B&K Precision BK8514 electronic load 1200W 120V 240A teardown - Page 1
Forum section: electronics / testgear

author: TopLoser
author_rank: Unknown
timestamp: July 22, 2015, 04:17:43 pm
content: This big heavy thing doesn't seem to want to be sold so i thought I'd open it up and see what's lurking inside. Good time to check that there is no obvious sign of magic smoke contamination!It's 20Kg of mostly aluminium heatsinks. Inside there are 4 big heatsink modules that upon closer inspection turn out to be 8 cleverly interleaved modules. Each module has 4 big IRFP250N MOSFETS (214W, 200V 30A rating) plus some control stuff. It all seems to be nicely put together, very serviceable. They've scratched the part number off the micro but everything else seems easily available if you needed to repair it.

author: bingo600
author_rank: Super Contributor
timestamp: July 22, 2015, 05:13:14 pm
content: That's some serious dimensioned electronics.Thanx Ian for the insight./Bingo

author: lukier
author_rank: Unknown
timestamp: July 22, 2015, 06:35:21 pm
content: Very interesting stuff. Rescaled BK/Maynuo/Itech load it seemsI'm thinking about building my own load, I got these nice big heatsinks (300x100mm) from Ian some time ago, I have a bunch of IRFP250N from eBay and I'm mostly inspired by Agilent 6060 schematics.My question is: what is correct current sense topology. Agilent 6060 model has separate control loop and current sense resistor per MOSFET and these current sense signals from multiple MOSFETs go into a summing amplifier to produce +IMON signal. In BK/Maynuo/Itech loads they have these loops of chunky metal wire (nichrome?) at the input and that's where they seem to get their total current measurement. What are the advantages of this method?

author: TopLoser
author_rank: Unknown
timestamp: July 22, 2015, 06:54:40 pm
content: Yes but each of the 16 channels on this load have their own current sense resistor as well. Each channel must regulate its 1/16 share and then the big loops at the end measure total current.

author: dom0
author_rank: Super Contributor
timestamp: July 22, 2015, 11:23:32 pm
content: Interesting. I would've expected more and more closely spaced, thinner fins on these fan/cooling aggregates. These seem rather few in number and relatively thick to me ... although they have these ... how do you call them? serrations?

author: TopLoser
author_rank: Unknown
timestamp: July 22, 2015, 11:33:47 pm
content: Lots of surface area on the heatsinks due to the serrated finish, makes the forced cooling a little easier..Might hook up a car battery to it tomorrow and stress it a bit!

author: Kjelt
author_rank: Super Contributor
timestamp: July 23, 2015, 06:45:16 am
content: Quote from: TopLoser on July 22, 2015, 04:17:43 pmBut the PCB clearly states MOSTET lolThanks for the tear down, very nicely put together and modulair so they can use it for all different configurations.For the DIY freak, replace the IRFP250 with the newer IRFP260 which can handle 49Amps and you have an extra safetymargin

author: nctnico
author_rank: Super Contributor
timestamp: July 23, 2015, 07:11:50 am
content: I don't like the copper(?) wire they soldered onto the board. Too much can go wrong in production with such a solution. A busbar would have been better.

author: krish2487
author_rank: Frequent Contributor
timestamp: July 23, 2015, 07:31:25 am
content: Tinned copper wire it would seem..I agree it serves little purpose other than to look impressive.@toploserI count 4 x IRFP250 + 1 x TO-220 (voltage regulator maybe) per pcb moduleand 2 such pcb modules interleaved to form a heatsink wind tunnel.and 4 such interleaved heatsink modules in total.That would make it 32 devices in total wouldnt it??I only ask because in another thread, theMG user has calculated 37.5 watts dissipation per device - which is consistent if it is 32 devices and not 16.16 devices would make it 75 watts per device (which while on the higher side, would still be manageable)

author: TopLoser
author_rank: Unknown
timestamp: July 23, 2015, 08:05:49 am
content: Quote from: krish2487 on July 23, 2015, 07:31:25 amoops yes, 4 MOSFETS per module, total of 32. Original post amended now, thank you.

author: nctnico
author_rank: Super Contributor
timestamp: July 23, 2015, 08:31:18 am
content: Quote from: TopLoser on July 22, 2015, 04:17:43 pmPerhaps people know it's a rebranded Maynuo. This one looks exactly the same:http://www.ebay.com/itm/Maynuo-M9714-USB-Programmable-DC-Electronic-Load-0-240A-0-150V-1200W-CC-CR-CV-CW-/111680125121

author: TopLoser
author_rank: Unknown
timestamp: July 23, 2015, 08:57:56 am
content: Quote from: nctnico on July 23, 2015, 08:31:18 amGood point, I reduced the price down to £850.

author: HighVoltage
author_rank: Super Contributor
timestamp: July 23, 2015, 09:06:23 am
content: Quote from: nctnico on July 23, 2015, 08:31:18 amI come to the understanding, that all BKP DC loads are made by the same guy that started Maynuo, including the brand new and revised 8600 series. No surprise.

author: lukier
author_rank: Unknown
timestamp: July 23, 2015, 12:47:15 pm
content: Quote from: Kjelt on July 23, 2015, 06:45:16 amWell, even at TopLoser bargin prices 850 GBP is way too much for a PhD student, especially for pretty much a bunch of MOSFETs and heatsinks(even small 300W DC loads are around >300 GBP, you can buy a scope, a lot more complex instrument, with that kind of cash).That's why I want to build my own and I have a lot of parts already (ADS1256 ADC, 16 bit DAC, big heatsinks, OP27s, AD8628s, everything either second hand or Aliexpress/eBay). Thanks for the tip with IRFP260, I've checked and they can be had cheaply at Aliexpress so I might consider using them instead.

author: Pinkus
author_rank: Frequent Contributor
timestamp: July 24, 2015, 09:03:08 am
content: QuoteCaution: ThegenuineIRFP260 will allow this nice high current/voltage. You never know if cheap ones on Ebay or Aliexpress are fakes and if they will release lots of magic smoke when stressed. I would always purchase critical parts (and these are the most critical parts in you application) at well known, reliable distributors.

author: lukier
author_rank: Unknown
timestamp: July 24, 2015, 09:22:19 am
content: Quote from: Pinkus on July 24, 2015, 09:03:08 amYeah, I know. That's always a gamble.In this case there might be something really dodgy going on. IRFP260N costs 4.15 GBP at Farnell (including VAT) per piece, while from China (eBay/Aliexpress) it goes for 1 GBP / pcs or less. That is a lot cheaper and therefore more suspicious

author: Pinkus
author_rank: Frequent Contributor
timestamp: July 24, 2015, 09:59:00 am
content: Check Digikey:http://www.digikey.com/product-search/en?keywords=IRFP260N%2025 of them costs USD 2.43 each; if you purchase less it is USD 3.95 each.