Author Topic: Why is BK Precision still in business? BK8500 programmable load OEM found  (Read 57686 times)

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Offline T4P

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #25 on: September 14, 2012, 08:43:34 am »
To think somebody said the previously unknown engineer who designed the Itech's then "stole" the documents and gone to maynuo ;)
It's just the same designer, it's his right to do whatever he wants

Going on taobao i see the maynuo only costs 400SGD for the 300W model and 256SGD for the 150W model

So BK's 8510 is TRIPLE the maynuo. Go figure
http://www.digikey.com/product-detail/en/8500/BK8500-ND/1135014
 

Offline saturation

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #26 on: September 14, 2012, 10:09:59 am »
That's great T4P, what's the shipping cost to you from China?  Can you post the full taobao link here?

To think somebody said the previously unknown engineer who designed the Itech's then "stole" the documents and gone to maynuo ;)
It's just the same designer, it's his right to do whatever he wants

Going on taobao i see the maynuo only costs 400SGD for the 300W model and 256SGD for the 150W model

So BK's 8510 is TRIPLE the maynuo. Go figure
http://www.digikey.com/product-detail/en/8500/BK8500-ND/1135014
« Last Edit: September 14, 2012, 10:17:19 am by saturation »
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Offline T4P

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #27 on: September 14, 2012, 10:23:24 am »
By EMS it's going to cost not much at all.
http://item.taobao.com/item.htm?id=14577117671

Well what i mean by not much is that it only weighs 3.5Kg so my agent charges 7$ for the first 500g and 4$ for the subsequent 500g
So it's 31 dollars. Cheap! Plus it's EMS ...

Anyway ITECH has one these mini loads
http://item.taobao.com/item.htm?spm=2013.1.0.113.55ffca&scm=1007.77.0.0&id=14764899428
« Last Edit: September 14, 2012, 10:26:06 am by T4P »
 

Offline saturation

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #28 on: September 14, 2012, 02:02:50 pm »
You're right T4P, I stand corrected.

As I wrote I checked with a taobao agent, and to the US worst case $100, still makes it cheaper. 

Taobao to me is closer to craigslist than ebay, so there is more risk, but its worth noting on this thread if one is interested in a low price.  I also checked the Array 3710 and 3711 prices versus local US sellers like Tekpowers or CSI, and the savings is between $100-200, but is not as great as getting Maynuo vs BKPrecision depending on shipping.

Have you purchased from taobao before, have you had any problems for delivery to SG?


By EMS it's going to cost not much at all.
http://item.taobao.com/item.htm?id=14577117671

Well what i mean by not much is that it only weighs 3.5Kg so my agent charges 7$ for the first 500g and 4$ for the subsequent 500g
So it's 31 dollars. Cheap! Plus it's EMS ...

Anyway ITECH has one these mini loads
http://item.taobao.com/item.htm?spm=2013.1.0.113.55ffca&scm=1007.77.0.0&id=14764899428
« Last Edit: September 14, 2012, 02:05:01 pm by saturation »
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Offline kmel

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #29 on: September 14, 2012, 06:55:46 pm »
By EMS it's going to cost not much at all.
http://item.taobao.com/item.htm?id=14577117671

Well what i mean by not much is that it only weighs 3.5Kg so my agent charges 7$ for the first 500g and 4$ for the subsequent 500g
So it's 31 dollars. Cheap! Plus it's EMS ...
If you are in an European country you can count on import duties: 20-25% and carrier custom handling charges.

If you like it real cheap: MAYNUO sells them to you if you buy 20.

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« Last Edit: September 14, 2012, 07:56:49 pm by kmel »
 

Offline T4P

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #30 on: September 14, 2012, 10:52:53 pm »
You're right T4P, I stand corrected.

As I wrote I checked with a taobao agent, and to the US worst case $100, still makes it cheaper. 

Taobao to me is closer to craigslist than ebay, so there is more risk, but its worth noting on this thread if one is interested in a low price.  I also checked the Array 3710 and 3711 prices versus local US sellers like Tekpowers or CSI, and the savings is between $100-200, but is not as great as getting Maynuo vs BKPrecision depending on shipping.

