Electrical Test Conundrum

[cobra kid]

I have a simple circuit. 30V AC, a 2kOhms resistor that gives a current of 15mA.

I know the circuit works as I have an independent data logger measuring current. For reasons left elsewhere.....I need to break in and measure current with a multimeter instead of the data logger. However, with the cables connected outside of the meter, current flows, with the cables connected to the multimeter, it stops. I've tried this with 3 meters now.

Internal resistance too high in the meters perhaps???

[Super Josh]

Have you moved the positive lead of the meter from the V to the current position?

[mr_fibuli]

You could try a clamp meter?

[brman]

is your cct literally as described? ie. simple power supply and resistor? If so, I would go with connecting the meter wrong as suggested above.
Also, how are you determining whether current is flowing or not? The data logger or the multimeter?

A picture of the setup (inc meter connections and setting) might help.

[cobra kid]

brman said:

is your cct literally as described? ie. simple power supply and resistor? If so, I would go with connecting the meter wrong as suggested above.
Also, how are you determining whether current is flowing or not? The data logger or the multimeter?

A picture of the setup (inc meter connections and setting) might help.

We have a HBM DAQ unit that has a current clamp. The DAQ unit is running in the background showing current output. This shows 15mA when the leads that I would put in the multimeter are joined together. As soon as plugged into the meter, the DAQ unit shows zero current.

I'll have a look at the multimeter settings.

Thanks.

[brman]

cobra kid said:

brman said:

is your cct literally as described? ie. simple power supply and resistor? If so, I would go with connecting the meter wrong as suggested above.
Also, how are you determining whether current is flowing or not? The data logger or the multimeter?

A picture of the setup (inc meter connections and setting) might help.

We have a HBM DAQ unit that has a current clamp. The DAQ unit is running in the background showing current output. This shows 15mA when the leads that I would put in the multimeter are joined together. As soon as plugged into the meter, the DAQ unit shows zero current.

I'll have a look at the multimeter settings.

Thanks.

Well, the bit in bold was going to be my next suggestion of things to try. So yes, that does imply the meter is causing the problem unless the circuit is more complex than you have suggested. Assuming it is a "normal" multimeter then it should be low enough impedance that it will not affect the current flow.

Actually, that begs another question. You say it is an AC power supply. What frequency? I had assumed mains frequency but you know what they say about assumptions.....

[brman]

btw. you might have noticed that the input impedance of the multimeter is not specified on the (milli)amps ranges. For current measurement it is normally specified as a "burden voltage". ie how much voltage is needed across the meter to measure a current. eg mV per A
Regardless, when measuring mA like you are you should see no noticeable drop compared to the 30V supply.

[cobra kid]

Just tried on DC from the power supply as well. No joy! How bizarre.

It really is as simple as one lead out of the supply, into the meter, out of the meter, into the resistance box, out of there and back into the supply.

[Piersman2]

Pictures man, pictures !!!

So we can see how much of a feckwit you're being!

[cobra kid]

Piersman2 said:

Pictures man, pictures !!!

So we can see how much of a feckwit you're being!

Chill! The cabling is a bit messy. I'll get round to it on Monday again. Off home in a few minutes.

Thanks for now.

[cobra kid]

Blue and red leads vanish off to the power supply. Tried different pos/neg combinations in both meters with different settings!

[57Ford]

Any chance of a close-up of the meters?
Also, I presume you’ve tested the resistance of your meter leads to make sure they aren’t broken?

[SS2.]

I don't think that Velleman meter measures AC current.

[Sheepshanks]

If the internal fuse has blown on the multimeters would that stop them working altogether?

[57Ford]

No current would flow so you would read zero. I’m more inclined to think either broken leads, operator error or as mentioned earlier, it’s not 50hz in which case a boggo meter won’t measure a true reading although the current would still flow so his other kit would still work.

[brman]

57Ford said:

No current would flow so you would read zero. I’m more inclined to think either broken leads, operator error or as mentioned earlier, it’s not 50hz in which case a boggo meter won’t measure a true reading although the current would still flow so his other kit would still work.

which is the symptoms reported. I think sheepshanks might have a point there.
I would say either a blown fuses (has someone tried using all the meters on the current setting to measure voltage?) or meters that don't measure AC current(although normally that would just give crap readings, not stop the circuit working??)

[TooLateForAName]

Did you change the terminal that the probes are connected to.

I cant see from the pictures but my guess is that you've put the meter into current mode but need to use the other terminal

[brman]

TooLateForAName said:

Did you change the terminal that the probes are connected to.

I cant see from the pictures but my guess is that you've put the meter into current mode but need to use the other terminal

It is hard to see but I think that on the meter he is currently using it is in the mA/V terminal which is correct, assuming he is on the mA range. The other one is for the 10A range.

[Piersman2]

I've got that exact same model of connected meter sat in front of me now.

It doesn't measure AC current, only DC.

Most 'cheap' meters don't do AC current. In fact, I'm not sure any that do.

ETA: That other meter , with the 4 connectors, may work. Leads should be connected across the common and the lower amp range connections (assuming 15mA is expected) - if the one there works as I've just seen on some google images.

Assuming that your wires are good, check the muli-meter gives an O ohm reading when you have the wires plugged into common/Ohm connectors and tough the ends together.

Edited by Piersman2 on Friday 17th May 17:28

[cobra kid]

You probably aren't interested......but it was several multimeters with blown fuses on the current side of things!!

Ohms law has been confirmed well and truly.