Maynuo M9812 hickup

[georges80]

Short story time

So, I want to run a quick battery test and don't have the isolated USB/Serial cable for the M9812 electronic load so decided to throw together a quick level shifter (to 5V) and use an old laptop running XP with a serial port.

Installed the Maynuo software and all good. The level shifter was just an npn transistor in each direction to take the RS232 to/from 5V levels. No isolation, but for my tests that was fine and the laptop runs on batteries anyway.

I decided to quickly test the shifter with my scope and managed to short the +5V to GND, this caused the M9812 to freak and the display went bizarro and of course I shut down, unplugged the serial adapter and located the short. Powered up the unit and the display was happy and everything "appeared" ok... but then I noticed when connected to a load (power supply for testing) it would show 0V and not the voltage that was present.

Hmm - did I blow something up? Open up the unit and measured the +12V, -12V and +5V regulators and all was good. Hmm, ok, remove a bunch of screws and unplug all the connectors (very nice job Maynuo!) and pull the main board out. I noticed there was a small trace from the +ve load lug that wandered off as a thinner trace - likely the sense line, so buzzed to find where it went. Obviously no blown traces, no obvious blown components, so it seemed I'd have to find where the +ve trace went and see if there was a dead component - though at this point I couldn't imagine why losing the +5V would blow anything up, other than possibly the regulator and that was working with the short removed.

Follow the trace and it ends up at a small signal relay. It's an Omron relay and pulled up the part number on digikey, it's got a single coil and is a non-latching DPDT. So, I measured the relay and the 2 normally closed contacts are open circuit - huh? I carefully applied 12V to the coil and the relay clicked and the 2 normally open contacts were then closed as they should be, ok, remove 12V to the coil and still open circuit on the normally closed side - ok, something not right here.

Digikey has the relay in stock so I added it to my next purchase queue and decided to desolder the relay (since it would need to be replaced) and then tested it on the bench. Again, the normally closed contacts are open circuit, so I cycle 12V on the coil a bunch more times and tap the relay and... it suddenly worked again. More tests and all ok. So, solder the relay back into the board and quickly assemble enough for testing and it all functions again.

Very weird and seems totally coincidental to the 5V short since I can't begin to imagine why both sides of the DPDT would fail to function in the normally closed position and then start working again. No apparent high current flows through the relay since it's just signal level currents given the resistor (values) that connect to the contacts.

Of course I'll still order the replacement from Digikey and have it on hand if the relay ever acts up again. At least it is obvious if it fails since the voltage display goes to zero even with a voltage present on the load terminals.

I've attached a picture of the specific relay. It is an Omron G6S-2 12VDC and in stock at Digikey.

And yes, I did finally get to run by battery test when everything was back together again

Hopefully Murphy can move on to the next engineer on his Santa list...

cheers,
george.

[Zucca]

Enjoy the story thanks! Congrats for the repair.
It happens to me all the time, regardless what kind of EE measure you need to do if you think you need X time it will end up eating away at least 2X.

[SeanB]

Those horrid little relays always fail on the NC contact as the only force applied is the return spring, and the contacts will oxidise and get the thin film that makes them appear open unless you have more than 24V and 50mA as a load. I have had them fail as well with intermittent contact like yours did, a sharp rap on the top and it will work again for a while.

[mij59]

Hi, thanks for sharing.

Got a similar problem with my REK RK8511.
The RK8511 is a Maynuo clone.

Tapping on the black relay causes a voltage spike on the output of the e-load.

Wondering if it would be better to replace the relay(s)  with  DPDT reed relay  ?

[HighVoltage]

This is good to know, if my Maynuo should ever fail.
Thanks for the great writeup.

[georges80]

SeanB, yes, I have seen switches/sensors on vehicles that carry only signal levels become intermittent and require 'cleaning' with a few hundred mA. I've read about switches/relays etc with minimum operating currents due to the required 'wetting' action that keeps the contacts 'clean'.

Certainly seems feasible the same problem occurs on this relay. I can see why they used a relay since it has to switch signal levels that could be up to the maximum voltage rating of the M9812 (150V) and a relay is a simple way to provide that switching. Though there is of course negligible current flow since it just feeds into various resistor dividers etc., for the unit to measure/display the voltage (required for the CV and CP modes as well).

Overall I was very pleased with the construction of the M9812 unit. Good layout inside, nice wiring bundles, connectors for everything going to the main board. The only heads up for folk that ever want to remove the main board is that there is a soft adhesive used on all the connectors - nice to ensure nothing accidentally unplugs in shipping/use, but something to be aware of when prying the connectors apart on the board.

cheers,
george.