BK 8550 electronic load current ripple

[adam.v]

Hello All,

I've just bought a BK Precision 8550 electronic load and I have a question regarding its current ripple. I haven't really used previously electronic loads (only made some simple resistive loads if needed for a short testing). I bought the load mainly for testing the different dcdc power supplies and LED drivers I build for my projects.
My problem is with the output current ripple: when I set the load in CC mode to let's say 50mA I can observe a ca. 10mA ripple in the load current. Let me show you an example. The test setup is the following: the load is connected to a Keysight E36313A power supply, and there is a Keysight 34465A DMM measuring the current.
Test: 5V output, 50mA CC set on the load (range 3A).
The average current measured by the DMM (10PLC): 50.195 mA. This is within the manufacturer specified ±(0.05% + 0.05% FS) CC accuracy. Below are some measurements with DMM digitizing:


Is it normal for a DC load to have this much (~10mA PP) current ripple? I plan to use the load for characterizing dcdc converters, and sometimes I want to use it in the 10-100mA range where this ripple can cause considerable error.
I also made this measurement with higher load setting like 500 mA, and even lower setting 1-2 mA (which is close to the minimum settable current). The ripple remained the same. If I set the load below 10mA this meant a ripple between ca. 0-10 mA. I tried also measuring a battery (just to be sure it's not my power supply) it didn't change anything.

Your help would be much appreciated. I can still return the load if this behavior seems to be not normal.

[BillyO]

Is the power supply also limiting at 50mA?

Also, try the load on constant resistance.  CC will hunt around a bit to keep up with what ever the power supply is doing, but I would not expect the Keysight to be that unstable, unless both the supply and the load were attempting to limit at the same current.

CR should not hunt around regardless of variance in current or voltage from the supply.  It should be very, vey quite.

[adam.v]

No, the PSU was set to 2A limit, it was in CV mode 5V.

I tried CR mode, with 100Ohm setting, and it produced the same result.

Just for reference I measured with the same conditions a 100Ohm resistor to check if not the DMM/PSU is causing the noise:


Looking at the graph I think it may be some AC power line noise, because the noise repeats nearly 50Hz.

[floobydust]

Repeat the test using a battery, your PSU might be tossing out too much noise. Check your grounding - are you using the PE binding post?

Otherwise, send the info to BK, they are pretty good answering support questions.

[nctnico]

Is it normal for a DC load to have this much (~10mA PP) current ripple? I plan to use the load for characterizing dcdc converters, and sometimes I want to use it in the 10-100mA range where this ripple can cause considerable error.

Unfortunately yes. The cause is likely the 50Hz magnetic field from the transformer coupling into the DC load control circuitry. Depending on the constructuon, you might be able to fix it. I had a similar problem with a Korad DC load: https://www.eevblog.com/forum/testgear/korad-kel2010-multicomp-mp710771-review/msg5039434/#msg5039434

BTW: That model is a rebranded Tonghui DC load.

[adam.v]

Thank you for your answers!

Repeat the test using a battery, your PSU might be tossing out too much noise. Check your grounding - are you using the PE binding post?

Otherwise, send the info to BK, they are pretty good answering support questions.

I mentioned in my previous post that I tried it with battery and the result was the same. Around 10mA noise. I have not used the PE binding post for the measurements with the PSU (I tried simple resistor loads, and there wasn't any 50Hz noise coming from it).

Unfortunately yes. The cause is likely the 50Hz magnetic field from the transformer coupling into the DC load control circuitry. Depending on the constructuon, you might be able to fix it. I had a similar problem with a Korad DC load: https://www.eevblog.com/forum/testgear/korad-kel2010-multicomp-mp710771-review/msg5039434/#msg5039434

Thanks, that's good to know. I think I'm not trying to fix it rather I will send it back. I am thinking about building a small electronic load myself, because generally I am testing low power circuits.

BTW: That model is a rebranded Tonghui DC load.

Yes, I saw that. I chosen the BK to have hopefully more stable firmware and better support. But for this price point I expected a better hardware design. (Another issue I haven't mentioned is that the encoder on the front is not debounced, so it is hard to navigate the menus sometimes.)

[Lexy]

I've bought the TH8401, which is to be the same as the BK8550. I've noticed the same current ripple as mentioned:



We see a current ripple of ~12mApp!

The source is a GPP-4323 set to 5V and current limit of 0.7A. A DMM6500 (in digitizer mode) is used to measure the current.

As mentioned by ntnico, i suspected the transformer as the culprit, so i've opened it:



As we can see, the transformer is in the front of the unit. Not the best place i think 

I've done a measurement with the transformer outside the case like this:



And see here, the ripple is reduced a fair amount to ~2mApp!




I'll try to move the transformer to the back of the unit and maybe add some extra shielding (mu metal?)