Convert 24AC to 12VAC/DC 1.5A  - Page 1
Forum section: electronics / projects

author: connectTek
author_rank: Regular Contributor
timestamp: March 26, 2020, 09:52:46 am
content: I need to design a CCT that converts 24ac to 12 VAC/DC, at 1.5amps, must be efficient and very small, approx 30mm X 30mm x 20mm h.Any ideas where to start?Cheers

author: Ian.M
author_rank: Super Contributor
timestamp: March 26, 2020, 11:06:14 am
content: As your application is so space constrained, start by getting 5+ years experience with switched mode PSU design and active power factor correction! Then you *MAY* be able to design a high frequency*buck converter that's good for 1.5A and is still small enough to leave space for a bridge rectifier and enough volume for large enough capacitors to ride through the intervals around the AC zero crossings where the input voltage is under 12V.Try using LTspice to sim a 24V RMS AC supply feeding a bridge rectifier, reservoir capacitor andconstant power loadto get an idea of how much capacitance you need at the input of the buck converter to keep its input voltage high enough to ride through the zero crossings. Assume you can get 80% efficiency out of the buck converter, set the load power in the sim to 22.5 watts (12V*1.5A/0.8 ).Edit: I've simmed it and the minimum cap is somewhere around 270uF, but its going to be a PITA finding one that's small enough for the rest of what you've got to cram in there, with enough ripple current rating and high enough temperature and reliability ratings. A larger (value) cap eases the buck converter design requirements by reducing its input voltage range, and a physically larger cap will run cooler and (like for like) be more reliable, so its a trade-off between volume for the cap and volume for the buck converter.* Less fussy, easier to work with lower frequency buck converters need larger magnetics and output capacitors for the same load current.

author: Zero999
author_rank: Super Contributor
timestamp: March 26, 2020, 01:17:33 pm
content: It should be doable. I wouldn't bother designing the SMPS from scratch.I'd go for a DC:DC converter module with a bridge rectifier and smoothing capacitor on the input. I did a quick simulation in LTSpcie, with a 20W constant power load, based on an efficiency of 90% for the DC:DC. Assuming the DC:DC converter works down to 16V, a 680µF capacitor will do: 470µF is adequate but we have to take into account ageing. Here's a link to one which is only 13mm*20mm and is rated to 50V.https://uk.rs-online.com/web/p/aluminium-capacitors/7111481/The DC:DC converter can also be tiny. Here's one which is only 17.5mm×11.5mm×9mm and rectifier diodes are quite small. Although it's only rated to 36V, so would be marginal for 24VAC input, assuming the voltage can be a bit higher than that.https://docs.rs-online.com/fc3c/A700000006631878.pdfhttps://www.mouser.co.uk/datasheet/2/348/rf201l2s-1017114.pdf

author: Ian.M
author_rank: Super Contributor
timestamp: March 26, 2020, 04:53:19 pm
content: Don't forget derating for temperature - that module would be right on the bleeding edge at 1.5A load current and high input voltage, and wouldn't even stand running at 70 deg C, so it would need *GOOD* cooling. Also, your choice of cap is inadequate as its only good for 770mA ripple current (@105 deg C) and even applying its temperature multiplier @70 deg C of 1.65, that only gets you 1.27A RMS, which is less than the simmed ripple current of 1.36A RMS. It will run hot, and the hotter it runs the worse its ripple current rating.

author: connectTek
author_rank: Regular Contributor
timestamp: March 26, 2020, 08:06:55 pm
content: Thanks guys.You've given me enough info to start.Thank again

author: Ian.M
author_rank: Super Contributor
timestamp: March 26, 2020, 08:41:36 pm
content: Don't forget to allow for the transformer regulation factor, annd also mains supply ('line') variation, which will affect the peak voltage seen by the buck converter. e.g. if the transformer has 15% regulation and is at low load current, its output voltage will be up to 15% higher than at full load current. This compounds with line variation so at a high line limit of +10%, you could get as much as 43V peak. Its going to be tougher than I'd like to find an off-the-shelf buck module that can handle that with a wide enough input voltage range.Low line obviously affects the peak voltage the reservoir capacitor charges to, which means it needs to be larger to store enough energy to ride through the zero crossings. Sim it at Vin 10% low, with worst case minimum capacitance (typ. -20% for a good grade of capacitor, and if its wet electrolytic, allow for further loss of capacitance due to ageing), and be prepared for an unpleasant surprise. To satisfy the ripple current requirement in limited volume, it may be easier to split up the reservoir capacitance between two (or more) capacitors.

author: Zero999
author_rank: Super Contributor
timestamp: March 26, 2020, 11:22:56 pm
content: Quote from: Ian.M on March 26, 2020, 04:53:19 pmApologies for not adding a disclaimer stating it was not a finalised design, but a rough sketch to prove it's doable.Of course the DC:DC converter I suggested will not do, but I'm sure there will be ones with a 45V input rating available. Failing that you'll be forced to roll your own, but there are plenty of ICs available, which can work up to 60V.Low ESR capacitors will be larger and it might make more sense to use to lower value capacitors in parallel, rather than one larger one to more efficiently fit the space available.

