DOA current measurement in new multimeter?

[Alaza]

Hello there,

I'm not very experiencing in breaking loops for measuring currents, but for my old Bang and Olufsen I wanted to calibrate the output quiescent currents among others.

So basically the manual says that I should put the meter in series with a given resistor, which I have done. But for some reason I do never get a reading on the meter. I have uploaded a picture of the configuration here:

**broken link removed**

The multimeter is a week old, and the resistor is connected between the two white wires. I have actually even tried connecting speakers and checking where there was sound playing in both channel after my initial try, but even then I don't get a reading on the meter? The manual says that I should expect a current of approximately 25mA, so the meter is set in the mA range, and measuring DC (also indicated on the display).

Is the meter just dead, or am I doing something basically wrong?

 

[Nigel Goodwin]

It looks like the meter is connected in parallel, not in series?.

 

[ronv]

Hard to tell, but I think I see 2 empty red sockets on the meter. Usually the leads will go to one red and one black.

 

[Alaza]

It looks like the meter is connected in parallel, not in series?.

In parallel? It goes from the PCB to the resistor, and from the resistor to the meter, and back the PCB?

Hard to tell, but I think I see 2 empty red sockets on the meter. Usually the leads will go to one red and one black.

The left one is also read, it just dictates the amount of current to be measured.

 

[ModemHead]

Check the fuse inside the meter.

Also, if you want to know the current through a resistor, measure the voltage across the resistor (without disturbing the circuit) then divide by the resistor value (I = V / R).

 

[Alaza]

I did that afterwards actually, it was also an excuse to try and do a current measurement since it's not something I usually need. And the current is off by 5mA or so, so I would need to fix that.

I reckon you don't loose any warranty for checking and changing fuses?

 

[kinarfi]

It may be that the resistor is in an AC part of the circuit and has no DC on it, Do you have the schematic with that resistor in it?

 

[Alaza]

It's a 22 ohm resistor in the output stage of the power amplifying stage, in the emitter of a BJT to ground?

I have opened the multimeter now, and the fuses are ceramic, so it's not visible whether they are open circuit, but I could think so, seeing that I had a minor sparkling short accident just after I bought it... stupid, I know.

But I guess I will be ordering both a couple of F500 and F10 fuses to be sure, and covered in the future.

Edit:

I just quickly reassembled the meter without the F500, and it is indeed an open circuit. I guess the problem has been identified then.

 

[ModemHead]

It's a 22 ohm resistor in the output stage of the power amplifying stage, in the emitter of a BJT to ground?

Should be DC, if the amplifier is quiet (no signal).

I have opened the multimeter now, and the fuses are ceramic, so it's not visible whether they are open circuit, but I could think so, seeing that I had a minor sparkling short accident just after I bought it... stupid, I know.

Yep, just be careful where you connect the meter when the leads are plugged into the current (A/mA) jacks. No "checking the amps in a battery."

 

[Alaza]

Aah, I wouldn't do that. The short I made was my clumbsy fingers being in the wrong place.

That actually leads me somewhere else. I was trying to simulate me a current dummy load for exactly the purpose of measuring batteries and the like, but I am uncertain of how to simulate a source like the battery, delivering a current varying with respect to R1. Can you help me?

**broken link removed**

 

[ModemHead]

I'm old-school and know very little of circuit simulators. But there are plenty of helpful people here on the forum who do.

In any case, you need to replace R1 with a reference voltage source, and replace X2 with an N-channel mosfet. The op-amp will then vary the gate voltage so that the voltage across R2 matches the voltage on its non-inverting input, thus regulating the current through X2 and R2.

 

[Alaza]

I didn't think it completely through. I've made the changes here:

**broken link removed**

Alright, I will see if anything happens here, otherwise I'll create a seperate topic on the matter.

 

[ModemHead]

Much better! I think you can simulate a battery with a voltage source and a low-value resistor in series.

 

[Alaza]

For some reason or the other, I can't get a current to flow even with this configuration:

**broken link removed**

... what on earth is wrong?

And I've tried with V2 at 1V for instance too.

 

[ModemHead]

I don't see a "simulated battery" or other source of current in that diagram... Also, try using 12V for the op-amp power supply. It can take up to 10V or so to fully turn on a run-of-the-mill MOSFET. Plus you need headroom for the voltage developed across R2. You can also use a NPN BJT in place of the MOSFET, just include a series base resistor to limit the base current.

 

[Alaza]

I'm not sure I follow you then. The source of currnet should be V2 resulting in a current flowing in R1?

 

[ModemHead]

I'm not sure I follow you then. The source of currnet should be V2 resulting in a current flowing in R1?

I'm sorry I don't see a V2 or an R1 in your latest diagram?

 

[Alaza]

Damn it, something went wrong with the link.

**broken link removed**

 

[ModemHead]

The polarity of V2 is backwards.

 

[Alaza]

With some focus I got it.

**broken link removed**

A1 is rotated 60% of the 50k. Now I just need to find out how to sweep the A1 resistor.