Easiest solution for a 24VAC power supply - Page 1
Forum section: electronics / beginners

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 25, 2014, 11:38:37 pm
content: I need a 24VAC power supply for a project. I salvaged an open frame PS from a printer thinking that would work, but it looks like something is bad on the board. Instead of 5V and 24V outputs, I get 12VDC across the board. So, the search continues. I realize I could buy a printer ac adapter and that would probably work, but it just seems like I should be able to make something work with all the crap I have. I've got loads of stuff for converting AC to DC (Triacs, SCRs, Rectifiers, regulators etc) but I've never tried doing much with AC unless I just used a doorbell transformer or something.SMPSs are beyond me. So, what's the easiest/cheapest way to get my project going? I have a box full of 24VAC motors and fans I'd like to put to use.

author: David_AVD
author_rank: Super Contributor
timestamp: April 25, 2014, 11:59:51 pm
content: If you need 24VAC, a simple transformer is all you need. Switch-mode power supplies are going to be DC on their outputs. You could tap onto the secondary of the switch-mode transformer of course (to get AC), but it will not be 50/60Hz.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 26, 2014, 12:23:32 am
content: An "AC power supply" is simply an old-school iron transformer. What's with "power supply"Are we missing something here? Are you vastly over-thinking this?You didn't mention how much CURRENT you need.Perhaps the "easiest solution" for a 24VAC "power supply" is simply a wall-wart!

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 26, 2014, 01:49:50 am
content: Wouldn't be my first time over-thinking somethingLike I said, SMPSs are beyond meOne advantage of SMPSs is that they transform voltage via a smaller transformer, making them more efficient and cheaper than transformers. It seemed reasonable to assume that some similar method is used for AC as well.Be that as it may... witness me lose my mind. I was inquiring about AC supplies, but I must correct myself... the fans are 24DC. I don't know why I was remembering them as AC. Maybe it was because I found the 24V to be unexpected due to being most familiar with computers and 12 volt systems. In any case, I still have need for 24VAC power, for reasons aside from the fans. If transformers are the only practical solution, then I'll go that route. Wall-warts are usually too small. I need a minimum of 50VA, preferably 100VA.pereczes, thanks for the links. Please, let me know how the boost modules work. Unbelievable how inexpensive that stuff is.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 26, 2014, 02:08:10 am
content: If you care to reveal what you are trying to do here, maybe we can help you.In any case, when you know what you actually need for sure, get back to us.

author: Teledog
author_rank: Regular Contributor
timestamp: April 26, 2014, 02:39:55 am
content: Found lots of 120/240 to 24VAC Xformers in old printer teardowns..A bit of work....

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 26, 2014, 12:52:07 pm
content: Quote from: Richard Crowley on April 26, 2014, 02:08:10 amYou've already helped. While Iamworking on a project at the moment, the intent of the OP was to also obtain general suggestions for the sourcing AC power supplies. DC power supplies sources were inadvertently suggested as well, and coincidentally likewise appreciated. I do know what I "actually need for sure." But, I thank you for your help.

author: theatrus
author_rank: Frequent Contributor
timestamp: April 26, 2014, 05:09:07 pm
content: Wall wart 24vac transformers are readily available as they're used in all home sprinkler valve supplies.

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 26, 2014, 05:16:57 pm
content: The sprinkler wall warts I've seen are below the 50 - 100 VA I'm looking for. Are you aware of any in that range?

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 26, 2014, 05:36:07 pm
content: 24V, 4A Center Tapped (12-0-12) Transformer (96VA)http://www.mpja.com/24V-4A-Center-Tapped-12-0-12-Transformer/productinfo/7845%20TR/

author: SeanB
author_rank: Super Contributor
timestamp: April 26, 2014, 05:44:56 pm
content: Look for DIN rail mounted power supplies, they are available in various power ratings, and you will find easily a 100VA unit. It will need a housing though like a small industrial electrical box, as the terminals are exposed for power in and out. Common on industrial equipment, I even have one sitting on a shelf that is unused, 100VA but with only a 230VAC input. Nice green painted German made transformer.

author: Fank1
author_rank: Regular Contributor
timestamp: April 26, 2014, 09:31:44 pm
content: Most old furnace controls are 24 VAC and the old furnaces will have a transformer in it.Run down to your local junk man or furnace repair man and ask for one.

author: IanB
author_rank: Super Contributor
timestamp: April 26, 2014, 10:53:43 pm
content: Quote from: chipwitch on April 26, 2014, 01:49:50 amIt's very hard to give concrete advice unless you can share what it is you want to supply power to.If 24 V AC means "24 V AC 60 Hz" then a transformer is the only realistic option. While it ispossibleto produce a regular 24 V AC supply with electronics it is not particularly convenient or practical to do so.If the device being powered is not fussy about what you feed it with, then there is the possibility of an electronic transformer. These are increasingly used to power low voltage halogen lighting. But they don't produce a clean 60 Hz voltage.Some devices are not fussy about whether you feed them AC or DC. If you have such a device you could supply it with 24 V DC instead of AC and it would still work.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 26, 2014, 11:19:47 pm
content: It appears that chipwitch is either trolling us or he/she doesn't know what they are doing enough to conduct an inteligent technical discussion.

author: TerminalJack505
author_rank: Super Contributor
timestamp: April 26, 2014, 11:25:55 pm
content: Quote from: Richard Crowley on April 26, 2014, 11:19:47 pm*intelligent<snicker>

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 27, 2014, 02:25:59 pm
content: Quote from: IanB on April 26, 2014, 10:53:43 pmWhat Ishouldhave said is my project indeed requires an AC power supply for reasons other than the fan. THAT issue was adequately addressed with the suggestion of transformers many posts back. And yes, they will work. While I expected there to be an alternative to bulky expensive transformers, apparently there aren't any practical ones. I get that. Thank you. Thanks even to those who have chosen to leave snarkish (read: "dickish") responses because I chose less than ideal words. I wasn't awareperfectionwas a prerequisite for asking questions on this, "BEGINNER" forum.Ian, you have often aided me in many of my questions and it should go without saying my previous remark wasn't directed at you.

