300V DC to 24V DC - Page 1
Forum section: electronics / beginners

author: Sam__
author_rank: Regular Contributor
timestamp: February 03, 2013, 03:13:33 pm
content: I was wondering what the best way of getting from mains rectified 330V DC to a usable 24-36V DC?I'm struggling to find step down transformers via the almighty google.thanks in advance.

author: M. András
author_rank: Super Contributor
timestamp: February 03, 2013, 03:34:08 pm
content: just buy the transformer for your line voltage and with a 24v output then add a bridgerectifier plus some caps then you can regulate it from there

author: Sam__
author_rank: Regular Contributor
timestamp: February 03, 2013, 03:49:26 pm
content: Sorry I should have said in OP that this is a purely theoretical question as I just took apart a computer power supply and saw it used 300V DC and was wondering how it got down from that to a lower DC voltage.

author: c4757p
author_rank: Super Contributor
timestamp: February 03, 2013, 03:53:08 pm
content: A custom transformer. Keep in mind you can't just stick 300VDCup a transformer, you'll blow the shit out of it. The computer power supply is an SMPS - it pulses the 300VDC to make AC at a controlled frequency and duty cycle. If you need to convert 240VAC to 24-36VDC just get a normal line transformer and put the rectifierafterit.

author: Sam__
author_rank: Regular Contributor
timestamp: February 03, 2013, 04:04:48 pm
content: Quote from: c4757p on February 03, 2013, 03:53:08 pmI knew it was a switchmode I was just confused as to how it got from 300DC to the lower voltages. Thanks a lot for explaining it. I may have to do a bit more reading on the subject, it's quite interesting.

author: mariush
author_rank: Super Contributor
timestamp: February 03, 2013, 04:12:25 pm
content: A switching power supply rectifies the 220-245v AC voltage to 310-350 (220-240 x 1.41) v DC, sometimes there's an Active PFC circuit that boosts the voltage to about 390-400v DC.Then there's a small controller which generates pulses of this voltage up to 50-120 thousands a second (52kHz - 110kHz) that are sent through the small smps transformer. The pulses are then rectified to DC voltage on the other side.On this secondary side there's another chip which monitors the output voltage of the transformer and depending on how low or high it is, it sends a signal to the primary controller to reduce or increase the number of pulses going into the transformer.So overall it's a very complex design.The simpler and easier design is to just use 220-240v AC and a regular transformer, which however will be much heavier and kinda expensive... If you want smooth 24v DC you get 20-24v AC that you can then rectify to DC and get 24v DC or more. Then you need some big capacitors (one or more 3300-4700uF 35v or 50v capacitors) to smooth out the voltage after it's rectified, and maybe even use a linear regulator to get the voltage really steady at 24v DC.

author: c4757p
author_rank: Super Contributor
timestamp: February 03, 2013, 04:20:19 pm
content: I wouldnotuse a 35V-rated capacitor for that. A 24VAC transformer will put out almost 34VDC, and especially if it's meant for a high load, it will easily soar a good bit higher than that unloaded. A 35V cap gives younosafety margin.

author: mariush
author_rank: Super Contributor
timestamp: February 03, 2013, 04:27:12 pm
content: I said 20-24v AC and 35 or 50v... 20 x 1.41 = 28v ... might peak to about 32v depending on your mains voltage.Also, some transformer manufacturers are nowadays listing the voltage for the secondary as it is after rectification, so if you get one on those that says 24v on secondary, it's actually 24-26v rectified, not 24 v AC.35v rated capacitors would be a bit risky, but if they're made by a good manufacturer, they should be OK...Of course 50v or even 63v would be better and safer, and someone should go for those if you can find them cheap and in stock at some good company like Mouser, newark, digikey etcHere's some transformers that will work to get a 24v DC output or slightly more, just for reference :http://goo.gl/bfnbU(goes to Farnell / Newark - their urls are super long and break down on this forum, so shortening them)Some of the transformers on the link above are 2 x 12v or 2x 15v , you can link the secondaries in series to get 1x24 or 1x30 that you can then rectify... There's also a separate section with toroidal transformers that you might wanna check out - those transformers are better for some types of usage but they're also a bit more expensive.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 03, 2013, 04:30:38 pm
content: A switchmode PSU designed for 230VAC should run fine on 300VDC. Two potential issues are if it has PFC, DC may confuse the PFC circuit, but that can probably be disabled quite easily.The other is the internal fuse will probably not be rated for 300VDC - Fuses for DC are a potential problem as if it blows & arcs, they don;t have the AC zero crossing to extinguish the arc.

author: IanB
author_rank: Super Contributor
timestamp: February 03, 2013, 04:42:06 pm
content: Quote from: mariush on February 03, 2013, 04:27:12 pmGotta watch the peak voltage carefully. Inthis postI show measurements of a "12 V 2 A" transformer and the unloaded output voltage is 15 V. If we assume a 24 V transformer of similar design would behave the same way we would have 30 V AC unloaded. Multiplying that by 1.41 would give 42 V DC, so the 50 V rated smoothing capacitor would definitely be indicated.

author: mariush
author_rank: Super Contributor
timestamp: February 03, 2013, 04:47:27 pm
content: I forgot to mention that a bridge rectifier has about 0.5-0.7v voltage drop per diode, so about 1-1.5v drop on the rectifier.But I agree with you, 35v may be too close for comfort.

author: codeboy2k
author_rank: Super Contributor
timestamp: February 05, 2013, 04:15:16 am
content: Quote from: mikeselectricstuff on February 03, 2013, 04:30:38 pm... and (I'm sure Mike and many others know this already, but I'll put it out there ..) that's why fuses on multimeters are special, and called HRC fuses (high rupture capacity). They are usually filled with sand to fill the gap and extinguish any high voltage DC arc.

author: Psi
author_rank: Super Contributor
timestamp: February 05, 2013, 06:50:28 am
content: Quote from: Sam__ on February 03, 2013, 03:13:33 pmDepends on the current needed.

author: nobody
author_rank: Newbie
timestamp: March 11, 2013, 05:05:46 pm
content: best to use is transformer and some caps