EEVblog #281 - BK Precision 8500 Electronic Load Teardown

[EEVblog]

My Beich has a 1M8 bleed resistor to earth, what does yours measure between the -ve input and earth?

No resistance, just 0.2uF

Dave.

[NiHaoMike]

Would you trust a membrane keypad with 500v on the contacts?

As long as the rubber is thick enough to provide enough insulation, why not? That situation comes up in many modern multimeters (including Flukes) as well as many mains operated appliances. In fact, the rubber buttons are probably thicker than the insulation on many smaller mains cables.

[eevblogfan]

hey

does any one has any sort of scheme of it ? I'd really like to understand the circuit and may even adopt some "out of the box" ideas ,

thank you in advance ! 

[electros6]

David sir , I am a big fan of yours. I am an Electronics hobbyist. You set me a right path . From my child hood I am very much interested in electronics , but I have no guidance . Even in my Engineering I hadn,t learn t as I learned form you. you are my role model.
     One small request . please do a blog on Dummy load ,about its constant power and resistance and do a review on BK Precision 8500 Electronic Load

Thanks you sir

[hax129]

After some browsing on eBay and www.aliexpress.com I found that these ITECH / B&K Precision series of instruments are sold under other brand names aswell:

Atten ATZ9711, ATZ9712 etc
Maynuo M9710, M9711, M9712 etc ($380 and up, including shipping)

I see no reason to pay a premium if the exact same device can be had for much less with another logo on the front. 

... so does anyone know if these are genuine OEM versions or just crappy ripoffs, not delivering what the specs promises? 

Thanks,
Henrik

[saturation]

Thanks to your comments Rufus, I've come up some interesting tidbits:

http://kb.bkprecision.com/questions.php?questionid=245
http://kb.bkprecision.com/questions.php?questionid=6

Many of these Chinese branded e-loads are not 3rd party safety or CE certified, FWIW.  However, I checked some 300W models from big names like Kikusui, most are CE certified, but often not more.  You can check status of any particular model from bigger names on their product pages such as from Agilent or Chroma.

Which comes down to the question, are any specifications written on the manual really true whether its safety or basic performance, when it comes to these China branded e-loads?  I think the bottom line for us is we need do what you did, test for ourselves, its the price of low cost.

The BK units tested here are at most CE.  So are equivalent Maynuo models.  But the Beichs, Arrays, et. al. have none at all.

The reason you get an isolated RS232/USB interface is that the TTL interface on the DB9 isn't isolated.

All the load internal circuitry is referenced to the -ve input. When you stick 500v on the +ve input (of the 500v version) and don't connect the -ve input all internal circuitry is sitting at 500v. Dave worries about clearance and creepage on a couple of tracks carrying mains while you should worry about clearance and creepage between every part of the internal circuitry and everything that is earthed.

Would you trust a membrane keypad with 500v on the contacts? Would you expect 500v to come out of the DB9 on the back? The remote sense terminals on the back will also have 500v. The trigger input is probably opto-isolated.

I commented in the other thread on loads about the BK manual having a page of safety drivel about mains operated equipment while not bothering to mention you can kill yourself with the DB9 on the back.

All these cheap Chinese loads must come from the same (quite old) design. My Beich also has a big bridge rectifier bolted on the heatsink. It has the two middle legs cut off and is connected with skinny wire. I assume it is used to sense heatsink temperature, hard to imagine several designers independently coming up with that solution.

[T4P]

The Array and the Maynuo is designed by the same guy as well the ITECH ( mostly findable in chinese markets only at a much lower price than what BK sells ) ones ( which makes BK DC electronic loads )
Well beich too, i'm pretty sure of that.
( BTW BK Precision rebadges their scopes )

[saturation]

On bolded items, thanks Rufus for the insights. 

This is a very concerning safety issue so I looked into it further and tested an Array 3710a. This issue raised would be a problem if the eloads are not floating, which they should be.  So 500V on +ve input would be problem if the source were referenced to earth.

1M8 bleed resistor alone from chassis to the system ground would limit current shock potential but it would not isolate the chassis from high voltage.

The reason you get an isolated RS232/USB interface is that the TTL interface on the DB9 isn't isolated.

All the load internal circuitry is referenced to the -ve input. When you stick 500v on the +ve input (of the 500v version) and don't connect the -ve input all internal circuitry is sitting at 500v.

I just tried it, and that is indeed correct. Not good at all...

My Beich has a 1M8 bleed resistor to earth, what does yours measure between the -ve input and earth?