Have you purchased from taobao before, have you had any problems for delivery to SG?
No issues at all, bought a 2kilo item and it came nicely and the second one was a mouse from a not so good dealer (95% rating) and it came with a razer mousepad!

If you are in an European country you can count on import duties: 20-25% and carrier custom handling charges.
Use tbcart. 20-25% won't jack it all the way to 1300$ anyway

Pick one
http://tbcart.com/Product/14577117671/%3CSpan+class%3DH%3Emaynuo+%3C+span%3E+Meier+nuo+M9712+programmable+electronic+loads+300W+0-30A+0-150V+package+postal+discounts/
http://tbcart.com/Product/15094663354/Meier+nuo+%3Cspan+class%3DH%3EMaynuo+%3C+span%3E+M8831+programmable+DC+power+%2830W+0-1A+0-30V%29/
http://tbcart.com/Product/14015175967/Meier+nuo+%3Cspan+class%3DH%3EMaynuo+%3C+span%3E+level+agent+LED+load+M9812+authentic+last/
 

Offline kmel

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #31 on: September 15, 2012, 06:14:44 am »
If you are in an European country you can count on import duties: 20-25% and carrier custom handling charges.
Use tbcart. 20-25% won't jack it all the way to 1300$ anyway

Pick one
http://tbcart.com/Product/14577117671/%3CSpan+class%3DH%3Emaynuo+%3C+span%3E+Meier+nuo+M9712+programmable+electronic+loads+300W+0-30A+0-150V+package+postal+discounts/
Ok... I pick the first one. $437.42 for the item, shipping to their warehouse and their handling fee.

If I pick the same item and seller via taobao.com: CNY 2000 = $322.58 for the item.
Agents handling fee should be about 10% or lower. I don't know the shipping fee to the agents warehouse, but I don't think it is the difference to $437.42. Or am I wrong? Can someone read taobao.com?

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Offline T4P

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #32 on: September 15, 2012, 08:06:46 am »
Agent fee.
 

Offline LaurenceW

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #33 on: September 19, 2012, 10:22:15 pm »
Guys, I got me a Maynuo M9712B 500V 300W electronic load, here in Blighty! Bought off fleabay, got nabbed for some customs fees, but still a good price.

Looking good so far. Some interesting features, some weak points easily fixed, one or two rough edges.  It works. I will do a teardown and review in a week or two.

Currently i have a calibration problem, so this will be a test of customer service, too!
If you don't measure, you don't get.
 

Offline saturation

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #34 on: September 19, 2012, 11:26:45 pm »
Great to hear.  Looking forward to your review.  I've just about completed a hardware teardown on the Array 3710a, so it would be great to compare the two.

When you add the review, please let us all know how much custom duties added to your cost too.

Curious to see what the differences you found compared to the BK Dave reviewed.

Guys, I got me a Maynuo M9712B 500V 300W electronic load, here in Blighty! Bought off fleabay, got nabbed for some customs fees, but still a good price.

Looking good so far. Some interesting features, some weak points easily fixed, one or two rough edges.  It works. I will do a teardown and review in a week or two.

Currently i have a calibration problem, so this will be a test of customer service, too!
Best Wishes,

 Saturation
 

Offline kmel

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #35 on: September 20, 2012, 03:15:57 am »
Guys, I got me a Maynuo M9712B 500V 300W electronic load, here in Blighty! Bought off fleabay, got nabbed for some customs fees, but still a good price.
Nice...  didn't you tell me that you didn't pay customs or taxes?

What did you get for the money besides the current load:
manual (engl./chin.), calibration certificate, CD with software,
galvanically insulated USB/RS232 interface cable,  power cord (UK/EU/US/CHI)?

