Just knocked up a quick box with a USB chopped in half and the data and GND wires re-joined inside. Then a take off for a volt meter from GND and VCC plus two connectors to the VCC in and VCC off for an Ammeter.

Making the Ammeter sit in series with the circuit and the volt meter in parallel.

ie:

When I connect this with no meters attached then any connected device clearly should not work as it will not get power so a jump lead between the ammeter connections puts everything back to normal and the USB device works....

Now if I connect a volt meter that works....

If I disconnect the jump lead and connect an ammeter across the connections the device and ammeter don't work.

Have I missed something really obvious?

I just want something to allow me to check that anything USB that I build will stay in spec.

 

It looks like it should work as you are only interrupting the power connections, the data needs to be clean. Did you try buzzing it out?

 

Useful gadget!

 

It looks like it should work as you are only interrupting the power connections, the data needs to be clean. Did you try buzzing it out?

Well exactly!

Nope I have checked the continuity between each point and everything is as it should be !

Very confused....

 

If I disconnect the jump lead and connect an ammeter across the connections the device and ammeter don't work.

Have I missed something really obvious?

What is the "device"? If it's smart, it may be shutting off it's draw on USB power when it sees a fault. Or if it sees insufficient voltage when it tries to negotiate a current draw.

Also, you've drawn the data lines on the outside pins. The outer pins are the power lines, the inner 2 are data.

 

What is the "device"? If it's smart, it may be shutting off it's draw on USB power when it sees a fault. Or if it sees insufficient voltage when it tries to negotiate a current draw.

Also, you've drawn the data lines on the outside pins. The outer pins are the power lines, the inner 2 are data.

Just ignore the positions I couldn't remember them when I was drawing the diagram that is why I labelled them. The device is actually something I have made and isn't shutting itself down. I really have no idea why it isn't working correctly at all

 

Well, when in doubt, drop back and PUNT!

Sounds like you built this device, is it still in breadboard stage? Cut the power from the USB cable at the device connector, and use another supply instead of the USB power. You should be able to monitor that power any way you wish.

 

I really have no idea why it isn't working correctly at all

So, you're saying there's a voltage drop across the ammeter, and no current is flowing thru it? Equals burnt meter.

 

More like some current is flowing, but the drop is too high for the device to work properly.

 

What sort of ammeter is it? It's probably meant to be driven by a 4-20mA current loop, and 5v won't do it.

 

It's a known prerequisite for nubes to blow the DMM's low current fuse. Prior to DMMs they weren't happy until their meter movements were bent at at 90°.

 

It's a known prerequisite for nubes to blow the DMM's low current fuse.

Exactly. The OP can check the fuse's integrity by using a continuity tester on the ammeter. If it's a class III or IV instrument, you're probably going to scream a bit when you find out how much those fuses cost.