How to buy a 9V 3A AC Adapter - Page 1
Forum section: electronics / beginners

author: bobcat2000
author_rank: Regular Contributor
timestamp: September 20, 2021, 02:54:06 am
content: Hello All,I am looking for a 9V 3A ac adapter. Reading reviews from Amazon by itself is an exercise.I see 2 of these. Is it bigger the better?Thanks!

author: ledtester
author_rank: Super Contributor
timestamp: September 20, 2021, 03:05:15 am
content: What are you going to use it for?The second one has a display and a knob for adjusting the voltage which is convenient but it also means the voltage could be changed if it gets accidentally bumped.The first one is better for an unattended, permanent installation - the second one would be better as a power supply for testing things out.

author: bob91343
author_rank: Super Contributor
timestamp: September 20, 2021, 03:07:15 am
content: I am very suspicious that these have poor, if any, regulation. You would be better off with an old CB 3 A power supply with an added regulator.Plus, I never buy from Amazon. Prices too high. Try ebay or Aliexpress. Maybe All Electronics has what you want.

author: bdunham7
author_rank: Super Contributor
timestamp: September 20, 2021, 03:14:54 am
content: Those are DC adapters. Which is it that you want and what will it be powering? (it all matters quite a bit)

author: rsjsouza
author_rank: Super Contributor
timestamp: September 20, 2021, 03:20:31 am
content: These adjustable models might be very sketchy when operating at their extreme capacity.I would trust a fixed output voltage from a good brand - for example, this 30WCUI SDI36-9-UD-P5is one of them.

author: mariush
author_rank: Super Contributor
timestamp: September 20, 2021, 04:32:59 am
content: Do you actually need AC output or DC output ? It's two totally different things.You can buy an adapter rated for higher current than 3A, that value is just the maximum the adapter can provide.Distributors of electronic components like Digikey Mouser Newark/Farnell have tons of such adaptersFor example, here's Digikey's selection of 9vDCadapters with current rating equal or higher than 3A (up to 100w, around 11a) :https://www.digikey.com/short/rvtf20vf

author: james_s
author_rank: Super Contributor
timestamp: September 20, 2021, 06:00:33 am
content: The term "AC adapter" is commonly used to mean any power adapter that takes in line voltage AC and outputs some lower, isolated voltage, whether AC or DC.I would look at Mouser or Digikey instead of Amazon. Real electronics suppliers tend to carry better quality stuff like this, with more reliable specs.

author: bobcat2000
author_rank: Regular Contributor
timestamp: September 20, 2021, 08:07:15 am
content: Thanks guys! I can see that there are many options.I need one for a dehumidifier that comes with a 9V 2.5A ac adapter. The cap inside bursted. It was running 24/7. Maybe that was why. The power brick ran quite hot drawing 2+ amp.The cost is the primary factor. Buying from Mouser may not be possible. The ac adapter is like $20 plus another $10 for shipping. That is $30+ expense. I can save the shipping if I add up $25 buck from Amazon.I also agree that the one with a dial is not idea.Amazon also has a bigger one with the selector. Is a longer one better than the small one?The dial or the selector is just there for changing the resistance to the switching circuit. Correct?

author: Gyro
author_rank: Super Contributor
timestamp: September 20, 2021, 09:01:21 am
content: Quote from: bobcat2000 on September 20, 2021, 08:07:15 amOk that clarifies - you want a 9VDCoutput mains adapter with a minimum output current of 2.5A.I would definitely go for one without a voltage selector for the reasons previously discussed. (the knob adjusts regulated output voltage, not resistance btw). I would also go for a rectangular laptop style rater than a 'wall wart' due to the weight - particularly with US outlets.If you want it to run cooler and with longer life then there's no harm in going for one with a higher current capability, for instance, 5A.P.S. Double check that the output connector polarity is correct (centre +ve etc). It's easy to screw up with those multi-plug kits.

author: ledtester
author_rank: Super Contributor
timestamp: September 20, 2021, 11:00:51 am
content: Quote from: bobcat2000 on September 20, 2021, 08:07:15 amIt is likely the adapter uses a switch-mode power supply inside and the dial/selector changes a feedback resistor to adjust the output voltage.

author: bob91343
author_rank: Super Contributor
timestamp: September 20, 2021, 05:30:57 pm
content: If the original one has a bad capacitor, why not do a repair job?

author: bobcat2000
author_rank: Regular Contributor
timestamp: September 20, 2021, 06:40:29 pm
content: I thought about fixing it. But that will be another $10 to buy a bag of cap. The AC adapter looks well made. The board looks burned front and back though.I would rather spend the money to just buy a new one for like $40 if I had to spend $10 here and there. Driving to the place to buy a parts will cost a lot of gas. The cost will add up.

author: james_s
author_rank: Super Contributor
timestamp: September 20, 2021, 07:14:31 pm
content: How about this?https://www.ebay.com/itm/164138851093?hash=item2637701b15:g:txkAAOSwN~Vee~rvDo check the output connector, most are center-positive but some are center negative, there is no standard and if you get it wrong it will usually damage your device.

author: Gyro
author_rank: Super Contributor
timestamp: September 20, 2021, 07:17:14 pm
content: Yeah, that's been running too hot, the board is delaminating. Caps wouldn't fix that.

