BK 8500 DC load - Erratic output - Page 1
Forum section: electronics / testgear

author: MarkL
author_rank: Unknown
timestamp: August 06, 2013, 05:14:19 pm
content: In doing some power supply testing, I've been having trouble using the BK 8500 to switch between two values in CR (constant resistance) mode. Once every few seconds, the unit appears to get the set values completely wrong, ranging from a much higher resistance, to what appears to be a short.To simplify things, I connected it to a 12V SLA battery and watched it with a current probe. I'm using fast ADC mode and transient switching to go between 20 and 25 ohms, each for 20ms.Most of the time it's a nice even square wave as it should be, but sometimes it loses its mind. See a couple of examples attached below.Slow ADC mode (the default) or a slower transient rate just makes it happen less often. Constant power mode also has a similar behavior, which isn't surprising since it probably has to do a similar calculation to determine the setpoint. Interestingly, under external computer control in CR mode, it doesn't happen at all. It looks to me like an internal race condition with transient mode.I've been going back and forth with BK Precision technical support for 3 months and this is my second unit. Their final answer yesterday is that if it's not working for your application, return it to your retailer. I was actually hoping they'd be interested in fixing it, or at least figuring out what's going on.Before I return it, can anybody else confirm if they do or don't see this on their BK 8500? I have FW v1.84, if it matters.Any other known good alternatives to the 8500? Maybe from Atten or Maynuo? (And maybe with better technical support?)

author: olsenn
author_rank: Frequent Contributor
timestamp: August 06, 2013, 05:21:06 pm
content: Funny; I just wrote a post on a similar issue earlier today with my Itech 8511 (which is the exact same machine as the B&K 8500). My issue is that if I operate in CV or CP mode and the source at any point in time becomes incapable of supplying the desired voltage or power, the device hangs, and once the conditions of the source are restored, the 8511 doesn't fix itself. I have to switch modes and re-enable the output to get it working again.I hadn't even noticed this problem until now because I usually only use CC mode. If they give you a new firmware, please make sure to post it here

author: Electro Fan
author_rank: Super Contributor
timestamp: August 06, 2013, 05:43:10 pm
content: Quote from: olsenn on August 06, 2013, 05:21:06 pmJust curious - can any Maynuo users take a look-see to determine if the Maynuo load behaves similarly or if maybe it is enough of a cousin rather than a direct brother/sister that perhaps the Maynuo doesn't exhibit such behavior? Thanks, EF

author: LaurenceW
author_rank: Frequent Contributor
timestamp: August 06, 2013, 06:36:28 pm
content: Hello MarkL, ElectroFan brings this post to my attention!Sorry to hear that you are having difficulties with your eLoad. I have a Maynuo 9712B (500V, 15A, 300W), and am happy to try and repeat your test and report back.However, from your description thus far, I am unable to figure out how to switch rapidly between tworesistances. What is this "Fast ADC mode"? My Maynuo has a "Transient" mode, which allows me to switch between twocurrents, but not resistances. I can find a sort-of way of doing this with the PC software, but of course that runs quite slowly. Also, driving the load from the PC software and altering the load between 20R and 25R appears to cause the load no problems that I can see on my 'scope.Certainly, nothing else that I've seen over several months with my Maynuo has ever casued it to lock up - but then, i haven't checked EVERY function.Can you give us a bit more info oin the settings? It may be there are more changes betwen the eLoad models that we thought.But an eLoad which even transiently presents a short circuit to the attached power supply is not good for business!

author: MarkL
author_rank: Unknown
timestamp: August 06, 2013, 07:35:43 pm
content: Quote from: LaurenceW on August 06, 2013, 06:36:28 pmAfter looking at the Maynuo manual, it doesn't say it supports a transient CR mode. The BK explicitly does support it. On the BK you would select CR mode, and then set up the resistance values and timing with the S-Tran button."Fast ADC mode" is another item that seems to be only on the BK unit. It controls the ADC update rate, which presumably is how often the output is measured and then adjusted to match the setpoint. The BK manual lists it but doesn't have any detail about it. I mentioned it because I changed it from the default and it does effect how often the glitches occur.Thanks for at least trying the manual CR mode on the Maynuo.Quote from: LaurenceW on August 06, 2013, 06:36:28 pmYes, quite. That's how we stumbled across this. There was a huge current spike coming out of our power supply during testing and it was going into current limiting mode. Fortunately, nothing died, but I would have liked to do short-circuit testing when we were ready for it.

author: MarkL
author_rank: Unknown
timestamp: August 06, 2013, 07:48:31 pm
content: Quote from: olsenn on August 06, 2013, 05:21:06 pmWell, at least I can say the BK 8500 hasn't locked up.But, I wouldn't hold your breath for any firmware updates from BK. Their engineering group hasn't even acknowledged this as a problem after >3 months. I was hoping someone here could confirm (or not) that they were also seeing this.It seems unlikely, but if no one else has this problem I could also ask for a third unit instead of returning it for a refund. Apart from the transient CR and CP issues, everything else seems to be ok.

author: Electro Fan
author_rank: Super Contributor
timestamp: August 06, 2013, 07:57:07 pm
content: What is the effective difference between this:MarkL: "I'm using fast ADC mode and transient switching to go between 20 and 25 ohms, each for 20ms."and this:LaurenceW: "driving the load from the PC software and altering the load between 20R and 25R appears to cause the load no problems that I can see on my 'scope."Seems like the ADC method is using software on the B&K and the the 2nd approach is using Mayuno software on the PC, but either way the resistance is being toggled.Thanks, EF

author: MarkL
author_rank: Unknown
timestamp: August 06, 2013, 08:07:31 pm
content: Quote from: Electro Fan on August 06, 2013, 07:57:07 pmRight. One toggling method is internal to the instrument, the other is done externally on a PC."ADC mode" is just a tweak setting on the BK. Internal vs. external transient control is the important difference. Internal control can also toggle a lot faster than banging the two settings over the external serial interface.

author: Electro Fan
author_rank: Super Contributor
timestamp: August 06, 2013, 08:28:44 pm
content: Quote from: MarkL on August 06, 2013, 08:07:31 pmOk, so it might be what you first suggested:"It looks to me like an internal race condition with transient mode."I'm guessing that the notion of an Electronic Load is maybe not real new (anyone know when the first ELs showed up?) but as with all of the newest test equipment (power supplies, etc, etc.) as the manufacturers put both more monitoring and more programmable controls (both internally and externally via a PC) into the design there are more opportunities for the equipment to behave in a way that the designers didn't fully anticipate, and hence the race conditions. This would in turn indicate that EEVers (and other "early adopters") who continually push forward with new use cases might want to pick their gear from manufactuers who are open to input and responsive. I'm increasingly of the opinion that the geograhic location of the manufactuer is not necessarily as important as their organizational commitment to fielding the input and responding to the input. It will be interesting to watch how the many manufacturers respond (or don't) to these types of issues.In most competitive industries the first player out of the gate with something innovative can enjoy a first mover advantage but other players can learn from the first mover and respond with something comparable or better.I can say that without a doubt if I was a product manager at any test equipment company on the planet I would be watching Dave's videos and reading this forum for product ideas and ways to win customer loyalty.