author: Biduleohm
author_rank: Contributor
timestamp: March 26, 2020, 11:53:01 pm
content: Quote from: Zero999 on March 26, 2020, 11:22:56 pmThere's afew of thembut the budget might not like it too much.25*25 mm should leave enough space for the brige rectifier and a few caps.

author: Zero999
author_rank: Super Contributor
timestamp: March 27, 2020, 10:08:46 am
content: Quote from: Biduleohm on March 26, 2020, 11:53:01 pmThose are also isolated, which is good but might not be needed.I had a quick look through Digi-Key to find something a bit cheaper, relaxing the maximum input voltage a bit and found the LMZ14202HTZ, which would require reflow soldering on a PCB, but it has a built-in shielded inductor. All that's required is a few resistors, capacitors and the bridge rectifier in this case.http://www.ti.com/lit/ds/symlink/lmz14202h.pdfhttps://www.digikey.com/product-detail/en/texas-instruments/LMZ14202HTZ-NOPB/LMZ14202HTZ-NOPBCT-ND/2533815Capacitor wise, I couldn't find 680µF 50V with a high enough ripple rating, which is small enough, so use two smaller ones in parallel.https://www.digikey.com/product-detail/en/nichicon/UPW1H391MHD/493-1910-ND/589651

author: connectTek
author_rank: Regular Contributor
timestamp: March 27, 2020, 10:25:25 am
content: Thanks again guys.Just a thought, can I convert the 24vac to 12vac using a small toroid transformer ( hand made)?Regulation is not to important

author: Ian.M
author_rank: Super Contributor
timestamp: March 27, 2020, 10:50:21 am
content: Yes, the LMZ14202HTZ should work as long as the 24V AC supply's total regulation + tolerance is better than 15%, and if its close to 15% use silicon diodes for the bridge rather than Schottky ones. However its going to be right on the edge with high line & light load.I doubt you could make a 50 or 60Hz 24V:12V transformer to fit the volume limit that can handle the current, even without the rest of the regulator circuit. Also 12V RMS will need a *MUCH* larger reservoir capacitor to ride through the zero crossings. An autotransformer to buck the AC by only 2V - 3V to get enough safety margin for the DC-DC converter would be smaller, but I'd be surprised if its small enough.

author: Zero999
author_rank: Super Contributor
timestamp: March 28, 2020, 12:02:23 pm
content: If regulation is unimportant and you're fine with winding your own transformer, then how about a Royer converter? Look up compact fluorescent or electronic transformer circuit and modify it to work of 24VAC, rather than mains. A rectifier and smoothing inductor/capacitor will need to be added to the secondary to convert it to DC. Use Schottky diodes for low voltage loss and fast switching.http://educypedia.karadimov.info/library/AN00048.pdfhttps://www.st.com/resource/en/application_note/cd00003902-electronic-transformer-for-a-12v-halogen-lamp-stmicroelectronics.pdfSuch a circuit can also be used to reduce the voltage to a suitable level for more common buck regulators, as well as providing isolation. See figure 15, page 12 of the TC33167 data sheet for an example.http://www.ic72.com/pdf_file/t/85248.pdf

author: mikerj
author_rank: Super Contributor
timestamp: March 28, 2020, 01:12:49 pm
content: Quote from: connectTek on March 27, 2020, 10:25:25 amDepends on how much current you want to draw from it. If the 1.5A applies to the 12VAC supply (i.e. 18VA), then the volume of the transformer will be larger than the space you have.

author: connectTek
author_rank: Regular Contributor
timestamp: March 28, 2020, 06:58:35 pm
content: Found these.https://www.avebiej.com/dm03-dcdc-855v-to-5v-12v-2a-power-supply-converter-step-down-module-batch-p-338454.htmWith a smd bridge rectifier , should be perfect.Thanks again guys for all your advice.Much appreciated for your time.Cheers

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 28, 2020, 10:05:26 pm
content: Exactly what is the load that can accept 12V AC or DC?

author: connectTek
author_rank: Regular Contributor
timestamp: March 29, 2020, 11:53:51 am
content: Either AC or DCGarden lights 24AC installed, Large property.GU5.3 LED Globes 12VAC 10W but I have tested one on DC , no problems.Customer wants to use this particular Globe as he has lots of them in storage.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 29, 2020, 01:16:48 pm
content: Tear one down. If it just uses resistive current limiting, two of them in series will run just fine on 24V.

author: Zero999
author_rank: Super Contributor
timestamp: March 29, 2020, 01:37:31 pm
content: Quote from: NiHaoMike on March 29, 2020, 01:16:48 pmI doubt it will work, the common circuit is a switched mode regulator and even if it does, it's a bad idea, because someone might change one of the lamps later on.Quote from: connectTek on March 29, 2020, 11:53:51 amIf he's replacing them all, then it would be cheaper to just replace the 24VAC transformer, with a 12VAC one, than faffing around with converting 24VAC to 12V.