Ordered a M9711 within the EU three days ago. Eagerly waitiing for a tracking ID... No tracking ID arrived, but the M9711 :-)
Currently i have a calibration problem, so this will be a test of customer service, too!
Can you elaborate the problem? I like to be prepared when my unit arrives.
And who is your customer service? Stantronic = MAYNUO distributor UK?
« Last Edit: September 21, 2012, 08:38:09 am by kmel »
 

Offline LaurenceW

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #36 on: September 20, 2012, 11:23:33 pm »
My general experience of customs duties is VERY HIT AND MISS. It depends on where the goods came FROM, who they were carried BY, and what the sender might have DECLARED on the CN22 customs form. And that's just the UK! Other countries no doubt differ. Your mileage WILL vary.

I thought I'd "got away" without alerting the customs' interest, KMEL, but a couple of days after my Maynuo load landed with me, and invoice for GBP£25 followed - that's £15 VAT , and £10 to the carrier for the charge to levy the VAT charge! Charges on charges - great. That's the key, of course, make a deal whereby there is something in it for the carrier, and for sure they will collect your country's taxes from you! >:(

OK - a few words on my Maynuo 300W 500V electronic load so far.

It came well packed and double boxed, with soft, "giving" packaging which will decelerate any delicate electronics to rest over a few mm of travel, so subjecting the items to lower "G" forces. Some firmer packaging (such as expanded polystyrene) I've seen in other instruments does not do this. GOOD
It came with the WRONG manual - FAIL
It came with the WRONG (wong?) Mains lead - FAIL! OK, I've got spares, but surely that is easy enough to get right?
It came with a propriety to USB interface which you need to hook the load up to anything else (GOOD - MAKE SURE you ask your supplier if that's included - it may not be as standard)
It DID NOT come with any PC software on CD - EVEN THOUGH that appeared in the eBay photo! FAIL But at least you can download it.
The PC software is pretty comprehensive, but the screen is probably designed  by a 12-year old who hasn't got a degree in man-machine interfaces - number display is CR@P - mimics 7-segment displays - WWHHAATT??? Should have stuck with a more readable Arial font, instead. NEAR FAIL
There are a number of mistakes in the printed specs - FAIL, but overall the accuracy is good within spec (which is pretty tight). Certainly, specs are 10X tighter than cheaper electronic loads. GOOD
The instrument did include a calibration sheet - Good
There are too many digits on the display! It's like measuring something with a ruler and declaring it to be 251.348mm long. It might be, or it might be 251.5mm long. You cannot tell with the measurement device in question, so the last digits are irrelevant.
The load's accuracy and performance are pretty good, but accuracy goes off at very low currents - below a few mA. You can manually compensate for this, though.
You can apparently adjust the electronic load's current setting, for example, in increments of 10uA, but the smallest incremental change that the load will actually resolve is 50uA. Another example of somewhat pointless resolution.
There are temperature sensitive fans, which vary continuously and smoothly according to the temperature demand - GOOD (not just on/off)
The display features four simultaneous measurements of Voltage, current, power and parameter under load control (voltage, current, power or resistance). Good
 The displayed character size is a little smaller than the ITECH/BK equivalents, as a result. You can get around this with the software app, if necessary.
The instrument is quite DEEP, but when you open it up, you'll see why - big, long heat sinks.
The case is of good quality overall. Standard tilt bail. Nice. Rubber bumper thingy at the front. I can take it or leave it (so can you - it comes off easy enough).
Most annoyingly, they have mounted the number pad buttons the wrong way up! Telephone style 1>9, rather than calculator style. You have to stop and think before keying a value in. Try rearranging your keyboard keys A>Z, and see what that does for your typing - FAIL
PROPER ON/OFF Switch - when it's off, it's off. GOOD
DO NOT RECALIBRATE THIS DEVICE (done through the front panel). I have £$%^&'d up the high current range as a result, and so far cannot get it back. FAIL (on the part of the user???) Waiting to hear from Maynuo...
Internal construction looks OK. It's no Agilent, but it will do. I don't get goosebumps plugging it into the mains... GOOD (enough)
There are no fewer that EIGHT paralleled MOSFETS sharing the load in my Maynuo. They don't look like they'll have a hard time doing their job.
Because the regulating elements are MOSFETS, the resistance of the load cannot go down to Zero. Mine goes down to about 0.25 Ohms
It is CEII marked (whatever that actually means), but the instrument does not carry any specs regarding isolation performance. The input is floating (not connected to earth), but how far away can I take it? 500V? Dunno
The computer interface NEEDS to use an external Propriety USB, 232 or 485 lead. (I've got the USB lead). These leads DO offer electronic isolation from the load - IMPORTANT
There is a rear-mounted BNC socket which carries a voltage proportional to the current flowing. Cheaper electronic loads skip this. This is useful for displaying current transients on a scope - GOOD. But again, Caution - this circuit is not isolated from the device under test.
Also rear-mounted wiring for separate load voltage sensing (to get around voltage drops of long cable at heavy currents) GOOD
Also Trigger inputs/outputs. Again, I've not seen these features on cheaper loads. GOOD
There are some great built-in battery test modes (cheaper electronic loads may skip these)
And the ability to program different currents or whatever at different times/durations, so that you can observe how your supply handles transient load changes. You can also set start up ramps (how quickly the load "comes on") - GOOD. Again, you may find these missing on cheaper loads.
YES, it's the same as the ATTEN, and YES, it is bl00dy similar to the ITECH/BK, but not the same. Probably a direct copy, from the country where the word "Copyright" means: "if you are going to copy something, copy it right"

