MAYNUO M9711 programmable DC electronic load - photos

[kmel]

Hi,

got my MAYNUO M9711 today. It's a programmable DC electronic load: 150W 0-30A and 0-150V.
Features and the datasheet you can find here: http://maynuo.com/english/xpro.asp?pid=51.

It was delivered with manual, CD with applications, calibration report, isolated TTL USB adapter, MAYNUO sticker and EU power cord.

As reader of eevblog I know, that I have to tear it down, before I can play with it.
So here are some quick photos...

--
kmel

[saturation]

Thanks kmel, this is very very helpful.  It essentially identical to the BK,but there are very small differences.  I've enclosed a screen grab from Dave's video so you can compare.

On the side vents, there are big windows on the Maynuo versus slotted vents on the BK.  A lot more airflow, but it reduces the structural integrity of the opening, but cheaper to make than slots.

There are no cutouts on the PCB for high voltage clearances at the inputs.

The ground wire is too close to the terminal, its actually touching V+.  Its also not optimally placed, it should be as close to the X-Y capacitors like it was on the BK version.

Those X or Y capacitors should be completely flush with the PCB to minimize arc potential, the feet are easily visible.

Only one large powered heatsink as expected, versus 2 in 300W version.  Seem is would be better installed on the right side to give those small ohm power resistors added cooling through the fan.

Those green 5 ohm power resistors seem a bit roughly made.

Its certainly a different class compared to the BK or the Array hardware construction.

[robrenz]

saturation, I get a not allowed error when I try to view your picture

[saturation]

Hi robrenz sorry, I was editing simultaneously, to get the photos close together!  try now.

ssturation, I get a not alowed error when I try to view your picture

[gerrysweeney]

I thought I would add to this, I have recently done a full video teardown of the Maynuo M9711 DC Electronic Load, may also be helpful

http://gerrysweeney.com/maynuo-m9711-dc-electronic-load-unboxing-teardown-and-quick-test/

Gerry

[Rufus]

I thought I would add to this, I have recently done a full video teardown of the Maynuo M9711 DC Electronic Load, may also be helpful

Thanks for the vid, nice to see the insides of another version of this cloned design.

The bridge on the heatsink is used for temperature sensing. Bizarre to use such a huge bridge to get two diodes but all these clones seem to come from the same quite old design which they more or less stick to including the tacky TTL serial interface. They seem to have a variety of processors and displays probably because they couldn't clone the firmware. I have a Beich 360v/30A/150W version which was cheaper and looks cheaper inside. It has an LCD display and I think an ATMega processor. It also has the huge bridge on the heatsink which is connected to the PCB by two skinny wires.

You worry about the serial interface being live - what about that BNC monitor?  I assume the body of that BNC is as live as anything you put on the inputs. At least you have to poke something in a hole to kill yourself with the serial interface.

These loads are dangerous mostly due to lack of documentation. Does the manual that came with it provide any warning?

[Kevin.D]

Hi Gerry ,just finished watching your nice teardown vid  , thanks that was interesting with some good info . .
On the question of the current equilizing resistors on the parrallel Mosfets ,yes they are required .I know that because I have just finished designing and building my own so had to find answers to the same problem .
The reason for this is that mosfet's do not have a  positve temp coefficient except at high drives (Vgs > 5V) and high currents (Id >20 A for power devices) .
At these levels then Rds on dominates and this value has a + temp coeff and increases with temp.
At a lower Vgs drives and lower currents then Vgs (Th) will dominate and this has a Negative temp coeff and gets lower with increasing temp ,so a small diff in Vgs(th)  between two Mosfets of say 0.5 Vgs can cause a difference of 10A Id at the same applied Vgs .

Check out the transfer characteristics of any power mosfet and notice at some piont the Vgs - Id current at different temps will cross over at some 'inflection' point from being negative to positve temp coeff.