author: james_s
author_rank: Super Contributor
timestamp: September 20, 2021, 07:24:27 pm
content: It may have been running hot due to failing caps, which of course will fail faster when it's hot. Replacing the caps probably would fix it, but whether it's worth it is certainly debatable.

author: bobcat2000
author_rank: Regular Contributor
timestamp: September 20, 2021, 07:56:11 pm
content: Come to think about this. Those dehumidifiers use Peltier chip inside. Can I just use a 12V 3A AC adapter instead of the 9V one that come with the unit?I have a bunch of 12V 3A adapters.Thanks!

author: james_s
author_rank: Super Contributor
timestamp: September 20, 2021, 10:05:01 pm
content: Maybe.The higher voltage will cause it to draw more power, so the cold side will get colder and may ice up, more importantly the hot side will get hotter. If it were mine and it was not some kind of valuable heirloom I would try it while monitoring carefully.

author: perieanuo
author_rank: Frequent Contributor
timestamp: September 21, 2021, 08:06:53 am
content: Quote from: bobcat2000 on September 20, 2021, 02:54:06 amhi, i guess you mean DC adapternot bigger, heavier is better. buy second one

author: Zero999
author_rank: Super Contributor
timestamp: September 21, 2021, 08:07:06 am
content: Quote from: bobcat2000 on September 20, 2021, 07:56:11 pmAssuming it's just a resistive load, the power will go up by a factor of nealry 1.8, as P = V2/R.How often do you use the dehumidifier? Is it on continiously? If so, you probably won't like my suggestion, but your best bet is to toss it and buy a more expensive compressor based one. Peltier coolers are cheap, but very inefficent and cost a lot to run, for the same cooling power, compared to a compressor refrigeration unit. If you use it a lot, you'll save money in the long run, by investing in a more efficient dehumidifier.

author: bobcat2000
author_rank: Regular Contributor
timestamp: September 22, 2021, 05:13:25 am
content: That was what I was wondering. I run the thing 24/7. It draws 9V 2.5A. So, it is about 23W/h. 23W/h x 24h x 30 days. = 17KW/month.Is my calculation correct?

author: james_s
author_rank: Super Contributor
timestamp: September 22, 2021, 05:51:16 am
content: KWh per month, but otherwise yes.

author: bobcat2000
author_rank: Regular Contributor
timestamp: September 22, 2021, 05:58:02 am
content: Hmmm... The compressor types use close to 800W/h. Not cheap. I think I will continue to look for a 9V 3A adapter instead. Cheaper.

author: bdunham7
author_rank: Super Contributor
timestamp: September 22, 2021, 05:30:34 pm
content: Quote from: bobcat2000 on September 22, 2021, 05:58:02 amHow much water does it extract from the air in a day?

author: james_s
author_rank: Super Contributor
timestamp: September 22, 2021, 05:53:52 pm
content: They don't have to be run continuously. If you look at it on a basis of energy consumed per quantity of water removed, the compressor type is much more efficient.

author: rsjsouza
author_rank: Super Contributor
timestamp: September 22, 2021, 07:29:58 pm
content: Indeed. I have a few small Peltier models and have been considering changing to a larger one. You run the smaller ones 24/7 and the bigger one for one hour or so. I had a borrowed one and it extracts much more water, which most probably saves you energy in the long run.

author: BrokenYugo
author_rank: Super Contributor
timestamp: September 22, 2021, 08:57:23 pm
content: There's a reason you only see peltier cooling in small applications, even the bigger powered camping coolers use a compressor, a rare case where the solid state solution is not the efficient one.Cheapest path to good safe AC adapters is picking through the box full of them you'll find at better thrift/junk shops, no name chinesium stuff should be avoided due to rampant exaggerations of capacity and electrical safety violations.

author: Zero999
author_rank: Super Contributor
timestamp: September 22, 2021, 09:34:07 pm
content: Quote from: bobcat2000 on September 22, 2021, 05:58:02 amPresumably you worked that out by looking at the power rating, on the label? If so, that's completely wrong, because the power rating, is the maximum. It will only turn on when the humidity increases beyond a certain set point, so whilst your Peltier dehumidifier is running 24/7, the compressor based one only needs to be on for a minute or so per hour, to extract the same amount of water.

author: bobcat2000
author_rank: Regular Contributor
timestamp: September 26, 2021, 12:29:40 am
content: Good point. The compressor models will not run 24/7, and they will get more water out of the air simply because they have a fan to push the air through the coil (I think). The peltier models only have a fan to cool the heat side of the chip. The cold side is just there doing nothing.A cheap one is like $160 which may last for 5 years. The 9V 3A AC adapter is like $20 which is about 10% of the total cost. Maybe not worth it.

author: Zero999
author_rank: Super Contributor
timestamp: September 26, 2021, 09:35:07 am
content: Quote from: bobcat2000 on September 26, 2021, 12:29:40 amNo, it's got nothing to do with fans. The thermodynamic efficiency of compressor based cooling is much higher, than a Peltier device. If there was no difference in efficiency, then Peltier coolers would be widely used in air-conditioners and refrigerators, but they're not because they're horribly inefficient.I'm not going to tell you whether it's worth it or not, just that just that you need to consider the: cost of energy and how long the device will last for, as well as the upfront cost.In my personal experience, I've found Peltier refrigerators to be less reliable, than compressor based units, but that's probably because they tend to be the cheaper end of the market, so built down to a price, rather than the technology itself.