Yeah - over all, quite happy with my purchase. But with just a little extra polish here and there, and more attention to detail and western standards, they could turn a good product into a GREAT product. C'mon, Maynuo...
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Offline T4P

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #37 on: September 21, 2012, 12:27:28 am »
"probably" a direct copy, NO!!! It's been said before! It's the same designer! How does a designer copy his own designs?

Anyway, i am placing a wish for the M9712 because of the battery test function, the other ones from Xiao Tony (except BK) do not have this
And ... the RS232 isolating cable is not standard, it's optional and it costs an extra 30USD if i buy it from china ... well
 

Offline kmel

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #38 on: September 21, 2012, 12:41:27 am »
I thought I'd "got away" without alerting the customs' interest, KMEL, but a couple of days after my Maynuo load landed with me, and invoice for GBP£25 followed - that's £15 VAT , and £10 to the carrier for the charge to levy the VAT charge! Charges on charges - great.
That's a bummer! But you should be lucky for the "low" declaration value and no customs duties.

OK - a few words on my Maynuo 300W 500V electronic load so far.
...
Many thanks for you first mini review and for the warnings.

YES, it's the same as the ATTEN, and YES, it is bl00dy similar to the ITECH/BK, but not the same. Probably a direct copy, from the country where the word "Copyright" means: "if you are going to copy something, copy it right"
Check out the reply #17 of this thread, I did a not-point-by-point comparison between them.
 

Offline Wartex

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #39 on: September 21, 2012, 01:47:36 am »
Very interesting information kmel.  How do you know Xiao Tony is/was the designer from these companies?  Do you have a reference or a source?   If true, he is a righteous dude, he knows his stuff.
I asked info@maynuo.com who the OEM of the M9710 is.
 
Some side issues, Atten also sells Array OEM, ATTEN ATZ3710A.
Thanks, fixed my listing.

BTW the MAYNUO M9710 often sold on ebay/aliexpress isn't for international market, only CHI. I didn't find a distributor who sells it.

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http://www.select-test.com/ sell them, this is where I played with them
 

Offline kmel

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #40 on: September 21, 2012, 08:30:45 am »
Anyway, i am placing a wish for the M9712 because of the battery test function, the other ones from Xiao Tony (except BK) do not have this

Today the M9711 arrived and the first thing I did was using the battery test function.

Grabbed an used LiPo battery, [I-set] constant current=100mA, [Shift-8] low voltage threshold: 3.3V, [On/Off].
Photo 1 is while the discharge is running.
Photo 2 is shortly after voltage arrived at 3.3V, display shows now the off-load voltage!
 

