now has a horrible offset (several mV, variable depending on connections and/or external circuit), although I checked it against my 121GW and it seems to measure well if you substract the offset. I thought it could have one of the uCurrents from the bad batch with oscillating LMV321's but mine seems to be the good one.

Do you think I might have toasted it?

[Kean]

Measure the shunt resistors (R1, R2, & R9) to check they are still in spec.
Also check the virtual ground (negative output jack) is at 1.5V with respect to the battery negative terminal (half battery voltage).

[Gyro]

You didn't indicate you supply voltage, but if it's much beyond the opamp battery supply voltage on the uCurrent then you're pretty sure to have damaged the MAX4239.

Before replacing parts I'd solder a pair of lowish leakage inverse parallel diodes across the input terminals of the uCurrent to provide it with some input protection -  it only has 270R resistor in series with the MAX4239 input, so each volt across the input above 3V battery supply will push about 3.7mA into it's input pin. The internal  ESD protection diodes of the MAX4239 are probably only good for about 5mA continuous.

[dmg]

You didn't indicate you supply voltage, but if it's much beyond the opamp battery supply voltage on the uCurrent then you're pretty sure to have damaged the MAX4239.

Before replacing parts I'd solder a pair of lowish leakage inverse parallel diodes across the input terminals of the uCurrent to provide it with some input protection -  it only has 270R resistor in series with the MAX4239 input, so each volt across the input above 3V battery supply will push about 3.7mA into it's input pin. The internal  ESD protection diodes of the MAX4239 are probably only good for about 5mA continuous.

Sorry, supply voltage was around 4.5V,  3xAAA cells.

Shunt resistors and 1.5V rail are OK both of them.  Now I'm seeing that offset is dependent on whether I touch the output wires or not. I mean, if I let the uCurrent stand on a wooden table with its current input shorted and I connect the multimeter (a 121GW, but I have other multimeter too and the results are the same) I see around 0.5 to 2mV offset, very noisy stuff. If I physically touch one of the wires then poof, offset goes to zero. 0.0000. I let the cable go and offset goes wild. Apart from offset the thing seems to be measuring well.... I mean, it linearly responds to change in current.

My issue looks a lot like the issue with the oscillating LMV321, but my LMV321 has the correct marking (i.e. it's the part that doesn't oscillate...)

[JS]

  If you don't have a ground reference in the whole circuit (as is likely in a battery powered device) it's good practice to hook it to ground in one end. I'm even doing so to improve the noise in resistance measurements (having a 121GW you are likely to have seen my and other people's posts about that)

  Looks like the µCurrent is fine, the offset you are seeing is just noise, which with an ungrounded, unshielded, high gain amplifier (as the µCurrent) you are likely to get.

  Run the same measurement as the problematic one but do two things, first ground some point of your measurment (µCurrent low seems fine, both lows are connected together IIRC) and second, start (as you always should) with the highest current range and go up by one. That way, if your measurement range is too low you jump up a range.

  While precision opamp inputs are pretty sensitive to abuse and even if they don't release the magic smoke, they will go out of specs. That usually happens when the BE junction is polarized backwards over the zener voltage of the junction, usually at about 6V, and higher currents to damage the device. 4.5V with some limiting resistance doesn't look so bad.

    I don't know what you are trying to measure, but if you have a 121GW you could just measure with it (unless you are going to the scope or some other instrumet) which already has low burden voltage, you can monitor it at the same time AND has better input protection, which is non existent in the µC, being one of the weak points of it. Notice that the fuses inside the 121GW would cost as much as the fuses in on offs and around the fuses to actually protect the whole thing you need to include include beefy, low leakage diodes, which is likely why dave decided not to go with them.

JS