Drawing too much current from AC to AC wallwart? - Page 1
Forum section: electronics / projects

author: K3mHtH
author_rank: Regular Contributor
timestamp: January 17, 2022, 12:56:51 am
content: Hey everyone,Ayy.. so... I recently shipped about a dozen little widgets that are powered by a 16VAC adapter. The voltage inside gets rectified and turned into a +/-15VDC rail (LM317/LM337). I was measuring the current draw of the actual rails and never bothered to measure the current draw out of the 16VAC adapter until one of the first recipients of my widget said that their wallwart died after 20 minutes.During testing, I left it on for weeks and my wallwart was fine so I never thought I was approaching the limit of the 16VAC adapter. Sure enough after testing it, I noticed I'm drawing 950mA from the wallwart that is rated for 1000mA.Wallwart data sheet:https://www.jameco.com/Jameco/Products/ProdDS/100108.pdfThe wallwart shows a thermal fuse, so I assumed if there ever was a problem that would trip and then return back to useable state after it cooled down, but this was not the case. I'm waiting for the blown wallwart to be returned so I can do an autopsy, but in the meantime I have a few questions:How bad is it that I'm drawing 950mA from a 1000mA rated AC wallwart? I know I'm not leaving a smart margin of error, and the widget definitely needs to be redesigned to lower the current - but I'm curious about the fate of the dozen widgets already out in the field.. maybe the one that blew was just a runt / weakest link? Or maybe I should expect all 12 to eventually give up the ghost?I know, I'm definitely stupid for not measuring this before shipping.

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 17, 2022, 04:21:01 am
content: Since a rectifier doesn't have a very nice power factor, you could be exceeding the VA rating of the wallwart by a lot more than you think. Can you post a schematic of the rectifier / filter cap portion of the circuit and how much current you are drawing from each +/-15VDC rail?

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 17, 2022, 04:41:49 am
content: If it's a very unbalanced load, it could be DC biasing the transformer which would cause it to heat up more than usual. Is the adapter getting hot to the touch?

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 17, 2022, 05:21:25 am
content: I would guess you are using a fullwave voltage doubler type circuit to get +22Vdc and -22Vdc to feed your LM317 & LM337. If it's something like the circuit attached (ignore cap values), all you can really pull would be around 280mA off each rail.

author: TheMG
author_rank: Frequent Contributor
timestamp: January 17, 2022, 05:29:23 am
content: The thermal fuse in those things is non-resettable. Once it's blown that's it, wall wart dead.Also keep in mind high ambient temperature can cause the transformer to overheat sooner. Same thing if the wall wart is buried underneath a pile of junk and the heat has nowhere to go.

author: Zero999
author_rank: Super Contributor
timestamp: January 17, 2022, 10:19:47 am
content: Quote from: TheMG on January 17, 2022, 05:29:23 amGoing from the ones I've opened: some have bimetal strips, which are resettable, others are just one-shot thermal fuses, but one should always assume the latter.Quote from: K3mHtH on January 17, 2022, 12:56:51 amHow much current does the DC side use?Is it balanced?You might be better off using a switched mode 15VDC power supply and a buck-boost to get -15V.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 17, 2022, 11:46:30 am
content: For the current the RMS current is relevant. With the usually poor powerfactor it makes a difference if the meter to measure the AC current is RMS or average reading.With a current drawn quite close to the maximum the supply would run quite hot and will have a higher failure rate.

author: K3mHtH
author_rank: Regular Contributor
timestamp: January 18, 2022, 05:39:29 am
content: Thanks for all the replies so far! Some replies and details:Quote from: Kleinstein on January 17, 2022, 11:46:30 amI'm using a Fluke 177 true RMS meter, so the AC current measurement should be accurate right?Quote from: Zero999 on January 17, 2022, 10:19:47 amThe +15V rail uses 275mAThe -15V rail uses 236mAQuote from: Kim Christensen on January 17, 2022, 05:21:25 amHi Kim, yup that is the circuit exactly.. but I think it's called a half-wave rectifier. And yup! I'm pulling dangerously close to 280mA from each rail (275mA on the positive side).Since I'm pulling 95% of the recommended current from these wallwarts and would love to be below 80% as a safety factor. I can't really see how that would be possible with the circuit.. I've tried a few SMPS solutions but have had noise issues - I will revisit those. But honestly at this point, I might just switch to a 16VAC 2000mA wallwart - which I found for North America, and will look into MOQ's for getting European versions made as well.

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 18, 2022, 06:25:37 am
content: QuoteYes, it is a halfwave for the positive supply and another half wave for the negative supply. Together, they load the transformer just like a full wave would.QuoteThat's the simplest to implement and probably the best solution at this point.

author: Zero999
author_rank: Super Contributor
timestamp: January 18, 2022, 11:20:27 am
content: Quote from: K3mHtH on January 18, 2022, 05:39:29 amI wouldn't worry about the current being 95% of the PSU's rating, which will result in 90% of the copper losses, compared to full load, as P = I2R. 41mA of DC in the transformer, is more of a problem.I'd advise using a 30VDC PSU and a rail splitter, such as the TLE2426.

author: magic
author_rank: Super Contributor
timestamp: January 18, 2022, 02:13:22 pm
content: Why would you do that?Dual half wave rectification is efficient when load isn't equal on the rails. Not sure if TLE2426 has thermal shutdown, it could probably cook itself on ±15V if something goes wrong and it could run hot even if everything is right.The only problem is that the rails don't come up simultaneously.

author: K3mHtH
author_rank: Regular Contributor
timestamp: January 18, 2022, 05:44:10 pm
content: Here's a scope shot of the 16VAC going into the instrument under full load. Interesting that there's some spikes on the direction changes.

author: Zero999
author_rank: Super Contributor
timestamp: January 18, 2022, 07:40:33 pm
content: Quote from: magic on January 18, 2022, 02:13:22 pmAdding a rail splitter and using a 30V PSU, would avoid DC in the transformer, which is the downside of dual half-wave rectification, when the output currents aren't balanced.The data sheet for the TLE2426 says it's short circuit proof, but it's only specified to 20mA, so I agree, it's not beefy enough. I suppose a couple of pass transistors could be added, to boost the output current a bit.

author: magic
author_rank: Super Contributor
timestamp: January 18, 2022, 08:16:30 pm
content: Quote from: Zero999 on January 18, 2022, 07:40:33 pmIs it an issue though?DC voltage on the primary is a known problem as it affects the magnetizing current.But is there any problem with asymmetric load current?I suppose mains output impedance is low enough that it doesn't cause much DC voltage to develop at power levels we are talking about here.

author: Zero999
author_rank: Super Contributor
timestamp: January 18, 2022, 09:34:08 pm
content: Quote from: magic on January 18, 2022, 08:16:30 pmYes, DC load current has a similar effect as DC voltage on the primary. It biases the core at DC, making saturation more likely, especially if it's run close to the maximum current, as is the case in the original poster's application.

author: K3mHtH
author_rank: Regular Contributor
timestamp: January 19, 2022, 06:35:15 pm
content: Quote from: Zero999 on January 18, 2022, 09:34:08 pmWould I see this on the scope shot I posted? It seems very symmetrical..Regardless, I have found some 2000mA wallwarts to replace the 1000mA one I've been using.. so I think I will switch to that and be happily at 50% of the rated current.

author: Zero999
author_rank: Super Contributor
timestamp: January 20, 2022, 09:46:45 am
content: Quote from: K3mHtH on January 19, 2022, 06:35:15 pmThat doesn't show the magnetic flux inside the core, just the secondary voltage.QuoteThat's much safer.