Offline saturation

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #41 on: September 21, 2012, 07:31:17 pm »
I've put a comparison of the 2 PCBs BK vs Maynuo, on kmel's photo post:

https://www.eevblog.com/forum/product-reviews-photos-and-discussion/maynuo-m9711-programmable-dc-electronic-load-photos/msg147455/#msg147455

regardless they are about ~95% identical.  There are some differences.  I'd be more cautious using the Maynuo on DC sources that may have superimposed AC.

Any eload that can plot a discharge curve on their PC software can be used to calculate AH.  While its convenient to have the test as push-button, the discharge curve gives you more information about the battery's overall health, in a simple quick view.  Most of China eloads have low-volt auto off and that can be set to the limit of a battery's working voltage, so the test terminates at that time.  If that function didn't exist, you'd have to manually discontinue the discharge or you'd over discharge the battery.



"probably" a direct copy, NO!!! It's been said before! It's the same designer! How does a designer copy his own designs?

Anyway, i am placing a wish for the M9712 because of the battery test function, the other ones from Xiao Tony (except BK) do not have this
And ... the RS232 isolating cable is not standard, it's optional and it costs an extra 30USD if i buy it from china ... well
« Last Edit: September 21, 2012, 07:35:52 pm by saturation »
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Offline saturation

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #42 on: September 21, 2012, 07:57:21 pm »
That's a good link, a "local" option of Maynuo availability in North America.

http://www.select-test.com/ sell them, this is where I played with them

Overall this sound great in terms of functionality.  Why does it go off on low currents and how do you compensate for it?

Why did you try to calibrate it?  But this is an important test as in order to maintain your eload yourself, one should gauge its requirements to be user serviceable.

The parallel MOSFETS are a must, but its not easy to say if the loads are being distributed to each MOSFET equally.  Over time, if not so, and one or several MOSFETs take the brunt of the work, it will fail more likely that others.

For isolation, check the X-Y capacitors to ground, its roughly its rated max voltage to earth.  You can measure creepage distances of the load traces + and - on the PCB and then the clearance from the chassis, which you'll use to estimate the risk of arc over from the PCB and possibly to earth.

http://www.ce-mag.com/ce-mag.com/archive/01/03/ProductSafety.html




OK - a few words on my Maynuo 300W 500V electronic load so far.

..The load's accuracy and performance are pretty good, but accuracy goes off at very low currents - below a few mA. You can manually compensate for this, though.
DO NOT RECALIBRATE THIS DEVICE ..
Internal construction looks OK. It's no Agilent, but it will do. I don't get goosebumps plugging it into the mains... GOOD (enough)
There are no fewer that EIGHT paralleled MOSFETS sharing the load in my Maynuo. They don't look like they'll have a hard time doing their job.
..It is CEII marked (whatever that actually means), but the instrument does not carry any specs regarding isolation performance. The input is floating (not connected to earth), but how far away can I take it? 500V? Dunno
The computer interface NEEDS to use an external Propriety USB, 232 or 485 lead. (I've got the USB lead). These leads DO offer electronic isolation from the load - IMPORTANT

Yeah - over all, quite happy with my purchase. But with just a little extra polish here and there, and more attention to detail and western standards, they could turn a good product into a GREAT product. C'mon, Maynuo...


Best Wishes,

 Saturation
 

Offline LaurenceW

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #43 on: September 21, 2012, 08:11:39 pm »
It's probably just semantics, T4P, because clearly the heritage of the Maynuo and Itech devices is common. But what I meant was that if we assume that the design belongs to the company which commissioned it (and not the designer), and then said designer goes off to another company and starts building ostensibly the same thing, then I guess he has copied (stolen?) his own handiwork, yes.

The point I really wanted to make was that a B&K may very well be an ITECH,  and an ATTEN  just a Maynuo (same thing, different badge), but a Maynuo device does NOT come from the ITECH production line. it is a copy, rip-off, clone, call it what you will. Hey, welcome to a short lesson in Chinese business ethics and intellectual property rights.

