Current draw and noise in a remote control glider power system

[Mike_K]

Hello all,

I was trying to measure the current draw of a small RC glider power system, and noticed a interesting noise phenomenon when trying to make the measurement.

Some basic info of the system:
The system consists of a basic RF receiver that powers 4x small RC servos.
The receiver (and thus servos) are being powered by a 1s lipo; at the time of the test the lipo was at about 4V (just about fully charged).

I made some observations/measurements in three different setup configurations:

Config A)
As a baseline without trying to make any measurements and the battery plugged directly into the receiver, the system behaves normally, without any noise affecting the servos.

Config B)
I then added some jumper cables between the battery and receiver to give me a point to place oscilloscope leads.
With the scope connected, it seemed like the servos were jumping around slightly (a sign of noise) and I read about a 320mV peak-to-peak ripple/noise signal with the scope.

Config C)
I then tried to measure the current draw by placing a multimeter (Fluke 107) in series with the positive battery lead.
When powering up the system, there is a lot more noise and I measure a peak-to-peak ripple/noise signal of up to 700mV,
and just visually it is clear that there is a lot of noise because the servos are randomly twitching and moving around.
The current measured in this config is about 200mA.

My theory is that the multimeter leads in Config C (and to a lesser extend the jumper wires in Config B) are acting as antenna,
adding noise to the signal. Does this theory seem correct?

Some questions I have:

1) Even with the noise, is the 200mA reading I am making still reliable? In practise the system will work like in Config A with the battery directly plugged into the receiver,
and in that case there was no appreciable noise. If the current measurement with noise is reliable, I don't really care about the noise, since the system will never have to operate in Config C.

2) The Fluke 107 I am using is not a very high performance meter and is more of an electricians meter. For example it doesn't have 'mA' or 'uA' capability, so I was making
the readings in 'A' reading mode. Is it possible that the meter is adding noise to the system? This seems very unlikely to me, but I would be curious to know if this is possible.

3) If my theory that the test leads are acting like antennae is correct, is there a better way to make measurements like this?

Thanks in advance for any feedback
Mike

[Manul]

This should have nothing to do with wires working as antenna. I would say, there is likely not enough capacitance for bypassing current ripple, so the system relies on battery having a low ESR and ESL. If you add wires, they add resistance and even more important - inductance. It is hard to say without seeing waveforms or seeing schematic and modules used. But it is definetely first idea which comes to my mind.

1. Likely correct, should not be off by more then a few percent even with high ripple.
2. Multimeter does not add noise by itself (at least not something what you can measure with your scope), but it adds some resistance and long wires (inductance). Speaking RF, it adds a loop, which can act as an antenna, yes.
3. Antenna theory is likely not correct (as I wrote above), but yes, better measurement can be made. You can add big enough low ESR capacitor on the power rail of the system and make measurement between battery and capacitor. In that way, system still sees low impedance and ripple will not go through multimeter wires, only more or less average value of current consumtion.

[Mike_K]

Thanks Manul,

At this point I am happy knowing the current measurement should be relatively accurate, but good to learn about the other things you mentioned.

Mike