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 27, 2014, 02:27:18 pm
content: Quote from: TerminalJack505 on April 26, 2014, 11:25:55 pmAnd, this weeks Glass House award goes to....

author: Witention
author_rank: Contributor
timestamp: April 27, 2014, 02:44:16 pm
content: I've purchased an outdoor LED lamp set - ships with a 24VAC 478Ma power supply - powers 20 "bulbs" each with 5 tiny LED'sAs these are for outside, I've also purchased (from Maplin Electronics - UK - part code N69NC) a 24VAC 2A IP rated external power supply.Obviously when I use this PSU - there is a "flicker" on the LED light string - I kind of forgot that there must be some form of additional conditioning electronics in the original PSU to give the LED's the DC (or appearance of DC!) they need.The manufacturer refuses to help (safety I guess and all that!) - but could someone please recommend something i can put in circuit to stop the flicker ?Many thanks in advance

author: G7PSK
author_rank: Super Contributor
timestamp: April 27, 2014, 05:31:04 pm
content: Try looking for Meanwell power supplies on Ebay. They are 100 to 250 volt to 24 volt SMPS rated at 4 or 5 amps.

author: miguelvp
author_rank: Super Contributor
timestamp: April 27, 2014, 08:16:24 pm
content: Let me get this straight.You first asked for 24 VAC output because of the motors, but then you found out the motors are 24 VDCBut you still need the 24 VAC for other things?If you know what you are doing, You can use a PC power supply and tap into the -12 VDC to 12 VDC to get the 24 VDC you need.For the AC you can tap to the un-rectified (edit) 24Vpp that feeds your positive rail.But be very careful with those power supplies touching the wrong thing, some of those caps in there can shock you pretty bad even if unpluged.And don't leave it on unattended.The safer way would be to use a lab power supply. The more projects you work on the more you'll appreciate it and you can current limit the output so if you make a mistake you can prevent ruining expensive components.

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 27, 2014, 08:54:34 pm
content: Quote from: miguelvp on April 27, 2014, 08:16:24 pmYes, I was wrong about the motors being AC. They are DC. And yes my project requires 24VAC for other parts of the circuit.QuoteThat might work. They're abundant and cheap. I didn't think about tapping the negative 12V. I use one for my bench supply. IIRC, the -12V was pretty low power. Positive 12V, however was high 15A, I believe.QuoteMost of my projects are deployed far from a lab bench, unfortunately. But, thanks for the suggestion tapping the AC rail. I've never actually opened one up to test. I'll look into that.

author: lewis
author_rank: Frequent Contributor
timestamp: April 27, 2014, 09:08:50 pm
content: If you need 24V ACandDC, use a normal iron transformer. Get one for whatever current rating you need. Then use the secondary AC to power your AC things, and rectify and smooth the AC to power your DC things. You'll get about 33V DC once you rectify and smooth it, so you might need a voltage regulator to get 24V DC precisely.This is the 'easiest way to get a power supply' (paraphrasing the thread title).

author: miguelvp
author_rank: Super Contributor
timestamp: April 28, 2014, 12:55:02 am
content: That will give you the positive rail for 12V, minus the diode drop, so around 11V DCBut yeah, you can get the 24 VAC out of a transformer like that directly out of the secondary.Something like this might do.C1, C2 & C3 should be around 200V caps.The transformer is a 120VAC to 48VAC, but this will be very crude.You end up with a positive around 11V rail and a negative one.Before the rectifier you have 24VAC, On the output of any of the secondary terminals respect to ground.Edit: Forget R1 the simulation just needs a resistor to work.Edit2: also on the transformer I used 25uH and 4uH to denote the 5:2 winding (squared Henrys) so it's just the ratio for a 5:2 transformer or a 120V to 48V AC transformer.

author: lewis
author_rank: Frequent Contributor
timestamp: April 28, 2014, 08:43:02 am
content: Does the OP need +/- 12V?

author: miguelvp
author_rank: Super Contributor
timestamp: April 28, 2014, 08:52:21 am
content: nope, just 24 volts, doesn't matter if ground split or not. But your full bridge gives 12 volts after rectification right?I did split mine so he/she can tap on the 24 VAC of any of the secondary outputs and also get the 24 VDC rectified.

author: lewis
author_rank: Frequent Contributor
timestamp: April 28, 2014, 09:06:29 am
content: Quote from: miguelvp on April 28, 2014, 08:52:21 amNope! It will give about 32V DC after rectification. Why did you think it was 12V?

author: miguelvp
author_rank: Super Contributor
timestamp: April 28, 2014, 02:55:43 pm
content: Quote from: lewis on April 28, 2014, 09:06:29 amBecause I'm an idiot and did my math with 120V pp instead of 300Vpp