It is interesting to see just how similar the two designs are. Aside from the display, when my own preference would be for the two-line (albeit smaller digit size) display of the Maynuo. Cant see the current output BNC on the back of the ITECH (which i guess was probably an easy add for Maynuo).

And I decided to calibrate it, Saturation, because I could! Or at least, because I thought I could. If it aint broke....   There is a "closed case" calibration routine that can be done from the outside, with a standard power supply and high accuracy volt/amp meters. But the instructions are suspect, and I think this is what has knocked the high current range out.
I'm now going to go away and read up on the ITECH manual, and see if I can learn more about my Maynuo!
« Last Edit: September 22, 2012, 12:20:10 am by LaurenceW »
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Offline LaurenceW

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #44 on: September 21, 2012, 11:41:18 pm »
OK, a further update from me on my Maynuo M9712B Load.

Bare in mind that these loads come in a series of power, voltage and current ratings, so you need to choose the right one. Mine needs to cope with 500V, 300W and 15A (but not all at the same time!!). So the output devices (in my M9712B model) are 8 mosfets, , IRFPG50. These are good for 1000V and each easily capable of sinking 4Amps without breaking into a sweat. Each Mosfet is separately driven through a dedicated driver IC and Gate series resistor. The design ensures the eight mosfets power share equally and reliably, by each driving into their own 0.2R series resistor. Drawing no more than 2A a device, I am happy that the power side of the Maynuo is conservatively designed. I can't see one device running away with an unfair chunk of the load, and committing suicide in that way.

Found it! In tiny print, in the corner of the board, I read

MN9711-1.7
20110815


That suggests that this board was laid out some 4 years after the ITECH variant. Clearly, a lot is exactly the same, but the processor logic and D/A, A/D stuff has changed a fair bit.

Hmmm- although the load terminals are floating with respect to mains ground, the board Earth connection is indeed mighty close to the negative side of the load- maybe just 2mm of PCB track, and no attempts at isolation slots in the PCB

(Just like the ITECH/BK model) I am not too keen to see AC mains running down the PCB to the mains switch, either. But at least it's routed so that it is fairly hard to touch with a finger.

Other interesting chips - a precision 2.5V reference TI REF5025, putting out 2.5000V, says my high resolution Fluke

A Burr-Brown 16 bit precision DAC DAC8831i, but i can't see any ADCs (a couple of likely suspects, but markings unclear)

An interesting mix of SMD components (some, but not all, ICs some, but not all resistors, smaller caps), but also through hole components (especially close tolerance resistors, transistors, caps and even some DIL IC's!).

Three small relays on board, which appear to select front or rear voltage sensing, and high/low current range (although none of the relays are in the main current path - Good).

Apart from my model being fully populated with twin heat sinks, it is the same as the photos on another thread, so I wont duplicate those.

Regular linear power supply for the circuit - I see +/- 12V, 5V and also about 40V DC for the VFD display. All conventional stuff. No fuse on the PCBs (only a mains input fuse, in the mains socket on the back of the case.

I mentioned a loss of accuracy at very low currents, so I shall explain more.  Let's look at the load operating in its most common CC (Constant Current) mode.

As well as the "set" current (that which you define) there is an "actual" current (that which is really flowing). When the two are the same - in other words, your external supply is able to source the current you demand, then the Maynuo is defined as "regulating", and it displays "CC". If the external power supply cannot deliver the required current, then the two current reading will differ, and the display adds "Unreg" to warn you. All OK so far. However, these two current displays each go to five decimal places ("resolving" to 10uA - allegedly). The two current displays will always tend to differ just a little bit, which is slightly distracting/annoying, but you just have to learn to ignore it.

At currents below 10mA, there is a slight and consistent offset between the set current and that which flows. It is around 0.25mA.  The trick is to use an external meter to set the maximum current. After that, the Maynuo seems to at least stick to the current set in this way.

Maybe it is asking to much of a piece of equipment capable of measuring 300W to also resolve fractions of milliwatts, and certainly the specifications make no claims in this department. But it is as well to be aware of the limitations of any piece of equipment.


