[highstream]

I've got a dac with a separate power supply line into its body, using a 2.1mm plug.  Its battery needs to be charged initially from a computer's USB port, but after that it can be continuously recharged either from the USB port or with an external charger (5-12v DC, 500-700mA).  After several months of using an external (9v, 600mA), the dac's battery went dead one day; that charger had gone dead.  Since then, and after recharging, the dac has yet to stay charged with an external adapter, running only off the USB port.  I've tried a few good adapters within specs, all of which have shown the correct voltage using a digital multimeter. That suggests something weird is wrong, but before sending the dac to be checked out, I've wanted to verify the adapters' current output, not just assume it's within range.  
 
My question is, to be accurate, does amperage have to be measured under load, with at least one lead hooked to the dac (e.g., ground or COM)?  It's not physically possible to open the dac - opening voids the warranty and is not a good idea in this case, so I've been measuring each adapter's output.  They are rated at 500-600 mA output, but the multimeter has been showing ~1A across their 2.1 mm plugs.  This made me wonder if I'm measuring incorrectly, that it really needs doing under load, i.e., with the dac involved.  Thanks,

 

[MisterX]

Sounds like a good time to read the manual that came with your meter.
 
 

 

[mcandmar]

You need to measure current in circuit, so with the +/- connect to the dac you basically need to split the + line and put the multimeter in the middle.  That however will only tell you the load the DAC is pulling, not what the adapter is capable of outputting.
 
I suspect when you bridged the +/- with the probes you were shorting out the adapter, and hence why you were seeing 1amp, but that will probably cook the adapter and kill it if left connected, not advised.

 

[wakibaki]

Dead or disconnected battery? Sounds likely. 
 
Yes, current (amps) must be measured under load. Normally you would break one of the wires from the charger to the DAC and connect the meter between the 2 free ends with the plug connected to the socket on the DAC. You can use a second plug and socket to make an adapter to avoid cutting any wires from the charger.
 
You cannot, and should not connect the meter across the plug from the charger as a very large current may flow and destroy the meter. You may have been getting away with doing this, but only because the current from the charger is self-limiting for whatever reason. If you were to do the same thing with a rechargable or primary (alkaline) cell the chances of destroying the meter are high unless it's rated for a high (10+A) current. A good ammeter will have a fuse, but it's not good practice to depend on or blow these.
 
w

 

[highstream]

Got it, thanks!  And didn't blow any multimeters.