Check out this short onsemi app note on mosfet temp stability . It will explain better than me
 http://www.onsemi.com/pub_link/Collateral/AND8199-D.PDF

[kxenos]

Has anyone found a cheaper source than this http://item.taobao.com/item.htm?spm=a230r.1.14.137.ZyNXal&id=8390155690 for a Maynuo 9712? 1980Y is roughly 245E or $320.

[gerrysweeney]

@Rufus, thanks for the info. From what I could tell the Bridge was wired directly to the input terminals so I assume it is reverse input protection, when I metered it out thats certainly what I could see. There is a TIP41C bolted to the heat sink, that was connected to an LM324 and that looked to me like the temperature sensor.  However, I did not really look too hard so I may well be wrong.

@kevotronic, thanks for the information, I did not know that about FET's that really useful to know, thank you. I plan to post a link on my blog article back to this thread but could I ask your permission to post your comment relating to the TCE of the FET devices?

Thanks

Gerry

[Rufus]

@Rufus, thanks for the info. From what I could tell the Bridge was wired directly to the input terminals so I assume it is reverse input protection

I posted this photo of the Beich a while ago but I can't find the thread.

The bridge is screwed to the heatsink below Q3. Two legs are cut off and skinny green and yellow wires take it to (I think) the connector just in front of the fan. There is nothing else on the heat sink but the 4 big MOSFETs and the fan definitely has heatsink temperature controlled speed.

Look at your circuit - reverse input current would just flow through the MOSFET intrinsic diodes anyway.


If you want an example of how tacky these things can be look at the photo. Notice the front panel components are mounted on a U shaped bit of aluminium. There are 6 hank bushes and screws on each side. What you can't see in the photo is a little plastic plate on each side which holds the front panel to the chassis and provides isolation. The front panel metalwork is connected to the input -ve and so is as 'hot' as whatever you put on the inputs (I guess because they couldn't achieve 500v isolation around the LCD and other front panel components). The kicker is the aluminium sleeve slides on from the back and wedges between the front panel metalwork and plastic bezel. What keeps the sleeve isolated from the front panel? A bit of green PVC insulating tape stuck along the top and bottom edges of the front panel metalwork, you can see the top edge in the photo. The sleeve isn't directly earthed either, just the other bits of aluminium it slides over don't have PVC tape.

You didn't answer my question about the monitor BNC. Is the body connected to the -ve input?

[gerrysweeney]

@Rufus, my apologies, yes the BNC is connected to the -Ve terminal of the input terminal, as you say it could be even more dangerous.  I guess the manual and chassis could do with some more obvious safety labels planted around it.

And yes you are absolutely right about the bridge rectifier, someone else also said the same thing.  The TIP41C is the driver for the fan, the LM324 controls the fan and the bridge rectifier senses the temperature of the heat sink. The whole fan circuit us also ground referenced to the -ve input terminal.

Gerry

[Kevin.D]

@kevotronic, thanks for the information, I did not know that about FET's that really useful to know, thank you. I plan to post a link on my blog article back to this thread but could I ask your permission to post your comment relating to the TCE of the FET devices?

Thanks

Gerry

Yes you can post it if you want. Good luck with the blog ,and the p.s.u your designing .

[gerrysweeney]

Yes you can post it if you want. Good luck with the blog ,and the p.s.u your designing .

Thats great, thank you.

Gerry

[saturation]

FYI, I think the partial review of the Beich unit is on the Array thread archives.  It may have copied elements of various Chinese eloads, but its overall design and assembly leaves much to be desired when other units exist that are better made.  Readers can compare photos between Beich, Maynuo and BK.

[Electro Fan]

Hi Gerry,

Just checking to see if you are still happy with your Maynuo Electronic Load?  I was thinking about going for a BK8500 but after see your video (Very nice! Congrats!) and reading some of the posts here it looks like the Maynuo might be the better product.  Any thoughts or recommendations?  Thanks, Electro Fan

PS, any chance the Agilent PS that failed during your Maynuo video might have had a problem due to something it didn't like about the Maynuo?