« Last Edit: September 22, 2012, 12:15:57 am by LaurenceW »
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Offline LaurenceW

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #45 on: September 22, 2012, 09:22:54 am »
A FURTHER UPDATE - and a bit of a GOTCHA - probably a general observation that applies to many electronic loads (Certainly it looks to apply to the Maynuo/ITECH/BK series).

In these types of electronic loads, the load control element is an array of high voltage and/or high current N-Channel MOSFETs all wired in parallel to some degree. This array acts like one giant MOSFET, present straight across the +/- load terminals.  There are no relays or other switches in the main current circuit.

Now, a reverse biassed N-Channel Mosfet is essentially a forward-biased PN semiconductor junction, and will conduct at or around 0.5V when reverse biassed. So even with the electronic load's own power supply switched off, the load terminals present a very low resistance short circuit path (I've measured well under 0.5 Ohm), if reverse biassed.

Here's the message. Don't go plugging a massive power supply across your electronic load the wrong way round - there is a chance you will blow something (either your test equipment or the tested equipment) up, before you've even powered up your electronic load!!
If you don't measure, you don't get.
 

Offline saturation

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #46 on: September 22, 2012, 01:24:38 pm »
Thanks a ton for this, LaurenceW, its very helpful.  Can you tell us a bit more about the heatsinks?  Its mandatory to distribute load to each MOSFET equally but I think that may be limited by the single large heatsink with the fan only on one end, is this the case?

Cooling may not be even.  The only way to test this is to load the unit enough so the fans are engaged, and check the temperature of both ends of the heatsink, is it equal, how big a thermal gradient exists?

MOSFET internal resistance rise with temperature.  When the fans are engaged, the FET nearest the fan are cooled first, so it takes more share of the load, and the more distant FET will be hotter and will take less of the load.  Also, ambient air enters the front, but hotter air from the cooling front FETs are blown back, so the rear FETs aren't ever the same thermal conditions as the front.

I think the derating the current limits from 8x4=32A of those IRFPG50  [I think its even higher, about 6A per device, thus 8x6 = 48A] to 15A, because the load balancing could be offset by the uneven cooling, but this only becomes apparent when the units are used nearer their specified limits.


OK, a further update from me on my Maynuo M9712B Load.

Bare in mind that these loads come in a series of power, voltage and current ratings, so you need to choose the right one. Mine needs to cope with 500V, 300W and 15A (but not all at the same time!!). So the output devices (in my M9712B model) are 8 mosfets, , IRFPG50. These are good for 1000V and each easily capable of sinking 4Amps without breaking into a sweat. Each Mosfet is separately driven through a dedicated driver IC and Gate series resistor. The design ensures the eight mosfets power share equally and reliably, by each driving into their own 0.2R series resistor. Drawing no more than 2A a device, I am happy that the power side of the Maynuo is conservatively designed. I can't see one device running away with an unfair chunk of the load, and committing suicide in that way.

Found it! In tiny print, in the corner of the board, I read

MN9711-1.7
20110815




If the ground wire looks like the Maynuo photo of kmel, you can simply reroute it to increase the clearances.  As for creepages, if it looks too close one can either avoid using the load in humid climates or cut a track with a dremel. 


..Hmmm- although the load terminals are floating with respect to mains ground, the board Earth connection is indeed mighty close to the negative side of the load- maybe just 2mm of PCB track, and no attempts at isolation slots in the PCB


« Last Edit: September 22, 2012, 01:30:57 pm by saturation »
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Offline saturation

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #47 on: September 22, 2012, 01:49:29 pm »
Good point.  This is a similar issue with the Array 3710a but only if powered on, input polarity reversed.  When powered off, it 500 ohms.  While the 3710a warns and beeps in case of reverse polarity, that its sinking the full load of the DUT should be made a warning sticker on the front of the unit.


A FURTHER UPDATE - and a bit of a GOTCHA - probably a general observation that applies to many electronic loads (Certainly it looks to apply to the Maynuo/ITECH/BK series).