I doubled checked the BK data sheet and manual and don't see any specification for maximum applied voltage on the inputs with respect to earth. I haven't seen that specified on any of the other Chinese clones either.

I checked the isolation on my Beich at 500v and it held up but it has no right to. In one place that isolation is provided by one thickness of pvc insulating tape separating two sheets of aluminium.

[konfu]

Hey folks,

I just found something nice on eBay and I just thought to myself "Wait a minute - that looks close to the BK Precision 8511". So I had a chat with the seller on ebay and just ordered a "Rek RK8511" electronic load for just about 250 EUR which is just around 312 AUD. Including shipping, customs, tax, ...

It arrived today and ... what should I say ... this seems to be a nice clone of the BK precision one. I've attached some pictures below.

Of course it is not as 120% well designed as the BK one: A little remaining flux on the PCB, some weird looking solder joints (I gonna fix this during next days), not such nice print on top of the PCB and so on. Really some pretty normal cheap Hong Kong stuff but hey - just 250 EUR and the same precision (0.05%) as the BK one. Great deal! Love it!

I actually didn't turn it on yet. I hope it just works the same way like the BK one. *crossing fingers*


One more thing: The name of the nice little µC on the board was not grinded away. It says "c8051F340 BCS038 1140" and that is a Silicon Labs MCU. Nothing spectacular but now we know what this really is ;-)

The handle bar is somewhat cheap but okay for a bench-top device.

Only the manual is a bit tricky: Nice chinese stuff. Can't read anything but "v3.0" and the nice little black&white pictures. I really hope it's working the same was like all the other devices so I can use other vendors manual

[LaurenceW]

Yes, there are a number of these floating about. They are SO similar that I imagine they are coming off the same production line, as the tooling looks identical (or stolen?). 

Have a GOOD LOOK in the corners of the PCB for any manufacturing reference numbers or brand names. What do you see?

Extra marks if you can figure out what the 8A Bridge Rectifier is doing, bolted on the heatsink. Clue: it is not connected to the load in any way, and is not part of the instrument's own internal power supply...

I have the 300W version of this, badged Maynuo. It features a smaller, but two-line, VFD, which displays four useful measurements at once, rather than the two larger measures on the "original" BK precision Load. Construction is OK.

You should be able to find either BK or Maynuo manuals on line - one or the other of those will probably cover your Load.

One tip. It is possible to recalibrate the instrument from the front panel alone, if you have appropriately calibrated external power an meters. DO NOT DO THIS - I had a play and it took me bloody AGES to get it back in spec. YOU HAVE BEEN WARNED.

[Teemo]

I wonder if BK Precision 8500 have variable adjusting with knob? The slower you turn the knob, the more adjusting precision you have. If you turn the knob slow, setting changes with really small steps, but if you turn the knob fast, the steps increase. So you can adjust fast and precise. It is not mentioned in datasheet, but this class of equipment logically should have that?

[SeanB]

Bridge rectifier is used as heatsink temperature sensor?

[jamesp15]

I wonder if BK Precision 8500 have variable adjusting with knob? The slower you turn the knob, the more adjusting precision you have. If you turn the knob slow, setting changes with really small steps, but if you turn the knob fast, the steps increase. So you can adjust fast and precise. It is not mentioned in datasheet, but this class of equipment logically should have that?

Yes, the BK Precision 8500 does this. 

[LaurenceW]

Honours to SeanB! The bridge rectifier in the Maynuo/Atten/BK series of electronic loads might be slightly over rated (8A current capacity, while passing only 1mA), but it is indeed a cheap, bolt-on (to the heatsink) linear temperature sensor. It drives the fan(s) according to actual heatsink temp, rather than load, so the fan is not "hunting" all of the time, like it does in some cheap power supplies.

HOWEVER, there is a GOTCHA in the higher power 300W versions of these supplies, wherein two fans and two heatsinks are used, BUT only the one is monitored by a bridge rectifier (temperature sensing diode). If one fan jams on the unmonitored side, the heatsink will get FLIPPIN' HOT with up to 150W being dissipated and no forced ventilation. The electronics won't pick this fault up, and damage will surely result.

Perhaps UNLIKE the BK loads, the Maynuo I have responds linearly incrementally (that is, the current setting changes by twenty steps, for every one complete rotation of the encoder knob), regardless of how fast you flick it. You can easily choose the size of the step (i.e which decimal position is to be incremented or decremented), however.