PSS, are you pretty sure the Maynuo is safe (not sure if the the consumer safety standards are quite the same in China as they are in the UK/Europe or the US)?

[gerrysweeney]

@Electro Fan, yes I am still happy with the Maynuo, its proven to be robust and reliable so far.  From what I understand all of the DC loads that look similar are all from the same design stable give or take so the differences really are brand and build quality.  I can't judge the build quality against other similar units because I have not had them to take apart and have a look but what I would say is the build quality of the Maynuo is very good, I could not find much to complain about.

The Agilent PSU failed because of a fault that I eventually found in the PSU so the Maynuo was not the cause, in fact using the DC load helped find the issue in the PSU. I covered the "re-repair" of the PSU here: http://gerrysweeney.com/my-e3634a-mystery-explosion-wtf/

As for the safety of the Maynuo, its safe so long as you are aware that although the load terminals on the front are mains isolated, the connectors on the rear panel are ground referenced to the input terminals.  Make sure that when you connect to the computer you use the supplied isolator cable and you should not have any issues.  This could have been designed better in the Maynuo for sure but at the end of the day its not that big a deal.

Gerry

[kado]

Hello,

could anyone tell me where to buy a
Maynuo E-load for a good price in the EU?

I can't find a (good priced offer) in Germany, any known
dealer in the UK?

Thanks for information
73 Karsten

[mcinque]

Spin electronics (http://www.spinelectronics.eu/), they don't keep that model in catalog but in 15 days they could order from Maynuo and ship it to you.

I doubt you can find a lower price from official distributors, maybe you can grab a 9710 model on aliexpress at less than a half of the price, with the same specifics. But be careful: it's a Chinese market model only, with no warranty and a "high repair rate" (this is what Maynuo says).

p.s. about the "cloned design": some months ago I've talked with a Maynuo representative about "who's the original between BK and Maynuo" and he explained me that the BK model was originally designed by an engineer that left BK (keeping all project designs) and founded Maynuo, making then some improvements to the original project (like 4 data fields onto display instead 2 of the BK). I don't know if this is the true, but this is the story

[casinada]

I heard or read similar things about the guy that designed the DC Loads for Array and BK went on his own and started Maynuo.
Now, where did I read that??

[saturation]

Its all over this forum's archives under Array or Maynuo.  You also need to include a 3rd brand, Itech.

I heard or read similar things about the guy that designed the DC Loads for Array and BK went on his own and started Maynuo.
Now, where did I read that??

[Circuitous]

If you want a Maynuo, you might get a price from Franky Tong, he's on this forum as Iloveelectronics.

[HighVoltage]

could anyone tell me where to buy a
Maynuo E-load for a good price in the EU?

I just got my Maynuo M9811 today
Shipped directly from China to Germany via FEDEX
Ebay name: wzspark0577
Deliverd in 5 days and no hassle with the German import customs
The price was Euro 310 plus 55 Euro FEDEX shipping cost

[mcinque]

I just got my Maynuo M9811 today
Shipped directly from China to Germany via FEDEX
Ebay name: wzspark0577
Deliverd in 5 days and no hassle with the German import customs
The price was Euro 310 plus 55 Euro FEDEX shipping cost

What about warranty?

[sotos]

I just got my Maynuo M9811 today
Shipped directly from China to Germany via FEDEX
Ebay name: wzspark0577
Deliverd in 5 days and no hassle with the German import customs
The price was Euro 310 plus 55 Euro FEDEX shipping cost

What about warranty?

Only luck.
I think no warranty, because to send it back and forth is difficult and expensive.

[iDevice]

Deliverd in 5 days and no hassle with the German import customs
The price was Euro 310 plus 55 Euro FEDEX shipping cost

I hope you don't forget the FEDEX's VAT and custom clearence fees invoice you will receive in the coming days/weeks...