In these types of electronic loads, the load control element is an array of high voltage and/or high current N-Channel MOSFETs all wired in parallel to some degree. This array acts like one giant MOSFET, present straight across the +/- load terminals.  There are no relays or other switches in the main current circuit.

Now, a reverse biassed N-Channel Mosfet is essentially a forward-biased PN semiconductor junction, and will conduct at or around 0.5V when reverse biassed. So even with the electronic load's own power supply switched off, the load terminals present a very low resistance short circuit path (I've measured well under 0.5 Ohm), if reverse biassed.

Here's the message. Don't go plugging a massive power supply across your electronic load the wrong way round - there is a chance you will blow something (either your test equipment or the tested equipment) up, before you've even powered up your electronic load!!
Best Wishes,

 Saturation
 

Offline nctnico

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #48 on: September 22, 2012, 06:18:14 pm »
MOSFET internal resistance rise with temperature.  When the fans are engaged, the FET nearest the fan are cooled first, so it takes more share of the load,
That is a common misconception. MOSFETs also suffer from thermal runaway (negative tempco) when operating in the linear region. Besides that there are large differences between MOSFETs. IOW it is absolutely necessary to balance the load between the MOSFETs.
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Offline LaurenceW

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Re: Why is BK Precision still in business? BK8500 programmable load OEM found
« Reply #49 on: September 22, 2012, 07:58:50 pm »
Well, the heatsink blocks are each comprised two fairly long (roughly 170mm) sections, that are interleaved with one another (take you hands flat, fingers extended, and interleave your fingers - that's the general idea, but with bigger air gaps between your fingers). These are then sealed into a square tube, with a fan bolted squarely to one end, and the other vented out the back of the case. All of the air that the fan draws in must pass down the heatsink tube, and out the back. There are two mosfets bolted onto each vertical side of the heatsink, making four per heatsink block, eight in the instrument (300W rated) overall.

All of the control electronics are both at the cooler (drafty!) end of all of this action, and also hardly putting out any heat themselves (less that 15W), so not under any threat.

I take your point about the devices running nearer the fan being to some degree cooler than the ones further down the heatsink, but to be honest my Mk1 temperature probe (Index finger) cannot detect a heat gradient. And anyway, it never gets uncomfortably hot. Even at 150W per bank, each fan is pushing a good deal of air out of the back, which eventually reaches around 30 degrees C above ambient.  And that's at the hottest end. Obviously, the Mosfet junction temps will be a little higher than this, but not greatly so. So the mosfets are still breezing it. The fans are on a bit of a mission by now, mind. Think: small hair dryer (which the Maynuo rather is, at this point...)

Blocking the air inlets while the Maynuo is at full chat causes the exhaust air temperature to spike! (I have not tested it to the claimed overtemp point). But opening the vents again very quickly causes the air temperature to revert to the previous value - around 53C. With all that air moving about, the rest of the outer case of the Electronic Load remains comfortably cool.

I cannot detect any significant variation or changing voltage drops in the series 0.2R resistors even over several minutes of full load operation. 100vDC at 3 Amps - OH YEAH!  Same results with 37V at around 7A. This suggests to me no shifting of duty between the mosfets, for any reason. I have no view on mosfets' temperature sensitivity, but I see no evidence of any such device behaviour impacting the instrument, overall.

OH! But I've just had a thought. There is a LOT of electronics between the two heatsinks, to do with the Mosfet gate drives. The gates could have all just been joined together, and then connected to the analogue drive voltage. But then, potentially one or more slightly lazier mosfets (with minutely different performance curves) might arguably have led to an uneven load distribution. But there are precision op amps , resistors and diodes scattered about, here, which now leads me to wonder whether this electronics isn't actively watching the individual mosfet current (as a voltage drop across each mosfet's 0.2R series resistor), and doing some load balancing that way. I am not about to do a further tear down and track out the mosfet gate drive electronics. but I could see that theory being the case.

(Electronic) Loads of fun!
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